Introduction
Arctic temperatures have increased more rapidly than the global
average during recent decades (Shindell and Faluvegi, 2009). While
increases in long-lived greenhouse gases certainly play a leading role
in Arctic warming, short-lived climate pollutants (SLCPs), such as
aerosols and tropospheric ozone, also have a substantial influence on
Arctic climate (Shindell, 2007; Quinn et al., 2008; Sand et al.,
2016). Black carbon (BC) has particularly attracted interest due to
its large influences on radiative forcing in the Arctic (AMAP,
2015). BC causes a heating in the atmosphere by absorbing solar
radiation, which is more efficient in the Arctic because of the high
surface albedo of snow and ice (Quinn et al., 2007). In addition,
the deposition of BC on snow and ice reduces the surface albedo and
results in faster-melting snow and ice sheets in the Arctic (Hansen
and Nazarenko, 2004; Flanner et al., 2007). Enhanced aerosol
concentrations can also increase cloud longwave emissivity and lead to
surface warming in the Arctic (Lubin and Vogelmann, 2006; Garrett and
Zhao, 2006). In the Arctic, air pollution and climate change are
strongly linked, and reductions in the concentrations of SLCPs could
contribute to mitigating Arctic warming (Quinn et al., 2008;
Arnold et al., 2016).
Aerosols in the Arctic show a distinct seasonal variation with
a maximum during winter and early spring and a minimum in summer
(Barrie, 1986). Arctic air pollution, including high concentrations of
aerosols and reactive gases (so-called Arctic haze), primarily
originates from anthropogenic pollutants transported from the northern
midlatitudes (Law and Stohl, 2007). The seasonal variation in
Arctic air pollution is caused by the enhanced transport of pollutants
from the midlatitudes, inefficient removal processes in winter and
spring, and increased wet scavenging during summer (Law and Stohl,
2007; Garrett et al., 2011).
Previous studies using chemical transport models (CTMs) and chemical climate
models (CCMs) revealed that these models had difficulty in reproducing the
seasonal variations in aerosols in the Arctic (Shindell et al., 2008; Koch
et al., 2009; Lee et al., 2013). Most models underestimated the concentration
levels of BC in the peak season, and the model-to-model differences were also
quite large (Shindell et al., 2008). This is caused by uncertainties in the
model treatments of transformation from hydrophobic to hydrophilic BC and
removal processes during long-range transport from source regions to the
Arctic. The seasonal variation in simulated BC in the Arctic is particularly
sensitive to parameterizations of BC aging (Liu et al., 2011; Lund and
Berntsen, 2012; He et al., 2016) and wet scavenging processes (Liu et al.,
2011; Bourgeois and Bey, 2011; Browse et al., 2012; Qi et al., 2017a, b).
This is consistent with observational analyses by Garrett et al. (2011) who
suggested that the wet scavenging process was dominant in determining the
seasonal variations in light absorption and light-scattering aerosols in the
Arctic. Although a recent model intercomparison study indicated that the
model performance of the BC simulations in the Arctic has improved, the
seasonal amplitude at the surface was too weak and the BC concentration
levels at the surface sites were still underestimated in the Arctic haze
season in many state-of-the-science models (Eckhardt et al., 2015). Mahmood
et al. (2016) pointed out that convective wet deposition outside the Arctic
influenced vertical distribution and seasonal variations in BC in the Arctic
by analyzing the same models used by Eckhardt et al. (2015). These
difficulties in the model simulation of Arctic BC are key uncertainties
in calculating the source contributions from important emission sources in
the northern midlatitudes and high latitudes.
In addition to the model representations of BC aging and removal
processes, it has been recently reported that missing emission sources
in the high latitudes significantly contribute to the underestimation
of simulated BC in the Arctic (Stohl et al., 2013; Huang et al.,
2015). Stohl et al. (2013) estimated that gas flaring in Russia that
is not treated in most inventories contributes 42 % to the annual
mean BC concentrations near the surface in the Arctic. Huang
et al. (2015) also showed that newly developed BC emissions for Russia,
which includes emissions from gas flaring, improved the model biases of
BC at the surface sites in the Arctic.
Previous efforts to investigate the source regions of BC in the Arctic were
made using a Lagrangian trajectory model (Stohl, 2006; Hirdman et al., 2010)
and chemical transport models (Koch and Hansen, 2005; Shindell et al., 2008;
Huang et al., 2010; Bourgeois and Bey, 2011; Wang et al., 2011, 2014; Sharma
et al., 2013; Qi et al., 2017c). These previous studies revealed that major
BC sources transported to the Arctic were anthropogenic emissions in Europe,
Russia, Asia, and North America. However, the relative importance among these
source regions is still rather uncertain or even contradictory because the
estimated contributions to Arctic BC vary in earlier studies (Wang
et al., 2014). For instance, while Lagrangian trajectory model analyses
suggested that northern Eurasia was the major source of BC near the surface
in the Arctic (Stohl, 2006; Hirdman et al., 2010), Koch and Hansen (2005)
estimated that the degree of the contribution from South and East Asia was
similar to that from Europe and Russia during winter and spring. In the
middle troposphere over the Arctic, some studies suggested that the
contributions from Europe and/or Russia were larger than or comparable to
those from Asia (Shindell et al., 2008; Huang et al., 2010; Sharma et al.,
2013), but other studies indicated that the contribution from Asia was
dominant (Koch and Hansen, 2005; Wang et al., 2011, 2014). This highlights
the need for and importance of a mechanistic understanding of transport pathways
and wet removal processes during long-range transport from individual major
source regions to the Arctic.
Previous studies have also reported that biomass burning emissions from
boreal forests in Siberia and North America and agricultural fires in Europe
have substantial influences on Arctic BC, especially from late spring to
summer (Stohl et al., 2006, 2007; Warneke et al., 2010; Matsui et al., 2011).
Stohl (2006) suggested that the contribution from Siberian forest fires to
the Arctic was greater than that from anthropogenic sources during summer.
Matsui et al. (2011) indicated that the biomass burning emissions in Russia
had the most important contributions of BC in the North American Arctic in
spring 2008, when severe fires occurred in Siberia. Emissions from fires in
boreal forests may increase under the future warm climate (Stocks et al.,
1998). Thus, it is important to investigate the contribution from biomass
burning emissions at relatively high latitudes to Arctic BC.
In this study, we investigated the long-range transport of BC from
various source regions and origins to the Arctic using a global
chemical transport model, GEOS-Chem, with a tagged tracer simulation for
the past 5 years (2007–2011). The tagged tracer method was used to
analyze detailed transport pathways and transport efficiencies of BC
from individual sources to the Arctic. We identified an important
geographic region where the inflow of BC from major source regions
into the Arctic occurred. This analysis also provides us with an
interpretation of the seasonal variation in Arctic BC and useful
diagnostics of the model performance to understand the possible causes
of model biases. We also quantitatively estimated the contributions of
emissions from various sources to BC concentrations and depositions in
the Arctic region.
