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

Abstract

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 α-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 (∼100 μg m–3), whereas the effect is less prevalent at lower loadings (∼20 μg 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).

Effects of the sample matrix on the photobleaching and photodegradation of toluene-derived secondary organic aerosol compounds

“…S6. Previous studies have found significant fractions of photo-recalcitrant material when SOA photolysis was performed in the organic particle phase, including for high-NO x toluene SOA (O'Brien and Kroll, 2019;Walhout et al, 2019;Baboomian et al, 2020;Pospisilova et al, 2021), and based on these studies we expect a photorecalcitrant fraction for the SOA studied here. When the photo-recalcitrant fraction was not constrained to zero, the obtained fit estimates a large photo-recalcitrant absorbance fraction (49 ± 5 %).…”

Monoterpene Photooxidation in a Continuous-Flow Chamber: SOA Yields and Impacts of Oxidants, NOx, and VOC Precursors

“…It is possible that the SOA production was actually enhanced due to synergy between ozonolysis and photooxidation and then lowered by photolysis, as some studies have suggested that the yield from monoterpene ozonolysis could be reduced in the presence of light. 52,53 However, a recent study suggested that the SOA loss due to photolysis is negligible at low aerosol loadings (∼20 μg m −3 ), 54 and the aerosol loadings in our experiments were even lower (∼10 μg m −3 ). In addition, the effect of photolysis has been observed most clearly in batch-mode experiments where SOA age continuously increased with time, while in continuous-mode experiments, aerosol has a distribution of age.…”

Radical-Initiated Brown Carbon Formation in Sunlit Carbonyl–Amine–Ammonium Sulfate Mixtures and Aqueous Aerosol Particles

“…This is important because most BrC photobleaching studies to date have been performed on bulk liquid solutions, 56−60 even if the brown carbon was initially formed in suspended aerosol. 59 The optical parameters of BrC from each experiment are compared to the recent atmospheric BrC classification scheme of Saleh et al 71,72 in Figure 5. It can be seen that BrC formed in dark, bulk-phase reactions between carbonyl species and AS (black triangle) does not match atmospheric BrC in absorptivity, expressed as log(MAC 405 ), and on average has a wavelength dependence (expressed as AAE 330−400 ) that is steeper.…”