A method for calculating the cross-border flux of microplastics in the surface water body of a sea area

Abstract

The application discloses a kind of sea surface water body microplastics cross-border flux calculation methods, it is related to microplastics technical field, including: model is constructed and output hydrodynamic data;Based on model output result, the normal direction surface grid residual flow velocity of boundary section is calculated;Combined with the water depth of boundary section surface grid, surface grid width, the flow of each surface grid each period is calculated;Based on surface grid microplastics abundance, the cross-border flux of each boundary section surface water body microplastics is calculated each surface grid each period flow, single period length;The cross-border flux of each boundary section surface water body microplastics in the study sea area is accumulated and summed, and the total cross-border flux of surface water body microplastics in the whole study sea area is obtained.The method of the application can not only be applied to the sea environment with complex flow state, but also can be compatible with single flow state river scene, which breaks through the limitation of traditional single flow state flux calculation.

Description

Technical Field

[0001] This invention relates to the field of microplastics technology, specifically a method for calculating the transboundary flux of microplastics in the surface waters of the ocean. Background Technology

[0002] Microplastics, a new type of pollutant attracting significant international attention, primarily accumulate in the ocean. Microplastic particles are not only marine pollutants but also readily adsorb toxic pollutants, and most remain suspended on the surface of oceans or rivers, hindering organisms' utilization of sunlight. Furthermore, microplastics can adhere to the surface of organisms or be ingested, affecting their growth and development and potentially harming human health through the food chain. Therefore, controlling microplastic pollution and protecting coastal aquatic environments is urgently needed, and establishing the response relationship between microplastics and the marine dynamic environment is key to solving the problem of marine microplastic pollution.

[0003] Microplastic transboundary flux in marine surface waters refers to the net total amount of microplastics transported through the surface waters of a given marine area over a period of time. Since microplastics in seawater are mainly concentrated in the ocean surface, calculating this transboundary flux can provide technical support for tracing the sources and controlling the total amount of microplastics in specific marine areas. Existing methods for calculating microplastic flux in water bodies primarily focus on rivers flowing into the sea, and their application is still limited by the singularity of river flow patterns. However, ocean currents are actually a combination of periodic tidal currents and non-periodic residual currents, making it difficult to directly apply methods for calculating river microplastic transboundary fluxes to marine environments with complex flow patterns. In particular, there remains a technological gap in quantifying the transboundary flux of microplastics in the surface waters of a specific marine area.

[0004] Based on this, a method for calculating the transboundary flux of microplastics in the surface water of the sea is now provided, which can eliminate the drawbacks of existing technical solutions. Summary of the Invention

[0005] The purpose of this invention is to provide a method for calculating the transboundary flux of microplastics in the surface water of the sea, so as to solve the problems in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for calculating the transboundary flux of microplastics in marine surface waters includes the following steps: Step S1: Construct a three-dimensional tidal current numerical model of the study sea area and complete the model accuracy verification. Output hydrodynamic data of the boundary section of the study sea area. The hydrodynamic data includes at least the surface grid velocity vector of the boundary section. The study sea area is generalized into a polygonal region. Each edge of the polygonal region is defined as the boundary section of the corresponding study sea area. Step S2: Based on the output results of the three-dimensional tidal current numerical model of the research sea area, calculate the residual flow velocity of each surface grid of the boundary section in the x and y directions. By solving the residual flow velocity in the x and y directions, the residual flow velocity and residual flow direction of the surface grid of the boundary section are obtained. Combined with the angle between the boundary section and the north direction, calculate the residual flow velocity of the surface grid in the normal direction of the boundary section. Step S3: Based on the residual flow velocity of the surface grid in the normal direction of the boundary section, and combined with the water depth and width of the surface grid in the boundary section, calculate the flow rate of each surface grid in the normal direction of the boundary section at each time period. Step S4: Collect surface seawater samples from the research area and detect the abundance data of microplastics in the surface seawater. Obtain the abundance of microplastics in the surface grid of the target section during the target time period through linear interpolation. Combine the flow rate, microplastic abundance of the surface grid, and duration of a single time period of each surface grid, and solve the cross-boundary flux of microplastics in the surface water of each boundary section. Step S5: Traverse all boundary sections of the study area, sum up the cross-boundary flux of microplastics in the surface water of each boundary section, and obtain the total cross-boundary flux of microplastics in the surface water of the entire study area, thus realizing the calculation effect of cross-boundary flux of microplastics in the surface water of the sea area. Step S6: Extract the residual flow velocity, microplastic abundance, and time-segmented microplastic transport of the surface grid in the normal direction of the boundary section. Based on the spatial coordinates of the surface grid, match the corresponding boundary section surface grid residual flow distribution map and microplastic abundance spatial distribution map. Divide the microplastic input / output section into microplastic input / output sections according to the microplastic abundance of the surface grid, and output the spatial partitioning source tracing results.

