Weakened Gas-to-Particle Partitioning of Oxygenated Organic Molecules in Liquified Aerosol Particles

Aqueous-phase chemistry plays an important role in secondary organic aerosol (SOA) formation. However, the mechanism of the interaction between aerosol physics and chemistry is still poorly understood. Here continuous measurements of the 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 nonrefractory PM2.5 during the whole campaign. These values are lower than those reported in previous studies. Aqueous-phase oligomerization from methylglyoxal (C3H4O2) to oligomers (C4-7H6-10O5) is a probable path of SOA formation. However, direct gas-to particle partitioning of some CxHyOz with a particle-gas partition coefficient of ?1.0 x 10(-5) is weakened when the aerosol water content (AWC) exceeds 15 mu g m(-3), corresponding to a relative glass transition temperature below 0.85 compared with the AWC of < 15 mu g m(-3). A phase transition from solid/semisolid to liquid, increasing the diffusivity of organics in particles, should explain the observed inhibition of gas-to-particle partitioning and, subsequently, the low OA mass fraction at high relative humidity in pollution events. This study provides new insights into the SOA formation mechanism.