Atmospheric Chemistry and Air/Surface Exchange of Ammonia in an Agricultural Region of the Southeast United States

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Title: Atmospheric Chemistry and Air/Surface Exchange of Ammonia in an Agricultural Region of the Southeast United States
Author: Walker, John Thomas
Advisors: Wayne Robarge, Committee Member
Abstract: Animal manure, synthetic fertilizer, and agricultural crops are the primary sources of atmospheric ammonia and together contribute approximately 64% of global emissions. Subsequently, mixed (i.e. animal and crop production) agricultural regions may experience high concentrations of ammonia in air and precipitation, along with elevated ambient inorganic PM[subscript 2.5] concentrations. In this study, the behavior of atmospheric ammonia is examined in eastern North Carolina where ammonia emissions from animal and crop production are high. Annular denuders are used to measure atmospheric concentrations of ammonia, acid gases, and inorganic aerosol. At a site influenced by high local ammonia emissions, annual ammonia and ammonium aerosol concentrations are 5.6 (±5.1) and 1.8 (±1.4) μg m⁻³, respectively. Variability in ammonia is highly correlated with ambient temperature, which is a controlling factor for ammonia emissions from animal manure. The mean concentration of total inorganic PM[subscript 2.5] at this site, which includes sulfate, nitrate, ammonium, and chloride, is 8.0 (±5.8) μg m⁻³. Sulfate, nitrate, ammonium, and chloride represent 53, 24, 22, and 1% of measured inorganic PM[subscript 2.5]. Ammonia contributes 72% of total ammonia + ammonium, on average. Equilibrium modeling of the gas + aerosol ammonium/sulfate/nitrate system shows that, under elevated ammonia, inorganic PM[subscript 2.5] is more sensitive to reductions in gas + aerosol concentrations of sulfate and nitrate relative to ammonia. Chemiluminescence technology is used as part of a micrometeorological flux measurement system to examine the air/surface exchange of ammonia over soybean at a second site with high local ammonia emissions. A mean flux of -10.8 ng m⁻² s⁻¹ indicates that the canopy was a net sink for ammonia, though emission fluxes occurred frequently during the late morning and early afternoon. The mean deposition velocity during the experiment was 3.3 mm s⁻¹. Measured deposition velocities indicate a large canopy resistance (median = 228 s m⁻¹), which is likely the result of very dry conditions. The net flux during the experiment corresponds to a dry deposition rate of 0.7 kg NH₃-N ha⁻¹ for the entire summer compared to wet deposition of 1.9 kg NH₄⁺-N ha⁻¹ at a nearby site during the same period. Dry deposition of ammonia accounted for approximately 0.3% of crop nitrogen requirements.
Date: 2005-04-28
Degree: PhD
Discipline: Soil Science

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