Analysis of meteorological conditions and diurnal variation characteristics of PM_2.5 heavy pollution episodes in the winter of 2015 in Hubei province

PM_2.5 pollution is still the main pollution type of winter air pollution in Hubei province, which shows obvious regional transport characteristics and the meteorological conditions of heavy air pollution in Hubei are different from that in northern China. Based on observational analysis and the WRF/Chem simulations using different emission scenarios, we investigate heavy PM_2.5 pollution episodes in Hubei province from December 2015 to January 2016 under "unstable" meteorological conditions. The processes of horizontal transport, suspended gathering and downward transport of pollutants are analyzed from the perspective of large-scale transport and atmospheric boundary layer dynamic process. The meteorological factors favorable for the occurrence of PM_2.5 peak value in the afternoon in Hubei province are revealed. The results show that the heavy pollution outbreak in Hubei province is mainly due to regional transport, with ground observations of PM_2.5 maximum corresponding to 10 m winds up to 8-10 m/s and strong northerly transport in the boundary layer below 1 km. Maximum pollutants transport occurs near 400 m height, where the important channel of pollutants transport is located, while there exists no obvious inversion layer below this level. This condition exhibits "unstable" characteristics. The large-scale conditions for heavy pollution episodes are as follows. Northerly winds are abnormally strong in the middle and lower reaches of the Yangtze River while winds are weak in the southern region; pollutants accumulate in the central plain. Strong winds in northern Hubei province are favorable for pollutants transport. Pollutants transport is mainly concentrated over northern and northeastern parts of Hubei, while northern Nanyang basin, Guanzhong area, southeastern Shandong, Anhui, Suzhou, etc. are potential sources of pollutants. The diurnal variation of PM_2.5 in Hubei exhibits a bimodal feature, the two peaks occur due to different weather conditions. The high concentration occurring from 21:00-24:00 BT is related to stable weather condition, while the high concentration from 11:00-14:00 BT is affected by regional transport and the planetary boundary layer height (PBLH). Before sunrise, the PBLH is lower, the stratification is stable, ascending motions occur, and pollutants transported from other regions suspend in lower levels near the height of 400 m; after sunrise, with the rising of the PBL, static stability in the boundary layer is destroyed, the high concentration of PM_2.5 descends downward due to dry deposition, resulting in peak value on the ground.