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

Anthropogenic emissions of NOx (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.