Atmospheric concentrations of ammonia and ammonium at an agricultural site in the southeast United States

In this study, we present 1 yr (October 1998–September 1999) of 12-hour mean ammonia (NH3), ammonium (NH4+), hydrochloric acid (HCl), chloride (Cl−), nitrate (NO3−), nitric acid (HNO3), nitrous acid (HONO), sulfate (SO42−), and sulfur dioxide (SO2) concentrations measured at an agricultural site in North Carolinaʹs Coastal Plain region. Mean gas concentrations were 0.46, 1.21, 0.54, 5.55, and 4.15 μg m−3 for HCl, HNO3, HONO, NH3, and SO2, respectively. Mean aerosol concentrations were 1.44, 1.23, 0.08, and 3.37 μg m−3 for NH4+, NO3−, Cl−, and SO42−, respectively. Ammonia, NH4+, HNO3, and SO42− exhibit higher concentrations during the summer, while higher SO2 concentrations occur during winter. A meteorology-based multivariate regression model using temperature, wind speed, and wind direction explains 76% of the variation in 12-hour mean NH3 concentrations (n=601). Ammonia concentration increases exponentially with temperature, which explains the majority of variation (54%) in 12-hour mean NH3 concentrations. Dependence of NH3 concentration on wind direction suggests a local source influence. Ammonia accounts for >70% of NHx (NHx=NH3+NH4+) during all seasons. Ammonium nitrate and sulfate aerosol formation does not appear to be NH3 limited. Sulfate is primarily associated ammonium sulfate, rather than bisulfate, except during the winter when the ratio of NO3−–NH4+ is 0.66. The annual average NO3−–NH4+ ratio is 0.25.