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ArticleThe Kuroshio Regulates the Air−Sea Exchange of PCBs in the
Northwestern Pacific Ocean
Min Yang, Xinyu Guo,* Miho Ishizu, and Yasumasa Miyazawa
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sı Supporting Information*

ABSTRACT: Calculating accurate air−sea fluxes for polychlorinated biphenyls (PCBs)
is an essential condition for evaluating their transport in the atmosphere. A threedimensional hydrodynamic-ecosystem-PCB coupled model was developed for thenorthwestern Pacific Ocean to assess the air−sea fluxes of four PCBs and examine theinfluences of ocean currents on the fluxes. The model revealed a fine structure in theair−sea flux that is sensitive to the Kuroshio, a western boundary current with a highsurface speed. Intense downward and upward fluxes (−23.6 to 44.75 ng m−2 d−1 for∑4PCBs) can be found in the Kuroshio region south of Japan and the Kuroshio
Extension east of Japan, respectively. In strong (weak) current regions, it takes ∼4 and∼1 days (1−3 and 3−12 days) for dissolved PCBs to reach an equilibrium in scenarios
where only air−sea exchange or only ocean advection is considered, respectively. In
strong current regions, the ocean advection has a shorter response time than the air−sea
exchange, indicating that dissolved PCBs from upstream carried by strong current caneasily change the downstream concentration by disrupting the equilibrium with original air−sea exchange and induce new air−sea
fluxes there. Therefore, strong western boundary currents should be correctly considered in future atmospheric transport models for
PCBs.
KEYWORDS: ocean model, seasonal variation, Kuroshio Extension, volatilization, western boundary currents■

INTRODUCTION

strong ocean currents on PCBs concentrations have not been
fully considered in atmospheric simulations of PCBs.
The Asia-Pacific region has the largest human populationand the largest manufacturing factories, thereby making it anarea where PCBs are used in large quantities.8 Global ocean
simulation of PCBs has estimated that approximately 30% of
the total atmospheric deposition takes place in the North
Pacific.7 However, most previous reports on PCBs havefocused on the Atlantic Ocean, Arctic Ocean, eastern Pacific
Ocean, and some coastal areas.9−11 There has likely been no
field observation of PCBs in the northwestern Pacific Ocean(NWPO) since the first report in 1989/1990.12
In the NWPO, a strong western boundary current, that is,
the Kuroshio, flows from low latitudes to midlatitudes, veers
east at 30° N, continuously flows along the southern coast ofJapan, and finally enters the open Pacific Ocean as theKuroshio Extension (KE; Figure S1 in Supporting Information). To reproduce a more realistic current, a horizontal gridsize of ∼10 km or finer is required for an ocean model.13 Some

The direction and magnitude of the air−sea diffusion flux ofpolychlorinated biphenyls (PCBs) depend on the concentration gradient between gaseous PCBs in the atmosphere anddissolved PCBs in water, which in turn depend on theprocesses influencing these concentrations.1 The oceanic
biological pump, current advection, and eddy diffusion affectthe distribution of PCBs in the surface ocean and therefore
affect the air−sea diffusion flux. The biological pump refers to
the removal of dissolved PCBs from the surface water via
phytoplankton uptake, which is favorable for increasing thedownward diffusion flux of PCBs from air to sea.2,3 Ocean
currents bring water with different concentrations of PCBs
from remote areas to a target area, thereby changing the local
concentration of PCBs. Eddy diffusion usually reduces thespatial gradient of PCBs concentrations.
An accurate air−sea flux is important for evaluating thetransport of PCBs in the atmosphere. Despite this fact, theocean is usually treated as a box module in most atmospheric
PCBs models, in which lateral transport by ocean currents is
not considered.4−6 In addition, the western boundary currents
in most oceans have widths of 100−200 km; as such, thehorizontal resolution of ∼100 km or more used in the global
ocean simulation of PCBs is too coarse to reproduce realistic
western boundary currents.7 Therefore, the effects of such
© XXXX American Chemical Society

