Long-term trend of visibility and its characterizations in the Pearl River Delta (PRD) region, China

Abstract

This paper suggests that the rapid increase in urbanization and human activities has important impacts on visibility in the Pearl River Delta (PRD) region. A long-term trend of visibility in Guangzhou (one of the largest cities in PRD) shows that between 1954 and 2006, there is a rapid change in visibility. Between 1954 and 1972, low visibility has the smallest occurrence (less than a few days per year). Between 1972 and 1980, the visibility condition is within a transition period, and the occurrence of low visibility increases rapidly. Between 1980 and 2006, the occurrence of low visibility remains very high (150 days year −1), suggesting that the city of Guangzhou is often under low-visibility conditions since the 1980s. In order to understand the causes of the low visibility in Guangzhou, we analyze physical and chemical characterizations of a low-visibility event occurring between 15 and 29 November 2005. The analysis suggests that high concentration of aerosol particles is a major cause for the low visibility. The threshold aerosol concentration that is corresponding to the low visibility (<10 km) is about 120 μg m −3. Further analysis indicates that the relationship between aerosol concentrations and visibility appears in a non-linearity correlation. When aerosol concentrations are very high (above 120 μg m −3), the change in visibility is not sensitive to aerosol concentration. By contrast, when aerosol concentrations are <120 μg m −3, the change in visibility is very sensitive to aerosol concentrations. This study also shows that absorbing aerosol particles (such as element carbon (EC)) play important roles in the reduction of visibility. In most cases, the extremely low visibility (<2 km) is often resulted from high EC concentrations (above 15 μg m −3). In averaged condition, small scattering aerosol particles (<1 μm in radius) have the largest contribution to the reduction of visibility (about 70%). By contrast, large scattering particles have the smallest contribution to the reduction of visibility. This analysis implies that the persistent low visibility is mainly resulted from small aerosol particles, including both scattering and absorbing particles. In order to improve the visibility in the Guangzhou region, the reduction of small particle emissions is urgently needed. This study suggests that there are two important steps to improve the visibility in Guangzhou. First, the aerosol concentration should be reduced to less than the threshold value (120 μg m −3). Second, a further decrease in aerosols from its threshold value will lead to a significant improvement in visibility due to the non-linearity relationship between aerosol and visibility.