Variation Characteristics of Atmospheric Visibility and Its Influence Factors in the background area of North China

In virtue of Mann-Kendall rank statistics, the trends and the abrupt variations of visibility were analyzed using the data of manual air horizontal visibility at the Shangdianzi meteorological station, Beijing from 1980 to 2014.In addition, factors affecting the trend of visibility were analyzed by exploring the relationships between visibility and meteorological elements, as well as atmospheric pollutants, at the background area of North China. The results showed that annual means of visibility experienced a significant decrease, the maximum and the minimum trends of the annual mean visibility were 3.4 km (10 a)^-1 in summer and 1.7 km (10 a) ^-1 in spring, respectively. The best visibility appeared in winter (38.1 km), and then in autumn (36.2 km).The worst values were observed in spring (32.8 km) and summer (31.4 km). The annual mean visibility showed no significant abrupt changes. In this analysis, the visibility had a negative correlation with relative humidity and wind speed, and a positive correlation with pressure, and a positive or negative correlation with temperature, which indicated that the influence of temperature on visibility has two sides. The further study pointed out that the degradation of visibility was mainly caused by the high concentration of aerosol particles (PM2.5) (R²=0.98, p < 0.01), which suggested that the concentration of PM2.5 should be controlled below 74 μg/m³ to make a good visibility (≥ 10 km) in Shangdianzi. Keeping the PM2.5 mass concentration equal or below the secondary standards of the National Ambient Air Quality Standard (75 μg/m³) is expected to perform good visibility (≥ 10 km) in the background area of North China.