Research On Size Distribution And Chemical Composition Of Fine Particles And Their Impact On Visibility In Urban Guangzhou

PM2.5 samples were collected by air sampler in 2008. Size distribution and number density of particles, mass concentration of PM2.5 and meteorological datas in Guangzhou were monitored by using the advanced equipments of atmospheric super station. In this research, chemical composition of PM2.5 samples was obtained, including water-soluble ions and carbon species concentration of the particles. Based on Mie theory and IMPROVE method, the contribution rate of the different size particulate matter and the major chemical composition of atmospheric particles on visibility were calculated. Besides, the reducing mechanisms of atmospheric visibility decline in urban areas, Guangzhou as a representative, are also illustrated in this study. The results show that:(1) Through the two-year monitoring of visibility in Guangzhou, the mean visibility declined from 7.8 km in 2007 to 7.2 km in 2008. Minimum visibility appeared in spring and winter, while the maximum appeared in summer. Visibility was significantly correlated with wind speed and temperature, however, it was negatively correlated with relative humidity. Wind speed and temperature provide a favorable condition for the spread of pollutants. Higher relative humidity will promote the formation of airborne fine particles. Also it will affect the turbidity, causing the decline of visibility directly.(2) Visibility was negatively correlated with the number density of particles and PM2.5 mass concentration.The absolute values of correlation coefficients between particle number density and PM2.5 mass concentration or relative humidity on hazy days were much higher than on normal days.(3) The results indicated that the particles number density reduced profoundly with the particle size enlargement. The mass concentration profile showed two peaks separately at 0.7μm and 2.5μm. Rainfall has a great impact on the scavenging of particles, especially coarse particles. The particles (size 0.5μm-1.0μm) remarkablely contributed to bext.It is the rising of concentration of particles (size 0.5μm-2.5μm) that induced a sharply decreased visibility on hazy days of urban Guangzhou.(4) The major chemical composition of fine particles which reduced atmospheric visibility are respectively sulfate, nitrate, organic matter, elemental carbon. Sulfate plays a leading role in spring and summer, and organic matter plays a leading role in the fall and winter. According to the statistics of the atmospheric extinction coefficient (bext) in 2008, the major chemical composition are respectively sulfate, organic matter, elemental carbon, nitrate in the contribution rate decreasing order.