【作者】 冯鑫媛；

【导师】 王式功；

【作者基本信息】 兰州大学 ， 气象学， 2009， 博士

【摘要】 沙尘暴作为一种灾害性天气不仅对工农业生产产生严重的直接危害,而且由此形成的沙尘气溶胶通过多种途径对气候、环境、生态和人体健康等多方面造成的滞后、持续、长期的间接危害更加严重。因此,在环境问题日趋突出的今天,深入研究沙尘暴对城市大气环境的影响具有重要意义。本文利用我国西北五省（区）及内蒙古自治区1954～2005年235站的沙尘暴观测资料、2000～2008年北方17个环保重点城市API指数资料、2005～2007年兰州市不同粒径大气颗粒物(TSP、PM₁₀、PM_2.5和PM_1.0)浓度监测资料以及2007年4～5月兰州大学半干旱气候与环境观测站（SACOL）PM₁₀浓度监测资料、铁塔梯度观测资料和超声资料,在分析近52年来我国北方中西部地区沙尘暴气候特征的基础上,探讨了沙尘天气造成的颗粒物污染对我国北方城市大气环境的影响,并较为全面、深入地分析了沙尘重点影响地区——兰州市的不同粒径颗粒物污染特征以及不同类型典型天气过程对其颗粒物污染影响的变化规律及其产生机制。主要结论如下:（1）我国北方中西部沙尘暴发生日数的年变化特征可划分为四大类型,即春季型、冬春夏型、春夏型和冬春型,它们分别占总数的29.7%、29.7%、27.1%和13.5%,除冬春型冬季沙尘暴发生最多外,其它类型均为春季最多。沙尘暴日变化和持续时间分布规律也分别划分为三种类型。沙尘暴的时间变化特征存在明显的地域差异,这一差异是某一时期内强风、沙尘源和不稳定层结三者在不同地区配置情况差异的综合反映。尽管近年来我国北方城市大气环境治理力度明显加大,北方多数环保重点城市的大气颗粒物污染状况依然严重,这主要与沙尘天气影响有关。春季沙尘输送对北方城市颗粒物污染的平均影响程度自西向东降低,其中对银川、兰州和西宁的影响程度最大。沙尘暴天气发生期间,各地颗粒物污染程度自强沙尘暴中心区向外围逐渐减轻。即使在并未出现沙尘天气的地区,在上游沙尘输送的作用下其颗粒物浓度也会受到较大影响。（2）沙尘重点影响地区——兰州市的不同粒径大气颗粒物(TSP、PM₁₀、PM_2.5和PM_1.0)污染特征分析表明,粗颗粒物(TSP和PM₁₀)浓度的年变化呈双峰型分布,特别是4月份出现的主峰值有别于其他城市;而细颗粒物(PM_2.5和PM_1.0)的年变化则呈单峰型分布,峰值在12月;这一差异反映出春季沙尘天气明显加剧兰州市粗颗粒物污染,而由于其风扩散效应则降低细颗粒物污染浓度。正常情况下（无沙尘）兰州市粗颗粒物日变化为双峰型;而沙尘天气影响期间,受沙尘天气的影响午后增加了一个浓度高峰,其日变化变为三峰型。另外,粗、细颗粒物在边界层大气中的稀释扩散过程也存在明显差异。上游地区大风沙尘天气是影响兰州市颗粒物污染状况的重要因素,其中青海地区和河西地区对其影响最大。（3）关于不同类型典型天气过程对兰州市不同粒径颗粒物(TSP、PM_＞10、PM_2.5～10、PM_2.5和PM_1.0)浓度影响的研究表明:沙尘天气发生时兰州市颗粒物污染严重,PM_2.5～10浓度升幅最大,不同粒径颗粒物浓度的变化规律存在差异。降水对颗粒物浓度的影响以湿清除为主,粗粒子的清除率明显高于细粒子。不同类型的降水对颗粒物的清除率存在明显差异,对流性降水对TSP、PM_＞10和PM_2.5～10的清除率是相同降水量锋面降水的数倍,这是由它们不同的产生机制和降水特征造成的。冷锋对颗粒污染物的影响也以清除为主,粗粒子的清除率较高。根据冷锋过境初期颗粒物浓度变化趋势,冷锋可分为先升后降型和直降型。先升后降的变化特征是冷锋对颗粒污染物与气态污染物浓度影响的重要差别之一。（4）为了更好地探讨典型天气过程中兰州地区颗粒物污染的形成机制,对沙尘、冷锋等不同类型典型天气过程中兰州地区大气边界层结构和湍流输送特征及其与颗粒物污染的关系进行研究,结果表明:沙尘天气过境前,边界层有较强逆温出现;随着沙尘天气爆发,逆温层被破坏,混合层迅速发展,边界层低层温度和湿度垂直梯度变得很小,而风速垂直梯度则增大;此时随着风速迅速增大,PM₁₀浓度骤增。沙尘过境期间垂直湍流输送强烈,经向和纬向动量均是向下输送的（其中经向输送较强）。上游有明显沙尘输送时,沙尘浓度峰值落后于风速峰值。先升后降型冷锋过境初期对颗粒物浓度的影响机制与沙尘天气类似。动量下传特别是经向动量下传是造成沙尘天气和先升后降型冷锋过境初期颗粒物严重污染的主要原因之一。更多还原

