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In order to understand the seasonal levels, formation mechanism and atmospheric chemical behaviours of water-soluble ions of PM 10  in the Yangtze River Delta (YRD) region, aerosol samples were collected from January 2nd to December 28th, 2017 at a WMO/GAW regional background station in Lin’an. The concentrations of PM mass and nine water-soluble inorganic ions were obtained. The annual average concentration of PM 10  was 59.9±33.9 μg m −3 , lower than those reported in previous studies, indicating air quality of YRD region was improved. Nine water-soluble inorganic ions was accounted for 30.2-45.1% of the total PM mass, while ammonium (NH 4  + ), sulfate (SO 4  2+ ), as well as nitrate (NO 3  - ) were the major ions which contributed 86.3% to total ions. The NO 3  -  concentration was lowest in summer but highest in winter, suggesting it was likely influenced by thermodynamics. The levels of SO 4  2-  in spring and winter were related to photochemical reaction and regional transportation. Except for the SNA, Ca 2+  was highest in four seasons likely due to sand storm and road fugitive dust. The annual mean ratio of [NO 3  - ]/[SO 4  2- ] was nearly to 1, indicating mobile and stationary sources were equally important in Lin’an. The mean nitrogen oxidation ratio (NOR) and sulfur oxidation ratio (SOR) were 0.22±0.13 and 0.41±0.13, respectively, suggesting secondary formation was significant in the atmosphere at the background station of YRD region.

Seasonal variations of water-soluble ions in PM10 at a WMO/GAW station in the Yangtze River Delta, China