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United States Department of Agriculture

Agricultural Research Service

Methyl Bromide
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1 - Background
2 - Chemical and Physical Properties
3 - Reactions with Stratospheric Ozone
4 - Solubility
5 - Henry's Law Constant
6 - Vapor Pressure
7 - Adsorption
8 - Diffusion Coefficient
9 - Air Sampling
10 - Field Experiments
11 - Transformation of MeBr in Water
12 - Transformation of MeBr in Soil
13 - Transport Model
14 - Simulating MeBr Volatilization
15 - Fumigation
16 - Post-Fumigation
17 - Further Reading
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.
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Last Modified: 10/20/2005
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