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United States Department of Agriculture

Agricultural Research Service

Methyl Bromide
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Background
 

Environmental Concerns

MeBr molecule In 1991, MeBr was identified as a potential ozone depleting compound (Chakrabarti and Bell, 1993). In 1992, on the Fourth Conference of the Parties to the Montreal Protocol, MeBr was officially added to the list of ozone depleting chemicals, with its production suggested to be frozen at the 1991 level, effective from 1995.
The inclusion of MeBr in the ozone-depleting chemicals list naturally brought this fumigant within the scope of the U.S. Environmental Protection Agency's (EPA) Clean Air Act, which has an amendment that mandates discontinuation of any chemical with an ozone depletion potential (ODP) greater than 0.2 at the beginning of 2001. The ODP index for MeBr was determined to be 0.60-70 in 1992 (UNEP, 1995); the ODP estimate was reduced to 0.4 in 1998. In March 1993, EPA announced that MeBr was scheduled for a phase out in the United States by the year 2001 (USEPA, 1993). The phase out date was later changed to the year 2005 (USEPA, 2000).
 
Over the last decade, there has been an increased research effort devoted to understanding of the effects of halogenated gases emitted into the atmosphere on the depletion of the stratospheric ozone layer. According to the Ozone Assessment Synthesis Panel of the United Nations Environmental Programme (UNEP), the hole in the Antarctic ozone layer is due primarily to increases in chlorine- and bromine-containing chemicals in the atmosphere. Even though most of the ozone loss is due to chlorinated compounds (90-95%, Watson et al., 1992), attention has been focused more recently on MeBr because stratospheric bromine is believed to be 40 times more efficient than chlorine in breaking down ozone on a per atom basis (Wofsy et al., 1975). Although the largest effects from ozone-depleting gases have been observed in the southern hemisphere, there are indications that atmospheric ozone is also decreasing in the northern hemisphere.
 
There is a great deal of uncertainty in estimates of the global MeBr budget. In the early 1990s, the ocean was viewed as a net source of MeBr. More recent global balances account for larger sinks than sources (Yvon-Lewis and Butler, 1997), with the ocean acting as a net sink of MeBr, the magnitude of which is being refined (Lobert et al., 1997; King et al., 2000). Soil fumigation is thought to contribute 32 Gg y-1 of MeBr to the atmosphere (~20% of the total MeBr source) (Yvon-Lewis and Butler, 1997). The oceans represent the largest known source of atmospheric MeBr, followed by fumigation (Butler, 2000). Other natural sources of atmospheric MeBr include biomass burning and production by plants, salt marshes, and fungi (Butler, 2000). In recent global budgets, only 60% of the MeBr sinks were accounted for by the quantified source terms and the "missing source" outweighed all other sources in the budget (Butler, 2000). Agricultural use of MeBr, including soil fumigation, may be responsible for 3-10% of stratospheric ozone depletion (USDA, 2001). The relative significance of each global source of MeBr, including that from agricultural uses, needs to be better quantified to assist in developing rational national and worldwide policy.
 

Economic Concerns

An economic assessment conducted by the U.S. Department of Agriculture indicated that the phase out of MeBr as a fumigant will have a substantial impact on many commodities because currently alternative control practices are either less effective or more expensive than MeBr (NAPIAP, 1993; Ferguson and Padula, 1994). A recent estimate of the annual economic loss to U.S. producers and consumers resulting from a ban of agricultural uses of MeBr is $500 million (Carpenter et al., 2000). In addition, at present there does not exist a single chemical alternative which can completely replace MeBr (Anderson and Lee-Bapty, 1992; Duafala, 1996). Under these circumstances, MeBr has become the topic of many heated discussions, and the 'methyl bromide issue' has received widespread attention (Anonymous, 1994; Noling and Becker, 1994; Taylor, 1994; Sauvegrain, 1995; Butler, 1995, 1996; Duafala, 1996; Thomas, 1996). The many unanswered questions have also stimulated extensive research on the environmental fate of MeBr, particularly on estimating the relative contribution from each source, obtaining accurate estimates for volatilization losses from fumigated fields, understanding the processes and factors that affect the volatilization, and identifying and developing emission-reduction techniques.
 
The USDA National Agricultural Pesticide Impact Assessment Program conducted one of the first assessments of the economic impact of eliminating MeBr (NAPIAP, 1993). This assessment determined that there would be a substantial adverse impact on the agricultural community and would be most strongly felt in two states, California and Florida, the primary users of MeBr. It was estimated that a MeBr phase-out for pre-plant soil fumigation would cause $1.5 billion dollars in annual lost production in the United States. This estimate ignored post-harvest, non-quarantine uses and quarantine treatments of imports and other future economic aspects such as lost jobs, markets, etc. The report predicted that the major crop losses would occur with tomatoes ($350 M), ornamentals ($170M), tobacco ($130M), peppers ($130M), strawberries ($110M) and forest seedlings ($35M).
 
More recently, the National Center for Food and Agricultural Policy (NCFAP) conducted an assessment of the economic implications of the methyl bromide ban (Carpenter et al., 2000). The NCFAP estimates a smaller economic loss of $479M to producers and consumers with the ban of preplant uses of methyl bromide. These losses are due to decreases in yield with use of alternative pest control strategies, increased production costs, changes in the marketing window in response to supply and demand, etc. The NCFAP estimates that losses of $235M may occur in annual crops (tomatoes, strawberries, peppers, etc.), $143M in perennials (orchards and grapes), and $101M in nurseries and ornamentals. As research on methyl bromide alternatives continues to progress and regulatory issues surrounding soil fumigation take effect, the economic impact of the methyl bromide ban will become more clearly defined.
 

Plant Production of Methyl Bromide

Production of MeBr by plants has been demonstrated in laboratory studies (Saini et al., 1995; Gan et al., 1998a). Floating plant leaf disks on solutions containing high concentrations of halide ions resulted in evolution of MeBr and other methyl halides (MeI, MeCl) (Saini et al., 1995). The capability of plant leaves to methylate halide ions and release methyl halides appears to be common among higher plants. A high proportion of species tested demonstrated the capability to methylate halide ions, with the family Brassicaceae being particularly efficient (Saini et al., 1995). Plants may contain a variety of methyl transferase enzymes that effect this reaction. Intact plants grown in soil containing BrG were also observed to produce MeBr. In a study by Gan et al. (1998a), several species from the Brassicaceae family were grown in soil containing 0.4 to 100 mg/kg BrG. Production of MeBr per gram of plant mass was correlated to the soil BrG concentration. No MeBr production was observed in BrG-treated soil without plants. No MeBr was observed in microcosms from which the aboveground plant mass was removed, indicating that MeBr production and/or release could not be caused by plant roots only. Because low concentrations of BrG are ubiquitous in soils, terrestrial plants may be an important component of the MeBr cycle.
 
Flux of MeBr and other methyl halides from rice paddies was measured by Redeker et al. (2000). Results indicated production of MeBr by rice paddies, although the study did not include a plant-free control, so it was not verified that plants were responsible for MeBr production. The flux of MeBr depended on the rice growth stage, and maximum MeBr flux occurred during heading and flowering. Plots in which the rice straw was incorporated into the soil had increased soil organic matter content and increased MeBr production. At one site, MeBr production was proportional to soil BrG concentrations. Muramatsu and Yoshida (1995) also observed production of methyl halide (methyl iodide) by rice plants, with seasonal variation in MeI production. Unplanted soil controls also produced measurable MeI, but both flooded and unflooded soils produced less MeI than the treatments containing rice plants.
 
Gan et al. (1998a) estimated a global production level of MeBr by rapeseed and cabbage plants to be 6.6 ± 1.6 Gg/yr and 0.4 ± 0.2 Gg/yr, respectively. Rice paddies may contribute approximately 1.3 Gg/yr (Redeker et al., 2000). These values are of the same order of magnitude as some anthropogenic sources of MeBr (Yates et al., 1998). It seems likely that the contribution of plants is important in the MeBr cycle, although it is often not accounted for in current MeBr budgets.

Chemical and Physical Properties
 
Some of the basic physical and chemical properties of MeBr are listed in Table 1. Due to its high vapor pressure, MeBr can readily penetrate many matrices and is extremely difficult to contain even in the laboratory. Since MeBr is colorless and virtually odorless at room temperature, even at potentially toxic concentrations, severe exposure can occur unknowingly (Yang et al., 1995). In commercial formulations of MeBr, various percentages (0.5 - 33%) of chloropicrin are added as a warning agent to protect workers and residents during and immediately after MeBr applications (and to assist in protecting plants from disease). However, it should be noted that since the vapor pressure of MeBr is many times that of chloropicrin, the safety of using low ratios of chloropicrin in the mixture as a warning agent is questionable (Van Assche, 1971). Methyl bromide is considered to be acutely toxic, with an 8-hr time-weight averaged limit for human exposure in air to be only 5 ppm (ACGIH, 1988). Acute toxicity to workers upon exposure to MeBr vapor has been a major concern in the many years of MeBr use, and is one of the reasons for some early modifications of its application method (e.g., use of surface tarp, mixing with chloropicrin, and use of buffer zones). Fatalities and injuries resulting from exposure to MeBr have been reported, but most incidents are related to structural fumigations rather than soil fumigations (Yang et al., 1995).
 
Table 1.  Selected Physical Properties of Methyl Bromide
 
Property Value
Synonym Bromomethane
Trade Name Brom-O-Gas,
Meth-O-Gas,
Terr-O-Gas
Molecular Weight, g mole-1 94.94
Vapor Density, g L-1 3.974
Dipole Moment, debye 1.8
Liquid Density, g cm-3  
  20 °C 1.676
  25 °C 1.737
Solubility, mg L-1  
  20 °C 16000
  20 °C 17500
  25 °C 13400
CAS Registry Number 74-83-9
Freezing Point, °C -93.7
Boiling Point (at 1.0 atm), °C 3.56
Koc, cm3/(g %Foc) 22
Log(Kow) 1.19
Vapor Pressure, mmHg  
  20 °C 1395 ±19
Henry's Law Constant  
  20 °C 0.23
  21 °C 0.30 ±0.02
   

Reactions with Stratospheric Ozone
 
It is known that bromine can catalytically destroy stratospheric ozone (Wofsy et al., 1975; Yung et al., 1980; McElroy et al., 1986; Salawitch et al., 1988; Anderson et al., 1989; Prather et al., 1990). Reactions involving bromine are believed to be responsible for 20-25% of the Antarctic 'ozone hole' that develops each austral spring (Anderson et al., 1989), which implies that a bromine atom is approximately 40 times more efficient than a chlorine atom in destroying ozone (Wofsy et al., 1975; Salawitch et al., 1988; Solomon et al., 1992). Methyl bromide is unique because it is a significant source of bromine to the stratosphere (Wofsy et al., 1975; Yung et al., 1980; Penkett et al., 1985; Cicerone et al., 1988; Schauffler et al., 1993). However, the case for restricting the use of MeBr is not clear-cut. Unlike the CFCs, atmospheric MeBr is not entirely contributed by human activities. Atmospheric MeBr has abundant natural and anthropogenic sources. Also, its sinks result not only from reactions with the atmosphere, but also from interaction with the oceans and land. Thus, estimating the contribution of MeBr fumigation (currently ~80% of the entire anthropogenic source) to the depletion of stratospheric ozone is much more complex than it is for other regulated halogenated compounds.
 
To justify the pending suspension of MeBr use in agriculture, it should be established that the known sources of atmospheric MeBr surpass the sinks, and the surplus is contributed by anthropogenic emissions. However, current estimates of global MeBr are out of balance, with sinks exceeding sources by a wide margin (Yvon-Lewis and Butler, 1997). The total atmospheric burden of MeBr is believed to be around 145 Gg y-1 (100-194 Gg y-1), and the concentration about 10 pptv, increasing at 0.1-0.3 pptv y-1 (Khalil et al., 1993; Singh and Kanakidou, 1993). The sinks currently thought to remove MeBr from the atmosphere include reactions with OH radicals in the atmosphere (accounting for ~86 Gg y-1 MeBr), removal by oceans (~77 Gg y-1), degradation in soil (42 Gg y-1 ) and uptake and degradation by plants. The relative strength of each of these sinks is not well quantified. The estimated lifetime of atmospheric MeBr is 0.7 y (range of 0.4 to 0.9 y), with a calculated ozone depletion potential (ODP) of 0.4 (range 0.2 to 0.5) according to the World Meteorological Organization's 1998 Scientific Assessment of Ozone Depletion (WMO, 1999).
 
