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Discussion papers
https://doi.org/10.5194/acp-2018-1142
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1142
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 15 Nov 2018

Research article | 15 Nov 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Contrasting physical properties of black carbon in urban Beijing between winter and summer

Dantong Liu1,2, Rutambhara Joshi2,5, Junfeng Wang4, Chenjie Yu2, James D. Allan2,5, Hugh Coe2, Michael J. Flynn2, Conghui Xie3, James Lee6, Freya Squires6, Simone Kotthaus7, Sue Grimmond7, Xinlei Ge4, Yele Sun3, and Pingqing Fu3 Dantong Liu et al.
  • 1Department of Atmospheric Sciences, School of Earth Sciences, Zhejiang University, Hangzhou, Zhejiang, China
  • 2Centre for Atmospheric Sciences, School of Earth and Environmental Sciences, University of Manchester, Manchester, UK
  • 3Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
  • 4School of Environmental Science and Engineering, Nanjing University of Information Science and Technology, Nanjing, China
  • 5National Centre for Atmospheric Science, University of Manchester, Manchester, UK
  • 6Department of Chemistry & National Centre for Atmospheric Science, University of York, York, UK
  • 7Department of Meteorology, University of Reading, UK

Abstract. Black carbon (BC) is known to have major impacts on both human health and climate. The populated megacity represents the 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 APHH (Air Pollution and Human Health) project, in both winter (Nov.–Dec. 2016) and summer (May–Jun. 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 coatings 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 coatings.

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 (PMF) of mass spectra from the SP-AMS. A method is proposed to isolate the BC from the transportation sector using a mode of small BC particles (core diameter Dc<0.18μm and coating thickness ct<50nm). This mode of BC highly correlated with NOx concentration in both seasons (~14ngm−3BCppb−1NOx) 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 (ct=50–200nm) contributing ~20–25% of rBC mass. Large uncoated BC particles (Dc>0.18μm and ct<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 contribution of BC thickly-coated with secondary species (ct>200nm) to the total rBC mass increased with pollution level in winter, but was minor in summer. These large BC importantly enhanced the absorption efficiency at high pollution levels – in winter when PM1>100μgm−3 or BC>2μgm−3, the absorption efficiency of BC increased 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.

Dantong Liu et al.
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This study provides source attribution and characterization of BC in the Beijing urban environment in both winter and summer. For the first time, the physically and chemically based source apportionments are compared to evaluate the primary source contribution and secondary processing of BC-containing particle. A method is proposed to isolate the BC from transportation sector and coal combustion sources.
This study provides source attribution and characterization of BC in the Beijing urban...
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