| 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,PM2.5and PM1.0)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, PM2.5and PM1.0)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(PM2.5and PM1.0)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,PM2.5～10,PM2.5and PM1.0)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 PM2.510concentration 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 PM2.5～10by 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.
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