The known sources of atmospheric MeBr include oceanic emissions, biomass burning, automobile emissions from leaded gasoline, and fumigation. Together, these emissions combine to produce 122 Gg y-1 of MeBr (range of 43 to 244 Gg y-1) (WMO, 1999). The 1998 Scientific Assessment of Ozone Depletion estimated oceanic MeBr emissions to be 60 Gg y-1, with the ocean acting as a net MeBr sink of -21 Gg y-1 (WMO, 1999). Recent research has indicated that the magnitude of the oceanic sink may be -11 to -20 Gg y-1 (King et al., 2000). Biomass burning (Manö and Andreae, 1994) is another significant, natural source of atmospheric MeBr, and its contribution is poorly quantified. Global emission of MeBr from biomass burning is estimated to be 20 Gg y-1 (range of 10 to 40 Gg y-1) (WMO, 1999). It has also been demonstrated that automobile exhaust from the combustion of leaded gasoline, which contains bromine compounds, can include measurable amounts of MeBr (Harsch and Rasmussen, 1977). Emissions from this source could range from 0 to 5 Gg y-1 (WMO, 1999). Additional potential MeBr sources which have recently been identified include production by plants (Gan et al., 1998a), salt marshes (Rhew et al., 2000), and fungi (Butler, 2000). Salt marshes may be a globally important source of MeBr (contributing 7-29 Gg y-1) (Rhew et al., 2000) and production of MeBr has been observed for a variety of plants (Gan et al., 1998a); therefore, plant sources may account for a large proportion of the "missing source" in current MeBr budgets.
 
Some anthropogenic emissions, such as fumigation of structures, perishables, and durables, are relatively well quantified, since nearly 100% of the applied MeBr is vented into the air during these fumigation processes. The use of MeBr for these fumigations accounts for about 15% of the total production. Trapping and/or decomposing MeBr during structural fumigation can drastically decrease atmospheric emissions of MeBr during these operations. Approximately 85% of the industrially-produced MeBr is used as a soil fumigant, equivalent to ~65 Gg y-1 in 1996. The actual discharge of MeBr from fumigated fields into the air is largely determined by the proportion of the applied MeBr that is emitted from the treated soil, which can be reduced through management practices (for example, Wang et al., 1997a; Yates et al., 1998; Gan et al., 1998d).
 
New measurements of the sources and sinks of MeBr are still being actively obtained, as evidenced by many recent reported studies. It is a fact that the relative contribution of MeBr used in fumigation practices is far from well quantified. Despite this, being a significant controllable source, the agricultural use of MeBr becomes a natural target for elimination. It is also assumed that due to a short atmospheric lifetime of less than 1 year, the effect of cessation of anthropogenic MeBr emissions on the restoration of stratospheric ozone will be nearly immediate. In comparison, all the chlorofluorocarbons (CFCs) have extremely long lifetimes, and with a complete elimination of emissions of these compounds, it may take many years, or even centuries, to reduce the atmospheric burden of the CFCs to an insignificant level (Butler, 1995).
 

Solubility
 
MeBr Solubility
 
 

Henry's Law Constant
 
It is known that the movement of a volatile chemical in soil is controlled by its distribution behavior over the soil–water–air phases. The reported Kh for MeBr at 20 EC varies from 0.24 to 0.30 (Siebering and Leistra, 1978; Gan and Yates, 1996), and changes with temperature. With a Kh of this magnitude, it can be expected that the movement of MeBr in unsaturated soil is mainly driven by its diffusion via the vapor phase (soil air). The temperature dependence of the Henry's Law constant for MeBr is shown in Figure 1, including Arrhenius equations and fitted parameters.
 
Figure 1. Henry’s Law Constant for Methyl Bromide as a function of temperature
Henry's Law Constant
 

Vapor Pressure
 
MeBr Vapor Pressure
 

Adsorption
 
The adsorption coefficient, Kd (ml/g), is important as a retaining force in slowing down MeBr transport through the soil. There are a few published measured or estimated Kd and Koc values for MeBr. The reported Koc ranges from 9 to 22 (Briggs, 1981; Karickhoff, 1981; Rao et al., 1985), which corresponds to a Kd of 0.09 to 0.22 in a soil with 1% organic carbon. Arvieu (1983) measured MeBr adsorption and desorption, and found different characteristics for soil with different organic matter contents. In organic matter-poor soils, the adsorption of MeBr is very weak unless the soil is very dry. In organic matter-rich soils, the adsorption is considerably greater. The same author also noted that the adsorbed MeBr became resistant to desorption. Gan and Yates (1996) observed that degradation of MeBr during the equilibration in adsorption studies might have contributed to the observed increased adsorption in soils with high organic matter content. A noticeable fraction of the spiked MeBr was degraded to Br during a 16-h shaking in organic matter-rich soils. This phenomenon may be also responsible for the irreversibility found in MeBr desorption isotherms (Arvieu, 1983). After correcting for the degraded fraction, MeBr adsorption became negligible in all the tested soils (Gan and Yates, 1996). MeBr can be considered to be a nonadsorbing chemical in soil with normal water content.
 

Diffusion Coefficient
 
The value of the diffusion coefficient of a chemical in the vapor phase is generally 104 times larger than that in the liquid phase (Jury et al., 1983). The diffusion coefficient can be estimated using a variety of methods (Reid et al., 1987), including the Fuller correlation
Diffusion Coefficient equation
 
where Dab is the binary diffusion coefficient (cm2/s), T is the absolute temperature (K), Mab = 2/(Ma-1 + Mb-1), Ma and Mb are the molecular weights of air and MeBr, respectively, P is the pressure (bars) and EL is obtained using the atomic diffusion volumes (Reid et al., 1987). Using this Equation yields an estimated diffusion coefficient for MeBr of 0.114 cm2 s-1 at 20°C and 1 atmosphere ambient pressure. The temperature dependence of the diffusion coefficient is shown in Figure 3 and appears to be nearly a straight line over the temperature range 0-60°C. The temperature dependence of the binary diffusion coefficient can be described using the Equation above or using activation energy and the Arrhenius Equation as shown in Figure 3.
 
Using a screening model, Jury et al., (1991) found that the movement of a chemical is dominated by vapor-phase diffusion if the air-to-water partition coefficient, or the Henry’s Law coefficient (KH) is »10-4. Since the KH for MeBr is approximately 0.25, transport in the vapor phase is important in describing the fate and transport in soil.
 
Diffusion Coefficient graph
 

Air Sampling
 
In the course of monitoring MeBr in workplace, field, and ambient atmospheres, sampling and analytical methods of different sensitivities and complexities have been developed. Depending on the sampling device that is used for collecting air samples, MeBr can either be in a contained atmosphere (such as canisters) or adsorbed on a solid adsorbent (such as activated carbon or a porous polymer) prior to analysis. For the past two decades, quantitation of MeBr has used gas chromatography exclusively, and electron-capture detectors (ECD) are usually selected over the other types of detectors due to its high sensitivity to halogenated compounds (Scudamore, 1988), although very high sensitivity is also found with photoionization detectors (PID) (Dumas and Bond, 1985). The main reported sampling and analytical methods for analyzing atmospheric MeBr are summarized in Table 2.
 
Container Methods
Among the container methods, steel canisters were used for sampling volatile toxic chemicals in air, such as MeBr, by Jayanty (1989) and Gholson et al. (1990), and good stability and sensitivity were achieved for all the selected analytes. Cryogenic preconcentration was required prior to the delivery of samples into the GC column. Yagi et al. (1993, 1995) used 500-ml canisters for sampling MeBr to obtain flux measurement under field conditions. Sampling with canisters is labor-intensive since the container has to be evacuated before sampling, and the contents must be cryogenically concentrated before injection, which limits the number of samples that can be collected and analyzed. Sampling with canisters is therefore not suitable for extensive sampling as needed in volatilization flux measurement under field conditions, though the sensitivity could be very high if a proper detector is used. Using canisters is also not compatible with active (flow-through) chambers that are used for continuous sampling of the atmosphere.
 
Another container method involves collecting an air sample using a gas-tight syringe, and injecting the contents directly into a gas chromatograph. In a study of the transport of MeBr in soil after fumigation, Kolbezen et al. (1974) used glass syringes to take and temporarily store soil air samples. The plungers were coated with a film of Triton X-100 to eliminate rapid leakage, and the needle was embedded in a MeBr-impervious sponge. Loss of MeBr was determined to be insignificant within 6 h, but 5-7% was lost after 22 h. The analysis was made by direct injection of the air sample in the syringe into a GC. This method has also been employed in small-scale laboratory experiments (for example, Gan et al., 1998a).
 
Adsorbent Methods
The most commonly used method for sampling atmospheric MeBr is pumping a relatively large volume of air through one or a series of adsorbent tubes. Methyl bromide in the air stream is trapped in the sample tube containing the solid adsorbent due to its high affinity to the adsorbent. Two types of adsorbent material have been recorded for use with MeBr: activated carbon (charcoal) (Eller, 1984; Woodrow et al., 1988; Lefevre et al., 1989; Gan et al., 1995a,b; Majewski et al., 1995; Gan et al., 1995a,b; Yates et al., 1996abc; Yates et al., 1997; Wang et al., 1997a), and porous polymeric adsorbent such as Tenax GC (Brown and Purnell, 1979; Dumas, 1982, Dumas and Bond, 1985; Krost et al., 1982).
 
Activated carbon or charcoal tubes are low in cost (about $1 each), can accommodate large sample volumes, and need minimum preparation before sampling. A typical charcoal tube consists of two adsorption beds: a primary bed (A) and a backup bed (B) in a sealed glass tube. The charcoal can be derived from either coconut or petroleum. Polyurethane spacers are used to separate the two adsorption beds, and a plug of glass wool is usually placed in front of the primary bed to hold the charcoal in the sample tube. Before use, a tube is broken at both ends, and then connected to a vacuum source to draw the air to be sampled into the tube. Depending on the sampled volume, air flow rate and MeBr concentration, multiple tubes connected in series may be required to eliminate loss through breakthrough (Gan et al., 1995a). The number of tubes should be increased when a high flow rate or a long sampling interval is used. Gan et al. (1995a) found that for a single 600 mg coconut charcoal tube at a flow rate of 100 ml min-1, a sampling interval of #2 h resulted in no breakthrough loss.
 
Methyl bromide adsorbed in charcoal tubes may be analyzed by two different methods: solvent extraction followed by injection from the solvent phase, and the so-called headspace-GC method. In solvent extraction, charcoal is transferred into a vial, a known amount of extracting solvent such as carbon bisulfide (CS2) is added into the vial, and the vial is sealed (Eller, 1984; Lefevre et al., 1989). After the solvent-charcoal mixture is mechanically shaken, an aliquot of the solvent is injected into a GC. This method has the drawbacks of manual sample preparation, and presence of other compounds in the final sample solution that may elute with or interfere with MeBr during chromatography (Gan et al., 1995b). This method allows for multiple injections of each sample so that multiple analytes may be measured using different methods or detectors.
 
An alternative method is the headspace-GC method. In headspace-GC analysis, the charcoal is equilibrated with an organic solvent in a closed headspace vial at an elevated temperature for a given period of time, and an aliquot of the headspace containing the analyte is then introduced into the GC column for detection. Benzyl alcohol is often used as the solvent due to its high boiling point (210EC) (Woodrow et al., 1988; Gan et al., 1995b). When the vial size, solvent volume, and equilibrating temperature and time are fixed, automated headspace injectors give high reproducibility and sample throughput. Gan et al. (1995b) found that the equilibration temperature and time in the headspace autosampler, the size of headspace vials, as well as the amount of solvent all had an effect on the signal output for a given sample. The sensitivity of analysis can thus be maximized by choosing an optimal combination of these parameters. For instance, to analyze a sample tube containing 600 mg coconut charcoal, the best conditions were determined to be: 9-ml headspace vials; 1.0 ml benzyl alcohol; 110EC equilibration temperature, and 15 min equilibration time (Gan et al., 1995b). Using this method, analysis of a MeBr-containing sample tube takes only 3-4 min, and as many as 300 samples can be analyzed within 24 h. This method is appropriate for analyzing samples from large-scale field studies measuring MeBr volatilization, when a large number of samples is required (Yates et al., 1996bc; Yates et al., 1997; Wang et al., 1997a). This method has the disadvantage of being destructive, where each charcoal sample can be analyzed with only a single injection.
 
There are many kinds of porous polymer adsorbent material that have been used for collecting volatile compounds in the air, and these include the Chromosorb series, the Porapak series, Ambersorb XE-340, and others. The most popular adsorbent, however, is Tenax-GC, which is a polymer of 2,6-diphenyl-p-phenylene oxide. Brown and Purnell (1979) estimated the safe sampling volume for MeBr to be 0.14 L for sample tubes packed with 0.13 g Tenax-GC. When coupled with a cryofocusing technique, the whole sample can be introduced into the GC column following thermal desorption, which greatly enhances the sensitivity. Detection limits of 500 pg/L (Krost et al.,1982) and 35 ng (Dumas and Bond, 1985) were reported when this method was used. Compared with charcoal tubes, polymer samplers need to be conditioned before sampling, the safe sampling volume is smaller, the cost is higher, and each analysis takes a longer time.
 
Other Methods
Other than the container method and the adsorbent method, cryogenic concentration in a cold trap has also been used for collecting MeBr (Kallio and Shibamoto, 1988; Kerwin et al., 1996). The cold traps include mixtures of dry ice-acetone, liquid nitrogen and dry ice-2-propanol. The weakness of this technique is the long time and many steps involved in handling one sample, but it is useful when sample throughput is not a factor and very low detection limits are sought.
 
When extremely high sensitivity is pursued, such as in the case of monitoring MeBr in ambient air, a technique called O2-doping could be useful (Grimsrud and Miller, 1978; Kerwin et al., 1996). Grimsrud and Miller (1978) first reported that addition of a fraction of O2 in the carrier gas drastically increased the sensitivity of ECD detection of halogenated methanes including MeBr. When 3-5% of O2 was added to the carrier gas, signal response was enhanced about 2 orders of magnitude for MeBr. Using cryogenic concentration and O2-doping, Kerwin et al. (1996) reported a detection limit as low as 0.23 pmol or 22 pg.
 

