Observational evidence for the dual roles of BC in the megacity of eastern China: Enhanced O3 and decreased PM2.5 pollution

Abstract

PM2.5 pollution has been alleviated significantly since implementing the Clean Air Action in China, whereas a concomitant increase in O3 pollution has been observed. In the megacity of China, haze episodes can be aggravated by the interaction between aerosols and the planetary boundary layer (PBL). Such interactions have been largely investigated in the middle and upper PBL and less so near the ground. Here, using observations from the ground and in the PBL during summer, we find that haze pollution has decreased notably, but O3 pollution has worsened in Nanjing. PM2.5, black carbon (BC), BC/PM2.5, O3 and planetary boundary layer height (PBLH) changed at rates of −0.34 μg m−3·month−1, 13 ng m−3·month−1, 0.08%·month−1, -0.06 μg m−3·month−1 and 1.5 m month−1, respectively (3.5 m·month−1excluding data from 2020) during 2015–2020. The height of the lower boundary of the temperature inversion layer decreased first and then increased at a rate of 5.4 m month−1 between 2017 and 2020. As the PM2.5 concentration has decreased by 42.4% over the last six years, aerosol extinction has weakened in the PBL. Subsequently, the solar radiation has strengthened near the ground, which is conducive to forming O3 and is mainly concentrated between 0 and 600 m. Due to the heating effect of BC, which can increase the temperature near the ground, the lower boundary of the inversion layer has been elevated, which further mitigates and aggravates the haze and O3 pollution, respectively.