Model description
We used GEOS-Chem version 9-02 as a global chemical transport
model (Bey et al., 2001). GEOS-Chem is driven by assimilated
meteorological data from the Goddard Earth Observing System (GEOS-5)
provided by the NASA Global Modeling and Assimilation Office
(GMAO). The model used a horizontal resolution of 2∘×2.5∘ with 47 vertical layers from the surface to
10 hPa. The dry deposition process in GEOS-Chem adopts
a standard resistance-in-series scheme as implemented by Wang
et al. (1998). Over snow and ice, BC dry deposition velocity is set
to 0.03 cm-1 to improve aerosol concentrations at the
Arctic surface sites as described in Fisher et al. (2011) and Wang
et al. (2011).
Emission inventories
For anthropogenic emissions of BC, GEOS-Chem originally used an
inventory from Bond et al. (2007) for 2000. Wang et al. (2011) indicated
that emissions in Asia and Russia were required to be doubled to
match them with observed BC over the Arctic. This doubling was done to
account for the emission increases since 2000 in Russia and China
(Wang et al., 2011). In this study, we adopted the BC emissions of
HTAP v2.2 (Janssens-Maenhout et al., 2015), which were developed for
the experiments of HTAP phase 2 for anthropogenic emissions. The
target year of HTAP v2.2 was 2010, and global annual emissions were
estimated to be 5.5 Tgyr-1, which was about 22 %
higher than those in Bond et al. (2007; 4.5 Tgyr-1). On
a regional basis, the emissions from China were 40 % higher than
those in Bond et al. (2007), and the emissions from Europe and North
America were 34 and 11 % lower than those in Bond et al. (2007),
respectively. As argued in recent studies, BC emissions from Russia
may be underestimated due to missing sources, such as gas flaring, and
have a significant impact on Arctic BC (Stohl et al., 2013; Huang
et al., 2015). Annual BC emissions in Russia were estimated to be
224 Ggyr-1 in Huang et al. (2015), which was about 2.5
times larger than those of HTAP v2.2. Our preliminary simulations found
that the model result replacing HTAP v2.2 emissions in Russia with the
inventory of Huang et al. (2015) improved the reproducibility of the
observed BC concentrations at the Arctic sites (see the Supplement, Fig. S1), and
thus we used this emission data set as the anthropogenic BC emissions
for Russia. For biomass burning emissions, we used GFED (Global Fire
Emissions Database) v3.1 with 0.5∘×0.5∘
of spatial resolution and daily temporal resolution (van der Werf et al.,
2010). In GFED v3.1 the BC emissions from biomass burning were globally
estimated to be 1.9 Tgyr-1 averaged for 2007–2011.
BC aging and wet scavenging schemes
In the standard GEOS-Chem model, 80 % of BC is initially emitted as
hydrophobic BC and then converted to hydrophilic BC with a constant
e-folding time of 1.15 day (Park et al., 2005). However, it is unknown
whether it is appropriate to adopt a constant value for the entire
atmosphere. Because this value was estimated from observations of
continental outflow near the source regions in the midlatitudes (Park
et al., 2005), it may be overestimated, especially in remote regions,
including the high latitudes. In this study, we implemented
a parameterization of BC aging developed by Liu et al. (2011) into
GEOS-Chem and tested this impact on BC concentrations over the
Arctic. This parameterization derives a timescale of BC aging based
on the number concentration of OH radical (Liu et al., 2011). In
remote areas, including the high latitudes, the aging time is expected
to be longer than in the midlatitudes near the source regions,
resulting in an increase in BC concentrations. Liu et al. (2011)
showed that the simulated seasonal variations at Arctic sites were
improved by implementing this parameterization due to the increases in
the BC concentrations during winter and spring.
Annual emissions of BC from (a) anthropogenic and
(b) biomass burning sources for the year 2010 and
2007–2011, respectively, and source regions for BC tracer tagging.
Wet scavenging processes are also important to simulate BC in the
Arctic region. The wet scavenging scheme for aerosols in GEOS-Chem is
originally described by Liu et al. (2001). Wang et al. (2011)
implemented several improvements for wet scavenging to distinguish
between liquid and ice clouds for in-cloud scavenging (rainout) by
comparing it with ARCTAS aircraft measurements over the Arctic. In
liquid clouds (T≥258 K), hydrophilic aerosols are assumed
to be incorporated in the cloud droplets. In the case of ice clouds
(T<258 K), the model assumes that hydrophobic BC can serve
as ice nuclei. However, the scavenging of BC by ice clouds is highly
uncertain (Wang et al., 2011). The assumption of 100 % of
hydrophobic BC can lead to an overestimation of BC scavenging in ice
clouds. We conducted a sensitivity simulation in which the scavenging
rate of hydrophobic BC was reduced to 5 % of water-soluble
aerosols for liquid clouds by following earlier model studies (Bourgeois
and Bey, 2011). We found that the reduced scavenging rate by ice
clouds improved the model reproducibility of BC at the Arctic sites in
winter and spring, as will be discussed in detail below.
BC tracer tagging by sources and regions
In the tagged tracer simulations, we distinguished the BC tracers by
source types (i.e., anthropogenic and biomass burning) and
regions. The horizontal definitions of source regions are shown
in Fig. 1. For the tagging of anthropogenic (AN) BC,
we divided the global domain into 16 regions (Fig. 1a). We separated
Europe, Russia, Asia, and North America to examine transport patterns
and contributions to the Arctic from the major source regions. Asia
was separated into three regions (i.e., East Asia, Southeast Asia, and
South Asia). East Asia was further divided into four regions: Japan,
the Korean Peninsula, North China, and South China. For biomass burning
(BB) emissions, we separated the model domain into 27 regions
(Fig. 1b). For boreal forests, Siberia was separated into six regions
based on vegetation types, and North America was divided into Alaska,
West Canada, and East Canada in addition to the United States.
We performed the tagged simulation for 5 years from 2007 to 2011
after a model spin-up of 6 months. The model simulation was
conducted as an offline aerosol simulation and used an improved wet
scavenging and aging process. The monthly average OH distributions for
the calculation of BC aging time were stored by the full-chemistry
simulation for each year.
To examine the role of wet removal during transport for each tagged BC
tracer, we estimated the wet scavenging ratio of BC. Using the wet
scavenging ratio, we discuss the differences in transport efficiency
among source regions and the roles of wet removal processes for the
seasonal variations in BC concentrations. We conducted an additional
simulation in which the wet scavenging processes were off, and thus BC
was removed from the atmosphere only by dry deposition at the
surface. The wet scavenging ratio of each BC tracer was estimated as
follows:
wet scavenging ratio (%)=(Cwetoff-Cctl)/Cctl×100,
where Cctl and Cwetoff are 6-hourly BC
concentrations of the control run and the simulation in which the wet
removal processes are off, respectively.
