Alkylperoxy Radical Photochemistry in Organic Aerosol Formation Processes
Alicia Kalafut-Pettibone, Joseph Klems, Donald R. Burgess Jr., William S. McGivern
Recent studies have shown that 254 nm light can be used to generate organic aerosol from iodoalkane/air mixtures via photodissociation of the C−I bond and subsequent oxidation of the single radical isomer. We examine organic aerosol formed from 1-iodooctane photolysis using high-performance liquid chromatography (HPLC) with derivatization to selectively probe carbonyl- and hydroxyl-containing molecules. Tan- dem mass spectrometry is used to examine the product distributions, which are found to be much more complex than the traditional low-NOx peroxy-peroxy oxidation mechanisms would justify. We propose that this difference is due to peroxy radical photochemistry that leads to reactions typically found only in combustion mechanisms: direct peroxy radical isomerization and the reverse dissociation reaction that leads to a vibrationally excited parent molecule that can scramble the location of the radical site. A branching ratio for these channel is estimated using a canonical representation of the internal energy distribution. Lifetime estimates using extrapolated ethyl peroxy absorption cross sections and the actinic flux near 310 nm show that peroxy radical photochemistry can play a significant role in defining the product spectrum of secondary organic aerosol in pristine (low-NOx) environments.