[0007] Furthermore, the construction of the three-dimensional tidal current numerical model for the research area in step S1 includes: Coastline data, water depth data, and boundary tidal level data are acquired. FVCOM or HEC-RAS hydrodynamic software is used for modeling. Hydrodynamic data of all boundary sections of the study area are extracted and output. The hydrodynamic data includes surface grid velocity vectors of the boundary sections, x-axis velocity time series of each surface grid, y-axis velocity time series, width of each surface grid, and water depth of the surface grid. The coastline data is obtained through field measurements or interpretation of remote sensing images, and the water depth data is obtained through field measurements or digital nautical charts. The measurement years of the coastline data and water depth data are matched. The model accuracy verification includes at least the verification of tidal current observation data from no less than two microplastic sampling survey stations and the verification of tidal level observation data from no less than three microplastic sampling survey stations at each boundary section of the study sea area. The verification duration of the tidal current observation data is no less than 25 hours, and the continuous verification duration of the tidal level observation data is no less than one month. The number of surface grids at each boundary section is no less than the number of microplastic sampling survey stations at the boundary section.

[0008] Furthermore, in step S2, the calculation process for the residual flow velocity in the x and y directions of each surface grid at the boundary section includes: Construct an orthogonal Cartesian coordinate system and decompose the velocity vector of the surface mesh at the boundary section along the x and y axes. In the numerical simulation, the formulas for calculating the residual velocity of the surface mesh at the boundary section in the x and y directions after decomposition include: , ; in, For the boundary section The residual flow velocity in the x-direction of each surface grid. The number of time steps in the numerical simulation and satisfying the formula , This represents the number of tidal periods in the numerical simulation. For the duration of the tidal cycle, This represents the time step of the numerical simulation. For the boundary section The first surface mesh The flow velocity in the x-direction at each time step. For the boundary section The residual flow velocity in the y direction of each surface grid. For the boundary section The first surface mesh The flow velocity in the y-direction at each time step.

[0009] Furthermore, the calculation formulas for the residual flow velocity and residual flow direction of the surface mesh at the boundary section in step S2 include: , ; in, For the boundary section The residual flow velocity of each surface grid, For the boundary section The direction of residual flow in each surface grid. For the boundary section The residual flow velocity in the x-direction of each surface grid. For the boundary section The residual flow velocity in the y direction of each surface grid.

[0010] Furthermore, the formula for calculating the residual flow velocity of the surface mesh in the normal direction of the boundary section in step S2 is as follows: ; in, The first normal direction of the boundary section Surface mesh Residual flow velocity during the time period For the boundary section The residual flow velocity of each surface grid, For the boundary section The direction of residual flow in each surface grid. The angle between the boundary section and the northward direction.

[0011] Furthermore, the calculation formula for the flow rate of each surface grid in the boundary section normal direction at each time period in step S3 is as follows: ; in, The first normal direction of the boundary section Surface mesh Traffic during a specific time period The surface water depth of the boundary section grid. The first normal direction of the boundary section Surface mesh Residual flow velocity during the time period For the first section on the boundary surface The width of each surface grid.

