ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-17-7459-2017Effects of the Wegener–Bergeron–Findeisen process on global black carbon distributionQiLingqiling@atmos.ucla.eduLiQinbinHeCenlinhttps://orcid.org/0000-0002-7367-2815WangXinhttps://orcid.org/0000-0002-8839-8345HuangJianpinghttps://orcid.org/0000-0003-2845-797XDepartment of Atmospheric and Oceanic Sciences, University of
California, Los Angeles, CA, USAJoint Institute for Regional Earth System Science and Engineering,
University of California, Los Angeles, CA, USAKey Laboratory for Semi-Arid Climate Change of the Ministry of
Education, College of Atmospheric Sciences, Lanzhou University, Lanzhou,
ChinaLing Qi (qiling@atmos.ucla.edu)21June20171712745974795August201628September201628March201730March2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/17/7459/2017/acp-17-7459-2017.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/17/7459/2017/acp-17-7459-2017.pdf
We systematically investigate the effects of Wegener–Bergeron–Findeisen
process (hereafter WBF) on black carbon (BC) scavenging efficiency, surface
BCair, deposition flux, concentration in snow
(BCsnow, ngg-1), and washout ratio using a global 3-D
chemical transport model (GEOS-Chem). We differentiate riming- versus
WBF-dominated in-cloud scavenging based on liquid water content (LWC) and
temperature. Specifically, we implement an implied WBF parameterization using
either temperature or ice mass fraction (IMF) in mixed-phase clouds based on
field measurements. We find that at Jungfraujoch, Switzerland, and Abisko,
Sweden, where WBF dominates in-cloud scavenging, including the WBF effect
strongly reduces the discrepancies of simulated BC scavenging efficiency and
washout ratio against observations (from a factor of 3 to 10 % and from a
factor of 4–5 to a factor of 2). However, at Zeppelin, Norway, where riming
dominates, simulation of BC scavenging efficiency, BCair, and
washout ratio become worse (relative to observations) when WBF is included.
There is thus an urgent need for extensive observations to distinguish and
characterize riming- versus WBF-dominated aerosol scavenging in mixed-phase
clouds and the associated BC scavenging efficiency. Our model results show
that including the WBF effect lowers global BC scavenging efficiency, with a
higher reduction at higher latitudes (8 % in the tropics and up to
76 % in the Arctic). The resulting annual mean BCair
increases by up to 156 % at high altitudes and at northern high latitudes
because of lower temperature and higher IMF. Overall, WBF halves the
model–observation discrepancy (from -65 to -30 %) of
BCair across North America, Europe, China and the Arctic.
Globally WBF increases BC burden from 0.22 to
0.29–0.35 mgm-2yr-1, which partially explains the gap
between observed and previous model-simulated BC burdens over land. In
addition, WBF significantly increases BC lifetime from 5.7 to ∼ 8 days.
Additionally, WBF results in a significant redistribution of BC deposition in
source and remote regions. Specifically, it lowers BC wet deposition (by
37–63 % at northern mid-latitudes and by 21–29 % in the Arctic),
while it increases dry deposition (by 3–16 % at mid-latitudes and by
81–159 % in the Arctic). The resulting total BC deposition is lower at
mid-latitudes (by 12–34 %) but higher in the Arctic (by 2–29 %). We
find that WBF decreases BCsnow at mid-latitudes (by
∼ 15 %) but increases it in the Arctic (by 26 %) while
improving model comparisons with observations. In addition, WBF dramatically
reduces the model–observation discrepancy of washout ratios in winter (from
a factor of 16 to 4). The remaining discrepancies in BCair,
BCsnow and BC washout ratios suggest that in-cloud removal in
mixed-phased clouds is likely still excessive over land.
Introduction
Black carbon (BC) effectively heats the atmosphere by absorbing solar
radiation and has been regarded as the second largest warming agent after
CO2 (Ramanathan and Carmichael, 2008; Bond et al., 2013; IPCC 2014).
Moreover, BC deposited on snow and ice reduces surface albedo and accelerates
melting (Flanner et al., 2007; He et al., 2014b; IPCC, 2014; Liou et
al., 2014). However, there are large uncertainties in estimating direct
radiative forcing of BC, mainly arising from the uncertainties in predicting
BC distribution (Bond et al., 2013). Current models in the project Aerosol
Comparisons between Observations and Models (AeroCom) underestimate aerosol
absorption optical depth (AAOD) of BC observed by the AErosol RObotic NETwork
(AERONET) and satellite by a factor of 1.6–4 (Bond et al., 2013) but
overestimates BCair observed in remote Pacific by a factor of
2–5 (Schwarz et al., 2010; Q. Wang et al., 2014; X. Wang et al., 2014).
Moreover, inter-model disagreement of BC loadings simulated by AeroCom models
is up to 2 to 3 orders of magnitude (Koch et al., 2009; Bond et
al., 2013). The large discrepancy with observations and large disagreement
among models are primarily attributed to wet deposition, which is the
dominant mechanism to remove BC from the atmosphere (Textor et al., 2006;
Koch et al., 2009; Bond et al., 2013) and consequently determines its
lifetime and atmospheric burden. The major process of wet scavenging is
in-cloud scavenging (Taylor et al., 2014), which occurs in two stages:
aerosol activation to form cloud droplets, and removal of droplets by
precipitation. The ability of a particle to be activated as a cloud
condensation nucleus (CCN) and thereby be scavenged by in-cloud scavenging
depends on its hygroscopicity, size, and super-saturation in the cloud (Ghan
et al., 2011). The partition of BC particles between condensed phase and
interstitial air in clouds is quantified by scavenging efficiency, which is
defined as the ratio of aerosol mass mixing ratio in cloud drops and ice
crystals to total aerosol mass mixing ratio in clouds (including aerosols in
interstitial air and in cloud drops).
The determining factors controlling BC scavenging efficiency in clouds are
the properties of BC particles, including their hygroscopicity, size, and
chemical composition (Sellegri et al., 2003; Hallberg et al., 1992, 1994).
Local changes of updraft velocity and critical super-saturation significantly
affect local BC scavenging efficiency. Such effects are also observed at
long-time averages. In mixed-phase clouds, the effect of cloud microphysics
on BC scavenging is considerably more complex. One complicating factor is the
so-called Wegener–Bergeron–Findeisen process (hereafter WBF; Wegener, 1911; Bergeron, 1935; Findeisen, 1938), where
water vapor transfers from liquid to ice phases when vapor pressure is
between the saturation vapor pressure over ice and water droplets. Liquid
cloud droplets evaporate and release the aerosol materials in the droplets
back into interstitial air, resulting in a slower scavenging of aerosols in
mixed-phase clouds. The water vapor evaporated from water drops deposit onto
ice surface and snow particles form. Accordingly, WBF leads to slower BC
scavenging and faster snow growth. Theoretical estimates show that snow
growth rate from WBF is a function of temperature (Pruppacher and Klett,
2010). As temperature lowers from 0 to -14 ∘C, snow growth rate is
estimated to increase drastically from 0 to 5.2×10-8
(gs-1) at 500 hPa (Pruppacher and Klett, 2010), and BC
scavenging efficiency at Jungfraujoch is observed to decrease from 0.6 to 0.2
(Cozic et al., 2007). From -14 to -25 ∘C, the estimated snow
growth rate from WBF varies in a relatively smaller range (4.8–5.5×10-8gs-1, Pruppacher and Klett, 2010), and BC scavenging
efficiency varies in 0.1–0.2 (Cozic et al., 2007). The two anti-correlated
trends indicate that WBF is a very important factor that explains the
observed temperature dependence of BC scavenging efficiency at Jungfraujoch.
Another process that affects BC scavenging in mixed-phase clouds is riming
(Hegg et al., 2011). Riming occurs when LWC is high and gravitationally
settling of snowflakes and ice crystals collect the water drops along their
pathways, thereby scavenging BC particles in the water drops. At Zeppelin, where
snow particles show predominantly rimed structures, BC scavenging efficiency
changes marginally (within 5 %) from summer (0.77) to winter (0.81) as
the average temperature lowers from -2∘C in summer to
-14∘C
in winter (Heintzenberg and Leck, 1994). The different trends of the
scavenging efficiency with temperature observed at Jungfraujoch and Zeppelin
indicate that WBF and riming are the major processes that determine BC
scavenging efficiency in mixed-phase clouds. Therefore, the decreasing of BC
scavenging efficiency with decreasing temperature at Jungfraujoch is mainly
attributed to WBF (Cozic et al., 2007). Recent studies have found that this
reduction of BC scavenging efficiency from WBF not only affects wet
scavenging of aerosols but also strongly affects cloud feedbacks and climate
sensitivities (Tan et al., 2016). Thus, it is critical to differentiate WBF and
riming process in model simulations.
