Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/15/2004
Publication Date: 2/1/2006
Citation: Meyers, T.P., Luke, W.T., Meisinger, J.J. 2006. Fluxes of ammonia and sulfate over maize using relaxed eddy accumulation. Agricultural and Forest Meteorology. 136:203-213. Interpretive Summary: The dry deposition of atmospheric sulfate and ammonia was measured over corn using the micro-meteorology method of relaxed eddy accumulation. This method estimates the flow of sulfate and ammonia onto a unit area of crop surface by sampling the atmospheric concentrations in the updraft and downdraft motions of the atmosphere as it passes over the corn canopy, the flow is proportional to the difference in concentration between the updraft and downdraft concentrations. The sulfate and ammonia in the gas phase were separated using annular denuders followed by Teflon filterpacks which trapped the particulate phase. Measurements were made when the leaf area index was low, less than one, and later when the canopy had a fully developed leaf area of greater than 3.5, this allowed an evaluation of the role of leaf area on deposition velocity. The daytime sulfate deposition measurements produced deposition velocities greater than 2 cm per second, which was larger than expected from short crops, but was consistent with other field measurements over forests. Large emissions of ammonia were observed for several days over a recently fertilized young stand of corn with a leaf area index less than one, following the application of urea containing fertilizer. Ammonia measurements over a fully developed corn canopy showed periods of both deposition and emission that were consistent with a plant ammonia compensation point of 2 micro-grams per cubic meter of air. If atmospheric ammonia concentrations were above 2 micro-grams per cubic meter the corn plant absorbed ammonia, if they were below this value the corn plant emitted ammonia. The deposition velocity for ammonia over fully developed corn for all the daytime studies averaged 5.6 cm per second, which is consistent with the assumption of a near-zero surface uptake resistance for corn leaves. These data will be valuable for scientists and modelers as they develop an expanded understanding of how ammonia cycles between the atmosphere and a growing crop, and to policy makers as they evaluate strategies to manage the fate of ammonia emitted from livestock production units.
Technical Abstract: 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 be 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 > 3.5) 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 <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 that were consistent with an ammonia compensation point of 2 ug m-3. For all the daytime runs over the fully developed canopy, the average deposition velocity for NH3 was 5.6 cm s-1 and is consistent with the assumption of a near-zero surface uptake resistance.