Field Experiments
 
Since the mid-1990s, there have been several experiments conducted to obtain information on MeBr emissions from typical agricultural operations. The results from these studies are summarized in Table 5. Various methods were used to estimate the emission rate, including: an increase in soil Br ion concentration as a result of MeBr degradation (Yates et al., 1996a), atmospheric flux method (Majewski et al, 1995; Yates et al., 1996b) and enclosed flux chamber method (Yagi et al., 1993; Yagi et al., 1995; Yates et al., 1996c). Every method has advantages and disadvantages that often make the interpretation of the experimental results somewhat difficult. However, for determining the total emission, all the methods should provide reasonably accurate results.
 
Yagi et al. (1993)
Yagi et al. (1993) conducted an experiment in Irvine, California to measure the MeBr emission from a fumigated southern California field using four passive flux chambers. MeBr was applied at a depth of approximately 25 cm depth and the soil surface was covered with low density polyethylene plastic film. The authors originally estimated that 87% of the total MeBr applied to the field escaped into the atmosphere. This estimate was revised to 74 ± 5% (Williams et al., 1999) by eliminating the data from a chamber that covered tarp material with a hole. The estimates of MeBr emissions measured during this study are the highest reported for MeBr injection at shallow depth and the soil surface covered with plastic. The high emission rate are probably due to a combination of factors such as: use of low density polyethylene plastic, which is permeable to MeBr vapors (Kolbezen and Abu-El-Haj, 1977), the high bulk density of the soil and the presence of a moist soil layer at 60 cm depth. This value is also higher than expected given other estimates based on mathematical models (Albritton and Watson, 1992; Singh and Kanakidou, 1993), but was similar in magnitude to the losses observed in glass-house studies (de Heer et al, 1983). To verify these results the authors returned to the field to collect Br information to provide mass balance information (Yagi et al., 1995).
 
Yagi et al. (1995)
The investigators conducted a second experiment in a nearby field using the same procedures as their first experiment (Yagi et al., 1993). For this experiment, high density polyethylene plastic was used to cover the field and 5 flux chambers was used to measure emissions. They found that only 34% of the applied MeBr escaped to the atmosphere. This value is more than 50% lower than the result of their first experiment, which included a low-density polyethylene tarp. Variability in the emission measurement is expected for several reasons: 1) only 10-15 samples of the volatilization rate were obtained during each 7-day experiment, generally at the high point during the day; 2) only a few soil samples were taken to measure Br concentrations and soil Br concentration has been shown to be highly variable (Jury, 1985; Yates et al. 1996a); and 3) degradation of soil MeBr is highly dependent of actual soil conditions. An a dditional source of variability may be the internal chamber temperature which has been shown to affect HDPE permeability. Yagi et al. (1993, 1995) did not correct their volatilization rates for this effect. The estimated 34% loss rate is within the 30-60% expected loss range noted in the Montreal protocol.
 
Majewski et al. (1995)
Two field experiments were conducted in Monterey County, California, from Oct 26 to Nov 4, 1992. The fields (Salinas clay loam) were separated by a distance of approximately 6 km. MeBr was injected at a depth of 25-30 cm and one field was covered with a standard high density polyethylene plastic film, the other left uncovered. The application rate for the tarped experiment was 392 kg/ha and for the bare soil experiment was 203 kg/ha. In both experiments the flux density was measured using the aerodynamic method (Parmele et al., 1972). The aerodynamic method produces a large-scale average volatilization rate and is relatively insensitive to small-scale variability which may occur when using chambers. Although an error analysis was conducted, insufficient information was obtained for a mass balance; therefore, there was no independent measure of the total emission. Majewski et al. (1995) found that 32% of the applied MeBr was emitted into the atmosphere from the tarped field during the first 9 days following application. This value is approximately the same as that from the second study of Yagi et al. (1995) and falls into the 30-60% range noted in the Montreal protocol (Albritton and Watson, 1992). For the bare soil experiment, approximately 89% of the applied fumigant was lost via volatilization.
 
Yates et al. (1996a,b,c)
Yates et al. (1996a,b,c) conducted an experiment at the University of California's Moreno Valley Field Station on a 4-ha field during August and September, 1993. The soil in this field is a Greenfield sandy loam. MeBr (99.5% purity) was applied at a depth of 25 cm, at a rate of 240 kg/ha, and the field was covered with 1 mil polyethylene plastic. Estimates of the MeBr emission rate and total loss were obtained using flux chambers, micro-meteorological methods and by estimating total loss from Br appearance. Using the micro-meteorological methods (e.g., aerodynamic, theoretical profile and integrated horizontal flux methods), the total emission was estimated to be 62% to 70% (±11%) of the applied MeBr. Data from the flux chambers give a total emission loss of about 59% of the applied mass and is from 3 to 10% lower than the estimates from the micro-meteorological methods. Cumulative emissions based on Br‾ appearance totaled 61% of the applied MeBr. A mass balance was calculated for each method used to estimate the flux (Table 5). The average mass recovery using all the flux methods was 103% (±10%) of the applied mass. The range in the mass balance percent (i.e., percent of applied mass that was measured) was from 97% to 108%. The averaged mass balance percent for the discrete aerodynamic method, which involved direct use of the measured data, was approximately 101%. The estimated 60% loss is at the high end of the range noted in the Montreal protocol and Reible (1994). This experiment was conducted under warm, dry conditions using multiple methods for measuring the volatilization rate. Since all methods produce supporting estimates, it is likely that 60% total loss value is correct and that large fractions of applied MeBr are lost when fumigation is performed under these soil and environmental conditions. The fraction of the applied MeBr mass volatilized in the experiment of Yates et al., (1996abc) is approximately double the value reported by Majewski et al. (1995) who estimated the total loss to be approximately 32%. This is probably due to regional differences in the climatic and soil conditions between the central coast and inland southern California. Lower temperatures in Monterey would cause a reduction in the diffusion through polyethylene plastic material (Kolbezen and Abu-El-Haj, 1977) and increase the residence time in the soil. This would facilitate greater MeBr degradation in the soil and reduce the total loss to the atmosphere. The range for total emissions described in Yates et al. (1996a,b,c) also differs from the results of Yagi et al. (1993, 1995) who reported values of approximately 87% and 34%, respectively, for experiments with a similar MeBr application methodology.
 
Yates et al. (1997)
The MeBr volatilization rate was determined in an 4 ha agricultural field after injection at 68 cm; results were compared to an earlier experiment where MeBr was injected at 25 cm and the surface covered with high-density polyethylene plastic (Yates et al., 1996a,b,c). Three independent methods were used to estimate the total MeBr volatilized after application, i.e., the appearance of soil Br, the flux chamber, and micro-meteorological methods. When injected deep in soils, the MeBr volatilization rate continued at relatively high values for more than 7 days after application. It was observed that the total MeBr mass emitted from the field was significantly less than the earlier experiment and this was attributed to deep injection, cooler air temperatures and smaller thermal gradients. The total emissions estimate obtained from soil Br content sampling was 21% of the applied MeBr. The estimates obtained from the direct flux measurements were found to range from 1.9% to 4.9%. The mass recovery ranged from 81% to 84% of the applied mass, with an average value of 82%. Comparison of the direct methods for measuring the volatilization rate with the estimate of total emissions from MeBr degradation suggests that for deep injection using only 2 shanks, the initially high MeBr gas pressure may cause localized evaporation of the liquid MeBr to play a significant role in the volatilization process. This process needs to be further studied to develop methods for controlling volatile losses.
 
Williams et al. (1999)
The emission studies of Yagi et al. (1993, 1995) and four additional experiments were summarized. Three field sites near Irvine, CA were used. Two experiments were conducted at Site I where the total emissions were 74 ± 5% (Yagi et al., 1993) and 63 ± 12%. The mass balance for this site was between 94 and 97%. At a second site, three experiment were conducted. The total emissions were somewhat variable with measurements of 36 ± 6%, 24 ± 5% and 45 ± 8%, respectively, in 1993, 1994 and 1995 experiments. The mass balances were from 86 ± 15% to 106 ± 11%. A sixth experiment was conducted at a third site and yielded 50 ± 9% total MeBr emissions with a 97 ± 13% mass balance. The authors found that the emission rate was highly dependent on the thickness of the plastic film used to cover the soil surface during fumigation. They also investigated the effect of soil carbon content, nitrogen content, and pH on emissions.
 
Wang et al. (1997a,c)
Smaller-scale outdoor studies (plot size ~17 m2) were conducted to determine MeBr emissions in untarped and tarped plots. Cumulative emissions were ≥60% for bare plots and plots tarped with high-density polyethylene when MeBr was injected at 25 cm. A large decrease in emissions was observed with the use of a low-permeability tarp (Hytibar), with volatilization losses <5% of the applied MeBr reported when the tarp remained in place for at least 10 days.
 

Transformation of MeBr in Water
 
Transformation or degradation of MeBr is an irreversible process that depletes MeBr from the soil-water-air system before it reaches the soil surface and volatilizes into the air. Extremely rapid transformation may deplete MeBr concentrations so quickly that efficacy is compromised. The actual transformation of MeBr in an agricultural soil is the sum of its hydrolysis in water, reactions with soil constituents, and decomposition by soil microorganisms.
 
Hydrolysis
Degradation of MeBr in water is important since it contributes to MeBr degradation in moist soil as well as to its fate in the overall environment. Based on its chemical structure, MeBr is an electrophile, and -Br is reactive as a leaving group and may participate in various nucleophilic substitution reactions (SN1 and SN2 types) in the environment. Water is a weak nucleophile, and therefore hydrolysis of MeBr in water is anticipated:
(Note: if you see a "ÿ" is should be an arrow "==>")
CH3Br  +  H2O  →  CH3OH  +  Br¯  +  H+ Reaction I
CH3Br  +  OH¯  →  CH3OH  +  Br¯ Reaction II
 
The reaction rate constants for reactions I and II are approximately 5 × 10-9 and 10-4 M-1s-1 respectively (Schwarzenbach et al., 1993). In pure water where the OH¯ concentration is extremely low, reaction I dominates, and the calculated pseudo first-order half-dissipation time (t1/2) of MeBr should be around 30 days. Mabey and Mill (1978) and Papiernik et al. (2000) report a t1/2 of 20 days, Arvieu (1983) reported a t1/2 of 46 d for MeBr in water at 20EC, and Gentile et al. (1989) reported t1/2 of 36-50 days in well waters at 18 EC. The relatively slow hydrolysis of MeBr in water was also noted by Herzel and Schmidt (1984). In an attempt to correlate MeBr hydrolysis and pH, Gentile et al. (1992) measured MeBr degradation in buffer solutions with pH 3.0 to 8.0, and found MeBr hydrolysis rates generally increased with increasing pH. However, in their experiments, they used buffer solutions comprised of phosphate and citrate, and apparently nucleophiles other than OH¯ caused the enhanced hydrolysis in solutions with elevated pH. From the rate constant of reaction II, MeBr hydrolysis rate should not increase significantly when pH is changed from 7 to 10.
 
In waters which are rich in nucleophiles, such as the supernatant of a salt marsh containing sulfide, MeBr degradation may be accelerated (Oremland et al., 1994a). The reaction produces methanethiol:
 
CH3Br  +  HS¯  →  CH3SH  +  Br¯
 
and further reaction with MeBr produces dimethylsulfide
 
CH3SH  +  CH3Br  →  (CH3)2S  +  Br¯  +  H+
 
MeBr was observed to degrade rapidly in anaerobic salt marsh slurries containing sulfide, with a reported transformation half life of /1 d. Production of methanethiol in slurries doped with sulfide exhibited very rapid reaction, with a MeBr half life of ~1 h (Oremland et al., 1994a). Accelerated transformation by MeBr in aqueous solution containing other nucleophiles (for example, aniline) has also been reported. Reaction with aniline in aqueous solution with a molar ratio of aniline:MeBr of 10:1 formed N-methylaniline and N,N-dimethylaniline with a MeBr transformation half life of 2.9 d (Gan and Yates, 1996).
 
The mechanism of photo-induced hydrolysis of MeBr in water was first reported by Castro and Belser (1981). When a pen-ray UV lamp emitting UV at 254 nm was used to irradiate MeBr-water solution in a 4-L closed flask, MeBr was gradually converted to methanol and Br¯. The following mechanism was proposed by these authors:
 
CH3Br  +  hv  →  (CH3Br)*  +  H2O  →  CH3OH  +  H+  +  Br¯
 
Photohydrolysis caused faster dissipation of MeBr in sunlight under sealed conditions (Castro and Belser, 1981), and under UV irradiation (Gentile et al., 1989). The significance of reactions with nucleophiles and UV in MeBr transformation in soil water, however, has not been investigated.
 
Hydrolysis of MeBr in aqueous solutions may bear limited significance in determining its fate as a water contaminant. The loss of MeBr in stirred and ventilated waters, or water that had a high surface-to-volume ratio, was found to be very rapid due to volatilization (Gentile et al., 1992). In a study to follow MeBr kinetics in surface water, Wegman et al. (1981) found that the average half-life for MeBr in surface water at a water temperature of 11 EC was only 6.6 h. Over its many years of use, contamination of water sources with the parent MeBr has never been a topic of concern.
 

