湖北2015年冬季PM2.5重污染过程的气象输送条件及日变化特征分析

PM_2.5污染仍然是湖北省冬季大气污染的首要污染类型,且具有明显区域传输特征,重污染过程的空气污染气象条件有别于华北地区,值得关注。采用WRF/Chem不同排放情景下的模拟结果,并结合观测分析,研究了2015年12月—2016年1月湖北省PM_2.5重污染过程的气象输送条件及日变化特征,从大尺度输送条件和局地边界层动力作用分析了外来污染物水平传输、悬浮聚集和向下传输的过程,并解释了该地区观测到的午后PM_2.5浓度特殊峰值的气象成因。结果表明,湖北重污染爆发以区域传输为主,地面观测PM_2.5极值对应10 m风速可达8—10 m/s,边界层0—1 km为较强偏北风输送,污染传输通量极值位于400 m高度附近,为重要传输通道,低空无明显逆温,重污染过程具有“非静稳”边界层气象特征。重污染形成的大尺度输送条件为,长江中下游及北部地区偏北风异常偏强,南部地区风速减缓,使污染物在中游平原堆积,鄂北边界风速越大,越有利污染输送增长。传输性污染主要来自偏北和东北方向的污染源输送,潜在源区贡献主要为途经偏北通道上的豫中、南阳盆地和关中地区,以及途经东北通道上的鲁、皖、苏等部分地区。PM_2.5浓度日变化双峰结构的天气成因不同,21—24时（北京时）峰值为静稳性污染,11—14时峰值为传输性污染。污染输送受大气边界层高度影响,日出前大气边界层高度较低,层结稳定并伴有上升运行,使得低空外来输送悬浮聚集在400 m高度附近;日出后随大气边界层高度升高,静稳层结被破坏,在干沉降作用下高浓度PM_2.5开始向下传输,并在午后地面形成峰值。 

Analysis of meteorological conditions and diurnal variation characteristics of PM2.5 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.