Airborne observations of ammonia and ammonium nitrate formation over Houston, Texas

Abstract

Anthropogenic emissions of NO x (nitric oxide (NO) + nitrogen dioxide (NO 2)), which in sunlight can be oxidized to form nitric acid (HNO 3), can react with ammonia (NH 3) to form ammonium nitrate particles. Ammonium nitrate formation was observed from the NOAA WP-3D aircraft over Houston during the 2006 Texas Air Quality Study with fast-response measurements of NH 3, HNO 3, particle composition, and particle size distribution. Typically, NH 3 mixing ratios over the urban area ranged from 0.2 to 3 ppbv and were predominantly from area sources. No NH 3 enhancements were observed in emission plumes from power plants. The few plumes with high NH 3 levels from point source emissions that were sampled are analyzed in detail. While the paucity of NH 3 data in emission inventories made point source identification difficult, one plume was traced to NH 3 release from an industrial accident. NH 3 mixing ratios in these plumes ranged from 5 to 80 ppbv. In these plumes, the NH 3 enhancement correlated with a decrease in HNO 3 mixing ratio and an increase in particulate NO 3 - concentration indicating ammonium nitrate formation. The ammonium nitrate aerosol mass budget in the plumes was analyzed to assess the quantitative agreement between the gas and aerosol phase measurements. The thermodynamic equilibrium between the gas and aerosol phase was examined for one flight by comparing the modeled dissociation constant for ammonium nitrate with NH 3 and HNO 3 measurements. The high levels of NH 3 in these plumes shifted the equilibrium toward favorable thermodynamic conditions for the condensation of ammonium nitrate onto particles. © 2010 by the American Geophysical Union.