Showing papers by "Ann M. Middlebrook published in 2010"

A large atomic chlorine source inferred from mid-continental reactive nitrogen chemistry

Abstract: Chlorine atoms can profoundly affect the composition of the atmosphere. Notoriously, as chlorofluorocarbons, they were implicated in ozone depletion in the stratosphere. New observations suggest that chlorine may be a more potent force lower down in the atmosphere than was thought. The presence of gaseous chlorine atom precursors in the troposphere is generally considered a marine air phenomenon. But measurements made near Boulder, Colorado, reveal significant production of atmospheric nitryl chloride (ClNO2) in a continental setting, 1,400 km from the nearest coastline. This finding, incorporated into model studies, suggests that nitryl chloride production in the contiguous United States alone — probably arising from anthropogenic pollutants — is at a level similar to previous global estimates for marine regions. The presence of gaseous chlorine atom precursors within the troposphere was thought only to occur in marine areas but now nitryl chloride has been found at a distance of 1,400 km from the nearest coastline. A model study shows that the amount of nitryl chloride production in the continental USA alone is similar to previous global estimates for marine regions. A significant fraction of tropospheric chlorine atoms may arise directly from anthropogenic pollutants. Halogen atoms and oxides are highly reactive and can profoundly affect atmospheric composition. Chlorine atoms can decrease the lifetimes of gaseous elemental mercury1 and hydrocarbons such as the greenhouse gas methane2. Chlorine atoms also influence cycles that catalytically destroy or produce tropospheric ozone3, a greenhouse gas potentially toxic to plant and animal life. Conversion of inorganic chloride into gaseous chlorine atom precursors within the troposphere is generally considered a coastal or marine air phenomenon4. Here we report mid-continental observations of the chlorine atom precursor nitryl chloride at a distance of 1,400 km from the nearest coastline. We observe persistent and significant nitryl chloride production relative to the consumption of its nitrogen oxide precursors. Comparison of these findings to model predictions based on aerosol and precipitation composition data from long-term monitoring networks suggests nitryl chloride production in the contiguous USA alone is at a level similar to previous global estimates for coastal and marine regions5. We also suggest that a significant fraction of tropospheric chlorine atoms6 may arise directly from anthropogenic pollutants.

An important contribution to springtime Arctic aerosol from biomass burning in Russia

Abstract: [1] Using aircraft observations and transport model calculations we determine the total amounts of various gas-phase and aerosol species in the Arctic due to distant biomass burning (BB) emissions. We find that for many climate-relevant species, including black carbon (BC) and organic aerosols, fires in Russia that typically occur during the critical springtime snowmelt can more than double the high seasonal Arctic atmospheric background that has built up in the winter months (commonly called “Arctic haze”). Decision makers have targeted BC, because it is expected to cause strong positive forcing over snow-covered surfaces yet is significantly shorter lived than greenhouse gases. These results demonstrate that BB is more important for the Arctic than previously believed and should be considered in any attempt to mitigate impacts.

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

Abstract: [1] 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.