Abstract: As an important modern manufacturing base in the middle of the Yangtze River Delta,Changzhou is one of the 25 monitored cities with severe air pollution .To investigate the pollution characteristics of water-soluble ions in atmospheric fine particulate matter (PM_2.5),PM_2.5 samples were collected in urban Changzhou for four months (spring (May 2020),summer (July 2020),autumn (September 2020),and winter (January 2021)).The mass concentration of PM_2.5 and water-soluble inorganic ions (WSIIs) were determined,and the chemical characteristics,seasonal differences and main sources of water-soluble ions were investigated by using the multiple-technique analysis,combined correlation analysis,HYSPLIT modelling and primary component analysis.The results showed that during the sampling period,the annual average concentration of PM_2.5 was 51.88 ± 20.47 μg/m³,which was 1.5 times higher than the annual average PM_2.5 concentration limit set by GB3095-2012.The seasonal ranking of PM2.5 concentration was in the order of winter > spring > summer > autumn.The annual averaged WSIIs was 24.37 ± 11.54 μg/m³.The relative contribution of WSIIs to PM_2.5 increased markedly in winter (56.24 %).NH₄⁺,NO₃^-,and Cl^- showed significant increases in winter,indicating the important influences from combustion emissions,vehicle emissions,and secondary sources.Sulfur oxidation rate (SOR) and nitrogen oxidation rate (NOR) analysis revealed that NOR (0.14 ± 0.07) was lower than SOR (0.40 ± 0.10),indicating a higher secondary conversion rate of SO₂ under the same atmospheric background.Seasonal disparity was found between SOR and NOR,indicating different dominant factors which affected the sulfate and nitrate formation,with SOR influenced by relative humidity and NOR by NH₄⁺ concentration and surrounding traffic sources.We conducted further analysis of characteristics of chemical compositions in different phases.From clean to pollution phase,SOR and NOR rose to a certain extent,suggesting a higher conversion rate of SO₂and NOx during pollution episodes.Comparison of the percentage contribution of ions in different pollution phases showed that SO₄^2- and Ca²⁺ were dominated during clean phase,while the increase of NO₃^- and NH₄⁺ contributed to the atmospheric pollution in Changzhou.This implied that nitrate and ammonia formation was potentially the predominant factor controlling the occurrence of PM pollution.Principal component analysis revealed that in winter,Changzhou was mainly influenced by combustion emissions (32.23%),vehicle emissions and secondary aerosols (29.97%),as well as soil sources (22.63%).In summer,the main influences were a mixture of secondary aerosols and combustion emissions (44.20%),soil and sea salt sources (23.51%),and industrial or combustion emissions sources.