Seasonal Variations in Particle Size Distribution and Water-soluble Ion Composition of Atmospheric Particles in Chengdu

Abstract

Size-resolved atmospheric particles were collected monthly from February 2012 to January 2013 on the campus of Chengdu University of Technology using the Anderson multi-stage impactor sampler. The mass concentrations of these samples and the contents of nine water-soluble ion species were analyzed. The results showed that the annual average concentrations of PM2.1 and PM11 in Chengdu during the sampling period were (125.9±56.14) μg·m-3 and (224.5±83.64) μg·m-3, respectively. The highest mass concentration of particulate matter was observed in winter, followed by spring while the lowest was in autumn. The average content of water-soluble ions in particulate matter in Chengdu was about 37.15 μg·m-3 and the order of water-soluble ion mass concentrations was SO42- > NO3- > NH4+ > Ca2+ > Cl- > Mg2+ > K+ > Na+ > F-. Meanwhile, SO42-, NO3-, and NH4+ were the major water-soluble ionic components that accounted for 78% of total water-soluble ions. Unimodal distribution was observed for SO42-, NO3-, and NH4+ mainly in fine particles, while Ca2+ and F- were concentrated in coarse particles and their distribution was also unimodal. Similar size distributions were found for Cl- and K+, as well as Mg2+ and Na+, both of which were bimodal. The content of water-soluble ions in both fine and coarse particles during winter and spring was significantly higher than that in summer and autumn in Chengdu. Based on the correlation analysis for major ions, SO42-, NO3-, and NH4+ in fine particles are likely in the form of (NH4)2SO4, NH4HSO4, and NH4NO3. In coarse particles, the major water-soluble ions (i.e. SO42-, NO3-, and Ca2+) might exist in the forms of Ca(NO3)2 and CaSO4. The secondary formation processes, soil dust, and biomass burning are the major sources of water-soluble ion species in the particulate matter in Chengdu.