Received: May 13, 2022
Revised: August 2, 2022
Accepted: August 3, 2022A

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ArticleFigure 1. Conceptual scheme of the coupled model for PCB simulation. CA is gaseous PCB in the atmosphere, CW is dissolved PCB in the water,
CWP is phytoplankton-bound PCB, and CWD is detritus-bound PCB. The fields in the boxes to the right are input data for PCB simulation.

regional persistent organic pollutants (POPs) models demonstrate relatively high horizontal resolution, which is a necessary
condition for resolving the fine structure in ocean
currents.10,14−16 However, a constant concentration of
atmospheric POPs is usually considered in such models,although this concentration is certainly nonhomogeneous inspace and time owing to differences in human activities.
To obtain an accurate air−sea flux, in addition to theinclusion of spatial and temporal variations in the atmosphericPOPs concentration, the ocean POPs model should also take
into consideration the influence of ocean currents on POPstransport. This requires the use of a high-resolution hydrodynamic model as the framework for ocean POPs modeling.
Furthermore, the presence of a biological pump effect on POPs
requires full coupling of an ecosystem module in the oceanPOPs model. Based on the above considerations, weestablished a fully coupled hydrodynamic-ecosystem-POPsmodel to simulate PCBs in the NWPO. We use this to examine
the seasonal variation and spatial distribution of the air−sea
diffusion flux of four selected PCB congeners and to clarify theregulatory effect of ocean currents on the air−sea diffusion flux.

■

and temporal variations in atmospheric PCB concentrations, aswell as the inclusion of wet deposition flux of atmospheric
PCBs, both of which were derived from the GEOS-Chemmodel.4 The other was the replacement of satellite-based
chlorophyll-a with ecosystem-model-based chlorophyll-a.
Because satellites only observe the surface layer, they cannot
provide a vertical profile of chlorophyll-a. In addition, theproduct of satellite chlorophyll-a is typically updated on a
weekly or monthly basis and is sometimes unavailable becauseof the influence of clouds. An ecosystem model can solve thesetwo problems.
The PCBs in our module are represented by four variables,that is, gas phase (CA) in the atmosphere, dissolved phase(C W ) in water, and a particle phase that contains
phytoplankton-bound (CWP) and detritus-bound (CWD) PCBs
in water. CA is the specified gaseous concentrations of PCBs
that enter the ocean through air−sea diffusion. CWP is the sumof PCBs adsorbed by the phytoplankton surface (CWPS) andtaken up by the phytoplankton matrix (CWPM).
The equations for PCBs in the ocean are given in Text S3,and the biochemical parameters and formulas for calculating
biological processes are presented in Text S4 and Table S1.
The primary processes are the uptake and release of PCBs by
phytoplankton, mortality of phytoplankton, sinking anddecomposition of detritus, and the degradation of the dissolved
PCBs due to photolytic and microbial processes.
Boundary Conditions for the PCB Module. In thisstudy, the atmosphere was the only source of PCBs in theocean.17 Other resources, such as the most PCBs from rivers
are adsorbed to particles and sink to the sediment near theriver mouth. A part of them can be transported to the sea shelf
but little can be transported to the open ocean. PCBs in theatmosphere enter the ocean through air−sea diffusion and wet
deposition, which were treated as the surface boundary
condition for the concentration of dissolved PCBs (CW) as
follows:

MATERIALS AND METHODS

The model domain covers the NWPO (10.5−62° N, 108−180° E, Figure S1) with a horizontal resolution of 1/12° (9.1−
4.4 km) and 47 vertical levels. PCBs were simulated using
three modules in this study. One is a hydrodynamic modulethat provides daily water temperature and current velocity inthree directions, and the horizontal and vertical eddy diffusivity
coefficients (see Text S1 in the Supporting Information). The
second module is an ecosystem module that provides daily
concentrations of phytoplankton and related biogeochemicalparameters (Text S2). The third module is a PCB module that
calculates the processes related to the PCB with specified
boundary conditions.
PCB Module. Mass exchange and chemical degradation are
the principal processes considered in this module. Mass
exchange processes include advection and diffusion, biogeochemical processes, and air−sea exchange. Figure 1
illustrates a conceptual sketch of the physicochemical processesof PCBs in the NWPO. The module has a framework similar to
the three-dimensional PCB model in the East China Sea.14
Compared with this,17 our PCB module contained two
additional improvements. One was the inclusion of spatialkh

