Analysis of Trends of Visibility and Its Characters during Air Seriously Polluted Days in Beijing

The visibility rules in Beijing were analyzed on the basis of historic data from 2005 to 2009, it showed that the annual mean visibility in Beijing was 10.17 km in the past five years and experienced a significant increasing trend, approximately 0.69 km·a-1(R2=0.99,p<0.01), and at the same time haze days decreased rapidly in the past four years. The decreased annual concentrations of PM2.5 (aerodynamic diameter ≤2.5 μm) were the main reason for the increased visibility. Visibility was the best in spring, better in winter and the worst in summer. In order to understand the causes of the low visibility in Beijing, the characteristics of the low visibility through a selected case in which the visibility rapidly changed to low visibility was analyzed. In this analysis visibility showed a negative correlation with relative humidity and a positive or negative correlation with wind speed, which suggested that the influence of wind speed to visibility was two-sidedness. The analysis also suggested that high concentration of aerosol particles (PM2.5) was a major cause for the visibility degradation. There was a significant anticorrelation between the concentrations of aerosol and the ranges of visibility (R2=0.93,p<0.01), which indicated that the concentration of PM2.5 should firstly be controlled below 30 μg·m-3 to keep a good visibility (>10 km) in Beijing, and the further reduction in aerosol concentrations will lead to an acceleration in improvement in visibility due to the non-linear relationship between aerosols and visibility. According to the diagnostic analysis result of environment process with weather pattern, PM2.5 and visibility had good relationship with the synoptic situation evolution. When the North China region was influenced by the high pressure gradient field after the front, the area of Beijing was affected by northerly winds. With a strong surface wind, atmospheric conditions favored for the diffusion of pollutants, resulting a lower concentration of particulate matter and a higher visibility. By contrast, when a mainland high pressure covered this area, the ground wind speed was weak and not conducive to the diffusion of pollutants. With accumulated local source and regional transportation, PM2.5 accumulated day after day from the valley value to the peak value, while the visibility decreased constantly. This study suggested the primary pollutant which should be controlled was PM2.5 to increase the visibility in Beijing.