Light attenuation cross-section of black carbon in an urbanatmosphere in northern China

Fine particulate matter (PM2.5) samples were collected over two years in Xi’an, China to investigate the relationships between the aerosol composition and the light absorption efficiency of black carbon (BC). Real-time light attenuation of BC at 880 nm was measured with an aethalometer. The mass concentrations and elemental carbon (EC) contents of PM2.5 were obtained, and light attenuation cross-sections (σATN) of PM2.5 BC were derived. The mass of EC contributed ∼5% to PM2.5 on average. BC σATN exhibited pronounced seasonal variability with values averaging 18.6, 24.2, 16.4, and 26.0 m2/g for the spring, summer, autumn, and winter, respectively, while averaging 23.0 m2/g overall. σATN varied inversely with the ratios of EC/PM2.5, EC/[SO42−], and EC/[NO3−]. This study of the variability in σATN illustrates the complexity of the interactions among the aerosol constituents in northern China and documents certain effects of the high EC, dust, sulfate and nitrate loadings on light attenuation.