Long-term Trends and Characteristics of Visibility in Two Megacities of Southwest China: Chengdu and Chongqing

摘要

From 1981 to 2010, visibility in Chengdu and Chongqing has been preserved at a rather low level with averages of 7.8±3.6 and 5.6±3.7 km, respectively. The ridit values showed that the best visibility periods in Chengdu and Chongqing mainly concentrated in 1980s and 1990s. During 2000s, PM_(10) levels in both cities degraded consistently to around 110 μg m~(-3) in 2011. Still, PM_(10) dominated the primary pollutants and was the major obstacle for air quality improvement in southwest China. Results of the correlation analysis and PCA suggested the complex chemical and synoptic mechanism involved in visual range formation, as visibility was significantly correlated with all the meteorological factors. Particularly, high pressure and relative humidity and low wind speed under stable weather system increased the low visibility frequency. Empirical models for visibility prediction demonstrated that visibility improvement was most sensitive to reduction in PM_(10) concentrations. The regression coefficients between observed and calculated visibility would be significantly improved when more data about PM_(2.5) and other gaseous pollutants were included in the models.