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Black carbon (BC) has important impact on climate and environment due to itslight absorption ability, which greatly depends on its physicochemicalproperties including morphology, size and mixing state. The sizedistribution of the refractory BC (rBC) was investigated in urban Beijingin the late winter of 2014, during which there were frequent haze events, through analysisof measurements obtained using a single-particle soot photometer (SP2). Byassuming void-free rBC with a density of 1.8 g cm<sup>−3</sup>, the mass of therBC showed an approximately lognormal distribution as a function of thevolume-equivalent diameter (VED), with a peak diameter of 213 nm. Larger VEDvalues of the rBC were observed during polluted periods than on clean days,implying an alteration in the rBC sources, as the size distribution of therBC from a certain source was relative stable, and VED of an individual rBCvaried little once it was emitted into the atmosphere. The potential sourcecontribution function analysis showed that air masses from the south to eastof the observation site brought higher rBC loadings with more thick coatingsand larger core sizes. The mean VED of the rBC presented a significant linearcorrelation with the number fraction of thickly coated rBC, extrapolating tobe  ∼ 150 nm for the completely non-coated or thinly coated rBC. It wasconsidered as the typical mean VED of the rBC from local traffic sources inthis study. Local traffic was estimated to contribute 35 to 100 % ofthe hourly rBC mass concentration with a mean of 59 % during the campaign.Lower local traffic contributions were observed during polluted periods,suggesting increasing contributions from other sources (e.g., coalcombustion and biomass burning) to the rBC. Thus, the heavy pollution in Beijingwas greatly influenced by other sources in addition to the local traffic

Size distribution and source of black carbon aerosol in urban Beijing during winter haze episodes