Transformation of MeBr in Soil
 
Hydrolysis in water is not the only pathway, and in many cases, not even an important pathway, that causes MeBr degradation in soil. This is evidenced by shorter t1/2 values obtained in soil degradation studies, particularly with organic matter-rich soils, as opposed to the t1/2 in water. Two other pathways, i.e., reaction with soil organic matter and microbial degradation, have been identified as contributing to MeBr degradation in soil.
 
Soil organic matter contains nucleophilic sites such as the -NH2 , -NH, -OH, and -SH functional groups. Methyl bromide may react with these groups via nucleophilic substitution, as in the hydrolysis reactions:
 
CH3Br  +  OM-NH2  →  OM-NH-CH3  +  Br¯  +  H+
 
CH3Br  +  OM-SH  →  OM-S-CH3  +  Br¯  +  H+
 
As a result of these reactions, soil organic matter is methylated, and inorganic bromide ion is released. The reaction of MeBr with soil organic matter is confirmed by the general observation of the close dependence of MeBr degradation on soil organic matter content (OM): in organic-matter-rich soils, degradation is consistently more rapid than in organic matter-poor soils (Brown and Jenkinson, 1971; Brown and Rolston, 1980; Arvieu, 1983; Arvieu and Cuany, 1985; Gan et al., 1994). For a soil containing 2.81% organic matter, 63 ppm of Br¯ was generated after exposure to 500 ppm MeBr in closed flasks for 24 h, while only 25 ppm of Br¯ was produced in a soil with 0.93% organic matter (Brown and Jenkinson, 1971). Arvieu (1983) studied the rate of MeBr degradation rate in 8 soils, and that t1/2 decreased with increasing organic matter content. In a soil with 0.23% organic matter, the t1/2 was 49 d; but in a soil with 5.11% organic matter, the t1/2 was shortened to only 3.6 d. After measuring MeBr degradation rates in selected soils, Gan et al. (1994) found that the MeBr degradation rate (based on Br¯ production) and soil organic matter content were highly correlated, and the measured correlation coefficient (r2) ranged from 0.95 to 1.00. Papiernik et al. (2000) also reported an increase in MeBr degradation rate with increasing soil organic matter content. Spiking soil samples with 14C-labeled MeBr resulted in the formation of non-extractable (bound) residues of MeBr, which increased as extractable MeBr decreased. These bound residues represented transformed (not sorbed) MeBr, as evidenced by the release of equimolar amounts of Br¯ for each mole of MeBr lost (Papiernik et al., 2000).
 
The reliance of MeBr degradation on soil organic matter content also has implications for MeBr degradation in subsurface layers. Since soil organic matter content normally decreases with increasing depths, MeBr degradation may be much slower in the deep layers, and the overall persistence could be much longer than what the degradation data generated from surface soils would suggest. This was verified in laboratory incubation studies using soils collected from 0 to 300 cm (Gan et al., 1994). In a Greenfield sandy loam, the t1/2 for MeBr degradation was about 8 d for the 0-30 cm layer, but increased gradually with depth to a t1/2 of 21 d for the 270-300 cm layer. This decrease closely corresponded to the decrease in soil organic matter content (Gan et al., 1994).
 
Biodegradation of MeBr has been documented for isolated bacteria, including the nitrifying bacteria Nitrosomonas europaea, Nitrosolobus multiformis, and Nitrosococcus oceanus (Rasche et al., 1990; Hyman and Wood, 1984), and Methylomonas methanica and Methylococcus capsulatus (Colby et al., 1975, 1977; Meyers, 1980; Oremland et al., 1994b). The nitrifier-catalyzing degradation suggests the involvement of ammonia monoxygenase, while the consumption of MeBr by the methane-oxidizing bacteria indicates that methane monooxygenases are responsible. Increasing the activity of nitrifying bacteria may increase the rate of biodegradation of MeBr in soil (Ou et al, 1997). Other bacteria capable of degrading MeBr have been isolated from soil. Miller et al. (1997) isolated a facultative methylotroph which could use MeBr as a source of carbon and energy. Oremland et al. (1994b) demonstrated that at high concentration, biodegradation of MeBr in methanotrophic soils was inhibited due to the toxicity of MeBr itself, but became significant at concentrations lower than 1000 ppm. Shorter et al. (1995) suggested that microbial degradation of MeBr at low concentrations (ppb) in surface soils may be important in removing MeBr from the atmosphere, thus reducing its lifetime in the atmosphere and lowering its ozone-depletion potential. They observed that MeBr removal from the headspace in closed systems containing soil was more rapid in live soils than in autoclaved soils, and the degradation rate decreased with the depth from which the soil was sampled, which corresponded to the methane oxidation activity of the soil. Hines et al. (1998) reported that at low atmospheric mixing ratios (5 pptv to 1 ppmv), rapid degradation of MeBr was effected by aerobic soil bacteria, resulting in half-lives on the order of minutes for a variety of soils. Since the initial concentrations around the injection point are normally in the order of 104 ppmv (Kolbezen et al., 1974) and the MeBr-degrading bacteria are low in population in normal agricultural soils, bacteria-mediated degradation may be insignificant under typical circumstances. However, studies have indicated that MeBr oxidation can occur in field-fumigated soil. High rates of 14C-MeBr oxidation to 14CO2 were observed in the first few days following soil fumigation where the MeBr concentration was >9.5 µg/g soil (Miller et al., 1997). This oxidation was inhibited by the addition of chloropicrin at concentrations >1.6 µg/g soil.
 

Transport Model
 
A common approach for simulating the fate and transport of MeBr for saturated and unsaturated water flow conditions, with consideration of variable soil temperature, includes descriptions for at least three governing processes: water flow, heat transport and fate and movement of MeBr. Programs exist that will numerically solve the nonlinear partial differential equations for one- and two-dimensional systems, non-equilibrium coupled transport of water, heat, and solute (in both liquid and gaseous phases) in a variably saturated porous medium. Degradation is usually described using a first-order decay reaction and, often, the degradation rate in each phase (liquid, vapor and solid) can be specified. The governing transport equations can be written as (Šimůnek and van Genuchten, 1994):
 
Water Transport
Water Transport equation
 
where θ is the volumetric water content [L3L-3], h is the pressure head [L] and Kij are components of the unsaturated hydraulic conductivity tensor [L t-1], and S is a sink term [t-1]; t is time, x is distance [L], and indices i and j represent the horizontal and vertical directions.
 
Heat Transport
Heat Transport equation
 
where Ch and Cw respectively, are the volumetric heat capacity for the porous media [Jm-3 K-1], liquid and λij, is the apparent thermal conductivity [Wm-1K-1].
 
Solute Transport
Solute Transport equation
 
where CL [M L-3], CS [M M-1], and Cg [M L-3] are solute concentrations for the liquid, solid, and gaseous phases, respectively; q is the volumetric flux density; μw, μs, and μg are first-order rate constants [t-1] for solutes in the liquid, solid, and gas phases, respectively; θ is the volumetric water content, ρ is the soil bulk density, η is the soil air content, S is the sink term in the water flow equation [t-1], Cr is the concentration of the sink term, Dijw is the dispersion coefficient tensor for the liquid phase [L2 t-1], and Dijg is the diffusion coefficient tensor for the gas phase.
 
Numerous computer programs have been developed to evaluate the effects of interacting processes and factors on pesticide movement through the root zone and to the groundwater. The approach used in developing the programs varies with the intended use of the model. Some of these include GLEAMS (Leonard et al., 1987); LEACHM (Wagenet and Hutson, 1987); PRZM (Carsel et al., 1985; 1998); PESTAN (Enfield et al., 1982); and SESOIL (Bonazountas and Wagner, 1984). Some of these models are not capable of predicting pesticide movement when water is applied in a controlled manner by furrow or subsurface drip irrigation systems. This has led to the development of process-based models which can be used to predict the transport in irrigated agriculture: CHAIN-2D (Šimůnek and van Genuchten, 1994), HYDRUS-2D (Šimůnek et al., 1996), and PESTLA (van den Berg and Boesten, 1997).
 
Volatilization Boundary Condition
For methyl bromide, critical processes affecting the fate and transport in soils are vapor diffusion and volatilization. Volatilization is an especially important route of dissipation due to MeBr's large vapor pressure and Henry's Law constant as demonstrated in recent field experiments (Yagi et al., 1995; Majewski et al., 1995; Yates et al., 1996b). Excessive volatilization is associated with many problems, such as a reduction in the amount of material available to control of pests and increased potential for contamination of the atmosphere. Emission losses to the atmosphere pose an increased risk to persons living near treated fields. When simulating MeBr emissions to the atmosphere, the approach used to describe the soil surface-atmospheric boundary condition strongly affects the simulated emission response.
 
The most common volatilization boundary condition used in current models is based on stagnant boundary layer theory (Jury et al., 1983). For this formulation, the volatilization rate, fs(t), is
Stagnant boundary equation
 
where CL and Cg, respectively, are the liquid-phase and gas-phase concentrations (M L-3) at the soil surface; Catm is the concentration in the atmosphere above the boundary layer; DE is the effective soil diffusion coefficient (L2/t); q is the Darcian water flux (L/t); h is the mass transfer coefficient (L/t); Dgair is the methyl bromide diffusion in air; and d is the boundary layer thickness (L). This approach assumes that a thin stagnant air layer occurs at the soil-atmosphere interface and chemical movement across the layer is due to vapor diffusion. The controlling parameter is the mass transfer coefficient, which is expressed as the ratio of the binary diffusion coefficient (i.e., air and MeBr) to the boundary layer thickness (Jury et al., 1983). A limitation of this approach is the estimation of the thickness of the stagnant boundary layer. Further, for some atmospheric conditions (e.g., changes in barometric pressure), it is likely that chemical transport occurs by both advection and diffusion. For these situations, assuming a stagnant boundary layer is inappropriate and more complex boundary conditions are required (Massmann and Farrier, 1992; Chen et al., 1995). An advantage of the stagnant boundary layer approach is that information about atmospheric conditions is unnecessary. However, adopting this boundary condition produces MeBr emission histories that are very regular and often do not resemble the erratic behavior commonly observed in the field (Majewski et al., 1995; Yates et al., 1996b; Yates et al., 1997).
 
A more accurate description of the volatilization process requires the coupling of soil-based processes with those operating in the atmosphere. This has led Baker et al. (1996) to develop an alternate formulation for the mass-transfer coefficient that includes atmospheric resistance terms.
 
Baker equation
 
where Re and Sc, respectively, are the roughness Reynolds and Schmidt numbers, u* is the friction velocity (L t-1), Ur is the wind speed at the measurement height (L t-1) and Φm is an atmospheric stability correction. This boundary condition depends on several aerodynamic parameters, such as the roughness Reynolds number, the Schmidt number, the friction velocity, the wind speed and an atmospheric stability term. The atmospheric resistance to diffusion near the soil surface and aerodynamic resistance from the diffusive layer to the measurement height affects the predicted emission rate. Further research is needed to evaluate the effectiveness of this approach in simulating the volatilization boundary condition, especially for agricultural fumigation.
 
Several studies have been conducted to determine whether conventional modeling approaches can accurately predict the rate of MeBr volatilization from bare soils (Wang et al., 1997b; Yates et al., 2002). Wang et al. (1997b) used CHAIN-2D to simulate methyl bromide emissions from a 3.5 ha field and compared the simulation results to emissions measured in a field experiment (Yates et al., 1996c). They found that the model simulated the total emission within a few percent of the measured value but the pattern of instantaneous emission rate was much more regular than the measured values and, at times, a value of the predicted volatilization rate could be very different from the measured value. Yates et al. (2002) conducted a similar study using the same experimental data and the volatilization boundary condition of Baker et al. (1996). They found that the predicted emissions had a more realistic temporal pattern compared a simulation based on the stagnant boundary layer. The total emissions were also within a few percent of the measured value.
 
When discrepancies occur, it cannot be determined whether the model or the measured values were in error since the measured volatilization rate is also subject to uncertainty (Majewski, 1997). Further research is needed to improve the accuracy of volatilization measurements and simulation models. Research is also needed to develop and test methods for coupling atmospheric and soil processes in models so that more accurate predictions of the volatilization rate can be obtained.
 

Simulating MeBr Volatilization
 
In typical applications, MeBr is applied to soil using metal shanks that cut into the soil. A nozzle is located at the rear of each shank that emits MeBr into the soil. The injection depth may vary from approximately 20 cm to 70 cm. As the tractor moves along the field, a layer of 0.025 mm (thick) by 3.5 m (wide) sheet high-density polyethylene (HDPE) plastic film is placed over the soil surface. For each sheet of plastic film, one side is buried in soil and the other is glued to the previous plastic sheet. This creates a series of panels down the field and a continuous plastic cover over the field. MeBr application rates range from 200 kg/ha to 400 kg/ha and large fractions (24% to 74%) of the applied chemical are lost to the atmosphere (Yagi et al., 1993, 1995; Majewski et al., 1995; Yates et al., 1996bc; Williams et al., 1999).
 