CW= FAW + FWPz(1)

where kh (m2 s−1) is the vertical eddy diffusivity coefficient,
FAW (pg m−2 s−1) is the air−sea diffusion flux, and FWP is thewet deposition flux (pg m−2 s−1). The wet deposition fluxes(FWP) and gaseous concentrations (CA) of PCBs in theatmosphere were derived from the GEOS-Chem globalB

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ArticleFigure 2. Distribution of surface dissolved CB153 concentrations (pg L−1) (a−d), surface particulate CB153 concentrations (pg L−1) (e−h), andair−sea CB153 diffusion fluxes (pg m−2 s−1) (i−l) in January, April, July, and October. Positive air−sea flux is from air to sea while negative flux isfrom sea to air.

atmospheric model,4 which was evaluated using observations
available in the NWPO (Figure S2, Table S2, details aredescribed in Text S5). The particle phase makes up only a verysmall fraction, while >99% for all four congeners is attributed
to the gaseous diffusion,4 thus the dry deposition of the particle
phase was not considered here.
The air−sea diffusion flux was calculated by following Dachset al. (2002),
iC
FAW = KAW jjj AkHy
CW zzz{Initial and Boundary Conditions and Calculation
Scheme of the PCB Module. The PCB module was solved
offline using the hydrodynamic and ecosystem modules.
Because we only wanted to reproduce the seasonal variationin the spatial distribution of four PCB congeners in the NWPO
and examine the mechanisms controlling the air−sea PCB flux,
we limited our calculation to a climatological forcing base; that
is, all forcings were repeated every year during the model
calculations. Climatological forcing included wind speed from
NCEP/NCAR every 6 h, daily ocean current velocity, andwater temperature from the hydrodynamic module,20 daily
phytoplankton biomass from the ecosystem module,21 monthly
atmospheric PCB concentrations, and wet deposition flux of
PCBs from the GEOS-Chem global atmospheric model. The
average period for climatological values of PCBs ranged from1979 to 2010, whereas that for the other variables ranged from1993 to 2018. In the Supporting Information, we present a
comparison of atmospheric PCB concentrations from GEOSChem and available observations.
The initial values for the concentrations of the dissolved,
phytoplankton, and detritus phases of PCBs in the ocean wereset at zero. Under the above climatological forcing, the model
was integrated for 10 years, and the results of last year wereanalyzed. During the 10 years of calculation, the model resultsin the upper 200 m layer were almost the same in the ninth
and tenth years. Hereafter, this calculation is referred to as thecontrol-run. In addition to the control-run, sensitivity
simulations were conducted to explore the role of ocean
currents in the air−sea diffusion flux in the NWPO.
CB153 was reported to be the most prevalent congener inmarine mammals in the NWPO22,23 and the Northern
Atlantic.24 Because of its remarkable influence on ecosystems
and humans, CB153, with high lipophilicity and persistence,25(2)where H′ is the dimensionless Henry’s law constant that
depends on the water temperature,18 and KAW (m s−1) is theair−sea mass transfer rate that depends on the wind speed and
H′. The relevant parameters and formulas for H′ and KAW arelisted in Table S3. A positive value of FAW indicates an input of
PCBs from the air to the ocean, while a negative value
indicates the volatilization of PCBs from the ocean to the air.
In this study, we chose four PCB congeners (CB28, CB101,
CB153, and CB180) for oceanic simulation because they havesubstantial differences in physicochemical properties.19 Congeners with lighter molecular weights, such as CB28, are much
more volatile and have a longer range transport in theatmosphere, having thus a smaller spatial concentration
gradient compared to CB153, CB180. We used wind speed
that was derived every 6 h from the National Centers forEnvironmental Projection/National Center for AtmosphericResearch (NCEP/NCAR) reanalysis data to calculate the air−sea diffusion flux. At the lateral boundary, the concentrations of
the dissolved, phytoplankton, and detritus phases of PCBs
were set at zero. At the bottom boundary, there was no flux for
any PCB phase through the sea.
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pg L−1 appeared in the Okhotsk Sea in April. The distributionof particulate CB153 has a similar pattern as the phytoplankton
biomass, which was also high in spring and summer in thesubarctic region.21 The large phytoplankton biomass insummer did not contribute to a high concentration ofparticulate CB153 (Figure 2g) because the high watertemperature in summer dampens PCB absorption by thephytoplankton.34
Given the large spatial variations of CB153 in seawater, themonthly mean concentrations of surface dissolved CB153 andparticulate CB153 were averaged across different latitudes(20°, 30°, 40°, and 50° N) across the open NWPO (lines in
Figure S1). The seasonal variation of dissolved CB153 alongthe 20°, 30°, and 40° N latitudes in the open NWPO wasslight, but was much larger at 50° N, with a difference of 0.35
pg L−1 between winter and summer (Figure 3a). The

