Source strength determination of a tracer gas using an approximate solution to the advection–diffusion equation for microplots

An approximate solution to the advection–diffusion equation was used to back-calculate the source strength from downwind concentration measurements of single and multipoint (4 and 16 points) tracer gas (SF6 and CH4) release experiments conducted over microplots over an open field. These microplots were 2 m×2 m and 4 m×8 m in size so that maximum downwind sampling distances were 4 m. Best estimates (within ±20%) of the source strength were obtained when downwind concentrations measured at or near the mean plume height and along the centreline of the plume were used in the model. Predictions of the source strength degraded when concentration observations made progressively further away (in the y and z sense) from the centreline of the diffusing tracer gas plume were used. For the small scale used in this study, the performance of the model was not sensitive to the number of point sources, source configuration or downwind sampling distance.