BC scavenging efficiency is typically prescribed as a constant (between 0 and
1) in global chemical transport models (CTMs) for computational efficiency
consideration and the limited understanding of the processes controlling the
partition of BC between interstitial air and condensed phases in mixed-phase
clouds (Textor et al., 2006). Textor et al. (2006) and Wang et al. (2011)
treated BC scavenging in mixed-phase the same as in warm liquid clouds. Stier
et al. (2005) used a scavenging efficiency of 0.40 for soluble Aitken-mode
aerosols and 0.75 for accumulation-mode aerosols in mixed-phase clouds, lower
(by 0.10) than their corresponding values in liquid-only clouds. Using the
same model, Bourgeois and Bey (2011) applied a substantially lower scavenging
efficiency (0.06) for both Aitken- and accumulation-mode aerosols in
mixed-phase clouds based on measurements from Henning et al. (2004). The
lower scavenging efficiency results in 5-fold higher BC burden in the Arctic
(from 0.75 to 3.7 Gg) and 3-fold longer BC lifetime (from 1.8 to
5.8 days). Liu et al. (2011) and Browse et al. (2012) also showed that BC
loading and lifetime are both very sensitive to scavenging efficiency. It is
clear that a systematic examination of BC scavenging efficiency and wet
deposition is warranted. To that end, recent comprehensive large-scale
measurements of BCsnow in North America (Doherty et al., 2014),
China (Huang et al., 2011; Ye et al., 2012; X. Wang et al., 2013, 2014; Zhang
et al., 2013), and the Arctic (Doherty et al., 2010) provide a unique
opportunity. Concurrent measurements of BC in fresh snow and rain
(BCsnow/rain) and BCair (Cerqueira et al., 2010; Mori
et al., 2014) provide better constraints on BC wet deposition.
BC scavenging efficiency varies as a function of BC aging in GEOS-Chem (Park
et al., 2005; Wang et al., 2011). Specifically, in warm and mixed-phase
clouds, hydrophilic BC particles are completely (100 %) incorporated in
cloud drops and serve as CCN, while hydrophobic BC particles remain in
interstitial air. In ice clouds, hydrophobic BC particles serve as ice
condensation nuclei, while hydrophilic BC particles are not scavenged. In this
study we investigate the effect of WBF on BC scavenging, its distribution in
air and snow, and the budget using GEOS-Chem. Specifically, we distinguish riming-
versus WBF-dominated in-cloud scavenging in mixed-phase clouds and
parameterize BC scavenging efficiency accordingly. We evaluate model results
of BC scavenging efficiency (Sect. 4.1), BCair (Sect. 4.2), BC
wet deposition fluxes (Sect. 4.3), BCsnow (Sect. 4.4), and BC
washout ratio (Sect. 4.5). We further discuss the WBF effects on global BC
budget (Sect. 5), followed by conclusions and implications (Sect. 6).
Observations
Figure 1 shows sites with measurements of BC scavenging efficiency,
BCair, BCsnow, and BC washout ratio in the Northern
Hemisphere.
Scavenging efficiency measurements
BC scavenging efficiencies in mixed-phase clouds are not well understood. In
mixed-phase clouds, BC is partitioned between condensed phase (water drops
and ice crystals) and interstitial air, which is crucial for accurate
estimates of the in-cloud scavenging of BC. Following Hallberg et al. (1992)
and references thereafter, the scavenging efficiency is defined as
rscav.=[BC]condensed[BC]interstitial+[BC]condensed,
where rscav. is BC scavenging efficiency,
[BC]condensed the mass mixing ratio of BC in condensed
phase, and [BC]interstitial the mass mixing ratio of BC in
the interstitial air.
There are eight surface sites that reported measurements of BC scavenging
efficiencies (Table 1 and Fig. 1). Cloud droplets and interstitial air were
collected through different inlets. Cloud droplets were collected by a
counterflow virtual impactor (CVI) (Ogren et al., 1985). Interstitial air was
sampled by impactor-type collectors such as annular-slit impactor, round jet
impactor, and mini-cascade impactor. Field calibration of the two inlets as
well as theoretical consideration and laboratory calibration showed that the
overall uncertainty of mass concentration of particles of the two phases in
clouds was close to 15 % (Sellegri et al., 2003). The scavenged fraction
was then computed from the comparison between cloud impactor samples and
interstitial aerosols (e.g., Hallberg et al., 1992, 1994; Heintzenberg and
Leck, 1994; Gieray et al., 1997; Hitzenberger et al., 2000, 2001). Long-term
measurements of BC mass mixing ratios in clouds require that the in situ
sites are located at high altitudes with frequent clouds. Only a few sites
meet these requirements. Thus, available measurements of BC scavenging
efficiencies are very limited.
BC measurements used in this study: scavenging efficiencies (purple
squares), concentration in air (red diamonds), concentration in snow (blue
circles), washout ratio (black triangles), and average snow depth (cm) for
2006–2012 (color contours).
Observed and GEOS-Chem-simulated scavenging efficiency of BC
(fraction of BC incorporated into cloud droplets and ice crystals).
SiteObservationsModel TimeReferencesRiming-only aWBFTbWBFIMFcJungfraujoch(46.5∘ N, 8∘ E, 3.85 km)0.600.900.590.48Jul–Aug 2004Cozic et al. (2007)Jungfraujoch0.05–0.100.290.110.10Feb–Mar 2004Cozic et al. (2007)Puy de Dôme(48∘ N, 2∘ E, 1.46 km)0.43±0.170.710.630.48Feb–Mar 2001Sellegri et al. (2003)Zeppelin(79∘ N, 12∘ E, 0.47 km)0.810.890.530.14May–Oct 1990–1992Heintzenberg and Leck (1994)Zeppelin0.770.410.110.03Oct–May 1990–1992Heintzenberg and Leck (1994)Mt. Sonnblick(47∘ N, 13.4∘ E, 3.11 km)0.74±0.190.670.260.10Apr–May 1997Hitzenberger et al. (2000)Po Valleyd(44.6∘ N, 11.6∘ E, 0 km)0.06 (0.01–0.30)–––Nov 1989Hallberg et al. (1992)Po Valleyd0.39 (0.31–0.57)0.550.530.46Nov 2011Gilardoni et al. (2014)Great Dun Fell(54.7∘ N, 2.5∘ W, 0.85 km)0.50±0.200.610.590.46Apr–May 1993Gieray et al. (1997)Rax(48∘ N, 16∘ E, 1.64 km)0.54±0.250.700.640.35Mar 1999–Apr 2000Hitzenberger et al. (2001)
a Simulation with riming only in-cloud scavenging of
BC in mixed-phase clouds. See text for details.
b Simulation with in-cloud scavenging of BC by WBF, parameterized
by temperature, in mixed-phase clouds. See text for details.
cSimulation with in-cloud scavenging of BC by WBF, parameterized
by ice mass fraction, in mixed-phase clouds. See text for details.
d Observations in urban fog.
The observed BC scavenging efficiencies increase with increasing distance
from source regions, from 0.06 in heavily polluted fog in Po Valley, Italy
(44.6∘ N, 11.6∘ E; sea level), (Hallberg et al., 1992) to
0.81 at Zepplin (79∘ N, 12∘ E; 0.47 km) in the
Arctic (Heintzenberg and Leck, 1994) (Table 1). The observed scavenging
efficiencies were vastly different at the Po Valley: 0.06 from Hallberg et
al. (1992) and 0.39 from Gilardoni et al. (2014). Reasons for the difference
are unclear. Freshly emitted BC particles are mostly hydrophobic and cannot
serve as CCN (Weingartner et al., 1997). Hydrophobic BC particles mix with
hydrophilic materials (e.g., sulfate, nitrate or soluble organics) during
transit and become hydrophilic and larger in size (Sellegri et al., 2003).
The incorporation of BC particles into cloud droplets via nucleation
scavenging is thus enhanced (Moteki et al., 2012; Taylor et al., 2014). Both
cloud dynamics (e.g., updraft velocity) and microphysics (nucleation,
condensation and coagulation) complicate and determine the partition of BC
particles between condensed phase and interstitial air in mixed-phase clouds
(Cozic et al., 2007). When riming occurs, large snow crystals collect cloud
water drops along their pathways and BC particles in these cloud water drops
are likewise removed (Heintzenberg and Leck, 1994; Hegg et al., 2011). BC
scavenging efficiency due to riming is thus similar to that in warm clouds.
