Quantitative Evidence for Organic Peroxy Radical Photochemistry
William S. McGivern, Joseph Klems
Quantitative evidence for the importance of alkyl peroxy photochemistry in the formation of secondary organic aerosol at 254 nm. Dodecanoic acid has been oxidized extensively in a flow cell by photolytically generated hydroxyl radicals to create particles. The resulting particles were collected and analyzed for composition, which shows much less multiply substituted parent molecules and much more decomposition product than expected from typical low-NOx oxidation mechanisms. Modeling studies were done to demonstrate this for two separate reaction times, and theoretical work from the combustion literature was used to estimate the branching of the photoexcited products. In addition, the alkyl peroxy radical absorption is sufficiently strong at 310 nm that the photochemical lifetime under pristine tropospheric conditions is similar to predicted lifetimes from peroxy-peroxy recombination reactions.