西安一次霾重污染过程大气环境特征及气象条件影响分析

Atmospheric characteristics of a serious haze episode in Xi'an and the influence of meteorological conditions

利用西安区域8个气象站点的气象观测资料及西安市13个环境质量监测站点的空气污染物浓度监测资料,对2013年12月16—25日西安地区一次长时间重污染霾天气过程的污染特征及成因进行了分析.结果表明此次霾重污染天气过程主要是一次在不利气象条件下形成的高浓度颗粒物污染事件,其中有54.6%的霾属于干霾,其余属于湿霾.气压场偏弱,气压梯度力小,风速小,弱冷空气形成的下冷上暖的稳定性层结等天气形势有利于霾重污染的形成与维持;弱的降温与相对湿度增大叠加,有利于气溶胶吸湿增长而加重霾的强度.关中盆地特有的喇叭口地形通风不畅,造成外来输送与当地排放的大量污染物堆积,为此次长时间霾发生提供了增强条件.低的混合层厚度抑制了垂直方向上的对流输送,严重削弱了大气垂直扩散能力,造成了大气中各类污染物浓度的大量积聚,是造成此次霾重污染过程的重要原因之一.城市污染加重热岛效应、热岛效应反过来通过热岛环流改变城市污染物传播扩散规律并加重污染,二者相互作用、互为增强条件.

The ground observation data from 8 meteorological stations and the air pollutant mass concentration data from 13 air quality monitoring sites in Xi＇an were used in this study to analyze the characteristics and cause a serious haze episode from 16 to 25 December 2013. It was concluded that the adverse weather condition was the main cause of the heavy haze,within which 54.6% was dry haze and the rest was wet haze. The weak pressure field,small pressure gradient-force,low wind speed and stable boundary layer formed by weak cold air were conducive to the formation and persistence of the haze process. The weak cooling and increasing relative humidity promoted aerosol hygroscopic growth and aggravated the haze intensity during this period.On the other hand,poor ventilation of Guanzhong Basin led to the accumulation of massive pollutants from regional transport and local emissions. Thinner mixed layer restrained the convective transport and thus weakened the diffusion ability of atmosphere in the vertical direction. Air pollution in the city aggravated heat island effect,while heat island effect conversely intensified the pollution by changing the pollution diffusion condition.