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Characterization of Natural and Affected Environments
Size-related physical properties of black carbon in the lower atmosphere over Beijing and Europe Shuo Ding, Dantong Liu, Delong Zhao, Kang Hu, Ping Tian, Wei Zhou, Mengyu Huang, Yan Yang, Fei Wang, Jiujiang Sheng, Quan Liu, Shaofei Kong, Pengyi Cui, Yuandong Huang, Hui He, Hugh Coe, and Deping Ding Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b03722 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019
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Environmental Science & Technology
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Size-related physical properties of black carbon in the
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lower atmosphere over Beijing and Europe
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Shuo Ding1,Dantong Liu1,*, Delong Zhao2,3,4,6*, Kang Hu1, Ping Tian2, Wei Zhou2,
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Mengyu Huang2, Yan Yang2, Fei Wang2, Jiujiang Sheng2, Quan Liu2, Shaofei
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Kong5,Pengyi Cui7, Yuandong Huang7, Hui He2, Hugh Coe8, Deping Ding2,3,4
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1Department
of Atmospheric Sciences, School of Earth Sciences, Zhejiang University, Hangzhou, China
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2Beijing
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3Beijing
Key Laboratory of Cloud, Precipitation and Atmospheric Water Resources, Beijing, China
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4Field
experiment base of cloud and precipitation research in North China, China Meteorological Administration, Beijing, China
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5Department
of Atmospheric Sciences, School of Environmental Studies, China University of Geosciences (Wuhan), Wuhan, China
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6Nanjing
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7School
of Environment and Architecture, University of Shanghai for Science and Technology, Shanghai, China
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8Centre
Weather Modification Office, Beijing, China
University, Nanjing, China
for Atmospheric Sciences, School of Earth and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK.
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Corresponding to:
[email protected],
[email protected] ACS Paragon Plus Environment
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Abstract
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The size-resolved properties of atmospheric black carbon (BC) importantly determine
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its absorption capacity and cloud condensation nuclei (CCN) ability. This study reports
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comprehensive vertical profiles of BC size-related properties over the Beijing area (BJ)
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and
Continental
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Europe
(CE).
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mass loadings over
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CE were in the range
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of clean background
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over BJ. For both
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planetary
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layer (PBL) and lower
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free troposphere, BC mass median core diameter over BJ during the cold season was
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0.21±0.02μm, larger than warm season over BJ and CE (0.18±0.01μm), which may
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reflect seasonal differences in emissions. The BC coatings were positively correlated
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with pollution level, with background BC having smaller coated count median diameter
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(0.19 ± 0.01μm). The modelled absorption enhancement (Eabs) due to coatings was
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1.23±0.14 for background but in the PBL following a linear expression
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(Eabs=0.13*MassBC,surface+1.26).The CCN ability of BC was significantly enhanced in
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the polluted PBL, due to both enlarged size and increased hygroscopicity. In polluted
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BJ at predicted supersaturations~0.08% half of the BC number could be activated,
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whereas in cleaner environment needs ~0.14%. The results here suggest the highly
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coated and absorbing BC can be efficiently incorporated into clouds and exert important
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indirect radiative impacts over polluted East Asia region.
BC
boundary TOC art
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Introduction
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Black carbon (BC) dominates the absorption of shortwave solar radiation in
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atmospheric particulate matter. This alters the atmospheric thermodynamics by its
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heating effects, especially in geographical hotspots that experience large emissions
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from anthropogenic or biomass burning sources1. Fresh BC particles are hydrophobic
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but can transform to be hydrophilic after aging thereby becoming cloud condensation
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nuclei (CCN)2-4. Previous studies reported that the BC layer present within clouds could
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cause important semi-direct effects
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incorporated into cloud droplets8.
