Abstract: Characteristics of spatial distribution of haze were analyzed using meteorological data at ground obtained from seventeen weather stations in Shijiazhuang region from 1970 to 2013.The results show that the reginal distribution of annual total haze days changes greatly from 1970 to 2013 in Shijiazhuang.The former distribution pattern is characterized by higher haze days appearing in the mid-eastern plain area and lower ones in the northwestern mountainous region.However,such distribution pattern gradually changes with higher haze days in the southwestern region and lower ones in the mid-eastern region.This kind of change becomes even more significant after 2010.With the rapid development of economy,the abundant mine resources in the western mountainous region have been exploited,which results in excessive emissions of pollutants such as SO₂,NO_X,and VOCS,and favors the conversion and formation of aerosols through photochemical reaction.In addition,pollutants can be transported from the southeastern region and accumulate at the mountain's front due to the block of the Taihang Mountains under a certain weather background condition.Therefore,besides the local pollutant emissions,regional transport of pollutants is another reason for severe pollution in the western mountainous region.The total number of days with the persistent haze events takes up more than 50% of the annual total number of the haze days.According to the classification of persistent haze event,when the duration of a haze event exceeds four (eight) days,it is defined as a medium (extreme) persistent haze event.The annual mean number of medium persistent haze events is 9.8,and that of extreme persistent haze events is 1.6.Taking the south-northeast railway line in Shijiazhuang as a boundary,extreme persistent haze events occur mostly in the mid-eastern and southern regions (Wuji,Zhengding,Gaocheng,Luancheng,and Zhaoxian),and less in the northwestern region (Pingshan,Xingtang,and Lingshou) and the eastern region (Jinzhou,Shenze,and Xinji).Medium persistent haze events occur typically in December and January and least from June to August,while extreme persistent haze events occur mostly from November to February,especially in January.The monthly mean occurrence number of extreme persistent haze event is the largest in January with 0.4 averaged from the study period,and less than 0.2 in other months,which means a small occurrence probability of haze in these months.

Key words: Haze day, Persistence, Haze events, Pollution emission