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Recent studies show that the rapid increases in urbanization and human activities in the PRD region have important impacts on regional air quality. In addition to local anthropogenic emissions which are major driving forces for poor air quality in this region, biomass burning in Southeast Asia has also important contribution on aerosol and ozone concentrations in the PRD region. In this paper, this effect is analyzed by using satellite data, ground measurements and models. MODIS aerosol optical depth (AOD) distribution in March 2006 shows a clear enhancement in AOD between Southeast Asia and the PRD region. With detail wind analysis, two distinguished conditions are classified, i.e., Condition-1 (PRD is under influence of the biomass burning from Southeast Asia) and Condition-2 (PRD is not under influence of the biomass burning from Southeast Asia). The characterizations of aerosol, UV, and ozone in Guangzhou city (located in the PRD region) under these two conditions are analyzed. The analyses suggest that aerosols and CO concentrations are higher in Condition-1 than in Condition-2; while the UV intensity and O3 concentrations are lower in Condition-1 than in Condition-2. This result indicates that in Condition-1, the enhanced aerosol concentrations from the Southeast Asia biomass burning produce reduction of UV intensity, and thus decreases the formation of ozone in Guangzhou. [Copyright &y& Elsevier]

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 (150daysyear−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 (<10km) is about 120μgm−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 120μgm−3), the change in visibility is not sensitive to aerosol concentration. By contrast, when aerosol concentrations are <120μgm−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 (<2km) is often resulted from high EC concentrations (above 15μgm−3). In averaged condition, small scattering aerosol particles (<1μm 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 (120μgm−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. [Copyright &y& Elsevier]