Mathematical modeling of seasonal variations in visibility in Hong Kong and the Pearl River Delta region

Seasonal variations in light extinction over Hong Kong and the Pearl River Delta (PRD) region in China in January, April, July and October of 2006 were studied using the MM5–CMAQ modeling system. A local empirical visibility regression formula, based on that used in the IMPROVE project (Malm et al., 1994), was introduced to examine the relationship between the predicted aerosol mass concentration and light extinction over the PRD region. The modeling system was able to reproduce the seasonal variations in sulfate, elementary carbon (EC), organic carbon (OC) and PM2.5 mass concentrations, but not aerosol nitrate. Compared to the IMPROVE formula, the local regression formula produced a more accurate prediction of the variation in seasonal light extinction over the inland PRD region, with a 13–27% improvement in model performance according to the normalized mean bias. The seasonal light extinction budget analysis suggested that hydrophilic sulfate, EC, and aged OC are the major contributors to regional visibility degradation over Hong Kong and the PRD. An emission reduction case study also suggested the importance of controlling vehicle and marine vessel emissions to improve regional visibility.