Interdisciplinary Studies on Environmental Chemistry—Marine Environmental Modeling & Analysis,
Eds., K. Omori, X. Guo, N. Yoshie, N. Fujii, I. C. Handoh, A. Isobe and S. Tanabe, pp. 197–204.
© by TERRAPUB, 2011.

Process Study on Numerical Simulation for Persistent Organic
Pollutants in the East China Sea
Jun ONO1, Daisuke TAKAHASHI2, Xinyu GUO 1,Shin TAKAHASHI1 and Hidetaka TAKEOKA1
1Center for Marine Environmental Studies (CMES), Ehime University,
Bunkyo-cho 2-5, Matsuyama 790-8577, Japan2Hydrospheric Atmospheric Research Center, Nagoya University,Nagoya 464-8601, Japan(Received 15 October 2010; accepted 16 November 2010)

Abstract—A three-dimensional/high-resolution transport model for persistentorganic pollutants (POPs) has been developed to investigate the dynamics ofPOPs in the East China Sea. The POPs model includes four compartments:
POPs in air, water, phytoplankton, and detritus. Biochemical processes
considered in this study are diffusive air-water exchange, phytoplankton
uptake of POPs, vertical sinking of phytoplankton, detritus from phytoplankton,and vertical sinking and decomposition of detritus. The POPs model is coupledwith an ocean circulation model that successfully reproduces the velocity fields
to include processes of advection and diffusion. We have conducted numericalsimulation for the PCB153 from the atmosphere and river. The model showed
the remarkable seasonal variability in PCB153. Concentrations of dissolved
and particulate PCB153 were higher in winter (January–March) and lower insummer (July–September). In coastal regions where chlorophyll a concentrations
are higher, horizontal and vertical distributions of dissolved and particulate
PCB153 concentrations were strongly affected by the effect of phytoplankton
uptake. The sensitivity experiments on the dynamics of PCB153 suggested thatthe seasonal variability in PCB153 was mainly caused by water temperaturefields. From the yearly mass balance of PCB153, most of the atmospheric input
by diffusive air-water exchange were removed by horizontal advection andwere accumulated to the sea bottom by vertical sinking. Massive amounts of thedissolved PCB153 from river influenced air-water and water-phytoplankton
exchange processes but its impacts were confined to regions near the mouth ofriver.
Keywords: East China Sea, persistent organic pollutants, polychlorinatedbiphenyls, seasonal variability

INTRODUCTION

Global contamination by persistent organic pollutants (POPs), such aspolychlorinated biphenyls (PCBs) and hexachlorocyclohexanes (HCHs), and its
impact on wildlife and ecosystem have been of concern over the last decade. POPs197

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are mainly characterized by persistence, long-range transport in the atmosphereand ocean, bioaccumulation, and toxicity. Thus understanding and quantifying
the dynamics of POPs are important to assess their environmental impact andfinal fate. In particular, the ocean is thought to be the global reservoir and ultimate
sink of POPs (Iwata et al., 1993). However, the dynamics of POPs in the ocean
has not yet been well understood. Numerical modeling studies (e.g., Dachs et al.,1999) suggested that not only the horizontal advection and diffusion by current
and turbulence but also the vertical transport of POPs by the phytoplankton,
which absorbs POPs and sinks to the deep water as detritus, are important tounderstand their dynamics inside the ocean.
Up to now, most numerical modeling of POPs was based on a simple box
model that was oriented to understand the processes. To know the spatial
distribution of POPs in the ocean, a three-dimensional transport model for POPs
based on realistic topography and ocean currents is necessary. Recently, Ilyina etal. (2006) has developed a realistic model for the POPs dynamics to investigate
the fate of γ-HCH in the North Sea. In addition, global modeling studies for PCBsand DDT have been reported by Kawai et al. (2009) and Stemmler and Lammel(2009), respectively.
In this study, we applied the POPs modeling to the East China Sea. Why do
we focus on the East China Sea? The East Asian countries are considered to bea potential source of POPs because of the enhancement of industrialization.
Nevertheless, the behavior of POPs in the ocean has been poorly understood. Thismay be partly because the observation for POPs is limited. We have conducted thePOPs observations in the East China Sea in December 2008, July 2009, andAugust 2010. Shimizu et al. (2009) reported the results in December 2008. Thesedata may be available for the model validation. The objective of this study is toassess quantitatively the POPs dynamics in the East China Sea (ECS), a marginal
sea surrounding by China, Korea, and Japan. Based on an ocean circulation modelthat provided flow field and turbulence parameters, we have developed a threedimensional high-resolution transport model for the POPs, in the ECS. As thefirst step towards the POPs modeling in the East China Sea, we examined theseasonal variability in PCB153 from the atmosphere and river.
MODEL DESCRIPTION