For example, at Zeppelin, where the riming process was typically dominant, BC
scavenging efficiencies in winter (0.77) (mostly mixed-phase clouds and ice
clouds) and in summer (0.81) (mostly warm liquid clouds) were within 5 %
(Heintzenberg and Leck, 1994). In contrast, when the WBF process occurs, ice
crystals grow at the expense of water droplets and hence BC particles inside
the water droplets are released back into the interstitial air, thereby
lowering in-cloud BC scavenging efficiency. The scavenging efficiency at
Jungfraujoch (46.5∘ N, 8∘ E; 3.85 km), where the
WBF process dominates in mixed-phase clouds, was higher in warm clouds (0.6)
in summer and substantially lower in mixed-phase clouds (0.05–0.10) in
spring (Cozic et al., 2007).
Cozic et al. (2007) reported comprehensive observations of BC scavenging
efficiency at Jungfraujoch, a site regularly engulfed by clouds (30 % of
the time) and far away from pollution sources. The site is well suited for
investigating continental background aerosols and clouds from a ground-based
platform. Cozic et al. (2007) examined the partitioning of BC in mixed-phase
clouds by sampling through two inlets, with one heated inlet collecting
aerosols in cloud drops, ice crystals and the interstitial air and the other
collecting only aerosols in the interstitial air. They found that the
scavenging efficiency of BC was influenced by LWC, BC content, temperature
and IMF. We use their results to parameterize the effect of WBF on BC
scavenging efficiency in this study (See Sect. 3).
BC in surface air
Surface BCair has been widely measured across the Arctic, North
America, Europe and Asia (Fig. 1). Observations in the Arctic are available
at Denali, Alaska; Barrow, Alaska; Alert, Canada; Zeppelin, Norway; and
Summit, Greenland (see details in Qi et al., 2017). We also use here
measurements of BCair at 178 sites as part of the Interagency
Monitoring of PROtected Visual Environment (IMPROVE; Malm et al., 1994;
http://vista.cira.colostate.edu/improve/) network in North America.
IMPROVE measurements were made every 3 days and 24 h averages were reported.
Additionally, we use BCair observations from East Asia in 2006
(X. Y. Zhang et al., 2008). Observations of BCair in Europe are
from the European Monitoring and Evaluation Programme (EMEP) network
(EMEP/MSC-W et al., 2014; http://ebas.nilu.no). We use here daily EMEP
measurements.
The thermal optical reflectance (TOR) combustion method is used to measure BC
concentrations by IMPROVE and EMEP network based on the preferential
oxidation of organic carbon (OC) and elemental carbon (EC) at different
temperatures (Chow et al., 1993, 2004). Heating protocols used by IMPROVE
network are as follows: the sample filter is heated stepwise at temperatures
of 120 ∘C (OC1), 250 ∘C (OC2), 450 ∘C (OC3), and
550 ∘C (OC4) in a non-oxidizing (He) atmosphere, and at
550 ∘C (EC1), 700 ∘C (EC2), and 800 ∘C (EC3) in an
oxidizing atmosphere of 2 % oxygen and 98 % He. Evolved carbon is
oxidized to CO2 and then reduced to CH4 for detection. The
pyrolyzed or charred OC is monitored by reflectance at wavelength λ=633nm. The portion of EC1 until the laser signal returns to its
initial value is assigned to pyrolyzed organic carbon (OP). EC is defined by
EC1 + EC2 + EC3 - OP. We use EC here to approximate the
concentration of BC. EMEP use different protocols. Samples were heated up to
either 850 ∘C (NIOSH) (and hence a fraction of EC may be combusted)
or 650 ∘C (EUSAAR_2; EUROPA, 2008). BC-like products of OC
pyrolysis can lead to uncertainty in measuring BC mass. The uncertainty is
estimated to be ∼ 20 % based on the repeatability and
reproducibility of the measurements (EMEP/MSC-W et al., 2014).
BC in snow
We use BCsnow (ngg-1) to constrain BC deposition on
snow-covered surfaces. There is now a comprehensive set of BCsnow
measurements, from sampling the full snowpack depth, in the Northern
Hemisphere (Fig. 1): the Arctic (Doherty et al., 2010), North America
(Doherty et al., 2014), northern China (Wang et al., 2013), and Xinjiang,
China (Ye et al., 2012). For direct comparison with model results, we merge
the observations in the same model grid cell. We exclude samples with obvious
contamination from dust, soil, or local emissions as indicated in the
observations. This leaves out a sample number of 334 from the Arctic, 158
from North America, 97 from northern China, and 47 from Xinjiang, China.
Doherty et al. (2014) grouped samples in North America into four geographic
regions based on land surface type and seasonal average snow water
equivalent: Canada, the Great Plains, the Pacific Northwest, and the
Intra-Mountain Northwest. Here we follow the same definitions. Wang et
al. (2013) defined three subregions of northern China: Inner Mongolia,
Northeast Border, and Northeast Industrial. We use the same definitions in
this study. The largest uncertainties of these measurements are uncertainties
of BC mass absorption cross section (-25 %), BC and non-BC absorption
Ångström exponent used to estimate BCsnow (∼ 50 %,
Doherty et al., 2010). Other uncertainties include instrumental uncertainty
(≤ 11 %) and under-catch correction (±15 %) (see details in
Doherty et al., 2010). The resulting overall uncertainty of the observed
BCsnow is < 60 %.
Washout ratio measurements
Washout ratio is a more easily measured parameter (compared to scavenging
efficiency) that characterizes wet scavenging of BC. It is defined as the
ratio of BC mass mixing ratio in fresh rain and snow to that in surface air
following Hegg et al. (2011),
rwashout=[BC]rain/snow[BC]air,
where rwashout is the washout ratio,
[BC]rain/snow the BC mass mixing ratio in fresh rain or
snow, and [BC]air the BC mass mixing ratio in surface air.
Washout ratio is an ambiguous metric for scavenging because it is rare that
surface BCair is representative of that at the altitude where BC
aerosols are scavenged. On the other hand, washout ratio does subsume a
number of individual processes such as in-cloud scavenging and below-cloud
scavenging to give an estimate of an overall assignment (Hegg et al., 2011).
Thus, unlike BC scavenging efficiency, which quantitatively describes the
partition of BC in condensed phase and interstitial air in clouds, BC washout
ratio is only a qualitative index for scavenging, which might partly explain
why we have such limited observations of washout ratios so far. During snow
season, washout ratio characterizes the riming- versus WBF-dominated snow
formation process and BC scavenging in mixed-phase clouds. The washout ratio
at Zeppelin, where snow particles show rimed structures, shows
that BC particles
are scavenged efficiently and the scavenging efficiency was ∼ 770 (Hegg
et al., 2011). However, at Abisko and Changbai, where pristine crystal
snow particles formed mainly from the WBF effect, BC was scavenged much less
efficiently than that in riming-dominated condition, resulting in a much
smaller washout ratio (∼ 150; Noone and Clark, 1988; Z. W. Wang et
al., 2014). This is because BC particles in cloud drops were released back to
the interstitial air and not subject to scavenging.
Observed and GEOS-Chem-simulated BC concentration in snow and rain
(µgL-1), BC concentration in surface air
(µgm-3) and the corresponding washout ratio.
SiteBC concentration Washout ratioTimeReferenceSnow or rainSurface air(µgL-1)(µgm-3)ChangbaiObservations145Wang et al. (2012)(42.5∘ N,Riming-onlya2810.574501Nov–Dec,128.5∘ E;WBFTb2320.603403Mar 2009–20120.74 km)WBFIMFc1780.643336LAVOObservations4.20.024229Mar 2006Hadley et al. (2010)(40.5∘ N,Riming-only13.70.021719121∘ W;WBFT18.70.0345231.73 km)WBFIMF18.30.048400AbiskoObservations2.4–77.10.03–0.9394Mar–Apr 1984Noone and Clark (1988)(68.3∘ N,Riming-only13.30.03148218.8∘ E;WBFT10.00.0512030.35 km)WBFIMF7.50.07296ZeppelinObservations13.90.030769Mar–Apr 2007Hegg et al. (2011)(79.0∘ N,Riming-only6.10.01944412.0∘ E;WBFT4.90.0411090.47 km)WBFIMF4.40.06362
a Simulation with riming only in-cloud scavenging of
BC in mixed-phase clouds. See text for details.
b Simulation with in-cloud scavenging of BC by WBF, parameterized
by temperature, in mixed-phase clouds. See text for details.
cSimulation with in-cloud scavenging of BC by WBF, parameterized
by ice mass fraction, in mixed-phase clouds. See text for details.
Figure 1 shows nine remote sites with concurrent measurements of
BCrain/snow and BCair to estimate washout ratio
(black triangles in Fig. 1). BCair and BCrain/snow
were measured at Cape Hedo (26.9∘ N, 128.3∘ E,;0.06 m) in
the East China Sea during 2011–2013. BCair was measured with an
integration time of 1 min using a filter-based absorption photometer.
