The haze pollution under strong atmospheric oxidization capacity in summer in Beijing: Insights into the formation mechanism of atmospheric physicochemical process
Authors: Zhao Dandan , Liu Guangjing , Xin Jinyuan , Quan Jiannong , Wang Yuesi , Wang Xin , Dai Lindong , Gao Wenkang , Tang Guiqian , Hu Bo , Ma Yongxiang , Wu Xiaoyan , Wang Lili , Liu Zirui , Wu Fangkun 
Published: January 2020

Abstract. Under strong atmospheric oxidization capacity, haze pollution in the summer of Beijing was the result of the synergistic effect of physicochemical process in the atmospheric boundary layer (ABL). The south/southwest areas generally ~ 60–300 km far away from Beijing were seriously polluted, in contrast to a clean situation in Beijing. The southerly winds moving more than ~ 20–30 km h−1 since early morning primarily caused the initiation of haze pollution. The PM2.5 level increased to 75 μg m−3 in several hours at daytime, which was simultaneously affected by the ABL structure. Additionally, the O3 concentration was quite high at daytime (250 μg m−3), corresponding to a strong atmospheric oxidation capacity. Numerous sulfate and nitrate were formed through active atmospheric chemical processes, with sulfur oxidation ratio (SOR) up to ~ 0.76 and nitrogen oxidation ratio (NOR) increasing from 0.09 to 0.26, which further facilitated the particulate matter (PM) level rising. Even so, the increase in sulfate was mainly linked by southerly transport. At midnight, the PM2.5 concentration sharply increased from 75 μg m−3 to 150 μg m−3 in 4 hours and stayed the highest level till the next morning. With the premise of an extremely stable ABL structure, the formation of secondary aerosols dominated by nitrate was quite intense, driving the outbreak of haze pollution. PM levels in the south/southeast of Beijing were significantly lower than that in Beijing over this time, even below air quality standards, thus, the contribution of pollution transport was almost gone. With the formation of nocturnal stable boundary layer of 0–0.3 km altitude, the extremely low turbulence kinetic energy (TKE) of 0–0.05 m2 s−2 inhibited the spread of particles and moisture, ending up with elevated levels of PM2.5 and relative humidity (~ 90 %) near the surface. Under quite high humidity and strong ambient oxidization capacity, the NOR rapidly increased from 0.26 to 0.60 and heterogeneous hydrolysis reactions at the moist particle surface were very significant. The nitrate concentration explosively increased from 11.6 μg m−3 to 57.8 μg m−3, while the concentrations of sulfate and organics slightly increased by 6.1 μg m−3 and 3.1 μg m−3, respectively. With clean &amp; strong winds passing through Beijing, the stable ABL was broken with potential temperature gradient turning to negative and ABL heights increasing to ~ 2.5 km. The strong turbulence activity with TKE of ~ 3–5 m2 s−2 notably promoted the pollution diffusion. The self-cleaning capacity of the atmosphere is always responsible for the dispersion of air pollution. Even so, reducing atmospheric oxidization capacity such as strengthening the collaborative control of nitrogen oxide (NOx) and volatile organic compounds (VOCs) was urgent, as well as continuously deepening regional joint control of air pollution.