Abstract
Abstract. The variation in the concentrations of ambient PM2.5 (particles with an aerodynamic diameter less than 2.5 µm) generally forms a continuous sawtooth cycle with a recurring smooth increase followed by a sharp decrease. The episode of abrupt decay of pollution is mostly meteorological in origin and is controlled by the passage of synoptic systems. One affordable and effective measure for quickly reducing PM2.5 concentrations in northern China is to wait for a strong wind to arrive. However, it is still unclear how strong the wind needs to be and exactly what kind of synoptic system most effectively results in the rapid decay of air pollution episodes. PM2.5 variations over the 28 pollution channel cities of the Beijing region are investigated to determine the mechanisms by which synoptic patterns affect the decay processes of pollution episodes. This work shows more obvious day-to-day variations in PM2.5 concentration in winter than in summer, which implies that wintertime PM2.5 variations are more sensitive to meteorological factors. There were 365 decay processes from January 2014 to March 2020, and 97 of them were related to the effective wet deposition. In total, 26 %–43 % of PM2.5 pollutant is removed by the wet deposition in different seasons. Two dominant circulation patterns are identified in summer. All the other three seasons have three circulation types (CTs), respectively. The three CTs in spring show the same patterns as those in autumn and winter. The circulation patterns beneficial to the decay processes all exhibit a higher-than-normal surface wind speed, a negative relative humidity anomaly and net outflow of PM2.5 from the domain. In addition, CT1 in spring, autumn and winter is controlled by northeasterly wind and features the most significant horizontal net outflow of air pollutants and effective upward spread of air pollutants to the free atmosphere. CT2 is the most frequent CT in autumn and winter, with the highest wind speed from the northwest, highest boundary layer height (BLH) and lowest relative humidity among the three CTs, all of which are favorable for the reduction of PM2.5 concentrations. In CT3, strong vertical wind shear within the boundary layer enhances the mixing of surface air pollutants, which is the extra cleaning mechanism besides dry and clean air mass inflow. PM2.5 concentrations show significant decreases of more than 37 %, 41 % and 27 % after the passage of CT1, CT2 and CT3, respectively. A dry airflow with a positive BLH anomaly and the effective horizontal outflow of air pollutants are the main reasons for the abrupt decay phase in summer. PM2.5 concentrations after the decay process show a significant decreasing trend from 2014 to 2020, reflecting successful emission mitigation. Emission reductions have led to a 4.3–5.7 µgm-3yr-1 decrease in PM2.5 concentrations in the 28 pollution channel cities of the Beijing region.
Highlights
PM2.5 pollution has become a severe threat and challenge in China, especially in the Beijing–Tianjin–Hebei (BTH) region, and has attracted significant concern regarding how to improve regional air quality (Che et al, 2019; Wang et al, 2015, 2019a; Xia et al, 2016; Zhang et al, 2018a; Mu and Zhang, 2014; Cai et al, 2017)
Because the JCT method is specialized in classifying daily mean sea level pressure patterns, which will ignore the thresholds of some other meteorological variables to some extent (Philipp et al, 2014), we only focus on the results of principal component analysis (PCA) hereafter
The T-mode principal component analysis (PCA) method was used to objectively classify the type of synoptic system dominating the decay phase of pollution episodes, as this method has an outstanding performance in terms of the reproduction of predefined types and temporal–spatial stabilities (Huth et al, 2008; Cavazos, 2000; Tie et al, 2015; Valverde et al, 2015; Xu et al, 2016)
Summary
PM2.5 (particles with an aerodynamic diameter less than 2.5 μm) pollution has become a severe threat and challenge in China, especially in the Beijing–Tianjin–Hebei (BTH) region, and has attracted significant concern regarding how to improve regional air quality (Che et al, 2019; Wang et al, 2015, 2019a; Xia et al, 2016; Zhang et al, 2018a; Mu and Zhang, 2014; Cai et al, 2017). Many studies took the smooth increase period of PM2.5 concentrations and abrupt decrease stage following it as a complete air pollution episode and investigated its development mechanism (Tang et al, 2016b; Zhang et al, 2018b; Sun et al, 2014; Zheng et al, 2015) It is still unclear how strong the wind needs to be, exactly what kind of synoptic systems can effectively terminate air pollution episodes in the BTH region, and what mechanism is responsible for the rapid reduction in PM2.5 concentrations in a few hours. This study will focus on the region covering these 28 pollution channel cities and reveal the synoptic circulation pattern that dominates the decay process of PM2.5 pollution events
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