The accuracy of this measurement has been estimated to be about 10 %
based on the consistency of the measured BC concentration by three methods,
including a filter-based absorption photometer, thermal–optical transmittance
method and single-particle soot photometer (Mori et al., 2014; Kondo et
al., 2011). BCrain/snow was measured with a system based on an
ultrasonic nebulizer, with an overall accuracy of about 25 % (Mori et
al., 2014). BCair and BCrain/snow were measured
concurrently in Europe at two rural background sites – Aveiro
(40.5∘ N, 8.6∘ W, 0.05 km) and K-puszta, Hungary
(47∘ N, 19.5∘ E, 0.2 km) – and two mountain sites,
Schauinsland, German (47.9∘ N, 7.9∘ E, 1.2 km) and
Sonnblick, Austria (47∘ N, 13.4∘ E, 3.1 km) in
2002–2004 (Cerqueira et al., 2010). Sampling of rain and snow mainly focused
on major precipitation events in order to collect large volumes over
short-time periods. Samples were collected on an event basis with a stainless
steel funnel connected to a pre-cleaned glass bottle. In order to minimize
dry deposition of particles, the collector was deployed when rain started to
fall and was removed immediately after filling or at the end of the event.
BCrain/snow was measured using the thermal–optical method described by
Castro et al. (1999). Weekly air samples corresponding to the precipitation
period were taken and BCair was determined by thermal–optical
method with the NIOSH protocol (Pio et al., 2007).
At site Changbai, China (42.5∘ N, 128.5∘ E;
0.74 km); LAVO, California (40.5∘ N, 121∘ W;
1.73 km); Abisko, Sweden (68.3∘ N, 18.8∘ E;
0.35 km); and Zeppelin, Norway (79.0∘ N, 12.0∘ E;
0.47 km), observations were taken only in spring or winter (Table 2).
At Changbai, snow samples were collected once per week during three winters
in 2009–2012 (Z. W. Wang et al., 2014). BCsnow was measured
using the thermal–optical method with IMPROVE protocol and BCair
was determined using a particle soot absorption photometer (PSAP). At LAVO,
seven precipitation samples were collected in March 2006 using an automated
rain sampler EcoTech with up to 95 % capture efficiency (Hadley et
al., 2010). BCrain was measured by a modified version of
thermo-optical analysis described in detail in Hadley et al. (2008).
BCair was measured by seven-wavelength Aethalometer with an
overall uncertainty of about ±30 %.
At Abisko, snow samples were taken in March and April in 1984 (Noone and
Clark, 1988). Snow samples were taken using a plastic spatula to scrape fresh
snow into polyethylene jars and then transported back to the laboratory. The snow
was transferred to a filtration apparatus, where it was melted and filtered.
The amount of BC on the filters was determined by optical analysis. The air
samples were measured using the integrating sandwich technique. At Zeppelin,
BCsnow and BCair were measured concurrently in April
and May 2007. BCsnow was concentrated by Nuclepore filters and
then determined using a multiwavelength spectrophotometer. Aerosol absorption
is measured by PSAP and BCair is computed using a mass absorption
cross section of 11 m2g-1 at 550 nm (Hegg et
al., 2011).
Model description and simulationsModel description
GEOS-Chem is a 3-D global chemical transport model driven with assimilated
meteorology from the Goddard Earth Observing System (GEOS) of the NASA Global
Modeling and Assimilation Office (GMAO). The GEOS-5 reanalysis meteorological
dataset is used to drive model simulations at
2∘latitude×2.5∘longitude horizontal
resolution with 47 vertical layers. BC aerosols are emitted by incomplete
combustion of fossil fuel, biofuel and biomass. Global anthropogenic
emissions from Bond et al. (2007) are used with Asian emissions from Zhang et
al. (2009). Previously missed gas flaring emissions are also included in this
study (Stohl et al., 2013; the flaring emission inventory is available at
http://eclipse.nilu.no/). Biomass burning emissions are from GFED3
emission inventory, with a small fire contribution included (Randerson et
al., 2012). About 80 % of the freshly emitted BC aerosols are assumed to
be hydrophobic (Park et al., 2003) and are converted to hydrophilic with an
e-folding time of 1.15 days, which yields a good simulation of BC export
efficiency in continental outflow (Park et al., 2005). Dry deposition of BC
is computed using a resistance-in-series method over all surface types
(Wesely, 1989; Zhang et al., 2001). Due to the lack of land surface module in
GEOS-Chem, we approximate BCsnow using BC deposition flux and
snow precipitation rate, following He et al. (2014a). More details are
provided in Qi et al. (2017).
Wet scavenging
Aerosol wet deposition in GEOS-Chem was first described by Liu et al. (2001).
It includes in-cloud and below-cloud scavenging in large-scale and convective
precipitation. In-cloud scavenging rate is parameterized following Giorgi and
Chameides (1986),
ϕ=-λ[BC]condensed,
where ϕ is in-cloud scavenging rate, λ the removal frequency
determined by precipitation forming rate, and
[BC]condensed BC mass mixing ratio in condensed phase,
including cloud water drops and ice crystals.
[BC]condensed is estimated as
[BC]condensed=[BC]total⋅rscav.,
where [BC]total is BC mass mixing ratio in clouds,
including BC in interstitial air and in condensed phase, and
rscav. the BC scavenging efficiency. In GEOS-Chem, it is assumed
that hydrophilic BC particles are 100 % incorporated in condensed phases,
while hydrophobic BC particles remain in interstitial air in warm liquid
clouds (Wang et al., 2011). rscav. is thus the fraction of
hydrophilic to total BC, which is determined by the initial fraction when
aerosols are emitted and the following aging process during transport. In ice
clouds, hydrophobic BC can serve as ice nuclei (Andreae and Rosenfeld, 2008),
and the resulting rscav. is the fraction of hydrophobic BC to
total BC.
In convective mixed-phase clouds, rapid updrafts bring water vapor to the
middle and upper parts of the clouds and the resulting environmental vapor
pressure is usually above the saturation vapor pressure of water. In this
condition, both water and ice grow and the WBF process is suppressed (Liu et
al., 2011). We assume no WBF effect in convective mixed-phase clouds. In
large-scale mixed-phase clouds, cloud microphysics, which determines the
rates of riming versus WBF, play a very important role in determining BC
scavenging efficiency. If the riming rate is much larger than WBF rate
(riming-dominated), most snow particles are formed from riming and show rimed
structures. BC particles in water drops are removed efficiently from the
atmosphere. In contrast, if the rate of WBF is much larger than riming rate
(WBF-dominated), most snow particles are formed from WBF and show pure
crystal structure. BC particles in cold water drops are released back into
the interstitial air and their removal is strongly slowed down. In the
control experiment, riming-only (default configuration of GEOS-Chem), it is
assumed that all snow particles are formed by riming process in mixed-phase
clouds, and rscav. is treated the same as that in warm liquid
clouds, which is determined solely by the hygroscopicity of BC (Table 4). In
experiments WBFT and WBFIMF, we distinguish riming-
versus WBF-dominated conditions and parameterize rscav. under
these two conditions. Following Fukuta and Takahashi (1999), we assume riming
dominates the in-cloud scavenging in large-scale mixed-phase clouds when
temperature is between 261 and 265 K and
LWC > 1.0 gm-3 because the terminal velocity of snow
particles was largest at 263 K and large LWC provided more water
drops for the falling snow particles to collect along their pathways based on
lab experiments. In this condition, hydrophilic BC particles in water drops
are brought to the surface by the rimed snow particles and removed from the
atmosphere, so the scavenging efficiency is simply the fraction of
hydrophilic to total BC. We assume that WBF dominates under other conditions
(258–261 and 265–273 K) in large-scale mixed-phase clouds and
rscav. follows observations from Cozic et al. (2007). In
experiment WBFT, rscav. is exponentially related to
temperature (Table 4, Cozic et al. 2007).
rscav.=0.03+0.661+exp(-T+9.32)6.77
In experiment WBFIMF, rscav. is computed using IMF
(Table 4; Cozic et al., 2007).
rscav.=0.05+0.92⋅exp(-8.95IMF)
Although the above two parameterizations of the WBF effect include the
determining factors of WBF rate, other variables that strongly affect the
local WBF rate are missing, such as local updraft velocity, local vapor
pressure, and distribution of cold water drops and ice crystals in mixed-phase
clouds. In a follow-up study, we couple a cloud-resolving model
with detailed cloud microphysics to GEOS-Chem to estimate the rate of WBF and
riming and to further investigate their roles in determining global BC
distribution.
Results and discussions
The primary goal of this study is to assess the impact of WBF on global BC
distribution. In this section, we compare BC distribution from GEOS-Chem with
and without WBF (Sect. 3.2). The differences can then be attributed to the
WBF effect. We present the comparison of BC scavenging efficiency in
Sect. 4.1. In Sect. 4.2, we show how WBF affects BCair. Following
this, we present the comparison of BC wet deposition fluxes (Sect. 4.3) and
BCsnow (Sect. 4.4). Finally, we show the effect of WBF on the BC
washout ratio.
