Aerosol liquid water in PM2.5 and its roles in secondary aerosol formation at a regional site of Yangtze River Delta.

• ALWC contributed 66.68% to PM 2.5 , with higher mass – based growth factor appearing when RH > 85%. • NO 3 – tended to form under higher ALWC conditions (RH > 80%), while SOC was favorable under lower ALWC levels. • ALWC was more sensitive to NO 3 – (R = 0.94) than SO 4 2– (R = 0.90) under heavy pollution. • SOC had a good correlation with ALWC (daytime: R = 0.84; nighttime: R = 0.54) with high atmospheric oxidation capacity (daytime: O X = 130 – 140 µg/m3; nighttime: O X = 100 – 140 µg/m3) in winter. Aerosol liquid water content (ALWC) plays an important role in secondary aerosol formation. In this study, a whole year field campaign was conducted at Shanxi in north Zhejiang Province during 2021. ALWC estimated by ISORROPIA-II was then investigated to explore its characteristics and relationship with secondary aerosols. ALWC exhibited a highest value in spring (66.38 µg/m3), followed by winter (45.08 µg/m3), summer (41.64 µg/m3), and autumn (35.01 µg/m3), respectively. It was supposed that the secondary inorganic aerosols (SIA) were facilitated under higher ALWC conditions (RH > 80%), while the secondary organic species tended to form under lower ALWC levels. Higher RH (> 80%) promoted the NO 3 – formation via gas-particle partitioning, while SO 4 2– was generated at a relative lower RH (> 50%). The ALWC was more sensitive to NO 3 – (R = 0.94) than SO 4 2– (R = 0.90). Thus, the self-amplifying processes between the ALWC and SIA enhanced the particle mass growth. The sensitivity of ALWC and O X (NO 2 + O 3) to secondary organic carbon (SOC) varied in different seasons at Shanxi, more sensitive to aqueous-phase reactions (daytime R = 0.84; nighttime R = 0.54) than photochemical oxidation (daytime R = 0.23; nighttime R = 0.41) in wintertime with a high level of O X (daytime: 130–140 µg/m3; nighttime: 100–140 µg/m3). The self-amplifying process of ALWC and SIA and the aqueous-phase formation of SOC will enhance aerosol formation, contributing to air pollution and reduction of visibility. [Display omitted] [ABSTRACT FROM AUTHOR]