Dependence of aerosol formation on OH radical sources under humid conditions from m-xylene

The OH radical plays the most critical role in secondary organic aerosols (SOA) formation. While multiple OH precursors exist in the atmosphere, their potential differential impacts on SOA production remain poorly understood. Here, two representatives of OH precursors (H₂O₂ and O₃) are selected to investigate the yield and chemical composition of SOA generated from m-xylene under humid conditions compared with dry conditions. Orbitrap mass spectrometry is used to elucidate the mechanisms on SOA formation from m-xylene-H₂O₂ and m-xylene O₃ photooxidation systems across humidity regimes. Experiments in a Teflon reactor revealed contrasting relative humid (RH) dependencies. In the m-xylene-H₂O₂ system, SOA yields decreased drastically from 29.8 % at 6 % RH to 3.8 % at 78 % RH. In contrast, SOA yields in the m-xylene-O₃ system exhibited no significant change, with 7.0-9.6 % at 5-6 % RH and 8.6-9.9 % at 68-87 % RH. It is attributed the suppression of the yield at elevated RH in the m-xylene-H₂O₂ system to excess OH concentrations derived from dissolved H₂O₂ photolysis in the condensed phase. It enhances fragmentation and carbon loss processes of SOA precursors, increasing their volatility and promoting gas-phase partitioning. Conversely, in the m-xylene-O₃ system SOA yields remained stable due to compensating pathways. Higher number concentrations, higher gas-phase OH concentrations, and aldol condensation and acetal oligomerization can promote the aerosol masses, and meanwhile more reacted m-xylene at elevated RH sustains the SOA yields. These findings highlight the critical role of the OH sources in realistic atmospheric processes and of the selection of OH sources in laboratory studies. The study provides key insights into SOA formation mechanisms influenced by OH sources, with implications for improving air quality models.