Chemical Composition and Light Extinction Contribution of PM2.5 in Urban Beijing for a 1-Year Period

Abstract

Daily PM_(2.5) samples were collected in Beijing across four consecutive seasons from June 2012 to April 2013. Major water-soluble inorganic ions, carbonaceous species and elements were analyzed to investigate their temporal variations and evaluate their contributions to visibility impairment over different seasons and under different pollution levels. The mass concentrations of PM_(2.5) ranged from 4.3 to 592.4 μg m-3, with an annual average of 112.4±94.4 μg m^(-3). The predominant components of PM_(2.5) were secondary inorganic ions (NH_4^+, NO_3^- and SO_4^(2-)) and carbonaceous compounds, which accounted for 45.9% and 24.1% of the total PM_(2.5) mass, respectively. Distinct seasonal variation was observed in the mass concentrations and chemical components of PM_(2.5). The average mass concentrations of PM_(2.5) were the highest in winter, followed by spring, and lowest in autumn. Light extinction coefficients (b_(ext)) were discussed over four seasons. (NH_4)_2SO_4 was the largest contributor (28.8%) to b_(ext), followed by NH_4NO_3 (24.4%), organic matter (19.5%), elemental carbon (7.4%), and coarse mass (7.2%), while fine soil, sea salt, NO_2 and Rayleigh made minor contributions, together accounting for 12.7% of b_(ext). During the polluted periods, the contributions of (NH_4)_2SO_4 and NH_4NO_3 to b_(ext) increased dramatically. Therefore, in addition to control primary particulate emissions, the reduction of their precursors like SO_2, NO_x and NH_3 could effectively improve air quality and visibility in Beijing.