Observed (black squares) and modeled (blue solid line for
standard scheme and red solid line for new scheme) seasonal
variations in BC mass concentrations at the Arctic sites. The plots
are monthly means and the error bars are standard deviations of
interannual variations. Measurements are averaged for 2007–2011 at
Barrow, Alert and Zeppelin, and for 2010–2014 at Tiksi. R and
RMSE indicate correlation coefficient and root mean square error,
respectively. The unit of RMSE is ngm-3.
Results and discussion
Model–observation comparison
The BC mass concentrations simulated by GEOS-Chem were compared with
measurements of equivalent BC (EBC) converted from aerosol light absorption
at four Arctic sites: Barrow, Alaska (156.6∘ W, 71.3∘ N;
11 ma.s.l.), Alert, Canada (62.3∘ W, 82.5∘ N;
210 ma.s.l.), Zeppelin, Norway (11.9∘ E, 78.9∘ N;
478 ma.s.l.), and Tiksi, Russia (128.9∘ E,
71.6∘ N; 8 ma.s.l.). Aerosol light absorption is observed
by particle soot absorption photometers (PSAPs) at Barrow, Alert, and
Zeppelin and by an Aethalometer at Tiksi. The measurement data at the Arctic
sites were obtained from the EMEP and WDCA database (http://ebas.nilu.no).
EBC is calculated from the particle light absorption coefficient with an
assumption of a mass absorption efficiency. In this study, the measured light
absorption coefficients with PSAPs have been converted to EBC mass
concentrations using the mass absorption efficiency of
10 m2g-1 (Bond and Bergstrom, 2006). The conversion to EBC
was internally performed by the Aethalometer for Tiksi.
Figure 2 shows the seasonal variations in BC concentrations simulated
with the GEOS-Chem standard scheme and our new scheme in comparison to
the observations at the Arctic sites. The observed seasonal variations
in BC at the Arctic surface sites show a maximum during winter and
early spring (i.e., Arctic haze season) and a minimum in summer. This
observed seasonal feature was relatively well simulated with the
standard scheme at the semi-quantitative level (the correlation
coefficients between the modeled and the observed BC (R) were
0.69–0.94). The new scheme yielded an increase in BC concentrations
with maximum effects in winter at all four Arctic sites. This is
consistent with the results of Liu et al. (2011) and Bourgeois and Bey (2011). By introducing the aging parameterization of Liu
et al. (2011), the lifetime of BC was increased due to a slower timescale of aging in the high latitudes. Reducing the wet scavenging
ratio by ice clouds also increased the lifetime of BC in the cold
season. The sensitivities by changing these parameterizations were
largest in winter because wet removal by ice clouds was the most important
in this season and the aging timescale, which depends on OH number
concentrations, also became longer than other seasons. The standard
scheme underestimated observed BC in winter and spring at Alert and
Tiksi. The model negative biases were reduced by the new scheme in
these seasons, and R values were improved from 0.89 to 0.92 at Alert
and from 0.935 to 0.944 at Tiksi (Fig. 2). At Barrow, while the new
simulation improved the negative biases in spring, the observed
concentrations were overestimated during winter. As a result, the
correlation coefficient was increased from 0.69 to 0.81, but root mean
square error (RMSE) was not improved by the new scheme at
Barrow. Whilst there was an improvement at Alert and Tiksi, the
observations at Zeppelin showed reasonably good agreement with the
standard simulation (R=0.89) rather than the new simulation (R=0.83). The new scheme yielded nearly double BC concentrations in
winter, while the observed BC concentrations were somewhat lower than
those at the other three sites. The sensitivities of aging and wet
removal by ice cloud processes at Zeppelin were larger than those at
Barrow and Alert, leading to the overestimation of the new scheme in
winter and spring. Previous model studies also showed similar
tendencies with larger BC concentrations in the European Arctic (i.e.,
at Zeppelin) than those in the North American Arctic (i.e., at Barrow
and Alert; Sharma et al., 2013; Stohl et al., 2013; AMAP, 2015). It
should be noted that the mass absorption efficiency used for the
conversion from the particle absorption coefficients to the EBC
concentrations has an uncertainty of at least a factor of 2 (AMAP,
2015).
We further compared the vertical profiles of BC concentrations over
the Arctic with the observations made during the Arctic Research of the
Composition of the Troposphere from Aircraft and Satellites
(ARCTAS) campaign in April 2008 (Fig. 3). Since the ARCTAS
aircraft campaign covered mainly the North American Arctic, the
observations made in the area north of 66∘ N were used. The
dates of flights used for the comparison were 8, 9, 12, 16, and
17 April. The model results by the standard and the new schemes were
analyzed at the grid closest to the locations and times of the
observations. The observed and simulated BC concentrations were
averaged for 1 km altitude intervals from the surface to
10 km of altitude. The observed vertical profile showed a maximum
in the middle troposphere at 5 km of altitude. Although the
standard scheme reproduced the increase from near the surface to the
middle troposphere and the decrease from 5 km to the upper
troposphere, the observed concentrations were underestimated by
24–42 % in the middle troposphere. The negative biases were
improved by the new scheme by increasing BC concentrations to
18–23 ngm-3 in the middle troposphere. These increases
by the new scheme were caused by the longer lifetime of BC in the high
latitudes as discussed above. Although the new scheme slightly
underestimated the observed BC concentrations from 3 to 7 km
of altitude, the model successfully captured the observed mean vertical
profile, including the peak in the middle troposphere and the
concentration level near the surface. The simulated vertical gradient
from the surface to the middle troposphere was slightly weaker than
that of the observations. One possible reason is that the upward transport
of BC was underestimated by the model.
Mean vertical distributions of observed and simulated (blue
solid line for standard scheme and red solid line for new scheme) BC
over the region of the ARCTAS aircraft campaign in April 2008. Black
squares and colored circles represent the median values. The error
bars indicate the 25th and 75th percentiles.
Scatterplots of annual mean BC concentrations modeled and
observed at the surface sites in North America, Europe, and East
Asia. Locations of the surface sites used for the comparisons
(right).
Distributions of seasonal mean concentrations (color) and
horizontal fluxes (arrows) at 1 km of altitude for selected
tagged BC tracers in winter (DJF), spring (MAM), and summer (JJA):
EUR-AN, RUS-AN, EAS-AN, and NAM-AN. Wet scavenging ratios are also
shown by solid lines. White lines indicate the source regions of BC
tracers.
