A MASS-BALANCE METHOD FOR NONINTRUSIVE MEASUREMENTS OF SURFACE-AIR TRACE GAS-EXCHANGE

A mass balance method is described for calculating gas production from a surface or volume source in a small test plot from measurements of differences in the horizontal fluxes of the gas across upwind and down wind boundaries, It employs a square plot, 24 m x 24 m, with measureme nts of gas concentration at four heights (up to 3.5 m) along each of t he four boundaries. Gas concentrations are multiplied by the appropria te vector winds to yield the horizontal fluxes at each height on each boundary. The difference between these fluxes integrated over downwind and upwind boundaries represents production. Illustrations of the met hod, which involve exchanges of methane and carbon dioxide, are drawn From experiments with landfills, pastures and grazing animals. Tests i ncluded calculation of recovery rates from known gas releases and comp arisons with a conventional micrometeorological approach and a backwar d dispersion model. The method performed satisfactorily in all cases. Its sensitivity for measuring exchanges of CO2, CH4 and N2O in various scenarios was examined. As employed by us, the mass balance method ca n suffer from errors arising from the large number of gas analyses req uired 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 atmosph eric stability or the shape of the wind profile, and is appropriate fo r flux measurement in situations where conventional micrometeorologica l methods can not be used, e.g, for small plots, elevated point source s, and heterogeneous surface sources. (C) 1998 Published by Elsevier S cience Ltd. All right reserved.