BC scavenging efficiencyComparison to observations
WBF improves the simulation of BC scavenging efficiency at sites where
mixed-phase clouds are frequent and WBF dominates in-cloud scavenging, for
example Jungfraujoch and Puy de Dôme (Cozic et al., 2007) (Table 1). At
Jungfraujoch, WBF reduces BC scavenging efficiency both in summer
(July–August, from 0.90 to 0.48–0.59) and in late winter and early spring
(February–March, from 0.29 to 0.10–0.11) and significantly reduces
model–observation discrepancies (50 to -20–0 % in summer and from a
factor of 3 to 10 % in late winter and early spring). At Puy de
Dôme, WBF brings the simulated BC scavenging efficiency (0.48 for
WBFIMF and 0.63 for WBFT) within the uncertainty
range of observations (0.43±0.17).
However, at sites where riming dominates in-cloud scavenging in mixed-phase
clouds, for instance Zeppelin (Hegg et al., 2011), accounting for WBF leads
to scavenging efficiencies considerably lower than observations (Table 1).
Riming-only reproduces the observed high scavenging efficiencies (0.81 in
summer and 0.77 in winter) at Zeppelin to within 50 %. Similarly, at Mt.
Sonnblick, an elevated site (3.10 km), the simulated scavenging
efficiency with riming-only (0.67) agrees with the observed values (0.74±0.19) within 10 % in April and May. WBF strongly reduces BC scavenging
efficiency (0.09–0.26) at the site.
At lower altitudes, where temperature is higher and mixed-phase clouds are
less frequent, WBF has a relatively weak effect, for example, at the Po Valley and
Great Dun Fell (Table 1). At the Po Valley, the measurements were in fog. We use
BC scavenging efficiency of the lowest clouds in GEOS-Chem for comparison.
All three model results – riming-only, WBFT, and
WBFIMF – agree with the observations (0.39) to within
20–60 %. At Great Dun Fell, WBF reduces BC scavenging efficiency by
less than 25 %.
To sum up, differentiating riming- versus WBF-dominated in-cloud scavenging
in mixed-phase clouds improves the comparison at sites where WBF dominates
but degrades the comparison at sites where riming dominates. We attribute the
discrepancy to several reasons. First, WBF is parameterized based on
observations at a single site (Sect. 3.2); extrapolating it to global scale
may introduce large uncertainties. Second, LWC, a key parameter that
separates the two conditions (Sect. 3.2), is biased high and associated with
large spatial discrepancies in GEOS-5 reanalysis (Li et al., 2012; Barahona
et al., 2014). Third, this separation is based on a lab experiment (Fukuta nd
Takahashi, 1999), while conditions in the real atmosphere are certainly more
complex. This calls for more extensive measurements of BC scavenging
efficiency in mixed-phase clouds to better understand the scavenging
processes.
GEOS-Chem-simulated monthly mean BC scavenging efficiency in the
Arctic (60–90∘ N), mid-latitudes (20–60∘ N) and the
tropics (0–20∘ N) in the boundary layer (0–2 km), the
lower troposphere (2–5 km) and the middle to upper troposphere
(5–10 km). Results are averages for 2007–2009.
In addition to the uncertainties in differentiating riming- versus
WBF-dominated in-cloud scavenging in mixed-phase clouds, uncertainties
associated with other processes that determine the hygroscopicity, size and
composition of BC particles also affect scavenging efficiency. Aged BC
particles (e.g., coated by hydrophilic species) with higher hygroscopicity
and larger size are more likely to be activated and serve as CCN (Wyslouzil
et al., 1994; Weingartner et al., 1997; R. Zhang et al., 2008), and the
scavenging efficiency is considerably higher than freshly emitted BC
particles. Sellegri et al. (2003) reported scavenging efficiencies of
0.39±0.16 for BC aerosols with diameters less than 0.3 µm
and hydrophilic material fractions less than 38 %. The scavenging
efficiency increased to 0.97±0.02 for particles with a diameter larger
than 0.3 µm and the fraction of hydrophilic material at 57 %
or higher. In this study, we assume 80 % of freshly emitted BC particles
are hydrophobic and externally mixed with co-emitted hydrophilic particles
(Cooke et al., 1999). However, field observations show that the fraction
systematically differs among urban plumes (∼ 10 %) and biomass
burning plumes (∼ 70 %) (Schwarz et al., 2008). The simple
assumption of 80 % hydrophobic BC for all sources thus carries
uncertainties for BC scavenging efficiency. Moreover, we assume hydrophobic
BC particles are converted to hydrophilic with an e-folding time of
1.15 days (Park et al., 2005). However, the conversion is much faster (a few
hours) in source regions where the concentration of hydrophilic materials is
high, while the conversion is much slower in remote regions (a few days) (He
et al., 2016). Therefore, the uniform conversion rate used in this study
might underestimate the scavenging efficiency near source regions. In
addition, faster conversion from hydrophobic to hydrophilic near sources
might cause more hydrophilic BC particles to be scavenged near sources and
thus alter the scavenging efficiency at remote regions. In addition, we
assume all hydrophobic particles serve as ice nuclei. This simplification
might also involve uncertainties in BC scavenging efficiency. First, current
field observations and lab experiments show contradictory result for the ice
nucleation ability of BC particles (Murray et al., 2012). Kamphus et
al. (2010) found that soot particles were not enhanced in the ice phase
compared to the background aerosol, while Cozic et al. (2008) found that the
black carbon mass fraction was enhanced from 5 % in the background to
27 % in ice residues. Gorbunov et al. (2001) found that hydrophilic soot
was 3–4 orders of magnitude more efficient at producing ice, while other
studies (e.g., Andreae and Rosenfeld, 2008, and references therein) found
that the ability of heterogeneous ice formation of pure hydrophobic soot
particles is reduced by the presence of organic materials or sulfuric acid.
Second, ice nucleation on soot particles is complex because soot particles
from different combustion sources have different ice nucleating abilities
(Murray et al., 2012, and references therein).
Seasonal variations of BC scavenging efficiency
Figure 2 shows model-simulated monthly mean BC scavenging efficiencies in the
Arctic, the northern mid-latitudes, and the tropics at 0–2, 2–5, and
5-10 km altitudes. The values are averaged for 2007–2009. BC
scavenging efficiencies in the Arctic show strong seasonal cycle below
5 km. If only the riming process in mixed-phase clouds is considered
(experiment riming-only), BC scavenging efficiency is determined exclusively
by its hygroscopicity (Wang et al., 2011). We find that more than 90 % of
BC particles in the Arctic are hydrophilic. In warm and mixed-phase clouds,
hydrophilic BC particles serve as CCN and are incorporated in cloud water
drops, while hydrophobic BC particles remain in the atmosphere (Wang et
al., 2011). Figure 2 shows that in the middle and lower troposphere
(< 5 km), where most clouds are warm and mixed-phase in summer, BC
scavenging efficiency is approximately the ratio of hydrophilic to total BC
(0.80–0.90). In ice clouds, hydrophobic BC particles serve as ice nuclei and
are removed with the falling snowflakes, while scavenging of hydrophilic BC
particles is suppressed completely (Wang et al., 2011). Consequently, when
ice clouds dominate in most of the wintertime, BC scavenging efficiency is
around the ratio of hydrophobic to total BC (∼ 0.10). WBF reduces BC
scavenging efficiency by 22–69 % in summer and by 63–85 % in
winter. In the upper troposphere (>5 km), where ice
clouds dominate year-round, BC scavenging efficiency likewise is around the
ratio of hydrophobic to total BC (∼ 0.1) and shows little to no
seasonal variation.
In the northern mid-latitudes, the seasonal cycle of BC scavenging efficiency is
weaker than that in the Arctic – the value in winter is much higher
(0.4–0.6) in the mid-latitudes as a result of higher temperature and lower
frequency of pure ice clouds (Zhang et al., 2010). WBF reduces BC scavenging
efficiency by 17–44 % in winter in the troposphere. The effect is
relatively weaker than that in the Arctic (63–85 % reduction). In
addition, the WBF effect increases with increasing altitude (from 0 at the
surface to 39–50 % in the upper troposphere), different from that in the
Arctic.
In the tropics, the seasonal cycle of BC scavenging efficiency disappears in
the lower troposphere in all three model experiments, for two reasons. First,
temperature is high throughout the year and clouds are mostly warm clouds.
Second, most of the tropical clouds are convective where strong updrafts
suppress WBF by bringing abundant water vapor to the clouds (Liu et
al., 2011). However, in the tropical upper troposphere, WBF reduces BC
scavenging efficiency by 33–47 % because the frequency of mixed-phase
clouds is higher than that in the middle and lower troposphere.