In addition to the Arctic region, we compared the model results with
measurements in the major anthropogenic source regions: East Asia,
Europe, and North America. For East Asia, we used BC data at nine
rural and remote sites in China during 2006 and 2007 from Zhang
et al. (2012). In addition, we used measurements at Fukue Island,
a remote site located in western Japan (Kanaya et al., 2016). For
North America, the data from the IMPROVE network for 2007–2011 were
used (http://views.cira.colostate.edu/fed). In this study, we selected 43 IMPROVE sites located above
1500 m of altitude for comparison. For Europe, we used
measurements at 12 sites by EUSAAR (European Supersites for
Atmospheric Aerosol Research) for 2007–2011. The measurement data at
EUSAAR sites were obtained from the EMEP and WDCA database
(http://ebas.nilu.no). Figure 4 shows the scatterplot of the
annual mean BC concentrations simulated by the model with the
standard and new schemes in comparison to the observations in
these three regions. The normalized mean bias (NMB) for East Asia was
-42 %, mainly because the model largely underestimated the
observations at two sites located in western China. Without these two
sites, the NMB for East Asia was improved to -19 %. For
North America, the simulated concentration levels were in good
agreement with the observations (NMB=-6 %). For
Europe, the model tended to underestimate the observations
(NMB=-33 %). The possible reasons for the
underestimations over East Asia and Europe are that BC emissions from
these regions are underestimated and removals are overestimated by
the model around the source regions. The differences between the
standard and new schemes were small in all three regions
(Fig. 4). This is because the BC aging time by the new scheme is similar
to that of the standard scheme (∼1 day) around the source
regions in the midlatitudes, and wet scavenging by ice clouds is not
so important in these regions. Because the BC concentrations tended
to slightly increase in the new simulation, NMBs were improved by the
new scheme from -14–-43 % to -6–-42 %
(Fig. 4). Overall, these model-to-observations comparisons showed
that our model simulations with the new scheme reasonably reproduced
the observed BC levels, horizontal and vertical distributions, and
spatial and temporal variabilities, thus demonstrating the model
capability to examine the long-range transport of BC to the Arctic
and its underlying physical and chemical mechanisms.
BC transport from anthropogenic sources to the Arctic
Figure 5 shows the horizontal distributions of tagged BC tracers for
major anthropogenic sources (ANs) and their fluxes at about
1 km of altitude in winter (DJF), spring (MAM), and summer
(JJA). The horizontal fluxes were calculated by multiplying 6-hourly
BC mass concentrations by horizontal wind speeds and were averaged for
3 months. East Asia (EAS-AN) was defined as the sum of Japan
(JPN-AN), the Korean Peninsula (KOR-AN), North China (NCH-AN), and
South China (SCH-AN). North America (NAM-AN) was defined by adding
Alaska and Canada (ALC-AN) to NAM-AN. BC originating from Russia
(RUS-AN) is widely distributed over the Arctic during winter and has
a large contribution (30–100 ngm-3) over almost the
entire Arctic region. The RUS-AN contribution showed a maximum in
central Siberia, which is a large source region of gas flaring
(Fig. 1; Huang et al., 2015). Northeastward winds prevailing over
western Russia and central Siberia (30–90∘ E) in winter
probably played an important role in the transport of Russian BC to
the Arctic (Fig. 7). Low precipitation in the cold season over Russia
also contributed to the effective transport to the Arctic due to
inefficient wet scavenging. Figure 7 shows that the precipitation
level was less than 1 mmday-1 over a large part of Russia
during winter. Horizontal distributions of the wet scavenging ratio are
also shown in Fig. 5. The wet scavenging ratio of RUS-AN was lower
than the other source regions, especially during winter. The
meteorological conditions in Russia during the cold season are
characterized by low precipitation and cold temperatures at the
surface. These meteorological conditions lead to ineffective removal
and hence effective transport from Russia to the Arctic in winter and
spring. In summer, the transport from RUS-AN to the Arctic was much
weaker than in the other seasons (Fig. 5). This is because
precipitation increased (1–4 mmday-1) over Russia,
leading to effective removal, and the circulation pattern also changed
to the southeastward winds at 30–90∘ E during summer
(Fig. 7). The seasonal variation in the large-scale circulation
pattern is caused by the intensified Siberian high during winter and
its replacement by low pressure in summer (Stohl, 2006). Strong
northeastward fluxes from Europe (EUR-AN) were seen at 1 km
of altitude in winter and spring. BC originating from EUR-AN was enhanced
over the European Arctic during winter (20–50 ngm-3) and
spring. The transport from Europe to the Arctic was also attributed to
northeastward winds blowing over northern Europe in the cold season
(Fig. 7). This result is consistent with previous studies, which showed
that high-latitude Eurasia (i.e., Russia and Europe) was an
important source region of BC at the surface in the Arctic (Stohl,
2006; Hirdman et al., 2010).
Same as Fig. 5 but for 5 km of altitude.
Distributions of seasonal mean precipitation (color) and
horizontal winds (arrows) of GEOS-5 at 1 km (a) and
5 km (b) altitudes in winter (DJF), spring (MAM), and
summer (JJA).
The horizontal fluxes of East Asian BC (EAS-AN) and North American BC
(NAM-AN) showed that long-range transport from East Asia and North
America to the Arctic was inefficient in the lower troposphere. In
winter, BC from East Asia was transported mainly southeastward by
northwesterly winds associated with the winter monsoon circulation,
which were dominant over northern China, Japan, and the northwestern Pacific
(Fig. 7). BC from EAS-AN had a contribution of
10–20 ngm-3 in the Eurasian and North American Arctic
during winter and spring. The NAM-AN contribution was estimated to be
5–10 ngm-3 in the North American Arctic during winter
and spring. The transport from EAS-AN and NAM-AN was also weak during
summer compared with the other seasons because precipitation increases
around the source regions (Fig. 7).
Longitude–height cross sections of mean net meridional fluxes
at 66∘ N for selected tagged BC tracers in winter, spring,
and summer: EUR-AN, RUS-AN, EAS-AN, and NAM-AN. Wet scavenging ratios
are also shown by solid lines.
