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Observational constraints on the photochemistry of non-acyl peroxy nitrates and organic nitrates on regional and global scales

  • Author(s): Browne, Eleanor Carol
  • Advisor(s): Cohen, Ronald C
  • et al.
Abstract

Reduced chemicals that are for the most part insoluble in water are emitted to the atmosphere from a wide range of natural and industrial processes. The self-cleansing nature of the atmosphere subsequently oxidizes these chemicals to products such as HNO3, CO2 and H2O. The oxidants in this chemistry are primarily OH, O3 and NO3 with an additional contribution from H2O2 in the liquid phase. The concentrations of these gas phase oxidants are regulated by the availability of NOx (NOx = NO + NO2) radicals. Consequently, understanding the sources of NOx and its removal from the atmosphere is crucial to understanding the composition of the atmosphere and thus air quality and climate. In this thesis I investigate processes controlling NOx concentrations in remote continental environments and in the upper troposphere. Using upper tropospheric data collected during the NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) experiment, I find that methyl peroxy nitrate (CH3O2NO2) is an important sink of NOx and that it may account for the long standing discrepancy between measured and modeled ratios of NO to NO2. Incorporation of CH3O2NO2 chemistry into a global chemical transport model shows that the formation of CH3O2NO2 has important impacts on atmospheric chemistry at temperatures below 240 K. Next, I focus on the lower troposphere over the remote continents and investigate the role of emissions from the biosphere as they affect the NOx removal rate. During the oxidation of biogenic organics, non-peroxy organic nitrates (molecules of the form RONO2 which will be referred to as total ANs) are formed. Using a simplified representation of low NOx, high biogenic volatile organic compound chemistry, I find that the reactions leading to formation of total ANs control the NOx lifetime over most of the continents. Comparison of these results to both ARCTAS observations and calculations using a regional 3-D chemical transport model (the Weather Research and Forecasting model with chemistry - WRF-Chem) confirm the importance of total ANs in determining NOx lifetime. In addition to their confirmation of total ANs as a NOx sink, the ARCTAS data imply that the lifetime of total ANs is shorter than that of HNO3. The chemical lifetimes and the products of oxidative chemistry of total ANs are not well known. I identify two mechanisms that are capable of explaining the short lifetime of total ANs, one of which returns NOx to the atmosphere and the other which removes it permanently. These two mechanisms are expected to result in different spatial patterns of NOx concentrations. The ARCTAS data is unique in its extensive coverage over the boreal forest where monoterpene emissions are especially high. I show that the observations imply that the reactions that produce monoterpene nitrates represent a NOx sink that is roughly equal to the rate of the OH+NO2 reaction to form HNO3. Using WRF-Chem I show how this chemistry affects OH, O3, and peroxy nitrate concentrations and describe how current uncertainties in our understanding of monoterpene nitrate chemistry affect predictions of the concentrations of those species and of NOx.

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