Main Influencing Factors of Visibility in Beijing

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 PM_2.5 concentration.PM_2.5 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 increases with the increase of wind speed.And correlations among visibility, PM_2.5 concentration and relative humidity are both negative, which means that visibility decreases with the increase of relative humidity and PM_2.5 concentration.But it is important to point out that once relative humidity reaches a certain threshold (80%), the correlation between visibility and PM_2.5 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 PM_2.5 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 PM_2.5 concentration in the wet haze events is needed to drop more for improving visibility than that in the dry haze event.