Fluxes of ammonia and sulfate over maize using relaxed eddy accumulation

The dry deposition of fine aerosol sulfate and ammonia was measured over maize using the relaxed eddy accumulation method (REA). This micrometeorological method determines the flux by sampling the concentrations in the “updraft” and “downdraft” eddy motions, with the flux being proportional to the difference in concentration between the “up” and “down” concentrations. The fine aerosol fraction and gas phase components were separated using annular denuders followed by filterpacks using Teflon filters. Measurements were made when the leaf area index was low (∼1) and later when the canopy was fully developed (LAI ∼ 4) to evaluate the role of LAI on the aerosol deposition velocity. The deposition measurements of sulfate aerosol yielded daytime deposition velocities larger than expected (∼2 cm s−1) from short crops but consistent with other field measurements over forests. Over a young fertilized stand of maize with a leaf area index of ∼1, large emissions of ammonia were observed for several days following the surface application of urea containing fertilizer. Measurements over a fully developed canopy with a leaf area index of about 4, showed both deposition and emission events.