UC Berkeley

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 NO_x (NO_x = NO + NO₂) radicals. Consequently, understanding the sources of NO_x 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 NO_x 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 NO_x 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 NO_x 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 NO_x, high biogenic volatile organic compound chemistry, I find that the reactions leading to formation of total ANs control the NO_x 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 NO_x 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 NO_x to the atmosphere and the other which removes it permanently. These two mechanisms are expected to result in different spatial patterns of NO_x 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 NO_x 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 NO_x.