【Abstract】 Dust storms,as a kind of disastrous weather,can cause serious immediate harms on industrial and agricultural production.However,the dust aerosol generated by them has much more harm which is lagged,continued and long-term on many aspects such as climate,environment,ecosystem and human health by several ways.As environmental problems attract more and more attention in our lives today,it has significant meaning to deeply study the influence of dust stroms on urban atmospheric environment.The data used in this study include:（1）the observation data of dust storms from 235 meteorological observation stations in the middle and west of northern China during 1954～2005;（2）the API data of 17-key environmental protection cities in northern China during 2000-2008;（3）the concentration data of PM with different sizes(TSP,PM₁₀,PM_2.5and PM_1.0)in Lanzhou during 2005～2007; （4）PM₁₀concentration,gradient observation data of meteorological tower and turbulent data observed at SACOL from April to May 2007.Based on the analysis of climatic characteristics of dust storms in the middle and west of Northern China during 52 years,the influence of particulate pollution caused by dust events on urban atmospheric environment in Northern China was evaluated.Lanzhou is one of the regions mainly affected by dust events.Therefore,both the variations of the concentrations of PM with different sizes in Lanzhou city and the influence of different kinds of typical weather processes on its PM concentrations and the influence mechanism were comprehensively and deeply studied.The main results are as follows:（1）The annual variations of the number of dust storm days in the middle and west of Northern China can be divided into four types,namely spring type, winter-spring-summer type,spring-summer type and winter-spring type,and they respectively account for 29.7%,29.7%,27.1%and 13.5%of the total number of stations.The dust storms of three types of them（except winter-spring type）most frequently occur in spring.The diurnal variations and the duration patterns of dust storms can be respectively divided into three types.These above classified results indicate that there are obvious regional differences in the temporal variations of dust storms.The differences are due to the match of the time in which strong wind,dust sources and thermal instability happen in different regions.Although more efforts have been devoted to the environmental protection and control in northern cities in recent several years,the particulate pollution of most key environmental protection cities in Northern China is still serious.This is associated with the influence of dust events.The degree of the influence of springtime dust intrusions on particulate pollution in northern cities decreases from west to east and the influence on Yinchuan, Lanzhou and Xining is the most serious.During a dust storm,the particulate pollution in the strong dust-storm occurrence regions is the most serious.In the regions where no dust events occurs,the PM concentration can also been influenced,which results from dust transports from upstream regions.（2）The variations of the concentrations of PM with different sizes(TSP,PM₁₀, PM_2.5and PM_1.0)in Lanzhou city（the region mainly affected by dust events）are analyzed.The result shows that the annual distribution of monthly average concentrations for coarse particles(TSP and PM₁₀)is bimodal with the highest peak in April,which is different from the situation in other cities not affected by dust events. However,the annual distribution for fine particles(PM_2.5and PM_1.0)is unimodal with the peak in December.This difference indicates that while dust events aggravate obviously coarse particle pollution in Lanzhou,they reduce the concentrations of fine particles（from local emissions）due to wind diffusion.Under normal conditions （without dust intrusions）,the diurnal distribution of coarse particle concentration in Lanzhou is bimodal.However,the distribution is trimodal during dust intrusions,with an extra peak in the afternoon.There are obvious differences in the diffusion behaviors in the atmospheric boundary layer between coarse and fine particles.Strong wind and dust intrusions from upstream regions have an important impact on particulate pollution in Lanzhou,and those from Qinghai and Hexi regions have the most serious influence.（3）The influence of different kinds of typical weather processes on the concentrations of PM with different sizes(TSP,PM_＞10,PM_2.5～10,PM_2.5and PM_1.0)in Lanzhou is evaluated.It can be found that,during dust events,the PM pollution in Lanzhou is very heavy and the relative increase rate of PM_2.5¹0concentration is the highest.There are differences in the temporal evolution of PM concentrations among particles with different sizes during dust periods.The major effect of precipitation events on PM concentration is wet scavenging.While precipitation reduces the concentrations of coarse particles,it sometimes increases the concentrations of fine particles.There are obvious differences in the scavenging rates of particles by different kinds of precipitation events.The scavenging rates of TSP,PM_＞10and PM_2.5～10by convective precipitation are several times as much as by frontal precipitation.This is due to their different formation mechanism and precipitation characteristics.The major effect of cold front on PM concentration is clearance and coarse particles can be cleared more efficiently.Based on the variations of PM concentrations during the initial stages of cold front events,cold front can be classified into increasing-first-and-decreasing-afterwards-type and continuous-decreasing-type. The characteristic—increasing first and decreasing afterwards—is one of the important differences in the influence of cold front on particulate and gaseous pollutant.（4）In order to deeply understand the influence mechanism of typical weather events on PM pollution,the characteristics of atmospheric boundary-layer structure and turbulent flux transfer during typical weather events such as dust and cold front events and their relation with particulate pollution are studied.The result shows that, before the dust events break out,there exists strong inversion in the atmospheric boundary layer.As the dust events break out,the inversion layer is broken and a mixing layer develops quickly.The vertical gradient of air temperature and humidity in the lower boundary layer decreases and that of wind speed increases.As the great increasing of wind speed,the PM₁₀concentration increases sharply.During dust periods,the vertical transfer of turbulence flux is very strong.The meridional and latitudinal momentum transports downward（the meridional momentum transportation is much greater）.If there is obviours dust transport from upstream regions,the peak of dust concentration can lag behind the peak of wind speed.The mechanism of the influence of increasing-first-and-decreasing-afterwards-type of cold front events on particulate pollution is similar to that of dust events.The momentum downward transportation（especially the meridional momentum transportation）is one of the major factors which cause the blowing dust and the heavy particulate pollution during dust events and increasing-first-and-decreasing-afterwards-type of cold front events.更多还原