Alkylperoxy Radical Photochemistry in Organic Aerosol Formation Processes

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 alkyl radical. We examine organic aerosol formed from the 1-iodooctane photolysis at this wavelength using high-performance liquid chromatography (HPLC) with derivatization to selectively probe carbonyl- and hydroxyl-containing molecules. Tandem mass spectrometry reveals that the product distributions are much more complex than a traditional low-NO_<i>x</i> peroxy–peroxy oxidation mechanism from a single parent isomer would justify. We propose that this difference is due to peroxy radical photochemistry, leading to two major channels: direct peroxy radical isomerization via internal H-abstraction and reverse dissociation to form alkyl radical and O₂. The complexity of the product spectrum is derived from both scrambling of the radical site in the alkyl radical and the additional oxidation of otherwise stable peroxy radicals as a result of the isomerization. A branching ratio for these channels 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 may play a role in defining the composition of atmospheric secondary organic aerosol formed in pristine (low-NO_<i>x</i>) environments