Photooxidation Yields of Organic Nitrates and the Influence of the Nitrate Moiety on Aerosol Formation

Organic nitrates occur throughout the atmosphere, routinely detected in rural, marine, and urban environments as gases and as components of particles. They can be formed in substantial quantities from the reactions of volatile organic compounds (VOCs) with OH radicals in the presence of nitrogen oxides (NOx), and from the reactions of alkenes with NO3 radicals. Formation of organic nitrates affects ozone concentrations by sequestering NOx, the catalyst required for ozone formation. Organic nitrates of sufficiently low volatility condense to form secondary organic aerosol (SOA), a major component of atmospheric aerosol, which affects surface albedo and cloud formation, and therefore climate. Since organic nitrates contribute to particle mass and affect NOx concentrations, quantitative understanding of the conditions governing their formation is needed for accurate atmospheric modeling of ozone and aerosol. In these studies, yields of organic nitrates were measured in environmental chamber reactions of C8-C16 n-alkanes with OH in the presence of NOx, and in reactions of 1-alkenes with NO3. Yield measurements of monofunctional organic nitrates, i.e. alkyl nitrates, were made to observe a yield plateau with respect to carbon number, which was predicted by previous studies, but never confirmed experimentally due to the analytical challenge posed by partitioning of large alkyl nitrates into chamber walls. A strategy to correct gas chromatographic (GC) alkyl nitrate measurements for gas-wall partitioning was developed, and a yield plateau was measured at ~30%. Particle-phase 1,4-hydroxynitrate yields were measured by analyzing particle filter extracts by liquid chromatography with UV absorbance and mass spectrometric detection (LC/UV/MS), and a yield plateau of ~13% was observed. These yield measurements were combined with over 140 previously reported measurements to update the parameterization of an existing yield estimation model for organic nitrate formation. The effect of the nitrate moiety on the decomposition of alkoxy radicals was investigated by reacting 1-pentadecene with NO3, and measuring aerosol yield and 1,2-hydroxynitrate, 1,2-carbonylnitrate, and organic peroxide product yields in particles. The branching ratio of decomposition/isomerization of the beta-nitrooxyalkoxy radical was extrapolated, and indicated that the nitrate moiety substantially enhances the decomposition of the alkoxy radical, inhibiting the formation of aerosol