was chosen as the primary objective congener for analysis in
this study. We also present the simulation results for CB28,
CB101, and CB180 in the Supporting Information forcomparison with CB153.

■

ArticleRESULTS AND DISCUSSION

Spatiotemporal Variations of CB153 in the NWPO.
The simulated surface dissolved CB153 concentration in the
NWPO had a mean value of 0.15 pg L−1 with a range of 0−
2.16 pg L−1. The surface dissolved CB153 concentrations
reported in previous studies had a range of 0.01 to 2.76 pg L−1
in the Northern Hemisphere surface ocean.9,26−29 The
dissolved PCBs concentrations of the ∑3PCBs (CB28,
CB101, CB153; 0−4.69 pg L−1) in December in the NWPO
were lower than those in the central Pacific (3.3−8.3 pg L−1),
which was attributed to the higher atmospheric PCBs
concentration in the central Pacific (Table S4).
The simulated surface particulate concentrations for theindividual CB153 and the total four PCB congeners(∑4PCBs) in the NWPO surface had ranges of 0−0.88 pg
L−1 and 0−2.27 pg L−1, respectively. The concentrations werecomparable to those observed from the North Atlantic30 (0.42
and 0.72 pg L−1), the North Sea31 (0.3−9.8 and 1.4−41.5 pgL−1), and the Baltic Sea32 (0.1−1 and 0.6−4.3 pg L−1) (Table
S5). From the above comparison, we can conclude that the
dissolved and particulate concentrations of PCBs on the
NWPO surface given by the ocean module are within the range
of known observations.
Figure 2 shows the distribution of surface-dissolved andparticulate CB153 concentrations in the NWPO. The
dissolved CB153 concentration displayed a general northward-increasing pattern, except for the highest value found incoastal waters around Japan and Korea from April to October(Figure 2a−d). The contour of the 0.1 pg L−1 for dissolved
CB153 was located around 30° N year-round, which can beconsidered as the boundary of subtropical and subarctic
regions. The seasonality did not coincide in the subarctic
region. In the area north of 50° N, the dissolved CB153
concentration was high in January and low in July; the Okhotsk
Sea presented the highest value of ∼0.5 pg L−1 in January andlowest value of <0.1 pg L−1 in July. In contrast, theconcentrations in the East China Sea, Japan Sea, and theKuroshio south of Japan were high in July and low in January;
the highest value was ∼2.5 pg L−1. The gaseous CB153
concentration in the marginal sea region was particularly highin summer, when its value was approximately 15 times higher
than that in winter (Figures S3, S4). The dissolved CB153
concentration in the subtropical open ocean was lower thanthat in marginal seas by at least 1 order of magnitude andshowed little seasonal variation owing to the smaller range ofseasonal variation in the water temperature and theatmospheric concentration of CB153 over the subtropicalregion. The details are shown in Figure S5. The spatial
distribution of dissolved CB153 was consistent with thatreported for the PCBs concentration inside fish.33 PCBs
concentrations in skipjack tuna were reported to reflect thepollution level in the seawater from where the fish werecollected. The PCBs concentrations in skipjack tuna showed
high values in the marginal seas and low values in the open
NWPO during 1997−2001.33
Particulate CB153 was concentrated in the subarctic region
north of 40° N (Figure 2e−h) where it showed a higher(lower) concentration in April (July). The highest value of 0.2

