北京一次持续霾天气过程气象特征分析

2013年1月10-14日，北京平原地区出现了水平能见度在2 km以下、以PM_2.5为首要污染物、空气质量持续5 d维持在重度以上污染水平的霾天气。综合分析此次霾天气过程的天气形势、北京地区常规和加密气象资料以及城郊连续观测的PM_2.5浓度资料。结果表明：此霾过程期间，北京高空以平直纬向环流为主，受西北偏西气流控制，没有明显冷空气南下影响北京地区，地面多为不利于污染物扩散和稀释的弱气压场；大气层结稳定、风速小(日平均风速小于2 m·s^-1)、相对湿度较大（日平均相对湿度在70 %以上）、逆温频率高强度大，边界层内污染物的水平和垂直扩散能力差；北京城区及南部的京津冀地区人类活动排放污染物强度大，在相对稳定和高湿的天气背景下，受地形和城市局地环流的影响，北京本地污染物累积和区域污染物输送以及PM_2.5细粒子在高湿条件下的物理化学转化等过程共同作用造成此次北京城区及平原地区污染物浓度快速增长并持续偏高，高浓度PM_2.5对大气消光有显著影响，造成低能见度和持续霾天气。

Meteorological characteristics of a continuous haze process in Beijing

A heaW haze event occurred in the plain area of Beijing from January 10 to 14,2013.During this period,the visibility was always less than 2 km;air pollution reached and exceeded severe level;the primary pollutant is PM2.5.The process was synthetically analyzed based on the synoptic situation,the conventional and intensive observational data from weather stations and the observational PM2.5 concentration from the urban and rural station.The results show that there exists relatively straight zonal circulation with west-northwest air current control and without obviously southward cold air in upper level over Beijing during this haze process.It is weak pressure system on the ground which is not favorable to diffusion and transportation of pollutants.The atmospheric stratification is stable; wind speed is low （the daily mean wind speed is less than 2 m· s-1） ; relative humidity （RH） is high （the mean daily RH is more than 70％） ;the frequency of temperature inversion is high and its intensity is strong.Thus,the ability of vertical and horizontal diffusion of air pollutants within the boundary layer is very weak.Because of the heavy emission intensity,the high humidity and stable weather background in Beijing-Tianjin-Hebei region,the concentration of pollutants are influenced by the complex terrain and local circulation.Combined actions of local pollutants emission and regional transportation as well as the physical or chemical reactions of fine particle PM2.5 make the pollutants concentration increasing sharply and maintaining with a high concentration in urban and plain areas of Beijing.The high concentration of PM2.5 has a significant impact on light scattering extinction,which results in low visibility and severe haze weather.