The POPs model includes four compartments (Gas phase POPs, Dissolved
phase POPs, Phytoplankton POPs, and Detritus POPs) that are linked by somephysical and biochemical processes, as shown in Fig. 1. Gas phase POPs dissolve
into the surface ocean through diffusive air-water exchange from the atmosphereto the ocean. In the ocean, a part of dissolved phase POPs are absorbed by thephytoplankton and the others are decomposed. The phytoplankton POPs isinvolved three processes: phytoplankton uptake, natural mortality, and verticalsinking. The natural mortality of phytoplankton transformed phytoplankton
POPs into detritus POPs. A part of detritus POPs are then transported to the deep
water by vertical sinking of detritus, and the others are transformed into dissolved
phase POPs by decomposition. The POPs model is coupled with a three-POPs Dynamics in the East China Sea

199Fig. 1. Schematics of the POPs model in the East China Sea.

dimensional ocean circulation model (Guo et al., 2003) that is based on thePrinceton Ocean Model and can reproduce well the seasonal variations of current
fields in the ECS. The velocity vectors in January are shown in Fig. 2, as an
example of the velocity fields in our model.
Figure 2 shows the model domain and bathymetry, which includes the East
China, Yellow, and Bohai Seas. The grid size is 1/18° and there were 20 levels inthe vertical. Monthly satellite chlorophyll a from Sea-viewing Wide Field-ofview Sensor (Sea WiFS) and wind stress from European Remote-Sensing Satellites(ERS-1 and ERS-2) are used to obtain the phytoplankton biomass and to calculate
the diffusive air-water exchange flux, respectively. Target compound in thisstudy is PCB153 that is one of the most bioaccumulation isomer of PCBs anddominant in the ecosystem of the ECS. Among initial and boundary conditions,concentrations of dissolved, phytoplankton, and detritus phase PCB153 were set
to 0 pg m–3, while concentrations of gas-phase PCB153 were set to 2.08 pg m–3(Jaward et al., 2004) that is the source of POPs in our model ECS.
To investigate what is the most important factor controlling the seasonalvariability in PCB153, we designed three sensitivity experiments, in which thetemporal variation of only one parameter was re moved while that of the others
was kept. Specifically, we removed the temporal variation in chlorophyll aconcentration (chlorophyll-run), wind speed (wind-run), and water temperature(temperature-run), respectively.

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Fig. 2. Model domain and bathymetry. Velocity fields at the surface in January were superimposedby vectors. The 20, 50, and 100-m isobath were also superimposed by solid line.

RESULTS AND DISCUSSION

The model results showed a remarkable seasonal variability in dissolved andparticulate PCB153 concentrations that are high in winter (January–March) and
low in summer (July–September) (Fig. 3). The surface flux by diffusive air-waterexchange is also higher in winter than in summer (not shown). We show thePOPs Dynamics in the East China Sea

201Fig. 3. Seasonal cycle of concentrations in dissolved-phase (solid line) and particulate-phase(shaded line) of PCB153, averaged for the whole modeling domain in the East China Sea.