The horizontal distributions of tagged BC tracers and their fluxes at
5 km of altitude are shown in Fig. 6, highlighting the long-range
transport of BC in the middle troposphere from individual source
regions. In the middle troposphere, BC originating from East Asia
(EAS-AN) was transported eastward and northeastward in winter and
spring. The eastward pathway from East Asia reached North America
across the North Pacific. BC from East Asia also spread northeastward
over the Sea of Okhotsk and eastern Siberia and reached the Arctic. It was
further transported eastward over the Arctic Ocean. BC from East Asia
had a contribution of 20–40 ngm-3 in the Eurasian Arctic
in winter and spring. In winter, northward winds blowing over the
Sea of Okhotsk, eastern Siberia, and the Bering Sea could play an important
role in the poleward transport of EAS-AN BC (Fig. 7). Although
seasonal mean northward winds in spring over these regions were weaker
than those in winter (Fig. 7), the contribution of East Asian BC in
spring was larger than that in winter (Fig. 5). This enhancement of
EAS-AN BC during spring was not sufficiently explained by only the
seasonal mean winds, suggesting that synoptic-scale disturbances on
shorter timescales had an important role in the poleward transport
from East Asia to the Arctic (Di Pierro et al., 2011). The patterns of
the horizontal fluxes suggested that EAS-AN BC was transported mainly
over the Sea of Okhotsk and eastern Siberia to the Arctic Ocean in winter
and spring. This transport pathway agreed with the results of Di
Pierro et al. (2011) that analyzed aerosol export events from East
Asia to the Arctic region using satellite observations. The vertical
profiles of aerosol observed by the CALIOP lidar onboard the CALIPSO
satellite showed that the pollution plumes were transported from East
Asia to the Arctic through eastern Siberia in the middle troposphere (Di
Pierro et al., 2011). The distribution of the wet scavenging ratio showed
that about 90 % of BC from East Asia was deposited before arriving
at the Arctic at 5 km of altitude during winter and spring. The
BC transport from East Asia was much weaker in summer than in
winter and spring. BC from North America (NAM-AN) was also transported
eastward and northeastward at 5 km of altitude during winter and
spring. In addition to eastward transport to Europe across the North
Atlantic, NAM-AN BC was transported from the eastern US to Greenland. The
contribution of BC from Russia (RUS-AN) in the middle troposphere was
much weaker compared with the lower troposphere, especially during
winter (Fig. 5). The stable conditions of cold temperatures near the
surface suppresses the upward transport of BC over Russia, especially
in winter. BC from Europe (EUR-AN) at 5 km of altitude was also
smaller than at 1 km of altitude.
Figure 8 shows the longitude–height distributions of the meridional
fluxes of BC from individual source regions at 66∘ N in
winter, spring, and summer. From these figures, we can identify
important regions where inflows of BC from major source regions to the
Arctic occur. Significant BC transport from EUR-AN toward the Arctic
was seen at 0–60∘ E below 2 km of altitude in winter
and spring. Transport from RUS-AN to the Arctic occurred mainly in the
lower troposphere at 30–90∘ E. During winter, low
temperatures at the surface lead to a thermally stable stratification
that reduces vertical mixing (Barrie, 1986). Due to the stable
conditions over Russia, the inflow from RUS-AN to the Arctic was
concentrated below 1 km of altitude during winter. A strong
inflow from EAS-AN to the Arctic was seen in the middle to upper
troposphere, and the low-level transport to the Arctic was weak in
contrast to EUR-AN and RUS-AN. BC from EAS-AN was uplifted during
long-range transport to the Arctic due to the large latitudinal
gradient in the potential temperature (Klonecki et al.,
2003). A strong poleward transport of EAS-AN BC occurred at
130–180∘ E at 3–8 km of altitude during
winter. Although the inflow from EAS-AN became slightly weaker than
that in winter, a similar structure to winter was also seen during
spring. This result was in good agreement with the observational study
by Di Pierro et al. (2011), which showed that the meridional transport
of aerosol originating from East Asia to the Arctic took place at
3–7 km of altitude. The Arctic lower troposphere is isolated by
the closed polar dome, which is formed by isentropic surfaces of lower
potential temperatures, and pollutants cannot easily penetrate into
the Arctic from outside of the polar front (Barrie, 1986). East Asia
is located south of the polar dome, and EAS-AN BC is emitted at
higher potential temperatures. As a result, the low-level transport of
East Asian BC into the Arctic was weak and it was transported at higher
altitudes (Klonecki et al., 2003; Stohl, 2006). In summer, BC
transport from EAS-AN to the Arctic was much weaker in the middle
troposphere and was confined in the upper troposphere. BC transport
from NAM-AN to the Arctic across 66∘ N was also seen in the
middle to upper atmosphere, and the inflow in the lower troposphere was
weak, similarly to EAS-AN. This is because North American BC is also
emitted at higher potential temperatures and was transported to the
Arctic above the polar dome. The inflow from NAM-AN to the Arctic
occurred mainly at 30–90∘ W at 3–8 km
of altitude. Pollutants exported from East Asia and North America
experience ascent transport by vertical mixing, such as warm conveyer
belts from the boundary layer to the free troposphere, and are
eventually transported to the Arctic in the middle to upper troposphere
(Klonecki et al., 2003).
Month–altitude cross sections of mean BC concentrations from
individual sources in the Arctic (66–90∘ N). Relative
contributions to total BC concentrations are also shown by solid
lines.
The distribution of the wet scavenging ratio at 66∘ N showed
that about 90 % of the EAS-AN BC was removed from the atmosphere
during long-range transport to the Arctic in winter and spring
(Fig. 8). This value is consistent with the transport efficiency
(i.e., the fraction of BC not removed during transport) from Asia
(13 %) derived from the BC/ΔCO ratio over
the Northern American Arctic observed during the ARCTAS spring
campaign (Matsui et al., 2011). The wet scavenging ratio of NAM-AN
(85–90 %) was similar to that of EAS-AN. The wet scavenging ratio
in the strong inflow regions of RUS-AN across 66∘ N
(30–90∘ E, below 1 km of altitude) was 30–50 %
during these seasons. Thus, the wet removal of the RUS-AN BC was much
less than that of EAS-AN and NAM-AN, leading to efficient transport
to the Arctic. The dry conditions with low precipitation in
high-latitude Eurasia reduce wet deposition and lead to a longer
lifetime of BC in the Arctic troposphere, especially in winter. The wet
scavenging ratio of EUR-AN BC at 66∘ N was estimated to be
40–80 % at 0–60∘ E below 2 km of altitude during
winter and spring.
Relative contributions from anthropogenic and biomass
burning emissions
Figure 9 shows the seasonal variations in the individual source
contributions averaged for the Arctic (66–90∘ N) from the
surface to 10 km of altitude. The total contribution from
anthropogenic sources other than the four major source regions
(Europe: EUR-AN, Russia: RUS-AN, East Asia: EAS-AN, and North America:
NAM-AN) was aggregated to OTH-AN. For biomass burning (BB), the
contributions from Russia (seven regions) and from Alaska and Canada
(three regions) were aggregated to SIB-BB and ALC-BB, respectively. The
total contribution from biomass burning sources other than SIB-BB and
ALC-BB was defined as OTH-BB. In Fig. 9, the relative contributions
from individual sources to the total BC concentrations are also shown.
