Highly time-resolved characterization of water-soluble inorganic ions in PM2.5 in a humid and acidic mega city in Sichuan Basin, China

- Characteristics of water soluble inorganic ions in high humidity and acidity area.
- Study based on high time resolution observation.
- Relatively humidity is the dominant factor effecting nitrate formation mechanism.
- Aerosol pollutions in Chongqing were mostly caused by local emissions.

To investigate the characteristics of water-soluble inorganic ions (WSIIs) in Chongqing, a well-known foggy and acid region in southwestern China, hourly real-time concentrations of five cations (Na+, K+, NH4+, Ca2 +, and Mg2 +) and six anions (F−, Cl−, NO2−, NO3−, PO43 −, and SO42 −) in PM2.5 during winter (from Dec. 18, 2015 to Mar. 20, 2016) in Chongqing were collected by applying In-situ Gas and Aerosol Compositions Monitor. The hourly total concentration of WSIIs was 38.5 μg/m3 on average, accounting for 57% of PM2.5 mass concentration. Secondary inorganic aerosols (NH4+, NO3−, and SO42 −) were dominant WSIIs, accounting for 91% of WSIIs mass. Compared to ten years ago, SO42 − concentrations were decreased by 31% but NO3− levels were doubled, likely indicative of sharply enhanced contribution to fine particle pollution from mobile sources over stationary sources. NO3− originated from the current fluxes of NH3 and HNO3 onto sulfate particles and/or from in-cloud processes were critical pathways under humid conditions in the study area. Water content and/or RH might be important factors controlling nitrate formation. Trajectory analysis manifested that aerosol pollutions in Chongqing were mostly caused by local emissions.

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