horizontal distributions of PCB153 in February in Fig. 4.
The high surface flux area is mainly formed in coastal regions from China toKorean Peninsula (Fig. 4a). Associated with the high surface flux, concentrationsof dissolved and particulate PCB153 were also higher (Figs. 4b and 4c).
Interestingly, in coastal regions where concentrations of chlorophyll a are high,dissolved PCB153 concentrations are lower but particulate PCB153 concentrations
are higher (Figs. 4b and 4c). This would be due to the uptake of dissolved phase
PCB153 by phytoplankton. Consequently, the sinking flux also becomes high incoastal regions by vertical transport of particulate PCB153 (Fig. 4d).
We investigate the vertical structure of PCB153 at northern latitude of 31°(not shown). The surface flux in winter is larger in coastal regions because ofstronger wind speed and lower sea-surface temperature. As a result, dissolved
PCB153 concentrations become higher in coastal regions but the maximum value
appears in offshore regions. This is because dissolved PCB is strongly absorbedby phytoplankton in coastal regions where the chlorophyll a concentration ishigh. In contrast, particulate PCB153 concentrations are higher at the coast. On
the other hand, these features cannot be seen in summer.
To examine what is the most important factor controlling the seasonalvariability in PCB153, we per formed three additional numerical experiments:
chlorophyll-run, wind-run, and temperature-run. These results imply that thetemporal change in water temperature fields contributes most to the seasonalvariability in PCB153 (Fig. 5). This can be caused by the variation of Henry’s law
constants with water temperature, which results in the direct change of the surface
flux and indirect change in the concentrations of dissolved and particulate
PCB153. We also estimated the yearly mass balance of PCB153 in the ECS.

Fig. 4. Seasonal cycle of concentrations in dissolved-phase (solid line) and particulate-phase(shaded line) of PCB153, averaged for the whole modeling domain in the East China Sea.

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Fig. 5. Horizontal distributions of (a) the surface flux (×10–3 pg m–2s –1) of PCB153 from theatmosphere to the ocean, (b) dissolved and (c) particulate PCB153 concentrations (pg m –3), (d)sinking flux (×10 –3 pg m–2s –1) of PCB153 to sea bottom, in February.

Atmospheric input by diffusive air-water exchange was about 35 kg per year, inwhich 19 kg was removed by horizontal advection outside the ECS and 16 kg wasaccumulated to the bottom of ECS by vertical sinking. We also investigated theimpact of river discharge on the PCB153 dynamics. Concentrations in thedissolved and particulate phases became high in summer associated with theincrease of river discharge. However, its impact was confined to regions near themouth of river.
CONCLUDING REMARKS

We have a three-dimensional/high-resolution transport model for persistentorganic pollutants (POPs) in the East China Sea to investigate the dynamics ofPOPs. The model is coupled with an ocean circulation model with good
reproducibility in the velocity fields. We have conducted some numericalsimulation of the PCB153 from the atmosphere and river. The model revealed
clear seasonal variability in PCB153. Concentrations of dissolved and particulate
phase were higher in winter (January–March) but lower in summer (July–September). Sensitivity experiments imply that the seasonal variability in PCB153
is mainly caused by that in the Henr’s low constant depending on water temperature.
From the yearly mass balance of PCB153, most of the atmospheric input bydiffusive air-water exchange were removed by horizontal advection andaccumulated to the sea bottom by vertical sinking. In addition, the impact of riverdischarge on the behavior of the PCB153 was limited to regions near the mouthof river. This study is a first step toward the realistic simulation of the POPstransport in the East China Sea. Our future effort will take account of following

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effects: (1) suspended particulate matter, (2) temporal and spatial variations ofthe POPs concentration in the atmosphere, (3) POPs concentrations at the open
boundaries, and (4) dry and wet depositions.
Acknowledgments—This study was supported by a Grant-in-Aid for the Global COE
Program in Ehime University from the Japanese Ministry of Education, Culture, Sports,
Science and Technology.
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J. Ono (e-mail: jo@sci.ehime-u.ac.jp)