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

Abstract

Abstract. 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 PM2.5 (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 PM2.5 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 m2 s−2 inhibited the spread of particles and moisture, ultimately resulting in elevated near-surface PM2.5 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 m2 s−2 notably promoted pollution diffusion. The self-cleaning capacity of the atmosphere is commonly responsible for air pollution dispersion. However, reducing the atmospheric oxidization capacity, 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.

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

“…They attributed this to stronger winter AHEs from elevated energy consumption for building heating systems (Lu et al, 2016), which readily accumulate in Beijing's frequently stable and shallow winter nocturnal boundary layer (Zhang et al, 2016), and more efficient nocturnal cooling in rural areas under frequent strong winter temperature inversions (Yang et al, 2013). Summertime nocturnal UHIIs are influenced more by the delayed release of heat stored within the urban fabric throughout the day (Oke et al, 1999;Wang et al, 2013), with a smaller proportion attributed to AHEs from residential air conditioning units (Zhao et al, 2018).…”

“…The UHI effect is well known to exacerbate and prolong extreme temperature events (Li et al, 2015;Jiang et al, 2019). As heatwaves are becoming more frequent in our warming climate (Krayenhoff et al, 2018;Zhao et al, 2018), the number of people in China left vulnerable to heat-related illnesses is increasing (Tan et al, 2010;Bai et al, 2014;Gu et al, 2016). Most at risk from illnesses such as heat stroke are the elderly (Gu et al, 2016), of particular concern in China owing to its ageing population (Li et al, 2016), and those without air conditioning (Zhao et al, 2018).…”

“…As heatwaves are becoming more frequent in our warming climate (Krayenhoff et al, 2018;Zhao et al, 2018), the number of people in China left vulnerable to heat-related illnesses is increasing (Tan et al, 2010;Bai et al, 2014;Gu et al, 2016). Most at risk from illnesses such as heat stroke are the elderly (Gu et al, 2016), of particular concern in China owing to its ageing population (Li et al, 2016), and those without air conditioning (Zhao et al, 2018). The latter is of most significance at night when residents are at home and the UHI intensity (UHII), the magnitude of the urban-rural near-surface air temperature difference, is strongest (Liu et al, 2007;He et al, 2020).…”

Comment on acp-2020-1273

“…3), the variation of PM2.5 concentration was more sensitive and responded rapidly to the wind shift from northerly to southerly, causing the PM2.5 concentration in Beijing to increase quickly from ~30 μg•m -3 to ~150 μg•m -3 , while southwest of Beijing (e.g., BD, SJZ, XT, and HD) the PM2.5 concentration increased rapidly to ~200 μg•m -3 . Research has revealed that the pollutant transport south of Beijing, especially in the southwest areas (the Taihang Mountains), is the most important contribution source of Beijing pollutants (Zhao et al, 2020). During the AP (approximately December 3; episode 3 in Fig.…”

Lidar vertical observation network and data assimilation reveal key processes driving the 3-D dynamic evolution of PM<sub>2.5</sub> concentrations over the North China Plain

“…As a developing country with the largest population in the world, China's air quality has exhibited an obvious improvement trend in recent years (Sun et al, 2015;Cao et al, 2017). Regional air pollution in China is still serious, however, especially the heavy aerosol pollution (HAP) caused by fine particulate matter (PM 2.5 ) in winter, which has attracted attention worldwide (Zheng et al, 2015(Zheng et al, , 2019Cheng et al, 2016). Therefore, providing a reliable distribution of the PM 2.5 concentration of HAP, especially at any time and at any height in a given region, is particularly important in the quest for avoiding public health problems and providing government policymakers with help in designing effective controls (Hu et al, 2015).…”