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

Abstract

The gas emission rate Q from an artificial 36-m² 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 σ_Q/Q = 0.36. This variability is lessened (σ_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.