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. Under a high atmospheric oxidization capacity, the synergistic effect of the physicochemical processes in the atmospheric boundary layer (ABL) caused summer haze pollution in Beijing. The southern and southwestern areas, generally 60–300 km away from Beijing, were seriously polluted in contrast to Beijing, which remained clean. Southerly winds moving faster than 20–30 km h −1 since the early morning primarily caused haze pollution initiation. The PM 2.5 (particulate matter with a dynamic equivalent diameter smaller than 2.5  µm ) level increased to 75  µg m−3 over several hours during the daytime, which was simultaneously affected by the ABL structure. Additionally, the O3 concentration was quite high during the daytime (250  µg m−3 ), corresponding to a high atmospheric oxidation capacity. Much sulfate and nitrate were produced through active atmospheric chemical processes, with sulfur oxidation ratios (SORs) up to ∼0.76 and nitrogen oxidation ratios (NORs) increasing from 0.09 to 0.26, which further facilitated particulate matter (PM) level enhancement. However, the increase in sulfate was mainly linked to southerly transport. At midnight, the PM 2.5 concentration sharply increased from 75 to 150  µg m−3 over 4 h and remained at its highest level until the next morning. Under an extremely stable ABL structure, secondary aerosol formation dominated by nitrate was quite intense, driving the haze pollution outbreak. The PM levels in the southern and southeastern areas of Beijing were significantly lower than those in Beijing at this time, even below air quality standards; thus, the contribution of pollution transport had almost disappeared. With the formation of a nocturnal stable boundary layer (NSBL) at an altitude ranging from 0–0.3 km, the extremely low turbulence kinetic energy (TKE) ranging from 0 to 0.05 m 2  s −2 inhibited the spread of particles and moisture, ultimately resulting in elevated near-surface PM 2.5 and relative humidity ( ∼90  %) levels. Due to the very high humidity and ambient oxidization capacity, NOR rapidly increased from 0.26 to 0.60, and heterogeneous hydrolysis reactions at the moist particle surface were very notable. The nitrate concentration steeply increased from 11.6 to 57.8  µg m−3 , while the sulfate and organics concentrations slightly increased by 6.1 and 3.1  µg m−3 , respectively. With clean and strong winds passing through Beijing, the stable ABL dissipated with the potential temperature gradient becoming negative and the ABL height (ABLH) increasing to ∼2.5  km. The high turbulence activity with a TKE ranging from 3 to 5 m 2  s −2 notably promoted pollution diffusion. The self-cleaning capacity of the atmosphere is commonly responsible for airpollution dispersion. However, reducing the atmospheric oxidizationcapacity, through strengthening collaborative control of nitrogen oxide( NOx ) and volatile organic compounds (VOCs), as well as continuously deepening regional joint air pollution control, is urgent.

atmospheric chemistry and physics

Haze pollution under a high atmospheric oxidization capacity in summer in Beijing: insights into formation mechanism of atmospheric physicochemical processes