Chemical composition and light extinction contribution of PM_2.5 in Urban Beijing for a 1-year period

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 (NH₄⁺, NO₃^– and SO₄^2–) 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. (NH₄)₂SO₄ was the largest contributor (28.8%) to bext, followed by NH₄NO₃ (24.4%), organic matter (19.5%), elemental carbon (7.4%), and coarse mass (7.2%), while fine soil, sea salt, NO₂ and Rayleigh made minor contributions, together accounting for 12.7% of bext. During the polluted periods, the contributions of (NH₄)₂SO₄ and NH₄NO₃ to bext increased dramatically. Therefore, in addition to control primary particulate emissions, the reduction of their precursors like SO₂, NO_x and NH₃ could effectively improve air quality and visibility in Beijing.