Insights into the Isoprene Photochemical Cascade

Citation

Paulot, Fabien Georges André (2011) Insights into the Isoprene Photochemical Cascade. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/C11Q-E180. https://resolver.caltech.edu/CaltechTHESIS:04292011-155605494

Abstract

Isoprene emissions are the single largest source of non-methane reduced carbon to the atmosphere. Proper simulation of the effect of its photooxidation on regional air quality and secondary organic aerosol formation requires detailed multi-generation and multi-phase photochemical mechanisms. In this work, I first demonstrate that CF₃O^- chemical ionization mass spectrometry (CIMS) is a powerful method to probe isoprene photochemical oxidation. Under high NO_x, isoprene nitrates and their photochemical products are directly observed and quantified. As isoprene nitrates are critical to the impact of isoprene photochemistry on ozone, these observations provide an important constraint on the effect of isoprene emissions on air quality in polluted and biogenic rich regions (e.g., Southeastern United States). The use of tandem CIMS in association with isotopically labelled hydroxy radicals is used to identify dihydroxyepoxides as major products of isoprene photooxidation in low NO_x conditions. This class of compounds may provide an important precursor for secondary organic aerosols under acidic conditions. The chemical mechanism derived from these laboratory observations is then incorporated in a chemical transport model to assess the impact of isoprene photooxidation on atmospheric chemistry. I examine how the photooxidation of isoprene impacts the budget of formic and acetic acids, two ubiquitous trace gases whose sources are poorly constrained. Comparison with observations from satellite, ground and aircraft platforms, reveal a major missing source that may originate from the aging of secondary organic aerosols. Finally, the consequences of the multi-scale interplay between NO_x and isoprene photochemistry on the tropical budget of ozone are examined using forward and adjoint sensitivity simulations.

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