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

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. [ABSTRACT FROM AUTHOR]