Nitrate-Enhanced Gas-to-Particle-Phase Partitioning of Water- Soluble Organic Compounds in Chinese Urban Atmosphere: Implications for Secondary Organic Aerosol Formation

To understand the gas-to-particle-phase partitioning process of watersoluble organic compounds (WSOCs) in the atmosphere, we conducted simultaneous measurements of gaseous WSOCs and particulate WSOCs in Shanghai during the summer of 2020. We found that pollution events (PEs) of PM2.5 during the campaign can be classified as sulfate-dominated (SD) or nitrate-dominated (ND) episodes. The gas-to-particle-phase partitioning coefficients (Fp) of WSOCs during ND episodes (0.36 +/- 0.04) were 40% higher than those during SD episodes (0.25 +/- 0.05). These results were largely driven by aerosol liquid water content and aerosol acidity during ND and SD episodes, respectively. Such bulk-level results are consistent with the partitioning behaviors of oxalic acid molecules during the PEs, demonstrating that the kinetics of the partitioning process differed for the two types of PEs. The thermodynamic model simulation for Fp and the trend analysis for PM2.5 composition revealed that the efficiency of the gas-to-particle-phase partitioning of atmospheric WSOCs throughout China in the past 20 years has increased with the increasing abundance of nitrate relative to sulfate, indicating that the formation of secondary organic aerosols in China will be more efficient in the near future because of the increasingly efficient partitioning of WSOCs.