Figure 3. Time series for surface concentrations of dissolved CB153(a) and particulate CB153 (b), and for air−sea diffusion fluxes (c)averaged along several latitudes across the open NWPO (lines in
Figure S1). The green, cyan, magenta, and blue lines show the results
at 20°, 30°, 40°, and 50° N, respectively. Positive air−sea flux is from
air to sea, while negative flux is from sea to air.

concentration of particulate CB153 increased with latitude,
and its value in the subarctic region was lower than that in the
dissolved phase (Figure 3b). A peak value of 0.12 pg L−1 wasobserved in April.
Spatiotemporal Variations of Other PCB Congeners.
In addition to CB153, we also conducted simulations for
CB28, CB101, and CB180 (Figures S5, S6, and S7,
respectively), which have diverse physicochemical properties.19
We chose CB28 for comparison with CB153 to represent twocongeners spanning a broad range of volatility and hydrophobicity. The mean concentration of surface dissolved CB28
in the NWPO was 0.47 pg L−1, which was three times higher
than that of CB153. The mean concentration of surface
particulate CB28 was similar to that of CB153, but the highest
value of CB153 (0.88 pg L−1) was approximately double thatof CB28 (0.43 pg L−1). Iwata et al. reported that the observed
dissolved phase of CB101 concentrations and CB(28 + 31),
which was a mixture of CB28 and CB31, were 0.1−3.1 and 2−
8.5 pg L−1 at several stations (Table S7). The observations
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were about 1−5 and 1−25 times higher than that in our model(0.1−0.7 and 0.1−0.9 pg L−1), respectively, which remained
consistent with the observed higher gas phase concentrations
at these stations (Table S7). The concentrations showed little
spatial variations for the observed CB(28 + 31) and CB101 but
showed a decreased trend from coastal region to open ocean in
our simulation (Figures S5, S6). Nevertheless, the seasonalvariation of dissolved CB28 at these stations in our simulation
was consistent with the observation of lower concentration insummer and higher concentration in winter.
The surface distribution and monthly mean series ofdissolved and particulate CB28 concentrations at several
latitudes (Figures S5a−h, S8a,b) were similar to those of
CB153 (Figure 2a−h, 3a,b); however, some differences werestill found. The concentration of dissolved CB28 in thesubarctic region was over 1.0 pg L−1, which was nearly doublethat of CB153. The concentration of dissolved CB28 in Japanand Korea was as high as 3.0 pg L−1, which was approximately
three times higher than that of CB153. Both the gaseousconcentrations and physicochemical parameters are responsible for the higher concentration of CB28 compared with thatof CB153. Sensitivity experiments, in which the gaseousconcentrations or the physicochemical parameters werechanged to those of another congener, indicated that thedifference in the gaseous concentration between thesecongeners had a larger influence (89%) on the dissolved
concentration of CB28 than the difference of physicochemicalparameters (64%). The details are shown in Figure S9.
Air−Sea Flux of CB153. Air−sea diffusion and wet
deposition are the two sources of PCBs in the NWPO.
Advection and eddy diffusion with the ocean outside the target
area, and self-degradation of the dissolved phase, are the two
sinks of PCBs in the NWPO. The daily amount of CB153
exchanged through the air−sea interface, the lateral boundaryof the model domain, and the self-degradation of CB153 in the
NWPO were calculated (Figure S10).
Because the wet deposition flux of CB153 is an order ofmagnitude smaller than the air−sea diffusion flux (Figure S11),
and the air−sea diffusion flux depends on the dissolved phase
concentration of CB153 in water, we examined the air−sea
diffusion flux in more detail. The air−sea diffusion flux of
CB153 was close to zero over the majority of the NWPO,
except for the western boundary area (Figure 2i−l). In thewestern boundary area, the air−sea diffusion flux in the Yellow
Sea, Japan Sea, and the Kuroshio region displayed remarkable
downward values in April with a maximum of approximately
0.03 pg m−2 s−1 (Figure 2j). This changed to an upward flux inthe summer and autumn in the Yellow Sea, southern Japan Sea,and the KE, and the maximum upward flux was as high as 0.03
pg m−2 s−1 in the KE. The upward flux indicates that theseregions act as secondary sources of CB153 in air. The KE eastof Japan, which possesses the strongest velocity in summer,
represents the most intense volatilization region (Figure 2k).
Figure 3c shows the monthly mean air−sea diffusion flux for
CB153 averaged at different latitudes (20°, 30°, 40°, and 50°N) across the open NWPO (lines in Figure S1). The flux along
40° N was close to zero in spring but demonstrated a large
upward value in summer (magenta line in Figure 3c). The timeseries along 50° N showed a reversal of the air−sea diffusion
flux direction from spring to autumn (blue line in Figure 3c).
The downward flux (positive) reached its maximum in
November owing to the highest gaseous concentration of
CB153 in November (Figures S4, S13), whereas the upward

