The Seasonal Characteristics of Regional Atmospheric Transport and Dispersion over the Pearl River Delta

After being emitted from the sources, air pollutants experience a series of atmospheric physical and chemical processes before reaching the receptor, in which the core process is the atmospheric transport, diffusion process and the clearance changes accompanied with them. Understanding the climatic background of atmospheric transport and diffusion of the Pearl River Delta would be helpful to determine the area affected by air pollutants, the regularity of air pollutants input, output and detention over this area, providing scientific basis for air pollution prediction, control and management. Based on the meteorological field of wind, temperature, humidity, geopotential height and precipitation from the NCEP/NCAR reanalysis for the period of 1985—2004, the HYSPLIT dispersion model and setting virtual air pollution sources approach are adopted to simulate the distribution of air pollutant, dispersive path and detention time over the Pearl River Delta, with full consideration of wet and dry removal processes. The seasonal characteristics and long term trends of atmospheric transport and diffusion process over this region are examined, and the main results are shown as follows. The significant differences of the distribution, diffusive paths and detention time of virtual air pollutants among four seasons suggest that there are remarkable seasonal variations of atmospheric transport and diffusion processes over the Pearl River Delta. In spring and summer, the air pollutant convergence zone is located at northwest side of the Pearl River Delta, while in autumn and winter it lies on the northwest side of the Pearl River Delta. Among four seasons, the convergence zone in summer is the strongest, followed by spring, autumn and winter. In spring and summer, the air pollutants are transported and dispersed to the mountainous area of the northwest and north side of the Pearl River Delta, respectively, while in autumn and winter, those are transported and dispersed along the relatively flat coast of western Guangdong to the west southwest. In autumn and winter, the atmospheric transport and diffusion rates are significantly higher than those in spring and summer. Pollutants stay over the Pearl River Delta for longer than 24 hours for 66.7% and 75.3% of all the air pollution events in spring and summer, respectively, while in autumn and winter, there are only 22.1% and 30.4%. The terrain effect is one of the possible factors that responsible for the weaker atmospheric transport and diffusion in spring and summer. For the period of 1985—2004, atmospheric transport and diffusion ability are weakest in 2004 and strongest in 1996 with significant annual differences, which may be caused by the variation of atmospheric systems.