陕西省霾天气变化特征及气候成因分析

通过对比选用14时实测值法利用1981—2016年全省96站14时相对湿度和能见度数据重建全省历史霾数据序列，分析区域内霾天气的特征及相关气象要素的特征，并探讨大范围持续霾天气过程对应的环流形势，研究霾天气过程的成因。结果表明：关中地区是我省霾天气的高发区，其次是陕南的汉中盆地，陕北出现最少。全省霾日数总体呈较明显的上升趋势，2000年后增加显著。从季节变化来看全省及各区域均为冬季霾日数最多，秋季次之，夏季霾日数最少。各区域均在5～8 km能见度、60%～70%相对湿度、静小风时易出现霾天气，西安及关中全年和四季在偏东北风向时霾天气频率较大。冬季地面风速小，相对湿度较大，近地面风场辐合及逆温层的存在，造成关中大气垂直扩散能力较弱，大气污染物发生堆积、累积效应易形成关中地区持续霾天气及严重大气污染。

Variation characteristics of haze weather and its climatic causes in Shaanxi Province

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 large scale continuous haze weather process in time and space,and studied the causes of haze weather process in order to provide scientific basis for governmental decision making 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 large scale 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.