Nitrate dominates the chemical composition of PM_2.5 during haze event in Beijing, China

Water-soluble inorganic ions (WSI), a major component of PM_2.5, often increased rapidly during the haze event in Beijing. Sulfate (SO₄^2-), Nitrate (NO₃^-), and Ammonium (NH₄⁺) are three main components of WSI. Since year 2015, sulfate concentrations in PM_2.5 has gradually decreased owing to the effective control of SO₂ emissions. However, the contribution of nitrate to PM_2.5 has significantly increased during haze events in Beijing at the same time. In this study, a highly time-resolved online analyzer (Monitor for Aerosols and Gases, MARGA) was employed to measure the WSI in PM_2.5 in Beijing from 5 February to 15 November 2017. Three typical haze events during this sampling period were investigated. During heavy pollution episodes in winter, nitrate concentrations increased from 7.5 μg/m³ to 45.6 μg/m³ (45.0% of WSI), while sulfate increased from 4.2 μg/m³ to 20.1 μg/m³ (19.8% of WSI). This indicated that nitrate is more important than sulfate as a driver for the growth of PM_2.5 during the period of heavy air pollution in winter. Nitrate also dominates the increase of WSI in the pollution episodes in autumn, with an average concentration of 52.5 μg/m³, and contributed up to 67% of WSI. The average concentration ratio of NH₄⁺ to SO₄^2- was higher in autumn (1.02) than that in summer (0.74) and close to that in winter (1.00). This is mainly because the emission control of coal combustion in Beijing and surrounding areas results in an NH₃-rich and SO₂-lean atmosphere, which promoted the formation of ammonium nitrate. Our study indicates that nitrate has become the most important component of WSI in PM_2.5 and is driving the rapid growth of PM_2.5 concentrations during heavy pollution episodes in Beijing. Therefore, more efforts shall be made to reduce the nitrogen oxide and ammonia emissions in Beijing and surrounding areas.