Comparison of tracer emission rates calculated from three different techniques

We compared tracer emission rates calculated from flux-gradient (FG) formulas with measurements from an integrated horizontal flux (IHF) method and a dispersion-model-based estimation (BLS) technique. A pre-emergent herbicide (active ingredient metolachlor) was applied to a bare soil field and was our tracer. Average concentration, windspeed, and stability were measured in the center of this large field and emissions rates calculated using FG formulas and the BLS technique. Concentrations were also measured at the upwind edge of the field, so that emission rates could be calculated using the IHF technique. We considered the IHF method as "truth" in the experiment (due to the limited number of assumptions in the method). We found that the FG formulas underestimated tracer fluxes by 30 to 40%. This suggests the turbulent Schmidt number (Sc) for tracers is approximately 0.6. The BLS-based estimate had better agreement with the IHF measurements.