A field experiment was conducted to investigate the environmental fate and transport of MeBr after soil fumigation. The experiment was designed to determine the dynamics of MeBr movement through soil, degradation and cumulative emissions to the atmosphere. MeBr was applied to a 3.5 ha field, at a depth of 0.25 m and a rate of 240 kg/ha. Meteorological measurements of incoming solar radiation, net solar radiation, air temperature gradient, wind speed gradient, wind direction, relative humidity, and barometric pressure were obtained, on site, at 10 min intervals during the experiment. Soil heat flux measurements were taken at 0.05 and 0.1 m below the soil surface and soil temperature was measured at 0.05-, 0.10-, 0.15-, 0.20-, 0.30-, 0.40-, 0.50-, 0.80- and 1-m depths. Periodically during the experiment, MeBr soil-gas samples were taken to a depth of 150 cm.
 
Several methods were used to estimate the MeBr volatilization rate: the flux chamber (Denmead, 1979), aerodynamic (Parmele et al., 1972), theoretical profile shape (Wilson et al., 1982) and integrated horizontal flux methods (Denmead et al., 1977). For the micrometeorological methods, an air sampling mast was used to hold charcoal sampling tubes at heights of 0.1, 0.2, 0.5, 0.8, 1.2, and 1.6 m above the field surface. These measurements provide the air concentrations and concentration gradients required to estimate the volatilization rate. Estimate of the cumulative emission were obtained by integrating the flux measurements and by calculating the difference between the applied mass and the MeBr mass degraded (e.g., Br‾ ion), several months after application. The cumulative emissions obtained from these methods varied from 58% to 70% of the applied MeBr mass. A complete description of the experimental procedures is given in Yates et al. (1996abc).
 
A one-dimensional numerical model was developed to simulate MeBr degradation and movement in soil, and volatilization into the atmosphere. Since shank-injected soil fumigation results in a planar source of MeBr at the target depth, a one-dimensional model is appropriate for simulating the fate and transport process. Three volatilization boundary conditions, with increasing complexity, have been explored: (1) volatilization under isothermal conditions, (2) volatilization in response to solar-driven temperature changes at the soil surface, and (3) volatilization from soil coupled to atmospheric processes.
 
The results are shown in the following figures. Note that the simulated volatilization are predictions, therefore, none of the parameters in the model where adjusted to improve the correspondence between simulated and measured volatilzation.
 
References:
Yates, S.R., Wang, D., Papiernik, S.K. and Gan, J. 2002. Predicting pesticide volatilization from soils, Environmetrics 13:569–578.
 
Stagnant Boundary Layer Theory, Isothermal
Stagnant Boundary Layer Theory, Isothermal
Figure 1. Methyl bromide volatilization as a function of time after injection (i.e., t=0.6 days). Open circles are measured volatilization rates and solid lines are simulated values for a constant mass transfer coefficient. The inset shows cumulative emissions.
 
Stagnant Boundary Layer Theory, Nonisothermal (i.e.,h (T))
Stagnant Boundary Layer Theory, Nonisothermal
Figure 2. Methyl bromide volatilization as a function of time after injection (i.e., t=0.6 days). Open circles are measured volatilization rates and solid lines are simulated values for a mass transfer coefficient that varies with ambient temperature. The inset shows cumulative emissions.
 
Coupling Volatilization to Atmospheric Processes (i.e., Baker et al., 1996)
Coupling Volatilization to Atmospheric Processes
Figure 3. Methyl bromide volatilization as a function of time after injection (i.e., t=0.6 days). Open circles are measured volatilization rates and solid lines are simulated values for a mass transfer coefficient that depends on atmospheric processes (i.e., Baker et al., 1996). The inset shows cumulative emissions.
 

Fumigation
 
 
Methyl Bromide Fumigation
 
Two months before fumigating with methyl bromide, the soil was ripped to a depth of approximately 0.75 m in both the north-south and east-west directions. This was followed by discing, to break up the soil large aggregates. Several weeks before applying methyl bromide, the field was irrigated and followed by a waiting period that lasted until the soil water content was near field capacity.
 
The morning that methyl bromide was applied, the field was cultimulched to break any remaining soil aggregates to further protect the plastic from punctures. After cultimulching, the surface soil had a bulk density of approximately 1.35-1.40 gm/cm3 and was easily compressed.
 
Methyl bromide was applied to the field by a commercial applicator using a tractor containing two noble plows mounted on the center section of the tool bar with four injection points evenly spaced along each plow. A standard straight shank was located at each end of the tool bar. There were a total of 11 injection points for each panel (e.g., the width of field covered by a single sheet of plastic), spaced laterally approximately 0.25 m apart. During the first north-south pass along the eastern side of the field, methyl bromide was applied to the soil and a 3.6 m (i.e., 12 feet) sheet of 0.025 mm (1-mil) high-density polyethylene plastic was rolled out from behind the tractor and a small portion of each edge buried with a small noble plow located at the end of the tool bar. When the tractor reversed directions for the next pass, one side of the plastic was glued to the previous panel near the buried edge and the other edge was buried. This application method creates a series of panels down the field and a continuous polyethylene cover over the field.
 
The depth of injection was approximately 0.25 m. The methyl bromide was applied to the field as 99.5% methyl bromide (CH3Br) and 0.5% chloropicrin (CCl3NO2) (EPA Reg. No. 8536-12-11220). The application rate was approximately 240 kg/ha (i.e., 215 lb/a) to an area of approximately 3.5 ha (i.e., 8.6 acres), for a total applied mass of 843 kg. The fumigation process took 6 1/2 h and began at 0800 h.
 
References:
Yates, S.R., Gan, J., Ernst, F.F., Mutziger, A., Yates, M.V. 1996. Methyl bromide emissions from a covered field. I. Experimental Conditions and Degradation in Soil. Journal Environmental Quality 25:184-192.
 
Yates, S.R., Ernst, F.F., Gan, J., Gao, F. and Yates, M.V. 1996. Methyl bromide emissions from a covered field. II. Volatilization. Journal Environmental Quality 25:192-202.
 
Yates, S.R., Gan, J. and Ernst, F.F. Methyl bromide emissions from a covered field. III. 1996. Correcting Chamber Flux for Temperature. Journal Environmental Quality 25:892-898.
 

Post-Fumigation
 
Removing Tarp After Methyl Bromide Fumigation
The plastic tarp was removed from the field 120 hours after application of methyl bromide. The process took several hours.
 
References:
Yates, S.R., Gan, J., Ernst, F.F., Mutziger, A., Yates, M.V. 1996. Methyl bromide emissions from a covered field. I. Experimental Conditions and Degradation in Soil. Journal Environmental Quality 25:184-192.
 
Yates, S.R., Ernst, F.F., Gan, J., Gao, F. and Yates, M.V. 1996. Methyl bromide emissions from a covered field. II. Volatilization. Journal Environmental Quality 25:192-202.
 
Yates, S.R., Gan, J. and Ernst, F.F. Methyl bromide emissions from a covered field. III. 1996. Correcting Chamber Flux for Temperature. Journal Environmental Quality 25:892-898.
 

Further Reading
 
Abdalla N, Raski DJ, Lear B, Schmitt RV (1974) Distribution of methyl bromide in soils treated for nematode control in replant vineyards. Pestic Sci 5:259-269.

ACGIH (American Conference of Governmental Industrial Hygienists) (1988) Threshold limit values and biological exposure indices for 1988-1989, Cincinnati, OH, p.26.

Albritton DL, Watson RT (1992) Methyl bromide interim scientific assessment (MontrealProtocol Assessment Supplement), United Nations Environment Programme (UNEP), Nairobi, Kenya.

Alexeef GV, Kilgore WW (1983) Methyl bromide. In: Gunther FA & Gunther JD, ed. Residue reviews. Residues of pesticides and other contaminants in the total environment, Vol 88, New York, Springer Verlag , pp 102-153.

Anderson JG, Brune WH, Lloyd SA, Toohey DW, Sander SP, Starr WL, Lowenstein M, Podolske JR (1989) Kinetics of O3 destruction by ClO and BrO within the Antarctic vortex-An analysis based on in situ ER-2 data. J Geophys Res 94:11480-11520.

Anderson SO, Lee-Bapty S (1992) Methyl bromide interim technology and economic assessment (Montreal Protocol Assessment Supplement), United Nations Environment Programme (UNEP), Nairobi, Kenya.

Angus JF, Gardner PA, Kirdegaard JA, and Desmarchelier JM (1994) Biofumigation:Isothiocyanates released from Brassica roots inhibit growth of the take-all fungus. Plant Soil 162:107-112.Br>

Anonymous (1980) Dutch press compaign to outlaw methyl bromide. Grower 93:3.

Anonymous (1994) Methyl bromide under fire. Environ Health Persp 102:732-733.

Arvieu JC (1983) Some physico-chemical aspects of methyl bromide behavior in soil. Acta Horticulturae 152:267-274.

Arvieu JC, Cuany A (1985) Effects of organic matter on the biological activity and degradation of methyl bromide in soil. EPPO Bull 15:87-96.

Baker JM, Koskinen WC, Dowdy RH (1996) Volatilization of EPTC - Simulation and Measurement. J Environ Qual 25:169-177.

Basile M, Lamberti F (1981) Bromide residues in edible organs of plants grown in soil treated with methyl bromide. Med Fac Landbouww Rijksuniv Gent 46:337-341.

Bonazountas M, Wagner JM (1984) "SESOIL" - A seasonal soil compartment model. Arthur D. Little, Cambridge, MA

Borek V, Elberson LR, McCaffrey JP, Morra MJ (1997) Toxicity of rapeseed meal and methyl isothiocyanate to larvae of the black vine weevil (Coleoptera: Curculionidae). J Econ Entomol 90:109-112.

Briggs GG (1981) Theoretical and experimental relationships between soil adsorption, octanol-water partition coefficient, water solubilities, bioconcentration factors and the parachor. J Agric Food Chem 29:1050-1059.

Brown AL, Burau RG, Meyer RD, Raski DJ, Wilhelm S, Quick J (1979) Plant uptake of bromide following soil fumigation with methyl bromide. Calif Agric 33:11-13.

Brown BD, Rolston DE (1980) Transport and transformation of methyl bromide in soils. Soil Sci 130:68-75.

Brown G, Jenkinson DS (1971) Bromine in wheat grown in soil fumigated with methyl bromide. Soil Sci Plant Anal 2:45-54.

Brown RH, Purnell CJ (1979) Collection and analysis of trace organic vapour pollutants in ambient atmosphere: The performance of tenax-GC adsorbent tube. J Chromatogr 178:79-90.

Brutsaert W (1982) Evaporation into the atmosphere. D. Reidel Pub., Dordrecht, Holland. 299 pp.

Butler JH (1995) Methyl bromide under scrutiny. Nature 376: 469-470.

Butler JH (1996) Scientific uncertainties in the budget of atmospheric methyl bromide. Atm Environ 30(8): i-iii.

Butler JH (2000) Better budgets for methyl halides? Nature 403:260-261.

Carey WW, Klausutis NA, Barduhn AJ (1966) Solubility of four gas hydrate formers in water and aqueous sodium chloride solutions. Desalination 1:342-358.

Carpenter, J., L. Gianessi, and L. Lynch. 2000. The economic impact of the scheduled U.S. phase out of methyl bromide. National Center for Food and Agricultural Policy (NCFAP), Washington, DC. (www.ncfap.org/pup/ methyl% 20bromide/mb.pdf)

Carsel RF, Imhoff JC, Hummel PR, Cheplick JM, Donigian AS (1998) PRZM-3:A Model for Predicting Pesticide and Nitrogen Fate in the Crop Root and Unsaturated Soil Zones; Users Manual for Release 3.0. U.S. EPA.

Carsel RF, Mulkey LA, Lorber MN, Baskin LB (1985) The pesticide root zone model (PRZM): A procedure for evaluating pesticide leaching threats to groundwater. Ecological Modeling. 30:49-69.

Castro CE, Belser NO (1981) Photohydrolysis of methyl bromide and chloropicrin. J Agric Food Chem 29:1005-1008.

Chakrabarti B, Bell CH (1993) The methyl bromide issue. Chem. & Ind. 24:992-995.

Chakrabarti B, Wontner-Smith T, Bell CH (1995) Reducing methyl bromide emissions from soil fumigation in greenhouses. Proceedings of the Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions, Nov. 6-8, 1995, San Diego, CA, Methyl Bromide Alternatives Outreach, Fresno, CA pp 25-1–25-3.

Chen C, Green RE, Thomas DM, Knuteson, JA (1995) Modeling 1,3-D fumigant volatilization with vapor phase advection in the soil profile. Environ Sci Technol 29:1816-1821.

Chitwood DE, Deshusses MA (2001) Development of a methyl bromide collection system for fumigated farmland. Environ Sci Technol 35:636-642.

Cicerone RJ, Heidt LE, Pollock WH (1988) Measurements of atmospheric methyl bromide and bromoform. J Geophys Res 93: 3745-3749.

Clendening LD (1988) A field mass balance study of pesticide volatilization, leaching and persistence, Ph.D. Dissertation, Department of Soil & Environmental Sciences, University of California, Riverside, 92521, 221 pp.