Articleflux (negative) reached its maximum in April. The surface
dissolved CB153 exhibited a consistent response to the air−sea
diffusion flux. For example, the concentration of surface
dissolved CB153 was lowest along 50° N in July before thereversal of the air−sea flux direction from upward to
downward, while the highest concentration along 50° N wasin December because of continuous strong deposition (Figure3a,c). More details regarding the mechanism of the air−sea
diffusion flux variations will be given later.
Air−Sea Flux of Other PCB Congeners. The distributionof the air−sea diffusion flux for CB28 was similar to that for
CB153. The flux in the western boundary region exhibited alarge downward value in winter and spring, shifting to an
upward value in the KE during summer and autumn. However,
the amplitude of CB28 flux (e.g., a maximum upward flux of
0.11 pg m−2 s−1) was larger than that of CB153 (Figure S5i−l).
Furthermore, there was little spatial difference in the seasonalvariation of CB28. The air−sea diffusion flux along several
latitudes showed large positive values in winter and smallvalues in summer (Figure S8c).
In addition to the difference in gaseous concentrations of
CB28 and CB153 in air, the discrepancy in the air−sea
diffusion flux between CB28 and CB153 was also influenced bytheir different physicochemical properties. The air−sea masstransfer rate (KAW) in eq 2 was determined by the masstransfer coefficient in water and air, which was calculated using
the Schmidt number and molecular diffusivity rates of thecongeners (Table S8). A smaller Schmidt number and larger
molecular diffusivity rate contributed to a larger KAW, andconsequently led to a larger air−sea diffusion flux for CB28
than for CB153. A summary of modeled results ofconcentrations for all four PCB congeners in the surface
NWPO, together with their air−sea exchange fluxes integrated
yearly, is listed in Table S9.
Influence of Ocean Currents on Air−Sea Diffusion
Fluxes of PCBs. Because gaseous PCBs in the atmosphere
were specified in this study, the intensification of air−sea
diffusion in the western boundary current area depends on theprocesses affecting the dissolved PCBs concentrations.
Advection and diffusion are the main physical processes,
while biological pump are the main biological processes
responsible for modulating dissolved PCBs in water. As shownin Figure S3, the distribution of gaseous CB153 concentration
was not homogeneous, and was approximately three times
larger in the area south of Japan compared with that in the KE.
In addition, the air−sea diffusion flux presented a band
distribution but with opposing directions in these two regions
in summer and autumn (Figure 2k, l). On the basis of theabove facts, we infer that the ocean current affects the air−sea
diffusion flux. To confirm this hypothesis, we conducted
sensitivity experiments on CB28, CB101, CB153, and CB180(Text S6).
The experiments confirmed that the Kuroshio current
moved a large amount of water with lower PCBs
concentrations from the upstream region to the area southof Japan, and transported water with high PCBs concentrations
from the south of Japan to the KE, resulting in an
intensification of downward (upward) air−sea diffusion fluxin the south of Japan (the KE). The air−sea flux of CB(28 +31) and CB101 at several stations (Table S7) were calculatedusing the observed paired data for air and water samples,12
which were −8.2−5.6 and −0.8−0.8 ng m−2 d−1, respectively.
The simulated air−sea flux of CB101 in our model (−0.4−0.8
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ng m−2 d−1) was consistent with the observation at thesestations, but the simulated flux of CB28 (0−2.4 ng m−2 d−1)
was 1−4 times lower compared to CB(28 + 31) in theobservation. Both in observation and simulation, CB101
showed weak outgassing in some stations, while CB28 was
mainly in the downward deposition, except at three stations inobservation.
The simulated air−sea flux of ∑4PCBs ranged between−23.6 and 44.75 ng m−2 d−1, which was an order of magnitudelarger than the air−sea flux in the central Pacific9 (−4.73 to−2.85 ng m−2 d−1) (Table S10). The strong western boundary
current (the Kuroshio) in the NWPO strengthened the air−sea
exchange of PCBs, whereas there were no such strong currents
in the central Pacific.
To quantitatively assess the effect of the strong westernboundary current on dissolved PCBs concentrations insummer (July−September) when the air−sea flux changed
largely along the Kuroshio and KE, we estimated the time forthe dissolved PCBs in surface water to reach equilibrium status(hereafter referred as response time) when only air−sea
exchange (Case 1) or only ocean advection (Case 2) was
considered (Figure 4). The temporal change rate of thelayer as the depth at which the temperature is 0.2 °C less than
the sea surface temperature,35 and obtained a mean summer
mixed layer depth of 10 m along the Kuroshio axis.
According to the above equations, the analytical solution for
CWm in Case 1 (eq 5) and Case 2 (eq 6) are as follows:

