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[MIS20-02] Estimating vertical distribution of microplastic particles under wind-driven and Langmuir turbulence by Large Eddy Simulation
Keywords:Large Eddy Simulation, Langmuir turbulence, microplastic particles
While microplastic particles consist of buoyant materials such as polypropylene (PP) and polyethylene (PE) are supposed to be suspended at the sea surface, these can be vertically dispersed in a water column under turbulent condition due to stormy weather and/or severe waves. The vertical distribution of microplastic particles in the open ocean may have strong effect on the frequency of biological removal processes (e.g., aggregation by detritus or ingestion by zooplankton), but is poorly understood particularly under severe condition due to difficulty of direct measurement. In the present study the vertical distribution of microplastic particles in an open ocean surface mixed layer was investigated using an idealized large eddy simulation (LES) model coupled with on-line Lagrangian particle tracking system under various wind and wave forcing.
The numerical code used in the present study is kinaco, a non-hydrostatic ocean model developed in AORI, University of Tokyo. We adopted the filtered structure function (FSF) model for the LES subgrid scheme and the effect of waves was introduced by the vortex-force term. The computational domain is horizontally 128 m x 128 m and vertically 80 m with periodic boundaries. We introduced two categories of plastic particles consist of PP and PE, whose density is set as 910 kg/m3 and 965 kg/m3, respectively. The diameter of particles is classified into 10 bins from 10 μm to 5.12 mm, and the buoyant terminal velocity for each size and material is estimated from the empirical formula derived by Kaiser et al. (2019).
The initial stratification is based on the CTD observation carried out around the Tsushima Island in August 2020. The model was forced by surface wind of 0, 5, 10, 20 m/s and wave amplitude of 0, 0.316, 1.0, 3.16 m (total 15 cases), and integrated for 48 hours. The result of each case shows strong dependency of vertical distribution on the particle size but less dependency on the material difference. While almost all particles of greater than 1.28 mm diameter floated at the surface, certain amount of smaller particles were dispersed in the mixed layer. However, no particle was transported below pycnocline even in the severest case with 20 m/s wind and 3.16 m wave amplitude.
To investigate the seasonal variation, we also performed experiments with using monthly stratification of the same location simulated by DREAMS-D ocean forecast system operated by RIAM, Kyushu university. As expected, the vertical distribution of particles has strong dependency on the stratification. In particular, substantial number of particles less than 320 μm diameter reached the domain bottom in the winter season. The result suggests the possibility of physical removal of fine microplastic particles of buoyant material under severe weather condition in winter seasons over continental shelves.
The present simulations also recorded ultraviolet (UV) radiation exposure for each particle. It is suggested that floating plastic debris will be decomposed into finer particles by UV exposure, but finer particles tend to stay underwater for relatively longer period and be protected from UV radiation. The quantitative data provided by the present study may contribute to the estimation of decomposition process of plastic debris in the open ocean.
The numerical code used in the present study is kinaco, a non-hydrostatic ocean model developed in AORI, University of Tokyo. We adopted the filtered structure function (FSF) model for the LES subgrid scheme and the effect of waves was introduced by the vortex-force term. The computational domain is horizontally 128 m x 128 m and vertically 80 m with periodic boundaries. We introduced two categories of plastic particles consist of PP and PE, whose density is set as 910 kg/m3 and 965 kg/m3, respectively. The diameter of particles is classified into 10 bins from 10 μm to 5.12 mm, and the buoyant terminal velocity for each size and material is estimated from the empirical formula derived by Kaiser et al. (2019).
The initial stratification is based on the CTD observation carried out around the Tsushima Island in August 2020. The model was forced by surface wind of 0, 5, 10, 20 m/s and wave amplitude of 0, 0.316, 1.0, 3.16 m (total 15 cases), and integrated for 48 hours. The result of each case shows strong dependency of vertical distribution on the particle size but less dependency on the material difference. While almost all particles of greater than 1.28 mm diameter floated at the surface, certain amount of smaller particles were dispersed in the mixed layer. However, no particle was transported below pycnocline even in the severest case with 20 m/s wind and 3.16 m wave amplitude.
To investigate the seasonal variation, we also performed experiments with using monthly stratification of the same location simulated by DREAMS-D ocean forecast system operated by RIAM, Kyushu university. As expected, the vertical distribution of particles has strong dependency on the stratification. In particular, substantial number of particles less than 320 μm diameter reached the domain bottom in the winter season. The result suggests the possibility of physical removal of fine microplastic particles of buoyant material under severe weather condition in winter seasons over continental shelves.
The present simulations also recorded ultraviolet (UV) radiation exposure for each particle. It is suggested that floating plastic debris will be decomposed into finer particles by UV exposure, but finer particles tend to stay underwater for relatively longer period and be protected from UV radiation. The quantitative data provided by the present study may contribute to the estimation of decomposition process of plastic debris in the open ocean.