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 transfer process remain unclear. We conducted simultaneous wintertime measurements of WSON in both gas and aerosol phases at an urban site in Chengdu, 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 ± 1.4 μgN m^–3) was about twice that of gaseous WSON (1.2 ± 0.9 μg N m^–3) and increased significantly as PM_2.5 increased. 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. The gas-particle partitioning coefficient (F_p(WSON)) showed a diurnal pattern with lower values during the day and higher values at night, which was enhanced by aerosol liquid water content (ALWC). Thermodynamic modeling using an S-curve analysis further revealed that ALWC was the key factor promoting the partitioning of WSON and amines into the particle phase, while the effects of pH and temperature were relatively weak. Furthermore, NH₄NO₃ was identified as the primary contributor to ALWC, suggesting that controlling NO_x and NH₃ emissions is crucial for reducing ALWC and subsequent WSON formation.