Atmospheric Chemistry and Physics | 2021
Dominant synoptic patterns associated with the decay process of PM2.5 pollution episodes around Beijing
Abstract
Abstract. The variation in the concentrations of ambient PM2.5 (particles with an\naerodynamic diameter less than 2.5\u2009 µm ) generally forms a continuous\nsawtooth cycle with a recurring smooth increase followed by a sharp\ndecrease. The episode of abrupt decay of pollution is mostly meteorological in\norigin and is controlled by the passage of synoptic systems. One affordable\nand effective measure for quickly reducing PM2.5 concentrations in\nnorthern China is to wait for a strong wind to arrive. However, it is still\nunclear how strong the wind needs to be and exactly what kind of synoptic\nsystem most effectively results in the rapid decay of air pollution\nepisodes. PM2.5 variations over the 28 pollution channel cities of the Beijing region are investigated to determine the mechanisms by which synoptic patterns\naffect the decay processes of pollution episodes. This work shows more\nobvious day-to-day variations in PM2.5 concentration in winter than in\nsummer, which implies that wintertime PM2.5 variations are more sensitive to\nmeteorological factors. There were 365 decay processes from January 2014 to\nMarch 2020, and 97 of them were related to the effective wet deposition.\nIn total, 26\u2009% – 43\u2009% of PM2.5 pollutant is removed by the wet\ndeposition in different seasons. Two dominant circulation patterns are\nidentified in summer. All the other three seasons have three circulation\ntypes (CTs), respectively. The three CTs in spring show the same patterns\nas those in autumn and winter. The circulation patterns beneficial to the\ndecay processes all exhibit a higher-than-normal surface wind speed, a\nnegative relative humidity anomaly and net outflow of PM2.5 from the domain.\nIn addition, CT1 in spring, autumn and winter is controlled by northeasterly\nwind and features the most significant horizontal net outflow of air\npollutants and effective upward spread of air pollutants to the free\natmosphere. CT2 is the most frequent CT in autumn and winter, with the\nhighest wind speed from the northwest, highest boundary layer height\n(BLH) and lowest relative humidity among the three CTs, all of which are\nfavorable for the reduction of PM2.5 concentrations. In CT3, strong vertical\nwind shear within the boundary layer enhances the mixing of surface air\npollutants, which is the extra cleaning mechanism besides dry and clean air\nmass inflow. PM2.5 concentrations show significant decreases of more than\n37\u2009%, 41\u2009% and 27\u2009% after the passage of CT1, CT2 and CT3,\nrespectively. A dry airflow with a positive BLH anomaly and the effective\nhorizontal outflow of air pollutants are the main reasons for the abrupt\ndecay phase in summer. PM2.5 concentrations after the decay process show a\nsignificant decreasing trend from 2014 to 2020, reflecting successful\nemission mitigation. Emission reductions have led to a 4.3–5.7\u2009 µ g m - 3 yr - 1 decrease in PM2.5 concentrations in the 28\npollution channel cities of the Beijing region.