Impact of HO₂/RO₂ ratio on highly oxygenated α-pinene photooxidation products and secondary organic aerosol formation potential

Abstract. Highly oxygenated molecules (HOM) from the atmospheric oxidation of biogenic volatile organic compounds are important contributors to secondary organic aerosol (SOA). Organic peroxy radicals (RO2) and hydroperoxy radicals (HO2) are key species influencing the HOM product distribution. In laboratory studies experimental requirements often result in overemphasis of RO2 cross-reactions compared to reactions of RO2 with HO2. We analyzed the photochemical formation of HOMs from α-pinene and their potential to contribute to SOA formation under high (≈1/1) and low (≈1/100) HO2/RO2 conditions. As HO2/RO2 > 1 is prevalent in the daytime atmosphere, sufficiently high HO2/RO2 is crucial to mimic atmospheric conditions and to prevent biases by low HO2/RO2 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 HO2/RO2 ratio was increased by adding CO, while keeping the OH concentration constant. We determined the HOM’s SOA formation potential, considering their fraction remaining in the gas phase after seeding with (NH4)2SO4 aerosol. Increase of HO2/RO2 led to a reduction in SOA formation potential, with the main driver being a ≈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 ≈30 % at HO2/RO2≈1/1. Particle phase observations measured an about according decrease in SOA mass and yield. Our study showed that too low HO2/RO2 ratios compared to the atmosphere can lead to an overestimation of SOA yields.