Due to the effective transport in the lower troposphere (Fig. 5), the
contribution from RUS-AN increased from late autumn to early spring
mainly below 2 km of altitude. It was largest near the surface
and decreased with altitude in these seasons (Fig. 9). This structure
reflected a thermally stable stratification by cold temperatures at
the surface during the cold season (Klonecki et al., 2003; Stohl,
2006). RUS-AN BC had a relative contribution of 40–70 % to
Arctic BC below 1 km of altitude except during summer. The
contribution from EUR-AN also increased below 2 km of altitude in
winter and early spring, accounting for 10–20 % of the Arctic
BC. EAS-AN BC increased with altitude from the surface and had the
largest contribution at about 5 km of altitude due to strong
poleward transport in the middle troposphere (Figs. 6 and 8). The
seasonal variation in the contribution from EAS-AN showed a maximum in
early spring (March) and a minimum during summer. The relative
contribution from EAS-AN was estimated to be 30–50 % in the
middle and upper troposphere in winter and spring. The contribution
from NAM-AN showed a maximum in winter at about 5 km
of altitude. Because BC from East Asia and North America located at
relatively lower latitudes was emitted at higher potential
temperatures, it was uplifted in the middle troposphere during
long-range transport to the Arctic (Klonecki et al., 2003). OTH-AN,
which consisted mainly of the anthropogenic sources in the northern
low latitudes and the Southern Hemisphere, had a contribution in the
upper troposphere above about 8 km of altitude. In contrast to
the anthropogenic sources, the contributions of biomass burning
emissions from SIB-BB and ALC-BB increased in summer because boreal
fires in Siberia, Alaska, and Canada increased from late spring to
autumn. The relative contributions of SIB-BB and ALC-BB were estimated
to be 20–40 and 30–40 %, respectively, during summer in
the lower troposphere.
Figure 10 shows the seasonal variations in the contributions from
individual sources to BC mass concentrations near the surface and at
about 5 km of altitude averaged for the Arctic region
(66–90∘ N). The wet scavenging ratios of the anthropogenic
sources (EUR-AN, RUS-AN, EAS-AN, and NAM-AN) are also shown to
highlight the role of wet removal processes in the seasonal variations
in Arctic BC. Near the surface, RUS-AN was a dominant contributor
of 40–70 % on a monthly basis, followed by EUR-AN (10–20 %)
and EAS-AN (5–15 %) in winter, spring, and autumn. Thus, the
contributions of anthropogenic sources were remarkably larger than
those of biomass burning sources during the seasons except
summer. SIB-BB and ALC-BB had a substantial contribution of
10–40 and 30–40 %, respectively, during summer, resulting
in a larger contribution from biomass burning than from
anthropogenic sources in this season. At 5 km of altitude,
EAS-AN was the most important, accounting for 30–60 % on
a monthly basis, followed by small but substantial contributions from
EUR-AN (10–20 %), NAM-AN (10–15 %), RUS-AN (5–20 %),
and OTH-AN (10–15 %) in winter, spring, and autumn. The
contributions of SIB-BB and ALC-BB were substantial in spring
(15–20 % from SIB-BB) and summer (10–30 % from SIB-BB and
15–30 % from ALC-BB). The biomass burning contribution was
comparable to that of the anthropogenic sources in summer.
Seasonal variations in mean BC concentrations (left axis)
from individual sources (a) near the surface and
(b) at 5 km of altitude in the Arctic
(66–90∘ N). Mean wet scavenging ratios (right axis) for
major anthropogenic source regions are also shown by solid lines:
EUR-AN, RUS-AN, EAS-AN, and NAM-AN.
Near the surface, the contribution from RUS-AN showed a large seasonal
variation with a maximum during winter (∼100 ngm-3)
and a minimum in summer (∼10 ngm-3; Fig. 10). BC
originating from Russia was the most important to the Arctic BC near the
surface, except during summer, and hence had a large influence on the
seasonal variation in the total BC concentration over the Arctic. The
wet scavenging ratio of RUS-AN had a large seasonal variation from
20 % in winter to 70 % during summer. Although the wet
scavenging ratios of all four anthropogenic sources (EUR-AN, RUS-AN,
EAS-AN, and NAM-AN) decreased during winter and increase in summer, the
amplitude of RUS-AN was the greatest among these sources. In addition,
the wet scavenging ratio of RUS-AN was the lowest among the major
anthropogenic sources in all seasons, leading to a significant
contribution to Arctic BC. The seasonal variation in the
contribution from EUR-AN near the surface was similar to that of
RUS-AN (Figs. 9 and 10). EUR-AN was the most important during winter with
a contribution of ∼20 ngm-3 to the Arctic. The wet
scavenging ratio of EAS-AN was the highest among the four major
anthropogenic sources and exceeded 90 % in all seasons near the
surface.
Budgets of BC from individual sources for the period
2007–2011.
Tracera
Emissionc,
Poleward flux across
Burden in the
Deposition to the
Lifetime, days
Ggyr-1
66∘ N (v>0), Ggyr-1
Arctic, Gg
Arctic, Ggyr-1
Wet
Dry
Global
Arctic
EUR-AN
353.7 (2.6)
76.1
0.9
18.2
4.8
6.4
14.2
RUS-AN
196.8 (22.2)
103.0
1.5
26.7
15.2
9.1
12.9
EAS-ANb
1844.9 (0.0)
175.4
1.9
10.4
1.9
6.4
57.5
NAM-ANb
342.2 (0.6)
45.5
0.5
4.5
0.8
5.7
34.1
OTH-ANb
2946.9 (0.1)
110.5
1.2
4.0
0.7
7.6
92.7
SIB-BBb
114.2 (4.9)
42.5
0.5
15.5
2.3
7.9
10.1
ALC-BBb
64.0 (5.6)
27.0
0.4
12.6
2.1
6.3
8.6
OTH-BBb
1718.3 (0.0)
21.9
0.2
1.3
0.1
8.0
57.9
Total
7580.9 (35.9)
601.8
7.1
93.1
27.9
7.3
21.3
a AN and BB indicate anthropogenic and biomass
burning sources, respectively. b EAS-AN (East Asia) is the sum
of JPN-AN, KOR-AN, NCH-AN, and SCH-AN; NAM-AN (North America) is the sum of
NAM-AN, and ALC-AN; OTH-AN is the sum of anthropogenic sources other than
EUR-AN, RUS-AN, EAS-AN and NAM-AN; SIB-BB is the sum of WRU-BB, S1-BB, S2-BB,
S3-BB, S4-BB, S5-BB, and S6-BB; ALC-BB is the sum of ALC-BB, WCA-BB, and
EAC-BB; and OTH-BB is the sum of biomass burning sources other than SIB-BB
and ALC-BB. c Values in brackets denote emissions from north of
66∘ N.
