Contrasting physical properties of black carbon in urban Beijing between winter and summer
Black carbon )BC) is known lo have major impacts on both human health and climate. The populated megacity represents (he most complex anthropogenic BC emissions where the sources and related impacts are very uncertain. This study provides source attribution and characterization of BC in the Beijing urban environment during the joint UK-China API III (Air Pollution and Human Health) project, in both winter (Nov. -Dec. 2016) and summer (May -.Inn 2017). The size-resolved mixing state of BC-containing particles was characterized by a single particle soot photometer (SP2) and their mass spectra was measured by a soot particle mass spectrometer (SP-AMS). The refractory BC (rBC) mass loading was around a factor of 2 higher in winter relative to summer and more variable coalings were present, likely as a result of additional surface emissions from the residential sector and favourable condensation in cold season. The characteristics of the BC were relatively independent of air mass direction in summer; whereas in winter the airmass from the Northern Plateau had a significant dilution effect resulting in less-coated and smaller BC, whereas the BC from the Southern Plateau had the largest core size and coalings
We combine two online source apportionment methods for the first time, by the physical method from the SP2, and the chemical approach using the positive matrix factorization i I'MI'i of mass spectra from the SP-AMS. A method is proposed lo isolate the BC from the transportation sector using a mode of small BC particles (core diameter D, -0 1 Sum and coating thickness ci <50nm). This mode of BC highly correlated with NO, concentration in both seasons (-14 ng nr* BC ppb'1 NO,) and corresponded with the morning traffic rush hour, contributing about 30% and 40% of the total rBC mass (35% and 55% in number) in winter and summer respectively. The BC from coal burning or biomass burning tended to dominate with moderate coatings (rr=50-200nm) contributing -20-25% of rBC mass. Large uncoatedBC particles (ft>0.18 um and c/<50nm) was more likely to be contributed by coal combustion, as these particles were not present in urban London. This mode was present in Beijing in both winter (-30-40% rBC mass) and summer (-40% rBC mass) but may be dominated by residential and industrial sector respectively. The conlrtbulion of BC thickly-coated wilh secondary species (r/>200nm) lo Ihe total tBC mass increased wilh pollution level in winter, bul was minor in summer These large BC importantly enhanced the absorption efficiency at high pollution levels-in winter when PM1--100 ugnr3orBC>2 ugm*1, the absorption efficiency of BC incieased by 25-70%. Reduction of emissions of these large BC particles and the precursors of the associated secondary coating will be an effective way of mitigating the heating effect of BC in urban environments.
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