Colby J, Dalton H, Whittenbury R (1975) An improved assay for bacterial mono-oxygenase: Some properties of the enzyme from Methylomonas methanica. Biochem J 151:459-462.

Colby J, Stirling DI, Dalton H (1977) The soluble methane mono-oxygenase of methylococcus capsulatus (Bath). Biochem J 165: 395-402.

Coosemans J, Van Assche C (1976) Investigations of the bromide concentration in Belgian greenhouse lettuce after methyl bromide disinfestation. Med Fac Landbouww Rijksuniv Gent 41:1361-1369.

CRC (1996) Handbook of Chemistry and Physics, 77th Ed. Lide DR (ed) CRC Press, New York, NY.

Cuany A, Arvieu JC (1983) Distribution patterns and nematocidal activity of methyl bromide in various soil conditions and methods of application. Acta Hortilculturae 152:277-287.

Daponte TLF (1995) Barrier films: Hytibar. Acta Horticulturae 382:56-66.

de Heer H, Hamaker Ph, Tuinstra, LGMTh, van der Burg AMM (1983) Use of gastight plastic films during fumigation of greenhouse soils with methyl bromide I. Significance of permeation and leakage for the emission into the outside air. Acta Horticulturae 152:109-126.

de Heer H, Hamaker Ph, Tuinstra LGMTh, van der Burg AMM (1986) Leaching of methyl bromide and bromide ions into surface water after fumigation of glasshouse soils. In: J.F. de Jong and L.G. Solbe (eds) Effects of land use on fresh waters: agriculture, forestry, mineral exploitation, urbanization. Chichester, England.

de Mello WZ, Hines ME (1994) Application of static and dynamic enclosures for determining dimethyl sulfide and carbonyl sulfide exchange in Sphagnum peatlands: Implications for the magnitude and direction of flux. J Geophys Res 99:14601-14607.

Denmead OT, Raupach MR (1993) Methods for measuring atmospheric gas transport in agricultural and forest systems. In: Harper LA, Mosier AR, Duxbury JM, Rolston DE (eds) Agricultural ecosystem effects on trace gases and global climate change, ASA Spec. Publ. 55, American Society of Agronomy, Madison, Wisconsin, pp 19-43.

Denmead OT, Simpson JR, Freney JR (1977) A direct field measurement of ammonia emission after injection of anhydrous ammonia. Soil Sci Soc Am J 41:1001-1004.

Duafala T (1996) Do we need further controls of agricultural methyl bromide? Atm Environ 30(8): iii-iv.

Dumas T (1982) Trapping low levels of methyl bromide in air or as residues at ambient and lower temperatures for gas chromatography. J AOAC 65:913-915.

Dumas T, Bond EJ (1985) Analysis of methyl bromide at ultra low concentration levels. J Agric Food Chem 33:276-278.

Dungan R, Gan J, Yates SR (2002) Accelerated degradation of methyl isothiocyanate in soil. Water, Air, Soil Pollut (in press).

Eayre CG, Sims JJ, Ohr HD, Mackey B (2000) Evaluation of methyl iodide for control of peach replant disorder. Plant Disease 84:1177-1179.

Eller, PM (1984) In: NIOSH Mannual of Analytical Methods, 3rd ed., US Department of Health and Human Services, Pub No 84-100, pp 2520-1-4.

Enfield CG, Carsel RF, Cohen SZ, Phan T, Walters DM (1982) Approximating pollutant transport to ground water. Ground Water 20:711-722.

Fallico R, Ferrante M (1991) Exposure to inorganic bromides from greenhouse crops where methyl bromide was applied for soil fumigation. Zbl Hyg 171:555-562.

Ferguson W, Padula A (1994) Economic effects of banning methyl bromide for soil fumigation. USDA Economic Research Service, Agricultural Economic Report 677, USDA, Washington, DC.

Fleagle RG, Businger JA (1980) An introduction to atmospheric physics, 2nd Edition. International Geophysics Series, Vol 25. Academic Press, New York, 432 pp.

Flessa H, Dörsch P, Beese F. (1995) Seasonal variation of N2O and CH4 fluxes in differently managed arable soils in southern Germany. J Geophys Res 100:23115-23124.

Fowler D, Duyzer JH (1989) Micrometeorological techniques for the measurement of trace gas exchange. In: Exchange of trace gases between terrestrial ecosystems and the atmosphere; Andreae MO, Schimel DS (eds) Wiley & Sons, Chichester, England, pp 189-207.

Fransi A, Pons R, Sala A, Vallejo VR, Bertran C (1987) Wheat and soil bromide dynamics after fumigation with methyl bromide in a mediterranean climate. Plant Soil 98: 417-424.

Gamliel A, Grinstein A, Katan J (1997) Improved technologies to reduce emission of methyl bromide from fumigated soil. Phytoparasitica, 25:S21-S30.

Gan JY, Anderson MA, Yates MV, Spencer WF, Yates SR (1995a) Sampling and stability of methyl bromide on activated charcoal. J Agric Food Chem 43:1361-1367.

Gan, J, Becker JO, Ernst FF, Hutchinson C, Knuteson JA, Yates SR 2000a. Surface application of ammonium thiosulfate fertilizer to reduce volatilization of 1,3-dichloropropene from soil. Pest Manag Sci 56:264-270.

Gan JY, Yates SR (1996) Degradation and phase-partition of methyl iodide in soil. J Agric Food Chem 44:4001-4008.

Gan JY, Yates SR (1998) Recapturing and decomposing methyl bromide in fumigation effluents, J Hazard Mat 57:249-258.

Gan J, Yates SR, Anderson MA, Spencer WF, Ernst FF (1994) Effect of soil properties on degradation and sorption of methyl bromide in soil. Chemosphere 29:2685-2700.

Gan JY, Yates SR, Becker JO, Wang D (1998d). Surface amendment of fertilizer ammonium thiosulfate to reduce methyl bromide emission from soil. Environ Sci Technol 32:2438-2441.

Gan JY, Yates SR, Crowley D, Becker OJ (1998c) Acceleration of 1,3-D degradation by organic amendments and potential application for emission reduction. J Environ Qual 27:408-414.

Gan, J, Yates SR, Ernst FF, Jury WA 2000b. Degradation and volatilization of the fumigant chloropicrin after soil treatment. J Environ Qual 29:1391-1397.

Gan JY, Yates SR, Ohr HD, Sims JJ (1997b) Volatilization and distribution of methyl iodide and methyl bromide after subsoil application. J Environ Qual 26:1107-1115.

Gan JY, Yates SR, Ohr HD, Sims JJ (1998a) Production of methyl bromide by terrestrial higher plants. Geophys Res Lett 25:3595-3598.

Gan JY, Yates SR, Papiernik SK, Crowley D (1998b) Application of organic amendments to reduce volatile pesticide emissions from soil. Environ Sci Technol 32:3094-3098.

Gan JY, Yates SR, Spencer WF, Yates MV (1995b) Optimization of analysis of methyl bromide on charcoal sampling tubes. J Agric Food Chem 43:960-966.

Gan JY, Yates SR, Spencer WF, Yates MV, Jury WA (1997a) Laboratory-scale measurements and simulations of the effect of application methods on soil methyl bromide emission. J Environ Qual 26:310-317.

Gan JY, Yates SR, Wang D, Spencer WF (1996) Effect of soil factors on volatilization of methyl bromide from soil. Environ Sci Technol 30:1629-1636.

Gardiner, JB, Morra MJ, Eberlein CV, Brown PD, and Borek V (1999) Allelochemicals released in soil following incorporation of rapeseed (Brassica napus) green manures. J Agric Food Chem 47:3837-3842.

Gentile IA, Ferraris L, Crespi S, Belligno A (1989) The degradation of methyl bromide in some natural fresh waters. Influence of temperature, pH and light. Pestic Sci 25:261-272.

Gentile IA, Ferraris L, Sanguinetti M, Tiprigan M, Fisichella G (1992) Methyl bromide in natural fresh waters: hydrolysis and volatilisation. Pestic Sci 34:297-301.

Gholson AR, Jayanty RKM, Storm JF (1990) Evaluation of aluminum canisters for the collection and storgae of air toxics. Anal Chem 62:1899-1902.

Goring CAI (1962) Theory and principles of soil fumigation. Adv Pest Control Res 5:47-84.

Grimsrud EP, Miller DA (1978) Oxygen doping of carrier gas in measurement of halogenated methanes by gas chromatography with electron capture detector. Anal Chem 50:1141- 1145.

Hague NG, Sood S (1963) Soil sterilization with methyl bromide to control soil nematodes. Plant Pathol 12:88.

Hamaker Ph, de Heer H, van der Burg AMM (1983) Use of gastight plastic films during fumigation of glasshouse soils with methyl bromide II. Effects on the bromide-ion mass balance for a polder district. Acta Horticulturae 152:127-135.

Harrison RM, Yamulki S, Goulding KWT, Webster CP (1995) Effect of fertilizer on NO and N2O fluxes from agricultural fields. J Geophys Res 100:25923-25931.

Harsch DE, Rasmussen RA (1977) Identification of methyl bromide in urban air. Anal Lett 10:1041-1047.

Hass HV, Klein L (1976) Influence of methyl bromide soil fumigation on fungicidal efficacy and bromide residues. Phytoparasitica 4:123-129.

Helweg A, Rasmussen AN (1982) Influence of soil fumigation with methyl bromide on bromide content in soil and in lettuce grown in the soil. Tidsskr Planteavl 86:461-469.

Hemwall JB (1959) A mathematical theory of soil fumigation. Soil Sci 88:184-190.

Hemwall JB (1962) Theoretical consideration of soil fumigation. Phytopathol 52:1108-1118.

Herzel F, Schmidt G (1984) The persistence of the fumigant methyl bromide in soil and water. Wasser Boden 36:589-591.

Hines, ME, Crill, PM, Varner, RK, Talbot, RW, Shorter, JH, Kolb, CE, Harriss, RC. (1998) Rapid consumption of low concentrations of methyl bromide by soil bacteria. Appl Environ Microbiol 64:1864-1870.

Hoffmann GM, Malkomes HP (1974) Bromide residues in vegetable crops after soil fumigation with methyl bromide. Agric Environ 1:321-328.

Hoffmann GM, Malkomes HP (1978) The fate of fumigants. In: Mulder D (ed) Soil Disinfestation pp 291-335.

Hollingsworth EB (1980) Volatility of trifluralin from field soil. Weed Sci 28:224-228.

Howard PH (1989) Handbook of environmental fate and exposure data for organic chemicals. Vol 1. Lewis Publishers, Chelsea MI.

Hutchinson CM, McGiffen ME, Ohr HD, Sims JJ, Becker JO (2000) Efficacy of methyl iodide and synergy with chloropicrin for control of fungi. Pest Manag Sci 56:413-418.

Hutchinson GL, Mosier AR (1981) Improved soil cover method for field measurement of nitrous oxide fluxes, Soil Sci Soc Am J 45:311-316.

Hyman MR, Wood PM (1984) Bromocarbon oxidations by Nitrosomonas europaea. In: Crawford RL, Hanson RS (ed) Microbial growth on C1 compounds. American Society for Microbiology, Washington DC, pp49-52.

Jayanty RKM (1989) Evaluation of sampling and analytical methods for monitoring toxic organics in air. Atmos Environ 23:777-782.

Jin Y, Jury WA (1995) Methyl bromide diffusion and emission through soil under various management techniques. J Environ Qual 24:1002-1009.

Jury WA (1985) Spatial variability of soil physical parameters in solute migration: A critical literature review. Research Report EA-4228, Electric Power Research Institute, Palo Alto, CA 66 p.

Jury WA, Farmer WJ, Spencer WF (1984a) Behavior assessment model for trace organics in soil: II. Chemical classification and parameter sensitivity. J Environ Qual 13:567-572.

Jury WA, Gardner WR, Gardner WH (1991) Soil Physics. John Wiley and Sons, Inc., New York, NY. pp 234-242.

Jury WA, Jin Y, Gan J, Gimmi T (1996) Strategies for reducing fumigant loss to the atmosphere. In: Seiber JN, Knuteson JA, Woodrow JE, Wolfe NL, Yates MV, Yates SR (eds) Fumigants: Environmental fate, exposure, and analysis. ACS Symposium Series 652, American Chemical Society, Washington, DC., pp 104-115.

Jury WA, Letey J, Collins T (1982) Analysis of chamber methods used for measuring nitrous oxide production in the field. Soil Sci Soc Am J 46:250-256.

Jury WA, Spencer WF, Farmer WJ (1983) Behavior assessment model for trace organics in the soil. I. Model description. J Environ Qual 12:558-564.

Jury WA, Spencer WF, Farmer WJ (1984b) Behavior assessment model for trace organics in soil: III. Application of screening model. J Environ Qual 13:573-579.

Juzwik J, Stenlund DL, Allmaras RR, Copeland SM, McRoberts RE. (1997) Incorporation of tracers and dazomet by rotary tillers and a spading machine. Soil Tillage Res. 41:237-248.

Kallio H, Shibamoto T (1988) Direct capillary trapping and gas chromatographic analysis of bromomethane and other highly volatile air pollutants. J Chromatogr 454:392-397.

Karickhoff SW (1981) Semi-empirical estimation of sorption of hydrophobic pollutants on natural sediments and soils. Chemosphere 10:833-845.

