People's work,life and travel could be affected by the air visibility especially low visibility which has harmful effects on society and traffic safety.In recent years,the cause and variation of visibility become a very important topic in the field of atmosphere and environment research.In light of this,an analysis is carried out on the visibility in Beijing using all available observations.And two typical low visibility events are picked out to analyze in detail.As for Beijing,the most relevant elements of visibility are relative humidity,wind speed and PM2.5 concentration.PM2.5 concentration reduces visibility by extinction for light.Water vapor in the air causes the atmospheric aerosol particles increasing through moisture absorption and changes its characteristics,which brings down the visibility.And when the wind velocity is low,with poor air pollutant diffusion conditions and accumulating particles,the visibility decreases obviously.So it could be concluded that atmospheric visibility depends on both meteorological conditions and air pollution,which differs in different areas because of geographical locations,climatic conditions,and the composition of particulate matter.In brief,the correlation between visibility and wind speed is positive,which means that visibility in creases with the increase of wind speed.And correlations among visibility,PM2.5 concentration and relative humidity are both negative,which means that visibility decreases with the increase of relative humidity and PM2.5 concentration.But it is important to point out that once relative humidity reaches a certain threshold (80％),the correlation between visibility and PM2.5 concentration declines,which results in the visibility mainly depending on relative humidity.Generally,the periodic variation of visibility is more consistent with the change of meteorological conditions,and the relationship between visibility and PM2.5 concentration is more complicated.In recent years,the result shows that the visibility in the northwest is obviously higher than that in the southeast including the city center.The annual spatial distribution of visibility turns out to decrease gradually from northwest to southeast.And the highest visibility in a year in Beijing happens in spring and the lowest is in summer.The maximum is in May and the minimum is in July.The lowest visibility in a day happens at about 0600 BT,which postpones in winter.The visibility peak happens at about 1600 BT,which advances in winter.Results show the probability of low visibility events is 62.14％ in Beijing.Dry haze happens more frequently than wet haze.In addition,as for the case studies,it should be noted that,when the wet haze event occurs,visibility in Beijing deteriorates more obviously than that in the dry haze event.And PM2.5 concentration in the wet haze events is needed to drop more for improving visibility than that in the dry haze event.%利用北京市道面自动气象站、国家级自动气象站等多种观测数据分析北京地区2007-2015年能见度及其主要影响因子,并挑选两次典型低能见度事件过程进行详细分析.从空间分布看,北京西北地区能见度明显高于中心城区和东南大部地区.从时间分布看,北京地区平均能见度最大值出现在5月,最小值出现在7月;日间的最低值多出现在06:00(北京时,下同)左右,冬季略向后推迟;最高值多出现在16:00前后,冬季略有提前.整体而言,2007 2015年北京地区发生低能见度事件的概率为62.14％,且发生低能见度的事件集中于1～5 km,霾事件中干霾、湿霾的发生频率分别为86.13％和13.87％.能见度的主要影响因子为相对湿度、风速和PM2.5浓度.其中,能见度与风速呈正相关,与相对湿度和PM2.5浓度呈反相关.需要指出的是,当相对湿度增加至80％,能见度受PM2.5浓度的影响程度在下降,而主要受相对湿度的影响.基于所选个例,当北京地区出现湿霾事件时,能见度的恶化程度远高于干霾事件,且PM2.5浓度需比干霾事件时下降得更低才能有效改善能见度.
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