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 (similar to 1) and later when the canopy was fully developed(LAI similar to 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 (similar to 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 similar to 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. (c) 2005 Elsevier B.V. All rights reserved.