Small contribution of conventionally-defined formation of secondary particulate matter to severe PM2.5 pollution in today’s northern China

Benefited from the tightening emissions of air pollutants, a large annual decrease in mixing ratio of SO₂ and a moderate decrease in PM_2.5 can be identified in northern China since 2014. However, a few extreme PM_2.5 pollution events still occur for sometimes during heating seasons, e.g., the 99th percentile value of PM_2.5 concentrations during the heating season in 2018 had exceeded 200 µg m^-3 therein. One unit of percentile value corresponds to approximately 30 hours. To reveal real causes of these extreme PM_2.5 pollution events, we define two technical terms in this study, i.e., 1) secondary particulate species formed in ambient air (conventionally-defined FSPM); 2) formation of secondary particulate matter in the fresh plumes during the initial several minutes (plume-processed FSPM). We also introduce a metric, i.e., PM_2.5/CO in unit of µg m^-3 / ppm. With these technical terms in mind, we then struggle to dissect real mechanisms causing the severe PM_2.5 pollution event in 11-14 January 2019 across norther China. A staircase function of ratios of PM_2.5/CO against PM_2.5 rather than a linear increase or decrease with PM_2.5 generally occurred through the event. However, in general, larger ratios of PM_2.5/CO were indeed observed with larger concentration of PM_2.5. Regarding frequently observed invariant ratios accompanying with large variations in PM_2.5, larger ratios are, however, probably not caused by conventionally-defined FSPM in PM_2.5. Alternatively, our further multiple-technical analysis results confirm plume-processed FSPM, followed by accumulation under poor meteorological conditions, dominatingly resulting in the severe event.