In the middle troposphere (at ∼5 km of altitude), the
seasonal variation in EAS-AN BC showed an increase in spring and
a decrease during summer (Figs. 9 and 10). Due to the large
contribution of EAS-AN, the total BC concentration also showed
a maximum in spring, which was different from the seasonal variation
near the surface (winter maximum). Although the wet scavenging ratio
of EAS-AN was the largest among the major anthropogenic sources, the
contribution from EAS-AN was dominant except during summer in the
middle troposphere. This is because the BC emission of EAS-AN is much
larger than that from the other sources as discussed below. Because
EAS-AN BC was uplifted from the lower troposphere to the middle and
upper troposphere during long-range transport, its contribution was
larger in the middle troposphere than near the surface. Although the
wet scavenging ratio of NAM-AN was slightly less than that of EAS-AN,
the contribution from NAM-AN was about 10 ngm-3 in winter
and spring and was smaller than that from EAS-AN. The contribution
from RUS-AN at about 5 km of altitude was much less compared with
that near the surface, especially in winter and spring (Figs. 9 and
10). Because of the thermally stable conditions over Russia in the
cold season, the upward transport of RUS-AN BC to the middle and upper
troposphere is suppressed. The contribution of EUR-AN in the middle
troposphere was also smaller than that near the surface.
Source contributions to the annual budget of BC in the
Arctic
In Table 1, we summarize the budgets of each BC tracer averaged for
2007–2011 (see Table S1 for more detailed source regions). The
annual total amount of the poleward BC flux from East Asia (EAS-AN)
across 66∘ N, which was calculated by 6-hourly concentrations
and northward winds (v>0), was estimated to be
175.4 Ggyr-1, corresponding to about 10 % of the
total emissions (1844.9 Ggyr-1). The deposition amount
of EAS-AN BC on the Arctic region (66–90∘ N) was
12.3 Ggyr-1, which was about 1 % of the EAS-AN
emissions. Thus, a large part of the EAS-AN BC transported to the
Arctic was transported outside of the Arctic without depositing onto
the surface within the Arctic. Although the fraction of BC from East
Asia transported to the Arctic was lower than the other
anthropogenic sources (EUR-AN, RUS-AN, and NAM-AN) due to effective
wet removal (Fig. 10), the inflow flux of EAS-AN was the largest among
the four major sources. This is because the emissions of EAS-AN are
much larger than those from the other source regions (Table 1). On the
other hand, the emissions from Russia (RUS-AN;
196.8 Ggyr-1) were relatively small among the major
anthropogenic sources, but the inflow flux was the second largest
(103.0 Ggyr-1). This is due to the effective transport
from Russia to the Arctic, especially during winter and spring (Figs. 5
and 10).
The global lifetimes of BC tracers, which were defined as the burden
divided by the annual total deposition, were estimated to be
5.7–9.1 days (Table 1). The average lifetime of 7.3 days agreed with
the value of the multi-model mean in the ACCMIP project (7.4 days; Lee
et al., 2013) and with those reported by previous studies (e.g.,
7.3 days from Koch and Hansen, 2005, and 5.9 days from Wang et al.,
2011). The BC lifetimes of each tracer in the Arctic
(66–90∘ N) were estimated to be 8.6–92.7 days. The lifetime
of EAS-AN BC in the Arctic (57.5 days) was longer than that of EUR-AN
(14.2 days) and RUS-AN (12.9 days) because East Asia, BC was
distributed mainly in the middle troposphere (Fig. 9) and its
deposition to the Arctic was smaller than EUR-AN and RUS-AN
(Table 1). The average lifetime of 21.3 days in the Arctic was close
to the 20.0-day the multi-model mean in the AMAP (Arctic Monitoring
and Assessment Programme) models (Mahmood et al., 2016).
Table 2 summarizes the relative contributions from individual sources
to the annual mean BC concentrations, burden, and deposition over the
Arctic (66–90∘ N). In Table 2, the tagged BC tracers were
aggregated to five anthropogenic and three biomass burning sources. As
expected from Figs. 9 and 10, Russia (RUS-AN) was the most important
contributor to the BC concentrations at the surface, accounting for
61.8 %. Europe (EUR-AN) had the second largest contribution at the
surface (13.4 %) among the sources. The relative contribution from
East Asia (EAS-AN) was estimated to be 8.0 %. This result is
similar to previous studies, which showed that northern Eurasia (Europe
and Russia) was the dominant source region and East Asia had a smaller
contribution at the Arctic surface (Shindell et al., 2008; Hirdman
et al., 2010; Sharma et al., 2013; Wang et al., 2014). The larger
contribution from Russia than Europe in this study is consistent with
recent studies using newly developed emissions, including gas flaring
(Stohl et al., 2013; Huang et al., 2015). The contributions from
biomass burning in Siberia (SIB-BB) and Alaska and Canada (ALC-BB)
were about 5 % at the surface. Thus, the contribution of
anthropogenic emissions was dominant at the surface over the Arctic,
accounting for 90 % of the annual mean.
Relative contributions from individual sources to the
annual mean BC concentrations at the surface and 5 km of altitude,
annual deposition, and burden in the Arctic (66–90∘ N)
(%).
Tracera
Surface
5 km
Burden
Deposition
EUR-AN
13.4
12.2
12.6
19.0
RUS-AN
61.8
9.8
21.0
34.7
EAS-ANb
8.0
40.6
27.4
10.1
NAM-ANb
3.1
10.4
6.9
4.3
OTH-ANb
2.9
10.9
17.0
3.9
SIB-BBb
5.2
8.5
7.0
14.7
ALC-BBb
5.2
4.3
4.9
12.1
OTH-BBb
0.4
3.3
3.2
1.2
a AN and BB indicate anthropogenic and biomass
burning sources, respectively. b EAS-AN (East Asia) is the sum
of JPN-AN, KOR-AN, NCH-AN, and SCH-AN; NAM-AN (North America) is the sum of
NAM-AN, and ALC-AN; OTH-AN is the sum of anthropogenic sources other than
EUR-AN, RUS-AN, EAS-AN, and NAM-AN; SIB-BB is the sum of WRU-BB, S1-BB, S2-BB,
S3-BB, S4-BB, S5-BB, and S6-BB; ALC-BB is the sum of ALC-BB, WCA-BB and
EAC-BB; and OTH-BB is the sum of biomass burning sources other than SIB-BB
and ALC-BB.
In the middle troposphere (5 km of altitude), East Asia (EAS-AN)
had the largest contribution of 40.6 % to the annual mean BC
concentration over the Arctic. Among the source regions in East Asia,
North China (NCH-AN) had the most significant contribution of
29.4 % (see the Supplement, Table S2). The dominance from East Asia in the
middle troposphere is consistent with previous studies (Wang et al.,
2011, 2014). The relative contribution from RUS-AN was 9.8 % at
5 km of altitude, which was much less than that at the surface
(62 %). Thus, the main contributor to Arctic BC differed with
altitude. This is because the transport pathways from individual
sources to the Arctic are different as described before (Figs.
