北京春季一次霾-沙天气污染特性与成因分析

Research on the Pollution Characteristics and Causality of Haze-sand Air Pollution in Beijing in Spring

2017年5月3～5日,北京发生一次特别的重污染过程,与之相配的气象条件较为特殊,对污染形态和成因展开研究.基于北京35个环境监测站和与之最近的35个自动气象站,获取本次污染过程的总体特征及PM_(10)、PM_(2.5)浓度与地面风场的匹配形态;利用MODIS和CALIPSO研究污染空间分布、输送路径、污染物类别;根据欧洲中期天气预报中心ECMWF第三代再分析资料ERA-Interim及风廓线雷达数据研究污染成因.以期以地-空立体监测技术手段配合气象条件得到本次污染特有的形态特征和影响因素.结果表明,利用以上多源数据,对本次污染进行立体观测和综合分析,能较好地反映污染特性和受制因素.本次污染骤然开始,陡然下降,持续约30h,整个过程PM_(10)和PM_(2.5)浓度高,分别可达600～1 000μg·m^(-3)和200～700μg·m^(-3).全过程分为三段,前半段、间歇期、后半段.前、后半段污染成因以及由此造成的PM_(10)和PM_(2.5)浓度在空间分布上各有特点.前半段主导风向为西北风,风速小,PM_(10)浓度空间差异小,在800μg·m^(-3)以上,而PM_(2.5)浓度空间差异大,南部和城区高,达600～700μg·m^(-3),其余地方低,在350～500μg·m^(-3).间歇期低层风向从西北风切变为南风,高层维持西北风,南部和城区PM_(10)浓度下降明显,到650μg·m^(-3),北部依然在800μg·m^(-3),而此时北部PM_(2.5)浓度甚至降到200μg·m^(-3).后半段主导风又回到了西北风,且风速激增,此时PM_(2.5)浓度空间差异小且同一站点的浓度均小于前半段,在250～500μg·m^(-3).而PM_(10)浓度又回到了800μg·m^(-3)的水平.说明本次过程属典型霾-沙混合型污染.在偏西气流的影响下,对北京污染的主要贡献是沙尘型的PM_(10),而在偏南气流下,对北京污染的贡献除了沙尘外,还有PM_(2.5).污染重的同时,风速也大,大气垂直运动交汇于大约2～3 km高度,在此高度层内有大量污染物累积.

From May 3 to 5,2017,a special heavy pollution event occurred in Beijing.The meteorological conditions associated with the heavy pollution were relatively special,so the pollution forms and causes were studied.The general characteristics of this pollution event were obtained based on data from 35 environmental monitoring stations in Beijing.Matching characteristics of PM10 and PM2.5 concentrations with ground wind field data from automatic weather stations closest to the environmental monitoring stations were analyzed.By using MODIS and CALIPSO data,the spatial distribution in the horizontal and vertical directions was obtained,and the transport paths and pollutant categories of the pollution were elucidated.The causes of the pollution were analyzed by using ECMWF ERA-Interim data and Wind Profiler radar data.It was hoped that the special morphological characteristics and influencing factors of the pollution could be obtained by means of ground-space monitoring technology combined with meteorological conditions.The results showed that pollution characteristics and constraints could be better reflected by stereo observations and comprehensive analyses based on the above multi-source data.The pollution started abruptly and dropped sharply,and the pollution process lasted for about 30 hours. The whole process was divided into the following three stages: the first half,intermittent period,and second half.The concentrations of PM10 and PM2.5 were high throughout the whole process,reaching to 600-1 000 μg·m^-3 and 200-700 μg·m^-3,respectively.The causes of pollution in the first half and second half and the resulting PM10 and PM2.5 concentrations were different in terms of the spatial distribution.In the first half,the dominant wind direction was northwest wind,and the wind speed was small.The spatial difference of PM10 concentrations was also small,with concentrations more than 800 μg·m^-3;meanwhile,the spatial difference of PM2.5 concentrations was great.The concentration of PM2.5 was high in the south and urban areas,reaching to 600-700 μg·m^-3,and it was low in other places,reaching to 350-500 μg·m^-3.During the intermission,the wind direction in the lower layer shifted from northwest wind to south wind,and the upper layer maintained northwest wind.The concentration of PM10 in the south and urban area decreased obviously to 650 μg·m^-3,and the concentration of PM10 in the north remained at 800 μg·m^-3.At this time,the concentration of PM2.5 in the north even dropped to 200 μg·m^-3.The dominant wind returned to northwest wind in the latter half,and the wind speed increased sharply.At this time,the spatial difference of PM2.5 concentrations was small and the concentration of PM2.5 at the same station was less than that in the former half,ranging from 250 to 500 μg·m^-3.The PM10 concentrations returned to the level of 800 μg·m^-3.The pollution process involved mixed pollution consisting of haze and sand.Under the influence of westerly winds,the main contribution to Beijing pollution was dust-type PM10 ,while under southerly flows,the contribution to Beijing pollution was not only dust,but also PM2.5.Heavy pollution was accompanied by high wind speeds.The vertical motion of the atmosphere converged at an altitude of about 2-3 km,which resulted in the accumulation of pollutants at this altitude.