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-NOx 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-NOx) environments.