Role of Hydroperoxyl Radicals in Heterogeneous Oxidation of Oxygenated Organic Aerosols

Heterogeneous oxidative aging of organic aerosols (OA) occurs ubiquitously in the atmosphere, initiated by oxidants, such as the hydroxyl radicals ((OH)-O-center dot). Hydroperoxyl radicals (HO2 center dot) are also an important oxidant in the troposphere, and its gas-phase chemistry has been well studied. However, the role of HO2 center dot in heterogeneous OA oxidation remains elusive. Here, we carry out (OH)-O-center dot-initiated heterogeneous oxidation of several OA model systems under different HO2 center dot conditions in a flow tube reactor and characterize the molecular oxidation products using a suite of mass spectrometry instrumentation. By using hydrogen-deuterium exchange (HDX) with thermal desorption iodide-adduct chemical ionization mass spectrometry, we provide direct observation of organic hydroperoxide (ROOH) formation from heterogeneous HO2 center dot and peroxy radicals (RO2 center dot) reactions for the first time. The ROOH may contribute substantially to the oxidation products, varied with the parent OA chemical structure. Furthermore, by regulating RO2 center dot reaction pathways, HO2 center dot also greatly influence the overall composition of the oxidized OA. Last, we suggest that the RO2 center dot + HO2 center dot reactions readily occur at the OA particle interface rather than in the particle bulk. These findings provide new mechanistic insights into the heterogeneous OA oxidation chemistry and help fill the critical knowledge gap in understanding atmospheric OA oxidative aging.