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5-7
or lead to evaporation of cloud water if it was
Both the size of refractory BC (rBC) core and the size including non-absorbing
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coatings
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impacts of BC in the atmosphere. The combination of both terms depends on the
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complex morphology and mixing state of BC. These have previously been
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mathematically treated as core-shell 11, partly encapsulated 12or externally mixed 13, all
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of which produce different results and each may provide reasonable approximations in
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different situations. The weighting between rBC core and coated size could vary at
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different stages of ageing process. The BC core is the essential part to determine its
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absorbing properties, and it could be sometimes used to determine the BC source
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signature 9, 14, 15. The amount of coating material associated with a BC particle and how
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the coating is associated determine how efficiently the incident light interacts with the
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absorbing core 16, 17. The addition of coatings can alter the initial hygroscopicity of BC18
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and also increase the particle size. The evolution of coated BC size could thus increase
8-10,
which are the uncoated and coated size, determine the properties and
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its ability of acting as cloud condensation nuclei, thereby determining its atmospheric
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lifetime 19.
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Although the mass concentration of BC has been widely measured globally20, there
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is still lack of measurements of the BC size distribution in both horizontal and vertical
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directions. The uncertainties in modelling BC mass loadings could largely result from
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the unresolved size distributions of BC 21. To incorporate the mixing state information
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of BC at different sizes into models could improve the realistic treatments of BC in
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terms of radiative effects 22 and CCN-activity 23. In addition, it is important to study the
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vertical distribution of BC properties because the location of BC in the atmospheric
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column and the vertical gradient of BC properties could importantly determine its
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impacts on the thermodynamic balance of the planetary boundary layer (PBL)
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Depending on the location of BC relative to cloud layers, the BC could either promote
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or inhibit cloud development 5.
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The aerosol properties over polluted megacity have been raised much attention, where
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their vertical profiles such as over the North China Plain were measured using a range
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of platforms
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properties over Beijing and compares these with those above continental Europe
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throughout the boundary layer and lower free troposphere. The results are used to
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examine the size distribution and size-related optical and hygroscopic properties of BC
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in detail.
25, 26.
24.
This study characterizes the vertical distributions of BC physical
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Materials and methods
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Aircraft platform
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The flights conducted over Beijing region used a King-Air350 aircraft with an
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average speed about 250 km/hr27, 28. The flights were conducted in 2018 during both
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warm (April to September) and cold (November to December) seasons. The sampling
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inlet system is a passive Isokinetic Aerosol Sampling Inlet (BMI, Brechtel
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Manufacturing Inc), which can deliver 150 lpm of sample flow at 100 m s-1 air speed
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and transmit particles with collection efficiency over 95% over a particle size range of
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0.01-6 μm. A silica drier was used in the sampling line before the instruments. This,
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combined with the constant cabin temperature (~25℃), guarantees the sampled aerosol
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was dry. Most flights were performed to avoid clouds but data collected when the
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aircraft was in cloud has been eliminated based on the measurements of cloud liquid
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water content. Most flights were conducted up to 3 km altitude. The flight tracks are
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shown in Fig. 1(a), and the aircraft was typically operated over the Shahe rural area to
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the northeast of central Beijing away which was ~20 km. The flights over continental
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Europe took place between April and September 2008 during the EUCAARI-
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LONGREX (European integrated Project on Aerosol Cloud Climate and Air Quality
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Interactions - Long Range Experiment) campaign
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campaign, a high pressure system continuously impacted western Europe and hence
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regionally-influenced urban pollutants were sampled.
29.During
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Meteorology
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Meteorological parameters including ambient pressure, temperature, relative
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humidity and wind speed/direction were in-situ characterized by an AIMMS-20
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(Aventech Research Inc), which was calibrated on an annual basis. The planetary
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boundary layer height (PBLH) is determined by a combination of factors (illustrated in
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Fig. S1): a low vertical gradient of potential temperature indicating the effective
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convective mixing and a temperate inversion on top of the PBL.
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Backward trajectory analysis was performed by HYSPLIT 4.0 model30 in ensemble
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mode using 1◦×1◦, 3-hourly GDAS1 reanalysis products. A 24h backward time is
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analyzed since the focus is on recent potential source influences.
