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Changes in Secondary Organic Aerosol Composition and Volatility Going from a Low to a High HO 2 /RO 2 Regime in α-Pinene Photooxidation

Year: 2025

Type: article

Abstract: High Resolution Image Download MS PowerPoint Slide The mechanisms of secondary organic aerosol (SOA) formation are not yet fully understood. The relative abundance of hydroperoxyl radicals (HO 2 ) and peroxy radicals (RO 2 ) affects SOA properties, but chamber experiments often underemphasize the role of HO 2 . To clarify their contribution, this study compares the composition and volatility of SOA formed by the hydroxyl radical (OH) oxidation of α-pinene under low and high HO 2 /RO 2 regimes with a constant OH concentration. The particle-phase was characterized with a Filter Inlet for Gases and AEROsols coupled to an iodide Chemical Ionization Mass Spectrometer (FIGAERO CIMS), and a CIMS with NO 3 – ionization was used for gas-phase measurements. High HO 2 /RO 2 conditions weakened the particle-phase monomer (C 10 ), fragment (C 4–9 ), and accretion product (C 11–20 ) signals by 34%, 29%, and 78%, respectively, compared to low HO 2 /RO 2 conditions. The only species with an increased signal (180%) was C 10 H 18 O 7 . The gas-phase changes align with those in the particle-phase within a factor of 2. Overall, the organic mass was reduced by 47% and 39% for particle and gas-phases, respectively. Bulk SOA volatility (log C *) increased slightly from −0.22 μg m –3 to −0.1 μg m –3, reflecting the suppression of low volatility accretion products but formation of high volatility hydroperoxide monomers. This study highlights the importance of HO 2 for SOA formation and model predictions.

Source: ACS ES&T Air

DOI: 10.1021/acsestair.5c00254