Gas-to-Particle Phase Transformation of Atmospheric Organics Amplified by Low Temperature and High Aerosol Water in Winter Haze of China

Gas-to-particle transformation of organic compounds is a key formation pathway for secondary organic aerosol (SOA), especially during haze periods in China. However, the drivers of this process remain poorly understood. On the basis of simultaneous hourly measurements of gas- and particle-phase water-soluble organic compounds (WSOC) at a rural site in the North China Plain (NCP) during winter 2023, we show at a bulk level that the partitioning of WSOC played an important role in SOA formation. Random forest analysis identified temperature, aerosol liquid water content (ALWC), and gas-particle conversion of NH3 as the dominant factors governing WSOC partitioning. Notably, high concentrations of ammonium nitrate (NH4NO3) under winter haze conditions increased ALWC, which in turn enhanced the partitioning of organics into the particle phase. Thermodynamic model analysis of formic acid confirmed the critical roles of low temperature and high ALWC in amplifying its gas-particle partitioning in the NCP winter. We found that the molar ratio of nitrate-to-sulfate in PM2.5 in China has continuously increased in the past few years due to strict SO2 emission control, which has resulted in atmospheric particles in the country becoming more hygroscopic and frequently dominated by liquid water. This aqueous-rich environment favors the gas-to-particle phase transformation of WSOC and thus promotes SOA formation, especially in winter haze periods. Our study highlights the synergistic effect of low temperature and aerosol water in driving organic partitioning to the aerosol phase, providing mechanistic insight into severe winter haze formation.