Ambient ammonia and ammonium aerosol across a region of variable ammonia emission density

We present 1 year of ambient ammonia (NH3), ammonium (NH4+), hydrochloric acid (HCl), chloride (Cl−), nitric acid (HNO3), nitrate (NO3−), nitrous acid (HONO), sulfur dioxide (SO2), and sulfate (SO42−) concentrations at three sites in the Coastal Plain region of North Carolina. The three sites, Clinton, Kinston, and Morehead City, are located in counties with total NH3 emission densities of 4800, 2280, and 320 kg NH3-N km−2 yr−1, respectively. Average NH3 concentrations were 5.32, 2.46, and 0.58 μg m−3 at Clinton, Kinston, and Morehead City, respectively. Average NH4+ concentrations were 1.84, 1.25, and 0.91 μg m−3, and total concentrations of inorganic (NH4++NO3−+ SO42−+Cl−) particulate matter with aerosol diameters <2.5 μm (PM2.5) were 8.66, 6.35, and 5.31 μg m−3 at Clinton, Kinston, and Morehead City, respectively. NH3 concentrations were highest during the summer at all sites, with summer-to-winter concentration ratios of 2.40, 5.70, and 1.70 at Clinton, Kinston, and Morehead City, respectively. NH3 concentrations were higher at night at the Clinton site, during the day at the Kinston site, and day vs. night concentrations were similar at the Morehead City site. NH4+ concentrations were highest during the winter at all sites, though this may not be representative of all years. Average daytime concentrations of NH4+ were similar to night values at all sites. NH4+ aerosol was primarily associated with SO42− at all sites, though the degree of SO42− neutralization was highest at Clinton and lowest at Morehead City. NH4+ aerosol formation appeared to be acid-gas-limited at the Clinton site during all seasons and during the spring and summer at the Kinston site. This study shows that agricultural NH3 emissions influence local ambient concentrations of NH3 and PM2.5.