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Title: A MASS BALANCE METHOD FOR NON-INTRUSIVE MEASUREMENTS OF SURFACE-AIR TRACE GAS EXCHANGE

Author
item DENMEAD, O - CSIRO CANBERRA AUSTRALIA
item Harper, Lowry
item FRENEY, J - CSIRO CANBERRA AUSTRALIA
item GRIFFITH, D - UNIV OF WOLLONGONG AUS
item LEUNING, R - CSIRO CANBERRA AUSTRALIA
item Sharpe, Ronald

Submitted to: Atmospheric Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: Trace gas emissions are difficult to evaluate accurately in source areas where emissions are variable in horizontal and/or vertical distribution. A system was developed whereby emissions can be measured in small plots which may also include animals. The system employs a square plot where vertical concentrations of the trace gasses are measured on all sides. The concentrations along with windspeeds and wind directions, are used to obtain the mass of gasses produced in the small plot. Gases measured have included methane, carbon dioxide, and nitrous oxide. Although the system is sensitive to light winds and variable wind directions which may create errors, it is non-disturbing, it has a simple theoretical basis, it is independent of problematic meteorological conditions (such as atmospheric stability or wind profile characteristics), and it is appropriate for measurement of trace gas emissions where conventional measurement methods cannot be used.

Technical Abstract: A mass balance method is described for calculating gas production from a surface or volume source in a small test plot from measurements of of differences in the horizontal fluxes of the gas across upwind and downwind boundaries. It employs a square plot, 24 m x 24 m, with measurements of gas concentration at four heights (up to 3.5 m) along each of the four boundaries. Gas consentrations are multiplied by the appropriate vector winds to yield the horizontal fluxes at each height on on each boundary. The difference between these fluxes integrated over downwind and upwind boundaries represents production. Illustrations of the method, which involve exchanges of methane and carbon dioxide, are drawn from experiments with landfills, pastures and grazing animals. Tests include calculation of recovery rates from known gas releases and comparisons with a conventioanl micrometeorological approach and a backward dispersion model. The method performed satisfactorily in all cases. Its sensitivity for measuring exchanges of CO2, CH4, anf H2O in various scenarios was examined. As employed by us, the mass balance method can suffer from errors arising from the large number of gas analyses required for a flux determination, and becomes unreliable when there are light winds and variable wind directions. On the other hand, it is non-disturbing, has a simple theoretical basis, is independent of atmospheric stability or the shape of the wind profile, and is appropriate for flux measurement in situations where conventional micrometeorological methods can not be used, i.e., for small plots, elevated point sources, and heterogeneous surface sources.