Mechanism of haze pollution in summer and its difference with winter in the North China Plain

Heavy haze pollution usually occurs in winter. However, according to the enhanced atmospheric boundary layer (ABL) field experiments conducted in the North China Plain (NCP) from 17 June to 6 July 2019, heavy haze pol-lution may also occur in summer, although with a lower probability. Winter haze pollution is significantly af-fected by adverse boundary layer meteorological conditions, whereas our study shows different mechanisms of summer haze pollution from that of winter. In summer, PM2.5 is distributed uniformly as a thick layer at a lighter pollution level; however, the PM2.5 column content in summer exceeds that in winter, suggesting that the better air quality in summer is mainly due to improved diffusion conditions. In summer, even under haze conditions, the ABL can develop over 1000 m and has a large ventilation similar to clean periods, which indicates both favourable vertical diffusion conditions and advection capability of the summer ABL. Unlike in winter, the heavy haze pollution in summer is often caused by regional transport which is related to local circulation. To ex-plore the influence of different scale systems on summer haze pollution, we applied the spectral analysis method to surface PM2.5 concentrations. Strong periodicity of PM2.5 concentrations is found in 4-9 d and 1 d, correspond-ing to the impacts of large-scale synoptic system changes and the ABL evolution, respectively. The influence of weather change is much stronger than that of the ABL evolution on PM2.5 concentrations in summer. The resulting changes in PM2.5 concentrations are approximately 45 mu g/m(3) and 15 mu g/m(3), respectively. There has been a consensus on the importance of emission control in winter. And this study shows that heavy haze pollution can also occur in summer and regional joint emission control should also be emphasized in summer. (C) 2021 The Authors. Published by Elsevier B.V.

Automated summary

The study reveals that heavy haze pollution can also occur in summer due to regional transport, different from winter. The fluctuation in PM2.5 concentrations in summer is more influenced by weather change than ABL evolution. Therefore, joint emission control at a regional level is necessary to address severe haze pollution in summer.