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 (NH 3), ammonium (NH 4 +), hydrochloric acid (HCl), chloride (Cl −), nitrate (NO 3 −), nitric acid (HNO 3), nitrous acid (HONO), sulfate (SO 4 2−), and sulfur dioxide (SO 2) concentrations measured at an agricultural site in North Carolina's Coastal Plain region. Mean gas concentrations were 0.46, 1.21, 0.54, 5.55, and 4.15 μg m −3 for HCl, HNO 3, HONO, NH 3, and SO 2, respectively. Mean aerosol concentrations were 1.44, 1.23, 0.08, and 3.37 μg m −3 for NH 4 +, NO 3 −, Cl −, and SO 4 2−, respectively. Ammonia, NH 4 +, HNO 3, and SO 4 2− exhibit higher concentrations during the summer, while higher SO 2 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 NH 3 concentrations ( n=601). Ammonia concentration increases exponentially with temperature, which explains the majority of variation (54%) in 12-hour mean NH 3 concentrations. Dependence of NH 3 concentration on wind direction suggests a local source influence. Ammonia accounts for >70% of NH x (NH x =NH 3+NH 4 +) during all seasons. Ammonium nitrate and sulfate aerosol formation does not appear to be NH 3 limited. Sulfate is primarily associated ammonium sulfate, rather than bisulfate, except during the winter when the ratio of NO 3 −–NH 4 + is ∼0.66. The annual average NO 3 −–NH 4 + ratio is ∼0.25.