Based on the monitoring data of air pollution in Xi’an City,Shaanxi Province,China,from 2015 to 2016,this paper analyzed the corresponding relationship between haze occurrence and the visibility and quality grade of the air by reconstructing the haze sequences with three different methods,namely,the daily value method,the hourly value method and the value at 14:00 PM method.This analysis helped to determine the value at 14:00 PM method as the method to reconstruct haze sequence and the reconstructing of the historical haze data series was done by using the data about the relative humidity and air visibility from 96 meteorological stations from 1981 to 2016 in order to provide a more reasonable basic data sequence for the environmental meteorological assessment.And then this paper analyzed the characteristics of haze weather and meteorological factors,discussed the circulation situation corresponding to the largescale continuous haze weather process in time and space,and studied the causes of haze weather process in order to provide scientific basis for governmental decisionmaking departments to protect the environment and formulate relevant policies.The results showed as follows: (1) The haze weather occurred frequently in Guanzhong area,followed by Hanzhong Basin in southern Shaanxi,and least in northern Shaanxi.The number of days with haze weather in the province showed a general upward trend,and was increased significantly after 2000.In the province as a whole and in every region of the province,it had the maximal number of days with haze weather in winter.The number of days with haze weather from November to the February next year accounted for 57.9% of the total annual days with haze weather. (2) The frequency of moderate haze (visibility: 5~8 km) and light haze (visibility: 8~10 km) was the highest in the province and all regions of the province,and the frequency of severe haze (visibility<3 km) was the lowest being less than 10%. (3) Wind is one of the important factors affecting haze weather.Haze weather is easy to occur under winter light wind condition.When Xi’an and Guanzhong area are under the effect of northeaster wind,there will be a high frequency of haze weather.(4) The relative humidity is another major factor causing haze weather,the highest frequency of haze weather occurrence was found with the relative humidity being 60%~70%,accounting for about 20% of haze weather occurrences,and the frequency of haze weather occurrence was lower when the relative humidity was less than 30%. (5) The largescale circulation situation in winter was relatively stable.The wind speed was small in Guanzhong area,and there were thermal inversion layers in the near surface atmosphere and wind convergence near Guanzhong area,which caused the horizontal and vertical diffusivity of the atmosphere over Guanzhong to become weaker.When the relative humidity was high,the atmospheric pollutants were accumulated and took chemical reaction.Under these conditions,it is easy to form continuous haze weather and cause serious air pollution in Guanzhong area.
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