To investigate the evolutionary characteristics and formation mechanisms of PM_2.5 in areas with high environmental capacity, Shenzhen, a representative megacity on the southeast coast of China, was chosen for analysis in this study. Continuous quantitative observations of the mass concentrations of PM_2.5, water-soluble inorganic ions (WSIIs) in PM_2.5 and relevant reactive gaseous precursors were conducted for 1 year at an urban site in Shenzhen. The differences in the meteorological factors and in the major air pollutants between the polluted and clean periods were compared in detail. Daily air quality index (AQI) values were used to distinguish these two periods. Using the ISORROPIA-II thermodynamic equilibrium model, aerosol pH and liquid water content measurements were obtained for the first time during both the polluted period and the clean period. The aerosol pH values during the polluted period and the clean period were determined to be 7.55 ± 0.58 and 4.23 ± 2.99, respectively. The secondary conversion efficiencies of SO₂ and NO₂ to sulfate and nitrate, respectively, were found to increase during the polluted period. The polluted period exhibited a combination of both high PM_2.5 and high O₃ pollution. In the clean period, the ozone concentrations remained at high levels despite a significant drop in PM_2.5 levels. The potential sources and transportation modes of air pollution in Shenzhen were also discussed.