[0012] Furthermore, the formula for calculating the transboundary flux of microplastics in the surface water of each boundary section in step S4 is as follows: ; in, For the first Transboundary flux of microplastics in the surface water of a boundary section Calculate the quantity for the total time period. This represents the number of surface meshes at the boundary section. The first normal direction of the boundary section Surface mesh Traffic during a specific time period For the boundary section after linear interpolation Surface mesh Microplastic abundance over time period for The calculation duration of the time period.

[0013] Furthermore, the formula for calculating the total transboundary flux of microplastics in the surface water of the entire study area in step S5 is as follows: ; in, To study the total transboundary flux of microplastics in the surface waters of the sea, To study the number of marine boundary sections, For the first Transboundary flux of microplastics in the surface water of a boundary section.

[0014] Further, in step S4, surface seawater samples from the study area are collected and surface seawater microplastic abundance data are detected. The surface grid microplastic abundance of the target section during the target time period is obtained through linear interpolation, specifically including: Several seawater surface sampling stations were set up at intervals along the boundary sections of the study sea area. Seawater samples were collected from the seawater surface. The samples were pre-processed and tested in accordance with the seawater sample processing specifications. Each sampling station was bound with the sampling station coordinates, sampling time period and measured microplastic abundance value to obtain a discrete station dataset. The seawater surface was defined as the water area from 0.1m to 1m below the sea surface. The measured microplastic abundance values ​​were subjected to time-series linear matching. The time-series linear interpolation method was used to uniformly interpolate the measured microplastic abundance values ​​to each calculation period of the three-dimensional tidal current numerical model of the study area, so as to obtain the time-series abundance of each sampling station in any calculation period. All sampling points and surface grids of the boundary section are projected along the direction of the section to the center line of the section and converted into the linear distance of the section. The microplastic abundance of each surface grid in the corresponding time period is obtained by using a one-dimensional two-point linear interpolation formula. After interpolation, the abundance range of all surface grids is verified, and gridded microplastic abundance data corresponding one-to-one with the hydrodynamic grid is generated, i.e., the surface grid microplastic abundance of the target section in the target time period.

[0015] Furthermore, the numerical sign of the total transboundary flux of microplastics in the surface water of the study area represents the transport direction; a positive value indicates that the water flows into the interior of the study area, and a negative value indicates that the water flows out of the study area.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention constructs a three-dimensional tidal current numerical model of the research area and extracts the surface grid velocity vector of the boundary section. Furthermore, it employs a multi-tidal cycle averaging method to separate the net transport residual flow in the normal direction as the core driving force for microplastic migration, thereby effectively filtering out the interference of tidal reciprocating flow. This allows the method to be applied not only to marine environments with complex flow patterns but also to river scenarios with single flow patterns, overcoming the limitations of traditional single-flow-pattern flux calculations and demonstrating broad applicability. In addition, this invention combines surface grid water depth, grid width, and time period duration to perform grid flow calculation, single-section microplastic cross-boundary flux calculation, and total cross-boundary flux accumulation and summation across the entire area, effectively improving the accuracy and operability of marine microplastic flux calculation. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the method steps of the present invention.

[0018] Figure 2 This is a schematic diagram of the system structure of the present invention.

[0019] Figure 3 Example 1 shows the residual flow distribution of the surface grid at the boundary section in May 2020.

[0020] Figure 4 Example 1 shows the residual flow distribution of the surface grid at the boundary section in October 2020.

[0021] Figure 5 Example 1 shows the spatial distribution of microplastic abundance in the surface mesh of the boundary section in May 2020.

[0022] Figure 6 Example 1 shows the spatial distribution of microplastic abundance in the surface mesh of the boundary section in October 2020.

[0023] Figure 7 This is a residual flow distribution map of the surface grid at the estuary section, as shown in Example 2.

[0024] Figure 8 This is a spatial distribution map of microplastic abundance in the surface mesh of the boundary section of Example 2.

