Simultaneous measurements of gas- and aerosol-phase water-soluble organic nitrogen in the winter urban atmosphere of Chengdu, China

Abstract

Partitioning gaseous water-soluble organic nitrogen (WSON) to the aerosol phase is a major formation pathway of atmospheric secondary organic aerosols (SOA). However, the factors influencing this WSON partitioning process remain unclear. We conducted simultaneous wintertime measurements of WSON in both gas and aerosol phases at an urban site in Chengdu (CD), located in the Sichuan Basin (SCB), China, to investigate the concentration, gas-particle partitioning processes, and driven factors of WSON. The average concentration of particulate WSON (2.3±1.4µgNm-3) was about twice that of gaseous WSON (1.2±0.9µgNm-3) and increased significantly as PM 2.5 increased. In winter, WSON readily partitions into the particle phase (FWSON=0.70±0.14), partitioning into the organic phase under warm, dry conditions and into aerosol liquid water under cold, humid conditions, with higher F WSON in the latter. Amines (methylamine, MA; dimethylamine, DMA) were predominantly present in the gas phase but exhibited enhanced partitioning into the particle phase during the PM 2.5 polluted periods. Amines preferentially partitioned into aerosol liquid water throughout the entire campaign, stably existing as amine-sulfate or amine-nitrate salts in PM 2.5 . The influence of aerosol liquid water content (ALWC) on the partitioning behavior of both WSON and amines was substantially greater than that of temperature ( T ), relative humidity (RH), and aerosol acidity (pH), as further confirmed by thermodynamic modeling with S-curve analysis. Furthermore, NH 4 NO 3 was identified as the primary contributor to ALWC, suggesting that controlling NO x and NH 3 emissions is crucial for reducing ALWC and subsequent WSON formation.