dissolved concentration in the surface ocean mixed layer inCase 1 (eq 3) and Case 2 (eq 4) is as follows:
CWm )

CWm
C
+ um Wm = 0txCWm = C*(1e t / ),

CWm = Cu(1e t / ),

=

=

hm
KAW

(5)

x
um

(6)

where Cu is the upstream dissolved concentration, Δx is thegrid spacing in the latitudinal direction for each grid point in
the model, denoting the distance for dissolved PCBstransported from the upstream to the target region. α and β
can be considered the response times for the local
concentration CWm to approach C* and Cu, respectively.
As shown in the “no-current” experiment, the air−sea
exchange was close to equilibrium over the entire region when
the current speed was zero. In the weak currents region such as
the subtropical region, it took 1−3 days to reach equilibrium
when only the air−sea exchange was considered (Figure 4a),while the time taken was 3−12 days in case 2 (Figure 4b). The
shorter response time to the air−sea exchange means that the
dissolved PCBs concentration in these region is mainly
controlled by the air−sea exchange and the advection did
not easily break the equilibrium of surface dissolved PCBs
concentration with the air−sea exchange. In the strong westernboundary current region, the response time of surface
dissolved PCBs concentration was ∼4 days in case 1 (Figure4a), whereas it decreased to 1 day in case 2 when only theocean current was taken into account (Figure 4b). Consequently, the dissolved PCBs from upstream carried by thestrong current can easily change the dissolved CB153 in thedownstream because the air−sea exchange needs more time torecover it to an equilibrium status. In other words, a strong
ocean current disrupts the original equilibrium in the “nocurrent” scenario and results in significant downward andupward air−sea fluxes in the area south of Japan and the KE,
respectively.
Implications. Owing to the high current speed, theKuroshio intensified the air−sea exchange in the westernboundary current region in summer by transporting large
amounts of water with high PCBs concentrations downstream.
The corresponding air−sea flux in the Kuroshio region isremarkable compared with that in weak current regions.
Notably, strong western boundary currents exist across theglobe, including the Kuroshio and East Australia Current in the
Pacific, the Gulf Stream and Brazil Current in the Atlantic, theSomali and Agulhas currents in the Indian Ocean. These
currents flow through coastal waters and transport highconcentrations of contaminants from nearshore areas to theopen ocean, which may play an important role in the global
air−sea exchange of PCBs. One important implication is thatthe effect of western boundary currents on the air−sea
diffusion flux of PCBs should be considered in atmosphericmodeling of the global distribution and fate of PCBs.
Current global fate simulations for PCBs are mostly basedon multimedia models, whose oceanic compartment is
simplified to a box model that ignores the direct effect ofocean current advection on PCBs. Some global ocean
simulations of PCBs do include the direct effect of ocean

Figure 4. Response time for dissolved PCBs to reach equilibrium in(a) scenario where only air−sea exchange was considered, (b)scenario where only ocean advection was considered.

