Atmospheric Chemistry and Physics | 2019
Contrasting physical properties of black carbon in urban Beijing between winter and summer
Abstract
Abstract. Black carbon (BC) is known to have major impacts on both human health and\nclimate. The populated megacity represents the most complex anthropogenic BC\nemissions where the sources and related impacts are very uncertain. This\nstudy provides source attribution and characterization of BC in the Beijing\nurban environment during the joint UK–China APHH (Air Pollution and Human\nHealth) project, in both winter (November–December\xa02016) and summer\n(May–June\xa02017). The size-resolved mixing state of BC-containing particles was\ncharacterized by a single-particle soot photometer (SP2) and their mass\nspectra was measured by a soot particle aerosol mass spectrometer (SP-AMS). The\nrefractory BC (rBC) mass loading was around a factor of\xa02 higher in winter\nrelative to summer, and more variable coatings were present, likely as a\nresult of additional surface emissions from the residential sector and\nfavourable condensation in the cold season. The characteristics of the BC were\nrelatively independent of air mass direction in summer, whereas in winter\nair masses from the Northern Plateau were considerably cleaner and contained\nless-coated and smaller BC, but the BC from the Southern Plateau had the\nlargest core size and coatings. We compare two online source apportionment methods using simultaneous\nmeasurements made by the SP2, which measures physical properties of BC, and\nthe chemical approach using the positive matrix factorization (PMF) of mass\nspectra from the SP-AMS for the first time. A method is proposed to isolate\nthe BC from the transportation sector using a mode of small BC particles\n(core diameter Dc µ m and coating thickness ct\u2009 \u200950\u2009nm).\nThis mode of BC highly correlated with NOx concentration in both\nseasons ( ∼14 \u2009ng\u2009m −3 \u2009BC\u2009ppb −1 NOx ) and\ncorresponded with the morning traffic rush hour, contributing about 30\u2009%\nand 40\u2009% of the total rBC mass (35\u2009% and 55\u2009% in number) in winter and\nsummer respectively. The BC from coal burning or biomass burning was\ncharacterized by moderate coatings (ct\u2009 = \u200950–200\u2009nm) contributing\n ∼20 \u2009%–25\u2009% of rBC mass. Large uncoated BC particles\n( Dc>0.18 µ m and ct\u2009 \u200950\u2009nm) were more likely to be\ncontributed by coal combustion, as these particles were not present in urban\nLondon. This mode was present in Beijing in both winter ( ∼30 \u2009%–40\u2009%\nrBC mass) and summer ( ∼40 \u2009% rBC mass) but may be\ndominated by the residential and industrial sector respectively. The\ncontribution of BC thickly coated with secondary species (ct\u2009 > \u2009200\u2009nm)\nto the total rBC mass increased with pollution level in winter but\nwas minor in summer. These large BC particles importantly enhanced the absorption\nefficiency at high pollution levels – in winter when PM 1 > \u2009100\u2009 µ g\u2009m −3 \nor BC\u2009 > \u20092\u2009 µ g\u2009m −3 , the absorption\nefficiency of BC increased by 25\u2009%–70\u2009%. The reduction of emissions of these\nlarge BC particles and the precursors of the associated secondary coating\nwill be an effective way of mitigating the heating effect of BC in urban\nenvironments.