[0025] Figure label annotations: 3D power flow numerical simulation module 10, cross-sectional normal residual flow calculation module 20, grid flow calculation module 30, microplastic abundance and flux calculation module 40, total flux summary module 50, layer coupling and segment partitioning module 60, storage and output module 70. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0027] To address the limitations of existing microplastic flux measurement techniques, which are mostly applicable to unidirectional, constant-flow rivers and cannot separate tidal currents from net residual currents, making it difficult to quantitatively calculate the total net transboundary transport of microplastics across multiple boundaries in complex tidal bays, this invention provides a method for calculating transboundary flux of microplastics in marine surface waters, considering that nearshore waters are characterized by a composite flow field of periodic tidal currents superimposed with non-periodic residual currents, and that microplastics are mainly concentrated in the surface water layer 0.1–1 m below the sea surface. Based on three-dimensional tidal current numerical simulation, this method extracts the surface grid velocity vector at boundary sections and separates the net residual current in the normal direction as the core driving force for microplastic migration. It performs layered calculations of grid flow rate, single-section microplastic transboundary flux, and total transboundary flux across the entire region. Furthermore, it couples grid-level hydrodynamics and microplastic spatial data to achieve visualized source tracing and partitioning of input / output sections. This method overcomes the limitations of traditional single-flow-mode flux calculations and can be applied not only to complex marine environments but also to river scenarios with single flow modes, demonstrating broader applicability. The specific technical solution is shown below.

[0028] Example 1 In this embodiment, as Figure 1 As shown, this invention provides a method for calculating the transboundary flux of microplastics in marine surface waters, specifically including the following steps: Step S1: Construct a three-dimensional tidal current numerical model of the study area and complete the model accuracy verification. Output hydrodynamic data of the boundary section of the study area. The hydrodynamic data shall include at least the surface grid velocity vector of the boundary section. The study area shall be generalized into a polygonal region. Each edge of the polygonal region shall be defined as the boundary section of the corresponding study area. The model grid inside the study area shall be densified. The grid spatial resolution shall be determined according to the model verification accuracy control requirements. Under the premise of meeting the model verification accuracy control requirements, the number of surface grids of the boundary section shall not be less than the number of microplastic sampling survey stations of the boundary section. Step S2: Based on the output results of the three-dimensional tidal numerical model of the research sea area, calculate the residual flow velocity of each surface grid in the x and y directions of the boundary section. The residual flow velocity and residual flow direction of the surface grid of the boundary section are obtained by solving the residual flow velocity in the x and y directions. Combined with the angle between the boundary section and the north direction, calculate the residual flow velocity of the surface grid in the normal direction of the boundary section. The residual flow is the long-term net flow velocity of seawater after filtering out the tidal reciprocating flow, which is the driving force for the migration of surface microplastics across the bay boundary. Step S3: Based on the residual flow velocity of the surface grid in the normal direction of the boundary section, and combined with the water depth and width of the surface grid in the boundary section, calculate the flow rate of each surface grid in the normal direction of the boundary section at each time period. Step S4: Collect surface seawater samples from the research area and detect the abundance data of microplastics in the surface seawater. Obtain the abundance of microplastics in the surface grid of the target section during the target time period through linear interpolation. Combine the flow rate, microplastic abundance of the surface grid, and duration of a single time period of each surface grid, and sum them up to obtain the cross-boundary flux of microplastics in the surface water of each boundary section. The cross-boundary flux of microplastics in the surface water of the boundary section refers to the net total amount of microplastics transported through the surface water of the boundary section within a certain period of time. Step S5: Traverse all boundary sections of the study area, sum up the cross-boundary flux of microplastics in the surface water of each boundary section, and obtain the total cross-boundary flux of microplastics in the surface water of the entire study area, thus realizing the calculation effect of cross-boundary flux of microplastics in the surface water of the sea area. Step S6: Extract the residual flow velocity, microplastic abundance, and microplastic transport volume of the surface grid in the normal direction of the boundary section. Based on the spatial coordinates of the surface grid, match the residual flow distribution map and microplastic abundance spatial distribution map of the corresponding boundary section surface grid. Divide the microplastic input / output section into microplastic input / output sections according to the microplastic abundance of the surface grid, and output the spatial zoning source tracing results to achieve spatial identification of pollution sources. It also includes: repeating steps S1 to S6 to simulate multiple scenarios such as spring tide, neap tide, high water season, and low water season, to obtain a dataset of microplastic cross-boundary flux in the research sea area under different seasons and tidal conditions. Based on multi-scenario operating condition flux datasets, the main transport channels and external input sources of microplastics in the research sea area are identified, and a total microplastic control and source tracing management plan for the research sea area is output.

