Impact of HO ₂∕RO ₂ ratio on highly oxygenated α-pinene photooxidation products and secondary organic aerosol formation potential

Highly oxygenated molecules (HOMs) from the atmospheric oxidation of biogenic volatile organic compounds are important contributors to secondary organic aerosol (SOA). Organic peroxy radicals (ROinline-formula₂) and hydroperoxy radicals (HOinline-formula₂) are key species influencing the HOM product distribution. In laboratory studies, experimental requirements often result in overemphasis on ROinline-formula₂ cross-reactions compared to reactions of ROinline-formula₂ with HOinline-formula₂. We analyzed the photochemical formation of HOMs from inline-formulaα-pinene and their potential to contribute to SOA formation under high (inline-formula≈1/1) and low (inline-formula≈1/100) inline-formula $M12inlinescrollmathmlchem{\mathrm{normal\; HO}}_{\mathrm{normal\; 2}}/{\mathrm{normal\; RO}}_{\mathrm{normal\; 2}}$ 50pt14ptsvg-formulamathimg2b4018409b4ed76ade91ea8e966f8959 acp-24-4789-2024-ie00004.svg50pt14ptacp-24-4789-2024-ie00004.png conditions. As inline-formula $M13inlinescrollmathmlchem{\mathrm{normal\; HO}}_{\mathrm{normal\; 2}}/{\mathrm{normal\; RO}}_{\mathrm{normal\; 2}}$ 50pt14ptsvg-formulamathimg8cd9c354b6d3062297ae8c19001649cf acp-24-4789-2024-ie00005.svg50pt14ptacp-24-4789-2024-ie00005.png  inline-formula> 1 is prevalent in the daytime atmosphere, sufficiently high inline-formula $M15inlinescrollmathmlchem{\mathrm{normal\; HO}}_{\mathrm{normal\; 2}}/{\mathrm{normal\; RO}}_{\mathrm{normal\; 2}}$ 50pt14ptsvg-formulamathimg2eb9e90b0bd9aa84933ade8700ded6ca acp-24-4789-2024-ie00006.svg50pt14ptacp-24-4789-2024-ie00006.png is crucial to mimic atmospheric conditions and to prevent biases by low inline-formula $M16inlinescrollmathmlchem{\mathrm{normal\; HO}}_{\mathrm{normal\; 2}}/{\mathrm{normal\; RO}}_{\mathrm{normal\; 2}}$ 50pt14ptsvg-formulamathimge1d7ab50b5885ac2f66055485c5143cf acp-24-4789-2024-ie00007.svg50pt14ptacp-24-4789-2024-ie00007.png on the HOM product distribution and thus SOA yield. Experiments were performed under steady-state conditions in the new, continuously stirred tank reactor SAPHIR-STAR at Forschungszentrum Jülich. The inline-formula $M17inlinescrollmathmlchem{\mathrm{normal\; HO}}_{\mathrm{normal\; 2}}/{\mathrm{normal\; RO}}_{\mathrm{normal\; 2}}$ 50pt14ptsvg-formulamathimg87d1411523dfc82614bf68f0ff2f9588 acp-24-4789-2024-ie00008.svg50pt14ptacp-24-4789-2024-ie00008.png ratio was increased by adding CO while keeping the OH concentration constant. We determined the HOM's SOA formation potential, considering its fraction remaining in the gas phase after seeding with (NHinline-formula₄)₂SOinline-formula₄ aerosol. An increase in inline-formula $M20inlinescrollmathmlchem{\mathrm{normal\; HO}}_{\mathrm{normal\; 2}}/{\mathrm{normal\; RO}}_{\mathrm{normal\; 2}}$ 50pt14ptsvg-formulamathimge5e8e165accc4799514d6e5795966a13 acp-24-4789-2024-ie00009.svg50pt14ptacp-24-4789-2024-ie00009.png led to a reduction in SOA formation potential, with the main driver being a inline-formula∼ 60 % reduction in HOM-accretion products. We also observed a shift in HOM-monomer functionalization from carbonyl to hydroperoxide groups. We determined a reduction of the HOM's SOA formation potential by inline-formula∼ 30 % at inline-formula $M23inlinescrollmathmlchem{\mathrm{normal\; HO}}_{\mathrm{normal\; 2}}/{\mathrm{normal\; RO}}_{\mathrm{normal\; 2}}$ 50pt14ptsvg-formulamathimg66226eec0ca5d58aa351b18e8a3becd8 acp-24-4789-2024-ie00010.svg50pt14ptacp-24-4789-2024-ie00010.png  inline-formula≈1/1 compared to inline-formula $M25inlinescrollmathmlchem{\mathrm{normal\; HO}}_{\mathrm{normal\; 2}}/{\mathrm{normal\; RO}}_{\mathrm{normal\; 2}}$ 50pt14ptsvg-formulamathimg866241d0f96f6d46374a213dee573c58 acp-24-4789-2024-ie00011.svg50pt14ptacp-24-4789-2024-ie00011.png  inline-formula≈ 1/100. Particle-phase observations measured a similar decrease in SOA mass and yield. Our study shows that too low inline-formula $M27inlinescrollmathmlchem{\mathrm{normal\; HO}}_{\mathrm{normal\; 2}}/{\mathrm{normal\; RO}}_{\mathrm{normal\; 2}}$ 50pt14ptsvg-formulamathimgedf73373e907de77c0d0711e75bc50a3 acp-24-4789-2024-ie00012.svg50pt14ptacp-24-4789-2024-ie00012.png ratios compared to the atmosphere can lead to an overestimation of SOA yields.