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

The mechanisms of secondary organic aerosol (SOA) formation are not yet fully understood. The relative abundance of hydroperoxyl radicals (HO₂) and peroxy radicals (RO₂) affects SOA properties, but chamber experiments often underemphasize the role of HO₂. 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 HO₂/RO₂ 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 NO₃ ^- ionization was used for gas-phase measurements. High HO₂/RO₂ conditions weakened the particle-phase monomer (C₁₀), fragment (C_4-9), and accretion product (C_11-20) signals by 34%, 29%, and 78%, respectively, compared to low HO₂/RO₂ conditions. The only species with an increased signal (180%) was C₁₀H₁₈O₇. 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 <i>C</i>*) 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 HO₂ for SOA formation and model predictions.