Reducing urban new particle formation as a plausible solution to mitigate particulate air pollution in Beijing and other Chinese megacities

With multi- and interdisciplinary approaches we show that atmospheric secondary particles are the dominating contributor to haze formation in terms of aerosol number, surface area and mass. Supported by our comprehensive observations in Beijing during 15 January 2018– 15 January 2020, we show that 80–90% of the aerosol mass (PM2.5) was formed via atmospheric reactions during the haze days and over 65% of the number concentration of haze particles resulted from urban new particle formation (NPF). Furthermore, the haze formation was much faster when the subsequent growth of newly formed particles was enhanced (rapid growth). We found that since the direct emissions of primary particles in Beijing has gone down significantly within recent years, all present-day haze episodes we preceded by a urban NPF event. We are also able to show that reducing the subsequent growth of freshly formed particles by a factor of 3-5 would delay the buildup of haze episodes by 1–3 days. Actually, this delay will decrease the length of each haze episode and the number of annual haze days could be approximately halved. The improvement can be achieved with targeted reduction of NPF precursors, mainly dimethyl amine, ammonia and further reductions of SO₂ emissions. Furthermore, reduction of anthropogenic VOC and nitrate emissions will slow down the growth rate of newly-formed particles and consequently reduce the haze formation. Our results show that the presence of haze decreases both boundary layer height and urban heat island intensity, which will further enhance haze particle number and mass concentrations over large spatial scales.