5–7). The transport from East Asia to the Arctic was characterized by
uplifting to the middle and upper troposphere during long-range
transport (Figs. 6 and 7). BC from Russia was transported to the
Arctic mainly in the lower troposphere due to the stable conditions
during the cold season (Figs. 5 and 7). In the context of air
pollution over the Arctic, BC from Russia and Europe is more important
due to the large contributions near the surface during the Arctic haze
season. In addition, BC in the lower troposphere effectively warms the
Arctic surface (Flanner, 2013). On the other hand, BC in the middle
troposphere is more important to radiative forcing at the top of the
atmosphere and causes atmospheric heating in the lower and middle
troposphere (Flanner, 2013). Thus, it is important to understand
altitudinally varying source contributions to Arctic BC because
the Arctic climate response is sensitive to the vertical distribution
of BC in the Arctic.
For the BC burden over the Arctic, the contribution from East Asia
(EAS-AN) was the most important and accounted for 27.4 % of the annual
mean. The second largest contributor to the BC burden over the Arctic
was Russia (21.0 %). This result is consistent with AMAP (2015),
which showed that the main contributors to the BC burden in the Arctic
were East and South Asia and Russia. Wang et al. (2014) also estimated
that East Asia and Northern Asia (consisting mainly of Russia) had the
two largest contributions of 23.4 and 22.6 %, respectively, to the
BC burden in the Arctic, which is consistent with this study. Bourgeois and Bey
(2011) showed that Siberia, Asia, and Europe had comparable
contributions to the Arctic BC burden. In this study, other
anthropogenic sources (OTH-AN) also had a significant contribution of
17.0 %. In OTH-AN, South Asia (SAS-AN) provided the most important
contribution of 8.7 % (see the Supplement, Table S2).
We also quantitatively estimated the relative contributions from each
source to the total deposition of BC to the Arctic region
(Table 2). The contribution from Russia (RUS-AN) was the largest
(34.7 %). The second largest was the contribution from EUR-AN
(19.0 %). Thus, the major sources of deposition on the Arctic
were identical to the dominant contributors to the BC concentrations
at the surface. This is similar to previous studies, which showed that
Europe and Russia provided the two largest contributions to BC
deposition to the Arctic, while East Asia contributed less to
deposition than to burden (Huang et al., 2010; Bourgeois and Bey,
2011; Sharma et al., 2013; Wang et al., 2014), although some studies
estimated a larger contribution from Europe than from Russia (Huang
et al., 2010; Sharma et al., 2013; Wang et al., 2014). The
contributions of biomass burning in Siberia (SIB-BB) and Alaska and
Canada (ALC-BB) were also important, accounting for 14.7 and
12.1 %, respectively. These values of biomass burning sources were
larger than their relative contributions to BC concentrations at the
surface (∼5 %). This is because BC deposition is enhanced
during summer due to increased precipitation, and the contributions
from SIB-BB and ALC-BB to the BC concentrations become large in this
season in contrast to the anthropogenic sources (Fig. 10).
We estimated interannual variations in relative contributions from
individual sources to Arctic BC and found that the results of each
year were similar to those of the 5-year averaged contributions (see the
Supplement, Table S3). The differences in the relative contributions from each
source to the BC concentrations between maxima and minima were lower
than 12 %. For BC total deposition, the relative contribution from
biomass burning in Siberia (SIB-BB) showed variation from 8.2 to
24.0 % (Table S3).
Conclusions
We investigated the long-range transport of BC from various source
regions and origins to the Arctic and quantified source contributions
using a global chemical transport model, GEOS-Chem, with a tagged tracer
simulation for 5 years (2007–2011). This study especially focused
on the transport pathways from the individual source regions to the
Arctic and the role of wet scavenging during long-range transport. For
tagging BC, we distinguished BC tracers by source types (anthropogenic
and biomass burning) and regions; the global domain was divided into
16 and 27 regions for anthropogenic and biomass burning emissions,
respectively.
We evaluated the simulated BC by comparing it with observations at surface
measurement sites in the Arctic and near large source regions in the northern
midlatitudes. The vertical profile of modeled BC was also compared with the
observations by the ARCTAS aircraft campaign over the Arctic. We introduced
a parameterization of BC aging into GEOS-Chem and changed the wet scavenging
ratio by ice clouds (T<258 K) to examine the sensitivities of these
processes to Arctic BC. By using these new schemes, the BC concentrations
were increased at the Arctic, especially in winter and spring. Although the
new scheme overestimated the observations at Zeppelin and Barrow, especially
during winter, model the negative biases in the cold season were improved at
Alert and Tiksi. The model also successfully reproduced the observed mean
vertical distribution of BC over the Arctic. Our simulations suggested that
there are remaining uncertainties in aging and wet scavenging processes, and
measurements are crucial to constrain the model representations of these
processes. Further model improvements of key processes, including
a microphysics-based parameterization of BC aging (Oshima and Koike, 2013; He
et al., 2016) and wet scavenging by mixed-phase clouds (Qi et al., 2017a, b),
are also important.
We examined detailed transport pathways from the individual source
regions to the Arctic and identified important regions where inflow
from the individual source regions to the Arctic occurred. Our
simulation showed that BC originating from Europe and Russia was
transported to the Arctic mainly in the lower troposphere during
winter and spring. In particular, BC transported from Russia is
extensively distributed over the Arctic in these seasons, leading to
the dominant contribution of 62 % to Arctic BC near the
surface in the annual mean. We also found that this contribution of BC
from Russia had a key role in the seasonal variation in the Arctic BC
at the surface. For the Arctic air pollution near the surface, BC
originating from anthropogenic sources in Russia and Europe was
important due to its large contributions during the Arctic haze
season.
In the middle troposphere, we found a large contribution from East
Asia to Arctic BC, which resulted from uplifting during
long-range transport. Our simulation demonstrated that BC from East
Asia was transported to the Arctic mainly through the Sea of Okhotsk and
eastern Siberia during winter and spring. We identified an important
region where a strong inflow from East Asia to the Arctic occurred
(130–180∘ E and 3–8 km of altitude at
66∘ N). The model simulation showed that the contribution
from East Asia to the Arctic had a maximum at about 5 km
of altitude in early spring. The efficiency of transport from East Asia
to the Arctic was smaller than that from other large source regions
such as Europe, Russia, and North America. However, the contribution of
East Asia was the most important to the middle troposphere (41 %) and
BC burden (27 %) over the Arctic because of large emissions from
this region. These results suggest that the main source of Arctic
BC differs with altitude.
The total contribution of anthropogenic sources to the BC
concentrations at the surface was dominant (about 90 %) compared
with that of biomass burning in the annual mean. However, for BC
deposition on the Arctic, the contributions of biomass burning
emissions from Siberia and Alaska and Canada became substantial
during summer and were important, accounting for 15 % (32 %) and
12 % (31 %) of the annual mean (during summer), respectively.