Insights on the Formation and Fate of Organic Nitrates in the Atmosphere from Field and Laboratory Observations

Citation

Teng, Alexander Pai-Chung (2017) Insights on the Formation and Fate of Organic Nitrates in the Atmosphere from Field and Laboratory Observations. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9RV0KRJ. https://resolver.caltech.edu/CaltechTHESIS:06042017-134711204

Abstract

Alkenes are oxidized rapidly in the atmosphere by addition of OH and subsequently O₂, leading to the formation of peroxy radicals. These peroxy radicals react with NO to form organic nitrates through a minor radical-terminating branching pathway. Over large regions of the continental boundary layer, the formation of organic nitrates control tropospheric ozone and the lifetime of NO_x. Laboratory investigations described herein show that the yield of nitrates through this pathway is larger than previously described for alkenes, and the yield increases with the number of heavy atoms. This result is used to interpret field observations taken over Houston in the summer of 2013. These measurements show that small alkenes still play a large role in ozone production more than a decade after they had been identified as a causal factor. In further studies, measurements of isoprene hydroxy nitrates (ISOPN) and hydroperoxides, formed from the OH oxidation of isoprene, are used to diagnose the complexities of reversible O₂ addition for allylic hydroxy isoprene radicals. It is shown that over most of the atmosphere, isoprene's peroxy radical isomers are in their equilibrium distribution. In this regime, hydroxy peroxy radical isomers comprise approximately 95% of the radical pool, a much higher fraction than in the nascent (kinetic) distribution. Intramolecular H-shift isomerization from the Z hydroxy peroxy radical isomers produced from OH addition to C₄ is estimated to be 4s^-1 at 297K. While the Z isomer is initially produced in low yield, it is continually reformed via decomposition of the hydroxy peroxy radicals. As a result, unimolecular chemistry from this isomer contributes as much as half of the atmospheric fate of the entire pool of peroxy radicals formed via addition of OH at C₄. In contrast, unimolecular chemistry following OH addition at C₁ is slower and less important. Field observations of alkyl nitrates over the Southeastern United States during the summer over forested environments show that there are still gaps in our understanding of the organic nitrate budget. The formation of isoprene hydroxy nitrates (ISOPN) is shown to be a dominant NO_x loss pathway during the day.

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