(a) Probability density function of observed (red line) and
GEOS-Chem-simulated (dotted: riming; dashed: WBFT; solid black:
WBFIMF) annual mean BC concentration in air
(µgm-3) and (b) observed and simulated annual BC
concentrations (black: riming; blue: WBFT; red:
WBFIMF; the percentage of annual BC concentrations locate outside
the 1:2 and 2:1 lines are in parentheses; solid line – 1:1 ratio line,
dashed lines – 1:2 (or 2:1) ratio lines). Data are for 2007–2009. See
text for details.
BC concentration in air
GEOS-Chem captures the probability density function (PDF) of annual
BCair at sites from IMPROVE and EMEP and in China and the Arctic
(Sect. 2.2) but overestimates the frequency of low BCair
(experiment riming-only) (Fig. 3a). WBF releases BC in cloud water droplets
back to the interstitial air and thus reduces BC scavenging efficiency and
leaves more BC particles in the atmosphere (Sect. 4.1). As such, including
the WBF effect increases BCair and improves the agreement with
observations compared with the control experiment, riming-only, particularly
for the low values (Fig. 3a). WBF reduces the fraction of simulated
BCair that is underestimated by more than a factor of 2 (from
47 to 28–35 %) (Fig. 3b). We use a ratio r to quantify the effect of
WBF on BCair,
r=[BC]WBF-[BC]riming-only[BC]riming-only,
where r is a fraction that describes the simulated changes in
BCair with WBF parameterizations relative to that with
riming-only, and [BC]WBF and [BC]riming-only are
simulated BCair with and without WBF at the measurement stations
(Sect. 2.2). The fraction r is much larger in the Arctic (62–140 %)
than in the northern mid-latitudes (0–40 %) (Fig. 4a) for several
reasons. First, the frequency of mixed-phase clouds is higher in the Arctic
(41–90 % from spring to fall) than in the mid-latitudes
(∼ 20 %) (Pinto 1998; Shupe et al., 2006; Zhang et al., 2010;
Morrison et al., 2012). Second, lower temperature and higher IMF in the
Arctic result in a stronger WBF effect. Third, WBF increases
BCair in the mid-latitudes and consequently the poleward
transport of BC. In addition, WBF increases BCair substantially
in winter and spring and hence delays the transition of high
BCair in winter to low BCair in summer (Qi et
al., 2017).
GESO-Chem-simulated fractional change to BC concentration, relative
to riming-only, as a result of WBF,
([BC]WBF- [BC]riming) / [BC]riming
that varies with (a) altitude and (b) latitude, averaged
for 2007–2009.
We find that r increases with increasing altitude from surface
(6–12 %) to ∼ 4 km (45–95 %) (Fig. 4b). This is
because as altitude increases, temperature decreases and IMF increases,
resulting in a stronger WBF effect and thus larger reductions of BC
scavenging efficiency in the mid-latitudes (Fig. 2). As a result, less BC is
scavenged and more BC particles remain in the atmosphere. Figure 5 shows
IMPROVE and GEOS-Chem-simulated monthly mean BCair. In summer,
the model underestimates BCair by 46–72 %. WBF increases
BCair and reduces the discrepancy to 35–58 % (by
5–55 %) from the surface to ∼ 4 km. The relative change in
BCair increases from surface (6–22 %) to above 3 km
(21–78 %). The largest discrepancy (54–58 %) is at
1.5–3 km, where the influence of fire emissions is significantly
underestimated (Mao et al., 2011, 2014). BCair is strongly
underestimated in winter as well and the discrepancy increases monotonically
with increasing altitude from ∼ 10 % at the surface to
∼ 70 % above 2.5 km in winter. WBF increases
BCair monotonically from 5 % at the surface to 80–156 %
above 2.5 km, reducing the discrepancy to within 30 %,
particularly at higher altitudes. Above 2.5 km, the discrepancy of
BCair decreases from 67–70 to 15–20 %. Cloud observations
show not much riming or graupel snow particles and simulations over Montana
and Nebraska in October–November suggest that rate of WBF is significantly
larger than that of riming (Smith et al., 2009; Niu et al., 2008). WBF has
little effect on BCair at sites in the lower troposphere in East
Asia and Europe, where temperature is high and mixed-phase clouds rarely
occur.
Observed and simulated precipitation (cm) and BC wet deposition
fluxes (mgm-2yr-1).
Precipitation (cm) BC wet deposition fluxes (mgm-2yr-1) ObservationsGEOS-5ObservationsModel Riming-onlyWBFTWBFIMFContinentalSchauinsland(47.9∘ N, 7.9∘ E; 1.20 km)157.4813821.220.418.3Sonnblick(47∘ N, 13.4∘ E; 3.11 km)208.2104.1106.15.55.1K-puszta(47∘ N, 19.5∘ E; 0.12 km)59.543.89.531.129.526.1Changbai(42.5∘ N, 128.5∘ E; 0.74 km)10.417.729.353.829.718.1Cape HedoCostal(26.9∘ N, 128.3∘ E; 0.06 km)198.5138.252.559.161.264.1andAzoresoceanic(38.5∘ N, 27.3∘ W; 0 km)113.257.654.65.77Aveiro(40.5∘ N, 8.6∘ W; 0.05 km)72.951.37.59.810.511.2TropicsSakaerat(14.5∘ N, 101.9∘ E; 0.04 km)119.4226.717.787.18786.9BC wet deposition fluxes
Table 3 shows observed and GEOS-Chem-simulated annual BC wet deposition
fluxes. GEOS-Chem captures the high deposition flux at Cape Hedo in the East
China Sea (52.5 mgm-2yr-1) and the low deposition flux
(5.0 mgm-2yr-1) at the Azores (within 40 %). Cape Hedo
receives outflow of East Asia (Mori et al., 2014), while the Azores is mainly
affected by clean marine air (Cerqueira et al., 2010). Wet deposition fluxes
at Schauinsland and Sonnblick are underestimated by ∼ 50 %. One
possible reason is the underestimated precipitation at the two sites. In
contrast, at K-puszta and Sakaerat, BC wet deposition fluxes are
overestimated by a factor of 2–5. At K-puszta, BC in precipitation is
overestimated, while BCair is underestimated (Sect. 4.4),
indicating that wet scavenging is too strong during transit to the site. At
Sakaerat, wet deposition is overestimated by a factor of 5.
IMPROVE-observed (red solid line) and GEOS-Chem-simulated (dotted:
riming-only; dashed: WBFT; solid black: WBFIMF)
seasonal variation in monthly mean BC concentrations (µgm-3)
for 2007–2009. Also shown are standard deviations of observations (error
bars).
(a) Probability density function of observed (solid red
line) and GEOS-Chem-simulated (dotted: riming-only; dashed: WBFT;
solid black: WBFIMF) BC in snow (ngg-1) and
(b) medians of observed and simulated BC in snow (ngg-1)
in the Arctic, North America (Canada, the Great Plains, the Pacific
Northwest, and the Rockies, as defined by Doherty et al., 2014)), northern
China (Inner Mongolia, Northeast Border and Northeast Industrial, as defined
by Wang et al., 2013), and Xinjiang, China. The regions are symbol-coded and
the simulations are color-coded (see text for details). Solid line – 1:1
ratio line; dashed lines – 1:2 (or 2:1) ratio lines.
WBF has opposite effects on BC wet deposition fluxes near source regions over
land and in remote regions over ocean. Over land, WBF reduces annual wet
deposition fluxes by ∼ 15 % at Schauinsland, Sonnblick and
K-puszta. This is because of reduced BC scavenging efficiency (5–45 %).
The largest effect of WBF is at Changbai, where WBF reduces BC wet
deposition flux (November–April) by 45–66 % (from 53.8 to
18.1–29.7 mgm-2), reducing the discrepancy from +84 to
-40–0 %. In contrast, WBF increases wet deposition fluxes at oceanic
sites Cape Hedo and the Azores and costal site Aveiro by 8–50 %, even with a
lower local scavenging efficiency (7–20 % reduction at the oceanic
sites). We find that the increase in wet deposition fluxes is mainly from
enhanced outflow from polluted land regions as a result of WBF. In the
tropics, WBF has a minimal effect on wet deposition flux (< 1 %), for
example at Sakaerat, because temperature at the site is above freezing
throughout the year and mixed-phase clouds are very rare.
GEOS-Chem simulations of global BC distribution.
