Different roles of nitrate and sulfate in air pollution episodes in the North China Plain

Different chemical substances make different contributions to particles with different diameters. It is important to determine which species play important roles to control air pollution. In this study, the concentrations of water-soluble inorganic ions, elemental carbon, organic carbon and metal elements in PM1 (particulate matter with a diameter less than 1 μm) and PM2.5 (particulate matter with a diameter less than 2.5 μm) in the Shijiazhuang suburban area were measured by synchronous sampling for one month in the summer of 2016. The results showed that the chemical components of particulate matter are mainly SO42−, NO3−, NH4+ and organic matter (OM), accounting for 14%, 8%, 11%, and 11%, respectively, in PM1 and 13%, 8%, 10%, and 9%, respectively, in PM2.5. PM1 and PM2.5 have similar sources, including secondary nitrate, secondary sulfate, industrial sources, motor vehicle sources, dust, biomass combustion and coal burning, which contribute 29%, 30%, 10%, 9%, 8%, 8% and 6%, respectively, to PM1 and 29%, 33%, 12%, 13%, 8%, 2%, and 4%, respectively, to PM2.5. Based on the analysis of a typical pollution episode, the concentrations of PM1 and PM2.5 increase synchronously in the explosive growth stage, with nitrate acting as the key chemical component. In the most polluted period, the concentration of PM2.5 continues to increase because of the increase in sulfate, while that of PM1 is almost constant. The results show that the prevention and control of PM1 and PM2.5 should be different in different pollution stages.