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Measurements on black carbon physical properties
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The mass of the refractory core (rBC) of an individual BC particle is measured by a
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single particle soot photometer (SP2, DMT Inc.)31, 32. The SP2 was calibrated using
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Aquadag (Acheson Inc., USA) and a factor of 0.75 was then applied to correct the
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calibration so that it is representative of ambient BC 33. The mass equivalent diameter
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of the rBC core (Dc) is obtained by assuming sphericity and a rBC density of 1.8 g cm-3
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34. The BC mass outside of the detectable range is obtained by extrapolating a lognormal
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distribution on the measured mass size distribution of the BC cores (Fig. 3(b)). The
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vertical profiles in this study are classified as low, medium and heavy pollution
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(denoted as LP, MP, HP respectively) according to the occurrence frequency of surface
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rBC mass loading, i.e. 0-0.4 μg m-3as LP, 0.4-1 μg m-3 as MP and >1 μg m-3 as HP.
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The coated BC size (Dp) is obtained by applying a Mie-look up table to match the
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modelled scattering cross section to the measured values9, 35. For a given time window,
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the mass (or count) median diameter (MMD or CMD) from an uncoated or coated BC
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size distribution is obtained as above and below which the mass (or number) population
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is equal.
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Fig. 2 shows an example of the distribution of the diameters of the coated BC
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particles as a function of uncoated BC core size of single particles for the
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flight20180426 in the PBL. Dp=Dc means there is no coating associated with BC, and
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the coating increases as Dp becomes larger than Dc. The bulk coating/rBC volume ratio
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(Vcoating/VrBC) is calculated as the total volume of coated BC divided by the total volume
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of uncoated rBC cores in a given time window, expressed by
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𝑉𝑐𝑜𝑎𝑡𝑖𝑛𝑔 𝑉𝑟𝐵𝐶
∑𝑖𝐷3𝑝,𝑖
= ∑ 𝐷3 ―1(1), 𝑖 𝑐,𝑖
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where Dp,i and Dc,iare the coated and uncoated diameters of ith BC particle respectively.
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In addition, the mass ratio of coating and rBC (MR) could also be obtained for each
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particle. Note that there is a retrieval rate of successfully SP2-measured Dp and this
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uncertainty is estimated by assuming two extreme scenarios for the particles missing
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Dp information (bottom panel of Fig. 2), as detailed in the Supplement. This uncertainty
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analysis is performed throughout the following calculations of optical and hygroscopic
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properties of BC.
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If the volume ratio of coating and rBC is known, the hygroscopicity parameter of a
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given BC-containing particle (κBCc) could be obtained by assuming a κcoating and κrBC=0,
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based on volume-weighted Zdanovskii–Stokes–Robinson (ZSR) rule36, expressed as,
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𝜅𝐵𝐶𝑐 = 𝑉𝑐𝑜𝑎𝑡𝑖𝑛𝑔 × 𝜅𝑐𝑜𝑎𝑡𝑖𝑛𝑔(2),
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The direct measurement on coating composition was not available in this study hence
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κcoating is not directly determined, thus we applied a value of κcoating=0.3 measured from
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a previous study37 and a reduced value of 0.2 for a sensitivity test. We only tested the
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sensitivity for κcoating3 is considered to have a core-shell structure leading to an absorption
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enhancement caused by a lensing effect. The calculated MAC of each particle is
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weighted by the total rBC mass to obtain the MAC in bulk, expressed as:
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𝑀𝐴𝐶 =
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where 𝑀𝐴𝐶𝑖 and 𝑚𝑟𝐵𝐶,𝑖 are the MAC and rBC mass for each particle respectively.
∑𝑖𝑀𝐴𝐶𝑖 × 𝑚𝑟𝐵𝐶,𝑖 ∑𝑖𝑚𝑟𝐵𝐶,𝑖
(3) ,
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Results and discussion
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rBC mass loading
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Fig. 1(c) shows the vertical profiles of rBC mass loading for all continental Europe
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and Beijing flights under different pollution levels. The rBC mass loading over
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continental Europe was generally of the same order of magnitude as that in light
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pollution levels over Beijing, with mean rBC mass loadings in the PBL and FT