[0029] In step S1, the construction of the three-dimensional tidal current numerical model for the research area includes: The target sea area is generalized as a closed polygon region, and each boundary line of the polygon is defined as an independent boundary section. The number of boundary sections in the sea area under study is counted. The density of the model grid inside the research sea area should be increased, and the number of surface grids at the boundary section should not be less than the number of microplastic sampling survey stations at the boundary section. Coastline data, water depth data, and boundary tidal level data were acquired. Hydrodynamic modeling was performed using FVCOM or HEC-RAS hydrodynamic software. Hydrodynamic numerical simulation is an existing technology and can be achieved using existing hydrodynamic software (such as HEC-RAS and FVCOM). The surface mesh water depth at the boundary section was set to 0.5m to match GB / T standards. The definition of the sea surface layer (0.1~1m) as specified in 17378.3 is used to extract and output hydrodynamic data for all boundary sections of the study area. The hydrodynamic data includes the surface grid velocity vector of the boundary section, the x-axis velocity time series and y-axis velocity time series of each surface grid, the width of each surface grid, and the surface grid water depth. The x-axis velocity time series and y-axis velocity time series are the component aggregation datasets of the instantaneous surface grid velocity vectors for all simulated time steps. A combination of a set of x and y instantaneous components at a single time step is the surface grid velocity vector at that time. Coastline data is obtained through field measurements or interpretation of remote sensing images. The spatial resolution, imaging time, and image quality requirements of remote sensing images shall be implemented in accordance with the provisions of HY / T 147.7. Water depth data is obtained through field measurements or digital nautical charts. The measurement years of the coastline data and water depth data are compatible. The measurement year of the water depth data should be close to the measurement year of the coastline data. If the acquisition time of the coastline data and the water depth data is no more than 5 years apart, the tidal current observation data and tidal level observation data of the whole area shall be collected simultaneously. Model accuracy verification shall include at least the verification of tidal current observation data from no less than two microplastic sampling survey stations at each boundary section of the study sea area, and the verification of tidal level observation data from no less than three microplastic sampling survey stations. The verification time for tidal current observation data shall be no less than 25 hours, and the continuous verification time for tidal level observation data shall be no less than one month. The verification of tidal current observation data shall be carried out during spring tides and neap tides. The accuracy control of model verification shall be carried out in accordance with the provisions of JTS / T 231.

