Deducing Ground-to-Air Emissions from Observed Trace Gas Concentrations: A Field Trial

The gas emission rate Q from an artificial 36-m 2 surface area source was inferred from line-average concentration C L measured by an open-path laser situated up to 100 m downwind. Using a backward Lagrangian stochastic (bLS) model, a theoretical C L /Q relationship was established for each experimental trial by simulating an ensemble of fluid-element paths arriving in the laser beam under the prevailing micrometeorological conditions. The diagnosed emission rates (Q bLS ) were satisfactory for trials done when Monin-Obukhov similarity theory gave a good description of the surface layer, but were poor during periods of extreme atmospheric stability ( | L | 2 m) and transition periods in stratification. With such periods eliminated, the average value of the 15-min ratios Q bLS /Q over n 77 fifteen-minute trials spanning 6 days was 1.02. Individual 15-min estimates, however, exhibited sizable variability about the true rate, with the standard deviation in Q bLS /Q being Q/Q 0.36. This variability is lessened ( Q/Q 0.22, n 46) if one excludes cases in which the detecting laser path lay above or immediately downwind from the source-a circumstance in which the laser path lies at the edge of the gas plume.