Heterogeneous OH Oxidation of Organic Aerosols: Searching for New Reaction Pathways under Various Environmental Conditions

The heterogeneous oxidation of organic aerosols (OA) in the atmosphere can alter their composition, size and physical properties, and hence have a great impact on the air quality, climate, and human health. In previous studies of heterogeneous oxidation of OA, flow tube reactors are commonly used as the tool to study the kinetics and mechanisms, which create an environment with high OH concentrations and short reaction time (seconds to a few minutes), to mimic the OH exposure level in the real atmosphere. However, it is questionable whether OH exposure is the appropriate metric to use to understand atmospheric heterogeneous oxidation of OA. Through these traditional studies, the heterogeneous oxidation kinetics were measured by the decay of parent OA under different OH exposure levels. The main oxidation mechanisms were found to be peroxy radical (RO2) self-reactions undergoing the Russell and Bennet-Summers mechanisms. In this study, we perform laboratory experiments to examine the heterogeneous oxidation kinetics and product compositions using a custom-designed continuous-flow stirred tank reactor (CFSTR). The residence time of CFSTR is much larger than typical flow tube reactors, also allowing for lower OH concentrations. We have also controlled gas-phase HO2 concentrations to study its role in the heterogeneous oxidation. We use adipic acid, succinic acid and citric acid as the OA surrogates to study the heterogeneous oxidation kinetics. The degradation of the parent OA compounds and oxidation products are measured in real time using a thermal desorption chemical ionization mass spectrometer (TD-CIMS) with iodide as the reagent ion. A gas chromatography mass spectrometer (GC-MS) and an electrospray ionization ion mobility spectrometry mass spectrometer (IMS-MS) are used offline to provide isomer-resolved characterization of the oxidation products with structural information. Using this experimental setup and suite of instruments, we can explore new reaction products and pathways centered on RO2. Through this work, more mechanistic insights into heterogeneous OH oxidation of OA under conditions that are closer to the real atmosphere can be revealed.