Contrasting physical properties of black carbon in urban Beijing between winter and summer
Authors: Liu Dantong , Joshi Rutambhara , Wang Junfeng , Yu Chenjie , Allan James D. , Coe Hugh , Flynn Michael J. , Xie Conghui , Lee James , Squires Freya , Kotthaus Simone , Grimmond Sue , Ge Xinlei , Sun Yele , Fu Pingqing 
Published: May 2019

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 (November–December 2016) and summer
(May–June 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 aerosol 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 the cold season. The characteristics of the BC were
relatively independent of air mass direction in summer, whereas in winter
air masses from the Northern Plateau were considerably cleaner and contained
less-coated and smaller BC, but the BC from the Southern Plateau had the
largest core size and coatings. We compare two online source apportionment methods using simultaneous
measurements made by the SP2, which measures physical properties of BC, and
the chemical approach using the positive matrix factorization (PMF) of mass
spectra from the SP-AMS for the first time. A method is proposed to isolate
the BC from the transportation sector using a mode of small BC particles
(core diameter Dc<0.18 µm and coating thickness ct < 50 nm).
This mode of BC highly correlated with NOx concentration in both
seasons (∼14 ng m−3 BC ppb−1 NOx) 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 was
characterized by moderate coatings (ct = 50–200 nm) contributing
∼20 %–25 % of rBC mass. Large uncoated BC particles
(Dc>0.18 µm and ct < 50 nm) were 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 the residential and industrial sector respectively. The
contribution of BC thickly coated with secondary species (ct > 200 nm)
to the total rBC mass increased with pollution level in winter but
was minor in summer. These large BC particles importantly enhanced the absorption
efficiency at high pollution levels – in winter when PM1 > 100 µg m−3
or BC > 2 µg m−3, the absorption
efficiency of BC increased by 25 %–70 %. The 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.