Aerosol Formation by Multiphase Reactions of Peroxide and Carbonyl Compounds over Beijing

The concentrations of peroxides and carbonyl compounds in the gas and aerosol phases were observed over urban Beijing in summer and winter 2022. Peroxide multiphase reactions were identified as dominant pathways for sulfate formation in fine particles (PM_2.5), and carbonyl compound multiphase transformation significantly contributed to secondary organic aerosol (SOA) formation, with the oxidative pathway dominating (> 80%) over the non-oxidative pathway. We develop the parameterization formulas for SOA formation from carbonyl compounds, showing that the rising atmospheric H₂O₂ level in Beijing significantly increased SOA formation rate via carbonyl compound multiphase reactions both in summer and winter in recent years. The mechanisms by which multiphase transformation of peroxides and carbonyl compounds promote secondary aerosol formation are suggested.