Aqueous-phase chemistry plays an important role in secondary organic aerosol (SOA) formation. However, the mechanism about the interaction between aerosol physics and chemistry is still poorly understood. Here continuous measurements of gas- and particle-phase composition of organics were conducted using an iodine-adduct time-of-flight chemical ionization mass spectrometer coupled with a filter inlet for gases and aerosols in Beijing to understand the physicochemical processes related to SOA formation. Our results showed that SOA contributed 65.9% to the OA mass, and 17.1% to the non-refractory PM_2.5 during the whole campaign, lower than that reported in previous studies. Aqueous-phase oligomerization from methylglyoxal (C₃H₄O₂) to oligomers (C_4-7H_6-10O₅) is a probable SOA formation path. However, direct gas-to-particle partitioning of some C_xH_yO_z with a particle-gas partition coefficient higher than ~1.0×10^-5 is weakened when the aerosol water content (AWC) is over 15 μg m^-3, corresponding to a relative glass transition temperature below 0.85 compared with the AWC<15 μg m^-3. Phase transition from solid/semi-solid to liquid, increasing the diffusivity of organics in particles, should explain the observed inhibition of gas-to-particle partitioning, subsequently, the low OA mass fraction at high RH in pollution events. This study provides new insights into SOA formation mechanism.

Ma wei

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