CWm
K= AW (C*thm

Article(3)(4)

where CWm denotes the mean concentration of dissolved PCBs
in the mixed layer, hm (m) is the mixed layer depth, C* (=CA/H′) is the dissolved concentration at equilibrium, um (m s−1) is
the mean velocity in the mixed layer and has a value of 1 m s−1
for the Kuroshio current in summer. We defined the mixed
F

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ACKNOWLEDGMENTS
We are grateful to Carey L. Friedman and Noelle Eckley Selinfor providing the atmospheric PCBs data. We thank Prof.
Hisato Iwata for providing the observation PCBs data. M. Yang
thanks the Ministry of Education, Culture, Sports, Science andTechnology, Japan (MEXT) for supporting her stay in Japan.
This study was supported by a Grant-in-Aid for Scientific
Research (MEXT KAKENHI, Grant No. 20KK0239).

current advection on PCBs. However, these global models use
a horizontal resolution of ∼100 km or more which is toocoarse to represent realistic western boundary currents. These
western boundary currents have a width of 100−200 km;
hence, a horizontal resolution of ∼10 km is necessary to obtaina realistic velocity (∼1 m s−1) in an ocean model. Coarse
resolution (∼100 km) induces a weak western boundary
current in an ocean model. Therefore, the effects of westernboundary currents on PCBs concentrations have not been fully
considered by the global ocean simulation of PCBs.
In this study, a three-dimensional hydrodynamic-ecosystemPCBs coupled model was developed for the northwestern
Pacific Ocean to assess the air−sea diffusion fluxes of four PCB
congeners and examine the influences of western boundary
currents on air−sea diffusion flux. This model has a horizontal
resolution of <10 km and can reproduce a realistic Kuroshio
Current and its extension. The model results indicate that theKuroshio is a necessary condition for the appearance ofdownward and upward fluxes of PCBs along the Kuroshio
south of Japan and Kuroshio extension, respectively. We
suggest that future atmospheric transport models for PCBs
should include realistic western boundary currents.

■■

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ASSOCIATED CONTENT

sı Supporting Information*

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.est.2c03459.
Model description supplementary section; discussion of
the influence of ocean currents on air−sea exchange flux;
parameters used in calculations of biological and air−sea
exchange processes; observations of gaseous and oceanic
concentrations; conditions of sensitivity experiments for
CB153; summary of model results of surface concentrations and air−sea fluxes for four PCB congeners;
additional figures as described in the text (PDF)■

ArticleAUTHOR INFORMATION

Corresponding Author

Xinyu Guo − Center for Marine Environmental Studies, Ehime
University, Matsuyama 790-8577, Japan; Application
Laboratory, Japan Agency for Marine-Earth Science andTechnology, Yokohama-City, Kanagawa 236-0001, Japan;
orcid.org/0000-0002-4832-8625; Phone: +81 (089)927-9824; Email: guoxinyu@sci.ehime-u.ac.jp
Authors

Min Yang − Graduate School of Science and Engineering,
Ehime University, Matsuyama 790-8577, Japan
Miho Ishizu − Center for Climate Physics, Institute for Basic
Science, Busan 46241, Republic of Korea; Pusan National
University, Busan 46241, Republic of Korea
Yasumasa Miyazawa − Application Laboratory, Japan Agency
for Marine-Earth Science and Technology, Yokohama-City,
Kanagawa 236-0001, Japan
Complete contact information is available at:https://pubs.acs.org/10.1021/acs.est.2c03459
Notes

The authors declare no competing financial interest.
G

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