Katan J (1992) Soil solarization research as a model for the development of new methods of disease control. Phytoparasitica 20:S133-S135

Kempton RJ, Maw GA (1972) Soil fumigation with methyl bromide: bromide accumulation by lettuce plants. Ann Appl Biol 72: 71-79.

Kempton RJ, Maw GA (1973) Soil fumigation with methyl bromide: the uptake and distribution of inorganic bromide in tomato plants. Ann Appl Biol 74:91-98.

Kempton RJ, Maw GA (1974) Soil fumigation with methyl bromide: the phytotoxicity of inorganic bromide to carnation plants. Ann Appl Biol 76:217-229.

Kerwin RA, Crill PM, Talbot RW, Hines ME, Shorter JH, Kolb CR, Harriss RC (1996) Determination of atmospheric methyl bromide by cryotrapping-gas chromatography and application to soil kinetic studies using a dynamic dilution system. Anal Chem 68:899-903.

Khalil MAK, Rasmussen RA, Gunawardena R (1993) Atmospheric methyl bromide: trends and global mass balance. J Geophys Res 98:2887-2896.

King DB, Butler JH, Montzka SA, Yvon-Lewis SA, Elkins JW (2000) Implication of methyl bromide supersaturations in the temperate North Atlantic Ocean. J. Geophys. Res. 105:19763-19769.

Knavel DE, Watkins H, Herron JW (1965) The influence of soil temperature, soil moisture and soil composition on the diffusion of methyl bromide. Am Soc Horti Sci 87:573-578.

Kolbezen MJ, Abu-El-Haj FJ (1977) Permeability of plastic films to fumigants. Proceedings of the International Agricricultural Plastics Congress, April 11-16, San Diego, CA. American Society for Plasticulture, Harrisburg, PA pp 1-6.

Kolbezen MJ, Munnecke DE, Wilbur WD, Stolzy LH, Abu-El-Haj FJ, Szuszkiewicz TE (1974) Factors that affect deep penetration of field soils by methyl bromide. Hilgardia 42:465- 492.

Krost KJ, Pellizzari ED, Walburn SG, Hubbard SA (1982) Collection and analysis of hazardous organic emissions. Anal Chem 54:810-817.

Lavergne JC, Arvieu JC (1983) Evolution and accumulation of inorganic bromide residues in soil and plant after localised and repeated applications of methyl bromide. Acta Horticulturae 152:289-296.

Lear B, Towson AJ, Miyagawa ST (1983) The value of leaching for removing inorganic bromide residues from soil after application of methyl bromide. Acta Horticulturae 152:305-313.

Lefevre C, Ferrari P, Guenier JP, Muller J (1989) Sampling and analysis of airborne methyl bromide. Chromatographia 27:37-43.

Le Goupil P (1932) Les propriétés insecticides du bromure de methyl. Rev Path Vég Ent Agric France 19:169-172.

Leistra M, Groen AE, Crum SJH, van der Pas LJT (1991) Transformation rate of 1,3-dichloropropene and 3-chloroallyl alcohol in topsoil and subsoil material of flower-bulb fields. Pestic Sci 31:197-207.

Lembright HW (1990) Soil fumigation: Principles and application technology. Suppl J Nematol 22:632-644.

Leonard RA, Knisel WG, Still DA (1987) GLEAMS: Groundwater loading effects of agricultural management systems. Trans. of the ASAE 30:1403-1418.

Livingston GP, Hutchinson GL (1995) Enclosure-based measurement of trace gas exchange: applications and sources of error, Chapter 2, In: Matson PA, Harriss RC (ed) Methods in Ecology- Biogenic Trace Gases: Measuring Emissions from Soil and Water. Blackwell Science, Cambridge, MA pp 52-97.

Lobert JM, Yvon-Lewis SA, Butler JH, Montzka SA, Myers RC (1997) Undersaturation of CH3Br in the southern ocean. Geophys Res Lett 24:171-172.

Ma QL, Gan JY, Becker JO, Papiernik SK, Yates SR (2001) Evaluation of propargyl bromide for control of barnyardgrass and Fusarium oxysporum in three soils. Pest Manag. Sci. 57:781-786.

Mabey W, Mill T (1978) Critical review of hydrolysis of organic compounds in water under environmental conditions. J Phys Chem Ref Data 7:383-415.

MaCartney L, Price TV (1988) Bromide residues in glasshouse soils in Victoria following bromomethane fumigation. Soil Biol Biochem 20: 393-397.

Majewski MS (1997) Error evaluation of methyl bromide aerodynamic flux measurements. In: Seiber JN, Knuteson JA, Woodrow JE, Wolfe NL, Yates MV, Yates SR (eds) Fumigants: Environmental fate, exposure, and analysis. ACS Symposium Series 652, American Chemical Society, Washington, DC., pp 135-153.

Majewski MS, Glotfelty DE, Paw KT, Seiber JN (1990) A field comparison of several methods for measuring pesticide evaporation rates from soil. Environ. Sci. Technol. 24:1490-1497.

Majewski MS, Glotfelty DE, Seiber JN (1989) A comparison of the aerodynamic and the theoretical-profile-shape methods for measuring pesticide evaporation from soil. Atmos Environ 23:929-938.

Majewski MS, McChesney MM, Woodrow JE, Prueger JH, Seiber JN (1995) Aerodynamic measurements of methyl bromide volatilization from tarped and nontarped fields. J Environ Qual 24:742-751.

Malathrakis NE, Sarris GE (1983) Bromide residues in greenhouse cucumbers and the possibility of reducing them. Acta Horticulturae 152:297-304.

Manö S, Andreae MO (1994) Emission of methyl bromide from biomass burning. Science 263:1255-1256.

Massicotte HB, Tackaberry, LE, Ingham, ER, Thies, WG (1998) Ectomycorrhizae establishment on Douglas-fir seedlings following chloropicrin treatment to control laminated-root rot disease: Assessment 4 and 5 years after outplanting. Appl Soil Ecology 10:117-125.

Massmann J, Farrier DF (1992) Effects of atmospheric pressures on gas transport in the vadose zone. Water Resourc Res 28: 777-791.

Masui M, Nukaya A, Ishida A (1979) Bromine uptake of some vegetable crops following soil fumigation with methyl bromide. J Japan Soc Horti Sci 48:55-60.

Masui M, Nukaya A, Ocura T (1978) Bromine uptake of muskmelon and cucumber plants following soil fumigation with methyl bromide. J Japan Soc Hort Sci 47:343-350.

Matthias AD, Blackmer AM, Bremner JM (1980) A simple chamber technique for field measurement of emissions of nitrous oxide from soil, J Environ Qual 9:251-256.

Maw GA, Kempton RJ (1973) Methyl bromide as a soil fumigant. Soil Fertil 36:41-47.

McElroy MB, Salawitch RJ, Wofsy SC, Logan JA (1986) Reductions of Antarctic ozone due to synergistic interactions of chlorine and bromine. Nature 321:759-762.

McHenry M (1994) A discussion of five portable soil drenching devices, three biocide injection methods and ten biocidal agents under field conditions. Proceedings of Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions. San Diego, CA, November 6-8, Methyl Bromide Alternatives Outreach, Fresno, CA pp 30-1–30-2.

Melichar MW (1995) Soil injection of 1,3-dichloropropene, alone or combined with chloropicrin and/or preemergent herbicides, for nematode, soil-borne diseases and weed control. Proceedings of Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions. San Diego, CA, November 6-8, Methyl Bromide Alternatives Outreach, Fresno, CA pp 51-1–51-2.

The Merck Index (1996) Budavari, S (ed) Merck, Whitehouse Station, NJ.

Meyers AJ (1980) Evaluation of bromomethane as a suitable analogue in methane oxidation studies. FEMS Microbiol Lett 9:297-300.

Miller LG, Connell TL, Guidetti JR, Oremland RS (1997) Bacterial oxidation of methyl bromide in fumigated agricultural soils. Appl Environ Microbiol 63:4346-4354.

Munnecke DE, Kolbezen MJ, Wilbur WD (1977) Types and thickness of plastic films in relation to methyl bromide fumigation. Proceedings of the 7th International Agricultural Plastics Congress, San Diego, CA, April 11-16 American Society for Plasticulture, Harrisburg, PA pp.482-487.

Munnecke DE, Van Gundy SD (1979) Movement of fumigants in soil, dosage responses, and differential effects. Ann Rev Phytopathol 17:405-429.

Muramatsu Y, Yoshida S (1995) Volatilization of methyl iodide from the soil-plant system. Atmos Environ 29:21-25.

NAPIAP (The National Agricultural Pesticide Impact Assessment Program) (1993) The Biologic and Economic Assessment of Methyl Bromide, USDA-NAPIAP, Washington, D.C.

Nazer IK, Hallak AB, Abu-Gharbieh WI, Saleh NS (1982) Bromine residues in the soil and fruits of certain crops after soil fumigation with methyl bromide. J Radioanal Chem 74:113- 116.

Nelson SD, Riegel C, Allen LH, Dickson DW, Gan JY, Locascio SJ, Mitchel DJ (2001) Volatilization of 1,3-dichloropropene in Florida plasticulture and effects on fall squash production. J Am Soc Hort Sci 126:496-502.

Noling JW, Becker JO (1994) The challenge of research and extension to define and implement alternatives to methyl bromide. J Nematol 26:573-586.

Olson SM, Noling JW (1994) Fumigation trials for tomatoes and strawberries in northwest Florida. Proceedings of Second Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions. San Diego, CA, November 6-8, Methyl Bromide Alternatives Outreach, Fresno, CA pp 6-1–6-4.

Oremland RS, Miller LG, Culbertson, CW, Connell TL, Jahnke L (1994b) Degradation of methyl bromide by methanotrophic bacteria in cell suspensions and soils. Appl Environ Microbiol 60:3640-3646.

Oremland RS, Miller LG, Strohmaier FE (1994a) Degradation of methyl bromide in anaerobic sediments. Environ Sci Technol 28: 514-520.

Ou LT, Joy PJ, Thomas JE, Hornsby AG (1997) Stimulation of microbial degradation of methyl bromide in soil during oxidation of an ammonia fertilizer by nitrifiers. Environ Sci Technol 31:717-722.

Papiernik SK, Gan JY, Yates SR (2000) Mechanism of degradation of methyl bromide and propargyl bromide in soil. J Environ Qual 29:1322-1328.

Papiernik SK, Yates SR (2002) Effect of environmental conditions on the permeability of high density polyethylene film to fumigant vapors. Environ Sci Technol (in press).

Papiernik SK, Yates SR, and Gan J (2001) An approach for estimating the permeability of agricultural films. Environ Sci Technol 35:1240-1246.

Parmele LH, Lemon ER, Taylor AW (1972) Micrometeorological measurement of pesticide vapor flux from bare soil and corn under field conditions. Water, Air, Soil Pollut 1:433-451.

Penkett SA, Jones BMR, Rycroft MJ, Simmons DA (1985) An interhemispheric comparison of the concentrations of bromine compounds in the stratosphere. Nature 318:550-553.

Porter IJ, Merriman PR, and Keane PJ (1991) Soil solarisation combined with low rates of soil fumigants controls clubroot of caulifowers, caused by Plasmodiophora brassicae Woron. Aust J Exp Agric 31:843-851.

Prather MJ, Watson RT (1990) Stratospheric ozone depletion and future levels of atmospheric chlorine and bromine. Nature 344: 729-734.

Rao PSC, Hornsby AG, Jessup RE (1985) Indices for ranking the potential for pesticide contamination of groundwater. Soil Crop Sci Soc Fl Proc 44:1-8.

Rasche ME, Hyman MR, Arp DJ (1990) Biodegradation of halogenated hydrocarbon fumigants by nitrifying bacteria. Appl Environ Microbiol 56:2568-2571.

Redeker KR, Wang NY, Low JC, McMillan A, Cicerone RJ (2000) Emissions of methyl halides and methane from rice paddies. Science. 290:966-969.

Reible DD (1994) Loss of methyl bromide to the atmosphere during soil fumigation. J Hazard Mat 37:431-444.

Reicosky DC (1990) Canopy gas exchange in the field: closed chamber. Remote Sensing Rev 5:163-177.

Reid RC, Prausnitz JM, Poling BE (1987) The properties of gases & liquids, 4th Ed. McGraw-Hill Co., New York, NY, 741 pp.

Rhew RC, Miller BR, Weiss RF (2000) Natural methyl bromide and methyl chloride emissions from coastal salt marshes. Nature 403: 292-295.

Riegel C, Dickson DW, Peterson LG, Nance JL (2000) Rate response of 1,3-dichloropropene for nematode control in spring squash in deep sand Soils. J Nematology 32:524-530.

Rolston DE (1986) Gas flux. In: Klute A (ed) Methods of soil analysis. Part 1. Physical and mineralogical methods; Agronomy Monograph 9, American Society of Agronomy and Soil Science Society of America, Madison, WI pp 1103-1119.

Rolston DE, Glauz RD (1982) Comparisons of simulated with measured transport and transformation of methyl bromide gas in soils. Pestic Sci 13:653-664.

Roorda AN, Eysinga JPNL, de Bes SS (1984) Bromine in glasshouse lettuce as affected by chemical soil disinfectants and steam sterilization. Acta Horticulturae 145:262-268.

