Investigation of the atmospheric boundary layer during an unexpected summertime persistent severe haze pollution period in Beijing
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Persistent severe haze pollution seldom happens in summer in Beijing. In this study, a persistent severe haze pollution event, defined as daily PM2.5 concentration higher than 150 μg m−3 on 5 consecutive days, was observed in Beijing from 26 to 30 in July 2010. It was not caused by crop residue burning during the harvest period. Weak pressure systems dominated at surface and implicated weak advection. Regional weak southerly winds with speeds of 2–3 m s−1 consistently brought pollutants from southern large-emission regions to Beijing. Surface convergence resulted from northerly winds prevailing in northern regions induced pollutants remaining in Beijing, which contributed to the maximum daily PM2.5 concentration on 26 in July. A continental high-pressure system persisted in the northwest of Beijing at 500 hPa, which led to significant sinking motion. Elevated inversion was found in the form of subsidence inversion, and this was confirmed by a skew T-logp diagram and vertical velocity analysis. It is much different from the vertical boundary layer structure in wintertime pollution period that surface-based inversion dominates. The subsidence inversion was an extremely stable layer with an average depth of hundred meters and strength of 1.4 °C. The capping effect of the inversion layer at low altitude of average 720 m limited vertical diffusion of pollutants and trapped them in a shallow layer, and thus, extremely high concentration of PM2.5 remained. WRF-Chem model simulation demonstrated that about 70% of PM2.5 was transported to Beijing from its southern regions.
This research was supported by National Key R&D Program of China (2016YFC0201505) and National Natural Science Foundation of China (91644212 and 41675142). The observational sounding data and skew T-logP diagram were obtained from the University of Wyoming. The ERA-interim data were from the European Center for Medium-Range Weather Forecasts. The surface meteorological data were collected from National Climatic Data Center. The authors thank Prof. Shanhong Gao in Ocean University of China for his contribution to the synoptic charts. The authors declare that they have no conflict of interest.
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