一次典型沙尘天气过程对甘肃空气质量的影响

A typical sand dust weather process and its influence on the air quality of Gansu

利用气象信息综合分析处理系统气象数据、ERA-Interim再分析资料,结合后向轨迹模式(HYSPLIT)、中分辨率成像光谱仪卫星数据和逐小时空气质量数据,分析了一次典型的甘肃沙尘天气过程的成因及其影响.结果表明,此次沙尘天气是由高空槽东移发展,冷空气南下,配合地面高压系统和锋面东移,冷锋后强气压梯度形成大风造成的;HYSPLIT模式模拟结果与气溶胶光学厚度分布显示沙尘天气沿西北路径传输,沙尘源地位于南疆地区,沿河西走廊向东南传输至陇中地区;沙尘天气发生前沙尘源地气温上升造成土壤湿度下降,低层大气处于不稳定状态,有利于起沙,沙尘期间各层水平风速均较大,有利于沙尘的水平输送;沙尘源地和武威沙尘加强区中低层的垂直方向皆为上升运动,有利于将沙尘源地的沙尘粒子带入高空,传输路径中的酒泉、张掖和兰州为下沉运动,有利于将沙尘带至近地面,形成沙尘天气.沙尘天气影响了甘肃绝大部分地区的空气质量,影响由西北向东南减弱,7个城市空气质量指数≥500的持续时间超过10 h,张掖达19 h,酒泉、武威和兰州达16h;沙尘天气对甘肃各城市污染物质量浓度的影响由西北向东南减弱,对甘肃中西部城市颗粒物质量浓度影响最大:武威w(PM10)最大值达8771μg/m3,是沙尘天气发生前的258倍,w(PM2.5)最大值为1516μg/m3,是沙尘天气发生前的95倍,沙尘天气期间风速增大,省内中西部城市气态污染物质量浓度下降,兰州w(SO2)由沙尘天气前30μg/m3以上,最高80μg/m3降至沙尘天气时的20μg/m3以下,w(NO2)由65μg/m3以上,最高123μg/m3,降至沙尘天气时的30μg/m3以下,沙尘天气结束后气态污染物质量浓度缓慢上升.

The cause of a typical sand dust weather process and its influence on air quality in Gansu Province were analyzed based on a group of data sets including meteorological information comprehensive analysis and processing system meteorological data,ERA-Interim reanalysis data,moderate resolution imaging spectroradiometer satellite data,hourly air quality data and hybrid single particle lagrangian integrated trajectory(HYSPLIT)model outputs.During the sand dust weather,the upper-level trough moved eastward and developed over northwest China to propel a cold air moving southward,the ground high pressure system and the front moving eastward.Strong wind near the surface was forced by a strong pressure gradient behind the cold front.HYSPLIT model was applied to retrieve the sand source and the simulation results combined with aerosol optical depth distribution showed that the dust source was located in southern Xinjiang and the sand dust transmitted through the Hexi corridor to the southeast of Gansu Province along the northwest path.Before the sand dust weather,temperature rose in the source resulting in a decrease of the soil moisture and instability of the lower atmosphere,which was conducive to sand dust emission.During the sand dust weather,the horizontal wind speed at all levels was relatively large,being favorable to the sand horizontal transportation.In the vertical direction,the upper and lower layers of the dust source and the enhancement zone such as Wuwei had an obvious ascending movement,being advantageous to bringing the surface dust particles to the upper air.Jiuquan,Zhangye and Lanzhou in the transmission path showed a descending movement,bringing the upper sand and dust in the path to the local surface to form a sand dust weather.The dust weather process affected the air quality in most parts of Gansu,and the impact weakened from the northwest to southeast.The duration of air quality index≥500 in 7 cities lasted more than 10 h,even up to 19 h in Zhangye and about 16 h in Jiuquan,Wuwei and Lanzhou.The greatest influence of sand dust weather on the mass concentration of pollutants had been found in the midwest cities.The maximum w(PM10)in Wuwei was up to 8771μg/m3,being 258 times more than the minimum concentration before the sand dust weather.The maximum w(PM2.5)was up to 1516μg/m3,being 95 times more than the minimum concentration before the sand dust weather occurred.The enhanced wind speed in the sand dust weather reduced gaseous pollutants concentration in the midwest.The w(SO2)decreased from the above 30μg/m3(up to 80μg/m3)to 20μg/m3 in Lanzhou in the process.The w(NO2)decreased from the above 65μg/m3(up to 123μg/m3)to 30μg/m3.The low-level concentration of the gaseous pollutants increased slowly after the sand dust weather.