Effect of ambient humidity on the light absorption amplification of black carbon in Beijing during January 2013
Abstract 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 2013. 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 (NF BC thick increased with increasing BC, reaching a plateau at ~80 90% when BC concentrations were =15 mg 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 NF BC thick which was likely due to the enhanced production of secondary aerosol under humid conditions.