[0030] In step S2, considering that seawater movement includes not only the periodic movement of tidal currents but also the residual current of non-periodic movement, seawater particles do not return to their initial position after one tidal cycle. Although the amount of residual current is not large compared to tidal currents, it directly indicates the movement and exchange of water and provides the driving force for the dilution, diffusion and transport of suspended and soluble substances in seawater. Therefore, this invention involves residual current calculation in the process of calculating the transboundary flux of microplastics in marine water bodies. Step S2 relies on the boundary section surface mesh velocity vector output in step S1 to complete the residual flow decomposition, flow direction solution, and normal projection transformation in layers. The calculation process for the residual flow velocity in the x and y directions of each surface grid at the boundary section includes: Based on the output results of the three-dimensional tidal current numerical model of the research area (such as the velocity vector of the surface grid of the boundary section), an orthogonal Cartesian coordinate system is constructed, and the velocity vector of the surface grid of the boundary section is decomposed along the x-axis and y-axis. In the numerical simulation, the arithmetic mean of the time-series velocity of the multi-tidal cycle is taken, and then the residual velocity of the surface grid of the boundary section in the x and y directions is obtained. The calculation formula includes: , ; in, For the boundary section The residual flow velocity in the x-direction of each surface grid, in m / s. The number of time steps in the numerical simulation and satisfying the formula The unit is one. This represents the number of tidal cycles in the numerical simulation, expressed in units of tidal periods. The duration of the tidal cycle, measured in seconds. This represents the time step of the numerical simulation, in seconds. For the boundary section The first surface mesh The flow velocity in the x-direction at each time step, in m / s. For the boundary section The residual flow velocity in the y-direction of each surface grid, in m / s. For the boundary section The first surface mesh The flow velocity in the y direction at each time step is expressed in m / s. The formulas for calculating the residual flow velocity and direction of the residual flow in the surface mesh of the boundary section include: , ; in, For the boundary section The residual flow velocity of each surface grid, in m / s. For the boundary section The direction of residual flow in each surface grid, in degrees. For the boundary section The residual flow velocity in the x-direction of each surface grid, in m / s. For the boundary section The residual flow velocity in the y direction of each surface grid, in m / s; The formula for calculating the residual flow velocity of the surface mesh in the normal direction of the boundary section is: ; in, The first normal direction of the boundary section Surface mesh The residual flow velocity during a given time period, expressed in m / s. For the boundary section The residual flow velocity of each surface grid, in m / s. For the boundary section The direction of residual flow in each surface grid, in degrees. The angle between the boundary section and the northward direction is expressed in degrees.

[0031] In step S3, the formula for calculating the flow rate of each surface grid in the normal direction of the boundary section at each time period is: ; in, The first normal direction of the boundary section Surface mesh Flow rate over a period of time, expressed in m³ / s. The water depth at the surface of the boundary section is in meters. The first normal direction of the boundary section Surface mesh The residual flow velocity during a given time period, expressed in m / s. For the first section on the boundary surface The width of each surface grid, in meters, is defined according to GB / T 17378.3 as the water surface range of 0.1m to 1m below the sea surface. Considering the representativeness of the sampling data, the surface sampling water depth is usually determined to be 0.5m. Therefore, the corresponding model surface grid water depth is also set to 0.5m, i.e., the surface grid water depth at the boundary section. It is 0.5m.

