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

[1] Anthropogenic emissions of NO_x (nitric oxide (NO) + nitrogen dioxide (NO₂)), which in sunlight can be oxidized to form nitric acid (HNO₃), can react with ammonia (NH₃) 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 NH₃, HNO₃, particle composition, and particle size distribution. Typically, NH₃ mixing ratios over the urban area ranged from 0.2 to 3 ppbv and were predominantly from area sources. No NH₃ enhancements were observed in emission plumes from power plants. The few plumes with high NH₃ levels from point source emissions that were sampled are analyzed in detail. While the paucity of NH₃ data in emission inventories made point source identification difficult, one plume was traced to NH₃ release from an industrial accident. NH₃ mixing ratios in these plumes ranged from 5 to 80 ppbv. In these plumes, the NH₃ enhancement correlated with a decrease in HNO₃ mixing ratio and an increase in particulate NO₃⁻ 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 NH₃ and HNO₃ measurements. The high levels of NH₃ in these plumes shifted the equilibrium toward favorable thermodynamic conditions for the condensation of ammonium nitrate onto particles.