| Animal manure, synthetic fertilizer, and agricultural crops are the primary sources of atmospheric ammonia (NH3) and together contribute approximately 64% of global emissions. Subsequently, mixed (i.e. animal and crop production) agricultural regions may experience high concentrations of NH3 in air and precipitation, along with elevated ambient inorganic PM2.5 concentrations. In this study, the behavior of atmospheric NH 3 is examined in eastern North Carolina where NH3 emissions from animal and crop production are high. Annular denuders are used to measure atmospheric concentrations of NH3, acid gases and inorganic aerosol. At a site influenced by high local NH3 emissions, annual NH 3 and NH4+ aerosol concentrations are 5.6 (+/-5.1) and 1.8 (+/-1.4) mug m-3, respectively. Variability in NH3 is highly correlated with ambient temperature, which is a controlling factor for NH3 emissions from animal manure. The mean concentration of total inorganic PM2.5 at this site, which includes SO4=, NO3-, NH4+, and Cl-, is 8.0 (+/-5.8) mug m-3. SO4=, NO3 -, NH4+, and Cl - represent 53, 24, 22, and 1% of measured inorganic PM 2.5. NH3 contributes 72% of total NH3 + NH 4+, on average. Equilibrium modeling of the gas + aerosol NH3/H2SO4/HNO3 system shows that under elevated NH3, inorganic PM2.5 is more sensitive to reductions in gas + aerosol concentrations of sulfate and nitrate relative to NH3. Chemiluminescence technology is used as part of a micrometeorological flux measurement system to examine the air/surface exchange of NH3 over soybean at a second site with high local NH3 emissions. A mean flux of -10.8 ng m-2 s-1 indicates that the canopy was a net sink for NH3, 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 NH3 accounted for approximately 0.3% of crop nitrogen requirements. |
