Black carbon aerosols in urban central China

Abstract

The first ever (to our knowledge), year-round measurements of Black Carbon (BC) aerosols in Hefei, an urban site of central China, from June 2012 to May 2013 are performed. The aim of this paper is to evaluate the black carbon in Hefei in terms of seasonal, monthly and diurnal variations, including their source identification. The annual mean BC mass concentration MBC is found to be 3.5±2.5μgm−3 in Hefei, while the aerosol optical depth shows a yearly average value of ~0.6. The seasonality of MBC depicts minimum values in the summer, moderate levels in the spring and fall, and maximum in the winter. The monthly average values of MBC vary threefold, ranging from the lowest average value of 2.0±1.0μgm−3 in July to the highest 6.0±2.6μgm−3 during January. Diurnal variations exhibit two BC peaks, corresponding to the morning and evening rush hours. Higher median BC concentrations are observed during haze episodes compared with non-haze periods, although low MBC is sometimes observed for high visibility, which is probably indicative of the aerosol scattering dominating diminished visibility. Based on trajectory analyses, the haze BC pollutions are mostly classified into three types from local areas, long-range transport from the Yangtze Delta, and transport from the North China Plain. The median MBC values for haze groups attributed to biomass burning from MODIS wildfire maps are higher than related groups that are not, which is indicative of the significant enhancement of BC aerosols due to agricultural biomass burning. The study suggests that aerosol absorption contributes more to the observed haze episodes in fall compared to other seasons.