Chemistry-triggered events of PM_2.5 explosive growth during late autumn and winter in Shanghai, China

To better understand the mechanism of PM_2.5 explosive growth (EG), we conducted concurrent measurements of gaseous pollutants, PM_2.5 and its chemical composition (inorganic ions, organic carbon, and element carbon) with a time resolution of 1 h in Shanghai in late autumn and winter from 2014 to 2017. In this study, the EG events, which are defined as the net increase in the mass concentration of PM_2.5 by more than 100 μg m^-3 within hours, are separately discussed for 3, 6, or 9 h. The number of EG events decreased from 19 cases in 2014 to 6 cases in 2017 and the corresponding PM_2.5 concentration on average decreased from 183.6 μg m^-3 to 128.8 μg m^-3. Both regional transport and stagnant weather (windspeed < 2.0 m s^-1) could lead to EG events. The potential source contribution function (PSCF) shows that the major high-pollution region is in East China (including Zhejiang, Jiangsu, Shandong, and Anhui Province) and the North China Plain. The contribution of stagnant conditions to EG episode hours of 55% (198 h, 156.9 μg m^-3) is higher than that of regional transport (45%, 230 h, 163.0 μg m^-3). To study the impact of local emission, chemical characteristics and driving factors of EG were discussed under stagnant conditions. The major components contributing to PM_2.5 are NO₃^- (17.9%), organics (14.1%), SO₄^2- (13.1%), and NH₄⁺ (13.1%). The driving factors of EG events are the secondary aerosol formation of sulfate and nitrate and primary emissions (vehicle emissions, fireworks, and biomass burning), but the secondary transformation contributes more to EG events. The formation of sulfate and nitrate is dominated by gas-phase oxidation and heterogeneous reactions, which are enhanced by a high relative humidity. The current study helps to understand the chemical mechanism of haze and provides a scientific basis for air pollution control in Shanghai.