DEDUCING GROUND-AIR EMISSIONS FROM OBSERVED TRACE-GAS CONCENTRATIONS: A FIELD TRIAL

Authors: FLESCH, T - UNIVERSITY OF ALBERTA; WILSON, L - UNIVERSITY OF ALBERTA; Harper, Lowry; CRENNA, B - UNIVERSITY OF ALBERTA; Sharpe, Ronald

Submitted to: Journal of Applied Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/14/2004
Publication Date: 3/1/2004
Citation: Flesch, T.K., Wilson, L.A., Harper, L.A., Crenna, B.P., Sharpe, R.R. 2004. Deducing ground-air emissions from observed trace-gas concentrations: A field trial. Journal of Applied Meteorology. 43:487-502.

Interpretive Summary: Canadian Scientists from the University of Alberta and Scientists from the J. Phil Campbell, Sr., Natural Resource Conservation Center in Watkinsville, Georgia have been working on measuring gaseous emissions from agricultural systems. Measurements are complicated by the low concentrations of atmospheric gases and structures such as barns, houses, lagoons, and fields that affect air movement. It is also difficult to verify the accuracy of measurement techniques because there is no accepted standard method. The collaboration has resulted in the development and testing of a new measurement technique called the backward Lagrangian stochastic (bLS) model. This procedure is less expensive and requires simpler equipment than the other techniques. Known quantities of gases were released and concentrations were measured directly above the release area and as far as 100 meters downwind. Emissions calculated using bLS were close to actual emissions when atmospheric conditions resulted in good mixing of gases. Comparisons were poor during periods of relative atmospheric stability (e.g. nighttime, low wind speed). With these observations removed, the calculated emissions and actual emissions over six days were similar. Variation between calculated and observed emissions was greater during short duration (15-minutes) comparisons. Variation could be decreased by increasing the distance between the measuring device and the source area. This puts the measuring device farther into the gas plume. Emission rates calculated with the bLS technique can be comparable to other techniques but with easier and less-expensive data collection and processing requirements.

Technical Abstract: The backward Lagrangian stochastic (bLS) model is a useful tool for estimating gas emissions, and compared with other techniques, it has the benefit of experimental simplicity, flexibility in the type and location of the gas concentration measurement, and applicability (in principle) even in disturbed flow conditions. Gas emission rate (Q) from an artificial 6x6 m2 surface area source was inferred from line-average concentration (CL) measured by an open-path laser situated up to 100m downwind. Using the bLS model, a theoretical CL/Q relationship was established for each trial, bu simulating an ensemble of fluid element paths arriving in the laser beam, under the prevailing micrometeorological conditions. The diagnosed emission rates (QbLS) were satisfactory for trials performed when Monin-Obukhov similarity theory (MOST) gave a good description of the surface-layer, but poor during periods of extreme atmospheric stability ( L 2m) and during transition periods in stratification. With such periods eliminated, the average value of the fifteen-min ratios of QbLS/Q over N = 77 fifteen-min trials spanning six days was 1.02. Individual 15-min predictions, however, exhibited sizeable variability about the true rate, the standard deviation in QbLS/Q being Q/Q = 0.36. This variability is lessened ( Q/Q = 0.22, N = 46) if cases are excluded in which the detecting laser lay above or immediately downwind from the source, a circumstance in which the laser path falls at the edge of the gas plume off the source.