ExperimentsBC scavenging efficiency in large-scale mixed-phase cloudsRiming-only (control)Same as that in warm clouds, rscav.=BChydrophylicBCtotal.WBFTSmaller value of rscav.=0.003+0.661+exp(-T+9.32)6.77 and rscav.=BChydrophylicBCtotalWBFIMFSmaller value of rscav.=0.05+0.92⋅exp(-8.95IMF) and rscav.=BChydrophylicBCtotal
GEOS-Chem-simulated BC deposition fluxes
( mgm-2month-1) in the Arctic (September–April), North
America (November–February), northern China (November–February) and western
China (November–February), averaged for 2007–2009.
a The minimum and maximum deposition fluxes.
b The deposition flux difference (WBF – riming-only) relative
to that from the riming-only simulation.
BC concentration in snow
Figure 6a presents the PDF of observed and GEOS-Chem-simulated
BCsnow in the Northern Hemisphere (Sect. 2.3). Observed
BCsnow shows a lognormal distribution and varies by 3 orders
of magnitude from a minimum of 1.8 ngg-1 in the Arctic to a
maximum of 4758 ngg-1 in northern China. The model (experiment
riming-only) underestimates the frequency of BCsnow at
8–80 ngg-1, while it overestimates the frequency outside the
range. More importantly, the observations have a single maximum but the model
shows a bimodal structure. WBF significantly improves the agreement between
observed and simulated distribution by increasing the frequency of
BCsnow at 8–80 ngg-1, resulting in a single
maximum. WBF decreases median BCsnow by ∼ 15 % (from
25.7 to 22.4–22.7 ngg-1) and improves the comparison with
observations (median: 19.1 ngg-1).
Figure 6b shows observed and simulated medians of BCsnow in the
nine subregions as defined in Sect. 2.3. Overall, GEOS-Chem captures the
spatial distribution of BCsnow from lowest in the Arctic to
highest in the Northeast Industrial region in northern China but
overestimates BCsnow in the mid-latitudes (by up to a factor of
3) and underestimates BCsnow in the Arctic (by 27 %). WBF
reduces BCsnow by 16–33 % in the mid-latitudes (discrepancy
reduced to within a factor of 2), while it increases BCsnow by
∼ 30 % in the Arctic (discrepancy reduced to within 15 %). The
improvements are due to the redistribution of BC deposition as a result of
WBF. WBF reduces BC deposition fluxes (by 12–34 %) in North America,
northern China, and Xinjiang, China, while it increases the flux in remote
Arctic by (7–21 %) (Table 5). In the mid-latitudes, WBF reduces BC wet
deposition fluxes (by 37–63 %), while it increases dry deposition fluxes
(by 3–16 %). This is because BCair in the boundary layer is
increased with the WBF effect (Sect. 4.2). The higher dry deposition flux
partly offsets the lower wet deposition, resulting in a reduction of
12–34 % in the total BC deposition flux. In the Arctic, BC wet
deposition flux decreases by 21–29 %, while dry deposition flux
increases substantially by 81–159 %, much larger than that in the
mid-latitude regions. As a result, the total deposition flux in the Arctic
increases by ∼ 20 %.
Even with the WBF effect, BCsnow is still overestimated across
much of the mid-latitudes. This indicates that BC scavenging over East Asia
and North America is likely overestimated in the model during snow season.
The exception is in Inner Mongolia and the Northern Industrial region in
China. In Inner Mongolia, snow samples were mixed with local soil and the
measurements of BCsnow were associated with very large
uncertainties (Wang et al., 2013). In addition, most of the snow samples in
this region were taken from thick drifted snow layers; therefore,
BCsnow does not correspond to BC deposition. In the Northern
Industrial region (median: 856 ngg-1, significantly larger than
the global median of 19 ngg-1), BC deposition is strongly
affected by emissions from local sources and dry deposition flux. At
Changbai, for instance, WBF significantly improves the simulation of wet
deposition flux (discrepancy lowered from +80 to -40–0 %, Table 3).
However, dry deposition flux at the site is underestimated by a factor of
5. Thus, the underestimate of BCsnow (by 34 %) in the
region is likely because of the excessively low BC dry deposition.
Observed and simulated BC washout ratio, BC concentration in surface
air and in snow at Cape Hedo in the East China Sea, Aveiro and K-puszta (rural
sites), and Schauinsland and Sonnblick (elevated sites).
Washout ratio of BC
Hegg et al. (2011) reported that now particles mostly showed rimed structures
at Zeppelin, resulting in high washout ratios (∼ 770, Table 2). Model-simulated washout ratio with riming-only (experiment riming-only) is in
agreement with observations to within a factor of 2 at Zeppelin. When snow
particles are pristine crystal formed mainly from WBF, for example, at Abisko
(Noone and Clark, 1988), the observed washout ratios tend to be significantly
lower (94 at Abisko and 145 at Changbai, Table 2). Modeled WBF reduces
the washout ratio by a factor of 5 (from 482 to 96) and significantly
lowers the discrepancy (from a factor of 4 to 2 %) at Abisko. WBF also
drastically reduces the discrepancies at LAVO (from factors of 3–5 to 2).
Global annual budget of BC. Ranges are given in
parentheses.
Model EmissionsDeposition Mass loading LifetimeReferences(Tgyr-1)(Tgyr-1) (day)DryWetTotal> 5 km(mgm-2)(%)Riming-only8.51.56.90.22215.7GOES-ChemWBFT8.51.66.80.29256.9WBFIMF8.51.76.70.35278.0WBT248–273 K8.51.66.80.30277.0This study258–268 K8.51.66.90.30256.7WBFIMF248–273 K8.51.76.70.36298.0258–268 K8.51.66.80.32267.0AeroCom I 6.3––0.25 (0.16–0.38)21 (18–41)7.3 (4.9–11.4)Schulz et al. (2006)AeroCom II 6.6–10.6––0.14 (0.07–0.31)––Myhre et al. (2013)AeroCom (median) 17––∼ 0.50–6.1Bond et al. (2013)GEOS-Chem 6.5––0.0894.2Q. Wang et al. (2014)GEOS-Chem 6.9––0.16124.4X. Wang et al. (2014)GEOS-Chem 10.8––0.2574.2He et al. (2016)
Figure 7 shows observed and GEOS-Chem-simulated monthly mean BC washout
ratios, BCrain/snow and BCair at four mountainous
sites in Europe and at Cape Hedo. We use only simulations when daily mean
precipitation is above the monthly median to compute monthly means, because
samples of BC in rain/snow were collected during major rain/snow events
(Cerqueira et al., 2010). At Sonnblick (3.1 km), a site that is
constantly in the free troposphere, washout ratios are overestimated by
orders of magnitude. This is because BCrain/snow is
overestimated,
while BCair is underestimated. WBF significantly reduces the
discrepancy of washout ratios, particularly in winter (discrepancy lowered
from factors of 4–16 to less that 4). The improvements are because WBF
reduces BCrain/snow (discrepancy reduced from a factor of 7
to a factor of 4) and increases BCair (discrepancy decreases
from -77 to -51 %). Remarkable improvement of washout ratio
simulation is also seen at Schauinsland (1.2 km). WBF lowers the
discrepancy of washout ratio in winter and spring from a factor of 2 to
∼ 20 %. However, this improvement is because of decreased BC in
snow, which degrades the comparison with observations. WBF does not affect
washout ratios at the three sea-level sites Aveiro (0.47 km),
K-puszta (0.19 km), and Cape Hedo (0.06 km). That is because
cloud processes have a rather limited effect on BC at the surface (Sect. 4.2).
Even with the WBF effect, BC washout ratios are still largely overestimated,
because BCair is underestimated and BCrain/snow is
overestimated, particularly in summer. These overestimates suggest that wet
deposition is likely too strong over Europe.
Global BC budget
Compared to AERONET observations of BC AAOD mainly over land, AeroCom models
(I and II) underestimate BC loading by 60–160 % (in South and Southeast
Asia it is a factor of 3 and 4) (Bond et al., 2013). Bond et
al. (2013) attributed the low bias to insufficient BC emissions. They then
scaled BC emissions up according to the discrepancy of modeled versus
observed BC AAOD and obtained a total global BC emission of
17 Tgyr-1, twice the median value used in the AeroCom models.
They reported BC loading of ∼ 0.50 mgm-2 after scaling
(Table 6). Our results suggest that the discrepancy can be partially
explained by WBF not being accounted for in the AeroCom models. WBF increases
global BC loading by 0.07–0.13 mgm-2 (by 32–60 %),
depending on the WBF parameterizations used (based upon either temperature or
IMF) (Table 6). Such increases are comparable to the median global BC loading
from the AeroCom II models (Myhre et al., 2013). With WBF, our results show
global BC loadings of 0.29–0.35 mgm-2, which is in remarkable
agreement with the AERONET-based estimates (with scaled-up BC AAOD) as
reported by Bond et al. (2013).