Rosenberg NJ, Blad BL, Verma SB (1983) Microclimate, The biological environment. John Wiley & Sons, New York, 495 pp.

Roughan JA, Roughan PA (1984a) Pesticide residues in foodstuffs in England and wales. Part I: Inorganic bromide ion in lettuce grown in soil fumigated with bromomethane. Pestic Sci 15:431-438.

Roughan JA, Roughan PA (1984b) Pesticide residues in foodstuffs in England and Wales. Part II: Inorganic bromide ion in cucumber, tomato and self-balancing celery grown in soil fumigated with bromomethane, and the 'natural' bromide ion content in a range of fresh fruit and vegetables. Pestic Sci 15:630-636.

Saini HS, Attieh JM, Hanson AD (1995) Biosynthesis of halomethanes and methanethiol by higher plants via a novel methyltransferase reaction. Plant, Cell Environ 18:1027-1033.

Salawitch RJ, Wofsy SC, McElroy MB (1988) Chemistry of OclO in the Antarctic stratosphere: Implications for bromine. Planet Space Sci 36:213-224.

Sauvegrain R (1995) Methyl bromide and the ozone layer: a responsible answer from the industry. Acta Horticulturae 382:51-55.

Schauffler SM, Heidt LE, Pollock WH, Gilpin TM, Vedder JF, Solomon S, Lueb RA, Atlas EL (1993) Measurements of halogenated organic-compounds near the tropical tropopause. Geophys Res Lett 20:2567-2570.

Schwarzenbach RP, Gschwend PM, Imboden DR (1993) Environmental Organic Chemistry. Wiley, New York, 1992.

Scudamore KA (1988) Fumigant analysis. In: (G Zweig and J Sherma, eds) Analytical Methods for Pesticides and Plant Growth Regulators Vol. XVI pp 207-261.

Shaw DV, Larson KD (1999) A meta-analysis of strawberry yield response to preplant soil fumigation with combinations of methyl bromide-chloropicrin and four alternative systems. HortScience 34:839-845.

Shorter JH, Kolb CE, Crill PM, Kerwin RA, Talbot RW, Hines ME, Harriss RC (1995) Rapid degradation of atmospheric methyl bromide in soils. Nature 377:717-719.

Siebering H, Leistra M (1978) Computer simulation of fumigant behavior in soil. In: D Mulder (ed) Soil Disinfestation. Elsevier North-Holland. pp 135-161.

Šimunek, J, Sejna M, van Genuchten M Th (1996) The HYDRUS-2D software package for simulating water flow and solute transport in two-dimensional variably saturated media. Version 1.0, International Ground Water Modeling Center publication IGWMC-TPS-53, Colorado School of Mines, Golden CO 167 p.

Šimunek, J, van Genuchten M Th (1994) The CHAIN_2D code for simulating two-dimensional movement of water flow, heat, and multiple solutes in variably-saturated porous media, Version 1.1. United States Salinity Laboratory Research Report #136. United States Salinity Laboratory, Riverside, CA 205 p.

Singh HB, Kanakidou M (1993) An investigation of the atmopheric sources and sinks of methyl bromide. Geophys Res Lett 20:133-136.

Solomon S, Mills M, Heidt LE, Pollock WH, Tuck AF (1992) On the evaluation of ozone depletion potentials. J. Geophys Res 97: 824-842.

Staerk H, Suess A (1974) Bromine content of vegetables and its accumulation after soil fumigation with methyl bromide, using neutron activation analysis. In: Comparative studies of food and environmental contamination, Proceedings of a symposium. IAEA, Vienna.

Stapleton JJ (2000) Soil solarization in various agricultural production systems. Crop Protection 19:837-841.

Taylor RWD (1994) Methyl bromide - Is there any future for this noteworthy fumigant? J Stored Prod Res 30:253-360.

Thomas W (1996) Methyl bromide-pesticide and environmental threat. Atmos Environ 30(8):i-ii.

Thompson RH (1966) A review of the properties and usage of methyl bromide as a fumigant. J Stored Prod Res 1:253-376.

United States Department of Agriculture (USDA) (2001) Atmospheric impact of agricultural use of methyl bromide. Methyl Bromide Alternatives. 7:1-2.

United Nations Environment Programme (UNEP) (1995) 1994 Report of the methyl bromide technical options committee. UNEP Ozone Secretariat, Nairobi, Kenya.

United States Environmental Protection Agency (USEPA) (1993) Protection of stratospheric ozone. Fed Regist 58(51):15014-15049.

United States Environmental Protection Agency (USEPA) (2000) Protection of stratospheric ozone: Incorporation of Clean Air Act amendments for restrictions in Class I, Group VI controlled substances. Fed Regist 65:70795-70804.

Valente RJ, Thornton FC, Williams EJ (1995) Field comparison of static and flow-through chamber techniques for measurement of soil NO emission. J Geophys Res 100:21147-21152.

Vanachter A, Feyaerts J, van Wambeke E, van Assche C (1981b) Bromide concentrations in water after methyl bromide soil disinfestation. II. Relation between leaching of methyl bromide fumigated greenhouse soils and bromide concentrations in the surrounding surface waters. Med Fac Landbouww Rijksuniv Gent 46:351-358.

Vanachter A, van Pee G, van Wambeke E, van Assche C (1981a) Bromide concentration in water after methyl bromide soil disinfestation. I. Relation between soil type, efficiency of leaching and bromide concentration in the leaching water. Med Fac Landbouww Rijksuniv Gent 46:343-349.

van Assche C (1971) Behavior and perspectives of chemical soil fumigation. Proc 6th Brit Insect Fumg Conf. pp 706-714.

van den Berg F, Boesten J (1997) User’s manual for version 3.1 of PESTLA. Unpublished report, DLO-Winand Staring Centre, Wageningen, The Netherlands.

van Wambeke E (1983) Efficiency increase of methyl bromide soil fumigation by admixture with methyl chloride or ameliorated tarps. Acta Horticulturae 152:137-145.

van Wambeke E, Vanachter A, van Assche C (1988) Measures for improving soil fumigation. Proceedings of the Brighton Crop Protection Conference - Pests and Diseases. British Crop Protection Council, Surrey, UK pp 199-204.

Wagenet RJ, Hutson JL (1987) LEACHM: Leaching estimation and chemistry model. Continuum Vol. 2. Water Res. Institute, Cornell Univ., Ithaca, NY.

Walker GE, Morey BG (1999) Effect of brassica and weed manures on abundance of Tylenchulus semipenetrans and fungi in citrus orchard soil. Aust J. Exper Agric 39:65-72.

Wang Q, Gan J, Papiernik SK, Yates SR (2000) Transformation and detoxification of halogenated fumigants by ammonium thiosulfate. Environ Sci Technol 34:3717-3721.

Wang D, Yates SR (1998) Methyl bromide emission from fields partially covered with a high-density polyethylene and a virtually impermeable film. Environ Sci Technol 32:2515-2518.

Wang D, Yates SR, Ernst FF, Gan J, Gao F, Becker JO (1997c) Methyl bromide emission reduction with field management practices. Environ Sci Technol 31:3017-3022.

Wang D, Yates SR, Ernst FF, Gan J, Jury WA (1997a) Reducing methyl bromide emission with a high barrier plastic film and reduced dosage. Environ Sci Technol 31:3686-3691.

Wang D, Yates SR, Gan J (1997b) Temperature effect of fate and transport of methyl bromide in soil fumigation. J Environ Qual 26:1072-1079.

Watson RT, Albritton DL, Andersen SO, Lee-Bapty S (1992) Methyl Bromide: Its Atmospheric Science, Technology, and Economics. United Nations Environment Programme, Ozone Secretariat, Nairobi, Kenya.

Wauchope RD, Buttler TM, Hornsby AG, Augustijn Beckers PWM, Burt JP (1992) The SCS/ARS/CES pesticide database for environmental decision-making. Rev Environ Contam Toxicol 123:1-164.

Wegman RCC, Greve PA, de Heer H, Hamaker Ph (1981) Methyl bromide and bromide-ion in drainage water after leaching of glasshouse soils. Water, Air, Soil Pollut 16:3-11.

Wegman RCC, Hamaker Ph, De Heer H (1983) Bromide-ion balance of a polder district with large-scale use of methyl bromide for soil fumigation. Food Chem Toxic 21:361-367.

Wesely ML, Lenschow DH, Denmead OT (1989) Flux measurement techniques. In: Lenschow DH, Hicks BB (eds) Global tropospheric chemistry: chemical fluxes in the global atmosphere. National Center for Atmospheric Research, Boulder, CO pp 31-46.

White JG, Hunt J (1983) Bromide residues in transplanted brassicas following methyl bromide treatment of seedbed and field. Ann Appl Biol 103:383-388.

Williams J, Wang NY, Cicerone RJ (1999) Methyl bromide emissions from agricultural field fumigations in California. J Geophys Res 104:30087-30096.

Wilson JD, Catchpoole VR, Denmead OT, Thurtell GW (1983) Verification of a simple micrometeorological method for estimating the rate of gaseous mass transfer from the ground to the atmosphere. Agric Meteor 29:183-189.

Wilson JD, Thurtell GW, Kidd GE (1981a) Numerical simulation of particle trajectories in inhomogeneous turbulence, I: Systems with constant turbulent velocity scale. Boundary Layer Meteor 21:295-313.

Wilson JD, Thurtell GW, Kidd GE (1981b) Numerical simulation of particle trajectories in inhomogeneous turbulence, II: Systems with variable turbulent velocity scale. Boundary Layer Meteor 21:443-463.

Wilson JD, Thurtell GW, Kidd GE (1981c) Numerical simulation of particle trajectories in inhomogeneous turbulence, III: Comparison of predictions with experimental data for the atmospheric surface layer. Boundary Layer Meteor 21:443-463.

Wilson JD, Thurtell GW, Kidd GE, Beauchamp EG (1982) Estimation of the rate of gaseous mass transfer from a surface source plot to the atmosphere. Atmos Environ 16:1861-1867.

Wofsy SC, McElroy, MB, Yung, YL (1975) The chemistry of atmospheric bromine. Geophys Res Lett 2:215-218.

Woodrow JE, McChesney MM, Seiber JN (1988) Determination of methyl bromide in air samples by headspace gas chromatography. Anal Chem 60:509-512.

World Meteorological Organization (WMO) (1999) Scientific Assessment of Ozone Depletion: 1998. Executive Summary. WMO, Geneva, Switzerland. http://www.al.noaa.gov/WWWHD/pubdocs/assessment98.html

Yagi K, Williams J, Wang NY, Cicerone RJ (1993) Agricultural soil fumigation as a source of atmospheric methyl bromide. Proc Natl Acad Sci USA 90:8420-8423.

Yagi K, Williams J, Wang NY, Cicerone RJ (1995) Atmospheric methyl bromide (CH3Br) from agricultural soil fumigations. Science 267:1979-1981.

Yang RSH, Witt KL, Alden CJ, Cockerham LG (1995) Toxicology of methyl bromide. 142:65- 85.

Yates SR, Ernst FF, Gan JY, Gao F, Yates MV (1996b) Methyl bromide emissions from a covered field II. Volatilization. J Environ Qual 25:192-202.

Yates SR, Gan JY (1998) Volatility, adsorption, and degradation of propargyl bromide as a soil fumigant. J Agric Food Chem 46:755-761.

Yates SR, Gan JY (1999) Methods for removing and decomposing methyl bromide from fumigation gases. Patent No. 5,904,909. Issued: May 18, 1999.

Yates SR, Gan JY, Ernst FF (1996c) Methyl bromide emissions from a covered field. III. Correcting Chamber Flux for Temperature. J Environ Qual 25:892-898.

Yates SR, Gan JY, Ernst FF, Mutziger A, Yates MV (1996a) Methyl bromide emissions from a covered field I. Experimental conditions and degradation in soil. J Environ Qual 25:184-192.

Yates SR, Gan JY, Wang D, Ernst FF (1997) Methyl bromide emissions from agricultural fields. Bare-soil, deep injection. Environ Sci Technol 31:1136-1143.

Yates SR, Wang D, Gan J, Ernst FF, Jury WA (1998) Minimizing methyl bromide emissions from soil fumigation. Geophys Res Lett 25:1633-1636.

Yates SR, Wang D, Papiernik SK, Gan J (2002) Predicting pesticide volatilization from soils, J Environmentrics (accepted 3/26/01).

Yücel S (1995) A study on soil solarization and combined with fumigant application to control Phytophthora crown blight (Phytophthora capsici Leonian) on peppers in the East Mediterranean region of Turkey. Crop Protection 14:653-655.

Yung YL, Pinto JP, Watson RT, Sander SP (1980) Atmospheric bromide and ozone perturbations in the lower stratosphere. J Atmos Sci 37:339-353.

Yvon-Lewis SA, Butler JH (1997) The potential effect of oceanic biological degradation on the lifetime of atmospheric CH3Br. Geophys Res Lett 24:1227-1230.

Zhang W, McGriffin Jr. ME, Becker JO, Ohr HD, Sims JJ, and Campbell SD (1998) Effect of soil physical factors on methyl iodide and methyl bromide. Pestic Sci 53:71-79.

 

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