Photodegradation of α-Pinene Secondary Organic Aerosol Dominated by Moderately Oxidized Molecules

Atmospheric secondary organic aerosol (SOA) undergoes chemical and physical changes when exposed to UV radiation, affecting the atmospheric lifetime of the involved molecules. However, these photolytic processes remain poorly constrained. Here, we present a study aimed at characterizing, at a molecular level and in real time, the chemical composition of alpha-pinene SOA exposed to UV-A light at 50% relative humidity in an atmospheric simulation chamber. Significant SOA mass loss is observed at high loadings (similar to 100 mu g m(-3)), whereas the effect is less prevalent at lower loadings (similar to 20 mu g m(-3)). For the vast majority of molecules measured by the extractive electrospray time-of-flight mass spectrometer, there is a fraction that is photoactive and decays when exposed to UV-A radiation and a fraction that appears photorecalcitrant. The molecules that are most photoactive contain between 4 and 6 oxygen atoms, while the more highly oxygenated compounds and dimers do not exhibit significant decay. Overall, photolysis results in a reduction of the volatility of SOA, which cannot be explained by simple evaporative losses but requires either a change in volatility related to changes in functional groups or a change in physical parameters (i.e., viscosity).

Automated summary

The study reveals that atmospheric secondary organic aerosol undergoes chemical changes when exposed to UV radiation, resulting in mass loss of molecules which are photoactive. However, not all oxygen-containing compounds decay when exposed to UV radiation, and the reduction in volatility may be related to changes in functional groups or other physical parameters.