The haze pollution under strong atmospheric oxidization capacity in summer in Beijing: Insights into the formation mechanism of atmospheric physicochemical process

Abstract

<p><strong>Abstract.</strong> 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<sup>−1</sup> since early morning primarily caused the initiation of haze pollution. The PM<sub>2.5</sub> level increased to 75 μg m<sup>−3</sup> in several hours at daytime, which was simultaneously affected by the ABL structure. Additionally, the O<sub>3</sub> concentration was quite high at daytime (250 μg m<sup>−3</sup>), 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 PM<sub>2.5</sub> concentration sharply increased from 75 μg m<sup>−3</sup> to 150 μg m<sup>−3</sup> 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 m<sup>2</sup> s<sup>−2</sup> inhibited the spread of particles and moisture, ending up with elevated levels of PM<sub>2.5</sub> 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<sup>−3</sup> to 57.8 μg m<sup>−3</sup>, while the concentrations of sulfate and organics slightly increased by 6.1 μg m<sup>−3</sup> and 3.1 μg m<sup>−3</sup>, respectively. With clean & 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 m<sup>2</sup> s<sup>−2</sup> 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 (NO<sub>x</sub>) and volatile organic compounds (VOCs) was urgent, as well as continuously deepening regional joint control of air pollution.</p>

Modelling spatiotemporal variations of the canopy layer urban heat island in Beijing at the neighbourhood-scale

“…This local-scale urban climate model has been used for studies in megacities of contrasting climates such as London (Hamilton et al 2014;Aktas et al 2017) and Kuala Lumpur (K. . As our simulations cover the two field campaign periods of the Atmospheric Pollution and Human Health in a Chinese megacity (APHH-China) programme , the results may assist the interpretations of related air quality measurement and modelling studies Biggart et al 2020;Squires et al 100 2020; Zhao et al 2020). ADMS-Urban studies in London focused only on specific urban developments (Hamilton et al 2014) and building materials (Aktas et al 2017), whereas in Kuala Lumpur the effects of AHEs were excluded (K. .…”