However, we find that, even with WBF, model results still have large biases
over land, with BCair biased low (Figs. 3 and 5),
BCsnow biased high (Fig. 6), and washout ratios too large
(Fig. 7). These remaining discrepancies likely point toward excessive wet
scavenging over land in the model. In North America, for instance,
model-simulated BCsnow is too high by ∼ 50 % (Fig. 6)
and BCair in winter (IMPROVE, Fig. 5) is low by up to
∼ 30 %. Additionally, model-simulated washout ratio at LAVO,
California, is twice the observed value. In Europe, model-simulated washout
ratios, particularly in summer, are excessively high, a result of overly high
BCrain/snow and too low BCair predicted by the model
(Fig. 7).
In contrast, compared to HIPPO over the remote Pacific, AeroCom models
overestimate BCair by a factor of 2 to 5 (Schwarz et al., 2010).
To narrow the gap between model results and HIPPO observations, previous
studies resorted to either enhancing wet scavenging or accelerating BC aging
near source regions. For example, Q. Wang et al. (2014) included scavenging
of hydrophobic BC in convective updrafts and hydrophilic BC in cold clouds
(< 258 K) by homogeneous freezing of solution droplets, neither of
which was accounted for previously (Q. Wang et al., 2014). X. Wang et
al. (2014) and He et al. (2016) used faster BC aging schemes, which led to
stronger wet scavenging close to source regions and consequently weaker
outflow from these regions. The BC loadings were estimated to be
0.08 mgm-2 (Q. Wang et al., 2014), 0.16 mgm-2
(X. Wang et al., 2014) and 0.25 mgm-2 (He et al., 2016), much
lower than those constrained from AERONET measurements
(∼ 0.50 mgm-2; Bond et al., 2013).
However, even with a faster aging scheme and stronger wet deposition,
simulated BCair is still biased high by a factor of 2–3 relative
to HIPPO observations over the remote Pacific. The remaining high bias is
likely a result of either excessive Asian outflow of BC or insufficient
scavenging of BC over the Pacific. At Cape Hedo in the East China Sea (directly
downwind of major sources in eastern China), model-simulated
BCrain is an order of magnitude too high, while BCair
is 50 % too large in October–January (Fig. 7). This overestimate of
wintertime outflow of BC from the region is likely the reason for the
overestimate of BCair over the Pacific in winter. Outside of
winter, simulated BCair and BCrain at the site both
agree with observations (within 50 %, Fig. 7). This suggests that the
overestimate of BCair over the Pacific is likely the result of
insufficient removal over the Pacific.
WBF results in more BC particles in the upper troposphere (Table 6). As a
result, there is a significantly higher fraction of BC loading above 5 km
altitude (from 21 to 25–29 %). This larger fraction, as expected,
enhances the top-of-the-atmosphere absorption forcing efficiencies (forcing per
aerosol absorption optical depth; Bond et al., 2013) because of larger solar
fluxes at higher altitudes (Bond et al., 2013; Samset and Myhre, 2011). The
aforementioned fraction (25–29 %) falls in the range of AeroCom I model
results (Schulz et al., 2006) but is 3 times higher than those
constrained by HIPPO observations (9–12 %) (Q. Wang et al., 2014;
X. Wang et al., 2014). Moreover, WBF increases BC lifetime from 5.7 to
6.9–8.0 days, an increase of up to 40 %. These longer lifetimes fall
within the range of the AeroCom I model results (4.9–11.4 days) but at the
higher end (Schulz et al., 2006). However, these lifetimes are nearly twice
as long as those constrained by HIPPO observations (Q. Wang et al., 2014;
X. Wang et al., 2014).
The temperature threshold for mixed-phase clouds and ice clouds is very
uncertain and controlled by processes such as the shattering of isolated
drops during freezing and the production of ice splinters during riming (Gayet et
al., 2009; Browse et al., 2012), which are not explicitly accounted for in
GEOS-Chem. To examine the sensitivity of BC distribution to various threshold
temperatures, we conduct additional simulations. In the standard simulation,
clouds are assumed to be mixed-phase at 258–273 K. In the
sensitivity studies, we vary the threshold between 268 and 248 K. The
results are summarized in Table 6. The resulting BC deposition, loading, and
lifetime are within 15 % of the standard simulation. This suggests that
our results are rather insensitive to the threshold temperature.
Conclusions and implications
We used GEOS-Chem (version 9-01-03) driven by GEOS-5 assimilated
meteorological fields to investigate the effects of WBF on global BC
distribution. Specifically, we evaluated model simulations against
observations of BC scavenging efficiencies, BCair, BC deposition
fluxes, BCrain/snow, and BC washout ratios. We distinguished
riming- from WBF-dominated conditions in mixed-phase clouds based upon
temperature and liquid water content following a lab experiment from Fukuta
and Takahashi (1999). We then related the WBF effect based on either
temperature or ice mass fraction following Cozic et al. (2007).
The model reproduced the observed low scavenging efficiencies of BC near
source regions and high scavenging efficiencies in remote regions. WBF
lowered BC scavenging efficiencies at all altitudes and significantly
improved the simulations at higher altitudes (such as Jungfraujoch,
3.85 km, and Puy de Dôme, 1.47 km). On average, in
northern mid-latitudes, WBF reduced BC scavenging efficiency by 17–44 %
in winter, depending on the WBF parameterizations used. In summer, the effect
increased with increasing altitude (from 0 at the surface to 39–50 % in the
upper troposphere). Across the Arctic, WBF reduced BC scavenging efficiency
by 22–69 % in summer and 63–85 % in winter in the lower and middle
troposphere. As a result, WBF increased BCair (< 4 km)
globally and halved the discrepancy (from -65 to -30 %). The
improvements were larger for sites at higher altitudes and latitudes.
The model captured the observed large BC wet deposition flux
(52.5 mgm-2yr-1) at Cape Hedo in the East China Sea and
comparatively low value (5.0 mgm-2yr-1) at the Azores in the
central Atlantic. WBF resulted in lower wet deposition fluxes near source
regions over land (by ∼ 15 %) but higher wet deposition fluxes over
remote oceans (by ∼ 30 %). In addition, WBF lowered BC deposition
fluxes (by 12–34 %) at mid-latitudes, while it enhanced the fluxes (by
2–19 %) in the Arctic. The former was because of the strong reduction of
BC wet deposition fluxes (by 37–63 %) and the latter was from the strong
enhancement of dry deposition fluxes (by 81–159 %). Thus,
BCsnow decreased (by 15 %) in mid-latitudes and increased (by
26 %) in the Arctic as a result of the WBF effect. Overall, WBF lowered
the discrepancy of BCsnow from 35 to 17 %, indicating that
WBF explained a large fraction of the high bias of the model results. In
addition, WBF significantly lowered the discrepancies of washout ratios of BC
in winter from a factor of 16 to 4. In summer, washout ratios were
overestimated by orders of magnitudes. WBF corrected a fraction of the
biases. Reasons for the large overestimate warrant further investigation.
WBF increased global BC loading by 60 % (from 0.22 to
0.35 mgm-2yr-1) and partially explained the low biases of
AAOD from the AeroCom models (Bond et al., 2013). In addition, WBF increased
the fraction of BC loading above 5 km (from 21 to 25–29 %) and
hence a larger absorption forcing efficiency of BC. BC lifetime was longer by
40 % (from 5.7 to 8.0 days) from the WBF effect.
On average, model simulations of BC scavenging efficiencies,
BCair, deposition fluxes, BCrain/snow, and washout
ratios improved significantly. However, the comparisons degraded at
riming-dominated sites, for example, Zeppelin. These results suggest that
more observations are needed to better differentiate WBF- versus
riming-dominated scavenging of BC. In addition, measurements of BC scavenging
efficiencies in mixed-phase clouds at different latitudes and altitudes
should be conducted, especially over the oceans, where there are scarce
measurements of BC scavenging efficiency. Finally, WBF and riming are
subgrid-scale processes that strongly depend on local variables, such as
local updraft velocity, local vapor pressure, distribution of cold water
drops and ice crystals in mixed-phase clouds and so on. Coupling a
cloud-resolving model with detailed cloud microphysics is necessary to better
estimate the rate of WBF and riming and to better identify their roles in
global BC distribution.
The data used in this study are available from the
corresponding author upon request
(qiling@atmos.ucla.edu).
The authors declare that they have no conflict of
interest.
Acknowledgements
This study was funded by NASA grant NNX14AF11G from the Atmospheric Chemistry
Modeling and Analysis Program (ACMAP). The GEOS-5 data used in this
study/project were provided by the Global Modeling and Assimilation Office
(GMAO) at NASA Goddard Space Flight Center. The authors thank Joshua P.
Schwarz, Betty Croft, Hongyu Liu for helpful discussions. We thank the two
reviewers for their constructive comments.
Edited by: Barbara Ervens
Reviewed by: two anonymous referees
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