Dominant synoptic patterns associated with the decay process of PM2.5 pollution episodes around Beijing
Authors: Wang Xiaoyan , Zhang Renhe , Tan Yanke , Yu Wei 
Published: September 2020

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 abrupt decay of pollution episode is mostly meteorological in origin, and is controlled by the passage of synoptic systems. One affordable and effective measure for the quickly reducing PM2.5 concentrations in northern China is to wait for 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 Beijing 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. 26 %~43 % of PM2.5 pollutant is removed by the wet deposition in different seasons. Two dominant circulation patterns are identified in summer, and the same three circulation types (CTs) are identified in the other three seasons based on the dry-day cases. The circulation patterns beneficial to the decay processes all exhibit a higher than normal surface wind speed, a negative relative humidity anomaly and positive divergence in the PM2.5 horizontal flux. 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, which promotes the abrupt reduction of local PM2.5 concentrations. CT2 is the most frequent synoptic pattern leading to decay processes in autumn and winter, and the domain region is located to the east of an anticyclone system. CT2 features a strong northwesterly wind of 2.98~3.88 m/s, the lowest relative humidity and the highest boundary layer height (BLH) among the three CTs, all of which are favorable for the reduction of PM2.5 concentrations. In CT3, a prevailing westerly wind anomaly occurs in the domain, with remarkable zonal divergence in the PM2.5 flux and strong horizontal wind shear in the near-surface under the boundary layer. PM2.5 concentrations show significant decreases of more than 37 %, 41 % and 27 % after the passage of CT1, CT2 and CT3, respectively. A dry air mass 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 μg/(m3.yr) decrease in PM2.5 concentrations in the 28 pollution channel cities of Beijing.