Effect of ambient humidity on the light absorption amplification of black carbon in Beijing during January 2013

Black carbon (BC) and its mixing state were measured with a ground-based single particle soot photometer in urban Beijing during the extremely polluted winter of w2013. Up to 70 +/- 14% of the BC-containing particles were thickly-coated during periods of haze, compared to 37 +/- 9% on non-hazy days. The thickly-coated number fraction (NFBC-thick) increased with increasing BC, reaching a plateau at similar to 80-90% when BC concentrations were >= 15 mu g m(-3) and visibility was <= 2 km. Regional inflows brought more aged, highly thickly-coated BC to Beijing during haze. The absorption coefficient showed a distinct linear correlation with BC concentration; the mass absorption efficiency (MAE) of BC was acquired, with an overall mean of 4.2 +/- 0.01 m(2) g(-1) at 870 nm. The MAE of BC amplified with increasing ambient relative humidity. This was largely explained by the increase in NFBC-thick, which was likely due to the enhanced production of secondary aerosol under humid conditions. (C) 2015 Elsevier Ltd. All rights reserved.