[0032] In step S4, considering that microplastics in seawater are mainly concentrated in the surface layer, surface seawater can be collected and brought back to the laboratory for testing to obtain surface seawater microplastic abundance data. The collection, storage and transportation of microplastic samples and laboratory analysis shall be carried out in accordance with the provisions of DB37 / T 4323. In this study, surface seawater samples were collected from the research area, and the abundance data of surface seawater microplastics was obtained. Linear interpolation was used to obtain the surface grid microplastic abundance of the target section during the target time period, specifically including: Several seawater surface sampling stations were set up at intervals along the boundary sections of the study sea area. If the width of the cross section varies greatly, at least two sampling points were set up on the narrow cross section, and one additional sampling station was set up every 1 to 2 km on the wide cross section. Seawater samples were collected from the seawater surface and the sample pretreatment and laboratory testing were completed in accordance with the seawater sample processing specifications. Surface seawater was collected using a stainless steel / glass water sampler and filtered on-site using a 0.45μm filter membrane under negative pressure to remove microplastic particles. The filter membrane was sealed, refrigerated, and protected from light to avoid contamination from plastic containers and tools. Simultaneously, on-site and laboratory blanks were set up to deduct interference values ​​introduced by containers and reagents. After digestion, flotation, and separation, microplastic particles were identified using microscopy and spectroscopy. The total number of particles was counted, and the microplastic abundance at a single station was calculated using the following formula: ,in, To measure the abundance of microplastics at the sampling stations, To detect the total number of microplastics on the filter membrane, To filter seawater volume; Each sampling station is bound to its coordinates, sampling time period, and measured microplastic abundance value to obtain a discrete station dataset. The sea surface layer is defined as the water area from 0.1m to 1m below the sea surface. The surface water area of ​​0.5m below the sea surface is uniformly selected to be consistent with the calculation depth of 0.5m above the surface of the three-dimensional tidal current numerical model of the study area, so as to eliminate the abundance systematic error caused by water depth. The sampling time covers the spring tide and neap tide of the target calculation period and is synchronously matched with the start and end time of the three-dimensional tidal current simulation to ensure that the measured data matches the hydrodynamic time series. Extracting the center coordinates of the surface grid of each boundary section in the three-dimensional tidal current numerical model of the research sea area. A grid spatial database is established. If sampling is conducted in multiple batches and at different time periods, a temporal linear interpolation method is used to uniformly interpolate the measured microplastic abundance values ​​to each calculation period of the three-dimensional tidal current numerical model of the study area. The results of the sampling stations in any calculation period are obtained as follows: Temporal abundance of time period The Grubbs test was used to remove anomalous abundance mutation sites to avoid interpolation distortion. Each boundary section is a narrow, elongated linear profile. One-dimensional linear interpolation along the section direction is used to project all sampling points and surface meshes of the boundary section onto the section centerline along the section direction, and convert them into linear distances of the section. linear distance of sampling points Abundance in corresponding time periods As interpolation control points, the projected distance to any surface mesh within the boundary section is used. The time period corresponding to each surface grid is obtained by using a one-dimensional two-point linear interpolation formula. Microplastic abundance over time period ; The calculation formula is: ,in, The nearest sampling point upstream of the grid projection location Abundance over time The nearest sampling point downstream of the grid projection location Abundance over time The cross-sectional distance is the nearest sampling point downstream of the grid projection location. The cross-sectional distance is the nearest sampling point upstream of the grid projection location; For the surface grid at both ends of the cross-section and beyond the outermost sampling point, a boundary constant extrapolation method is used, that is, the abundance of the sampling point at the outermost edge of the cross-section is directly taken. After interpolation, the abundance range of all surface grids is verified: the maximum and minimum abundance values ​​of all grids in the cross-section do not exceed ±10% of the measured sampling abundance range. If they do, the sampling points are re-densified and re-measured to generate gridded microplastic abundance data corresponding one-to-one with the hydrodynamic grid, that is, the surface grid microplastic abundance of the target cross-section in the target time period. Each surface grid Each calculation period Matching unique microplastic abundance The cross-boundary flux of microplastics in the surface water of each boundary section can be directly substituted into the calculation formula to participate in the flux calculation. The formula for calculating the transboundary flux of microplastics in the surface water at each boundary section is as follows: ; in, For the first Transboundary flux of microplastics in the surface water of a boundary section, in units of microplastics. The quantity is calculated for the total time period, in units of items. This represents the number of surface meshes on the boundary section, expressed in units. The first normal direction of the boundary section Surface mesh Flow rate over a period of time, expressed in m³ / s. For the boundary section after linear interpolation Surface mesh Microplastic abundance over a period of time, expressed in units of cells / m³. for The duration of the time period is calculated in seconds.

[0033] In step S5, considering that the study area is generally formed by multiple boundary sections, the total amount of microplastics entering and leaving the surface water of the entire study area within a certain period of time can be obtained by calculating the cross-boundary flux of microplastics at each boundary section and summing the results. The formula for calculating the total cross-boundary flux of microplastics in the surface water of the entire study area is as follows: ; in, To study the total transboundary flux of microplastics in the surface waters of the sea, the unit is individual microplastics. To study the number of marine boundary sections, the unit is "unit". For the first The transboundary flux of microplastics in the surface water of each boundary section, expressed in units of microplastics; the total transboundary flux of microplastics in the surface water of the studied sea area. The numerical sign represents the direction of transport; a positive value indicates that the water flows into the interior of the study area, while a negative value indicates that the water flows out of the study area.

[0034] In step S6, structured data for each surface mesh of all boundary sections is extracted from the calculation data in the above steps, such as single mesh. Single time period Matching boundary section normal direction surface mesh residual flow velocity Surface mesh microplastic abundance Grid-based time-segmented microplastic transport volume, etc. The symbols represent the direction of transport. This indicates that water carrying microplastics flowed into the research area. This indicates that water carrying microplastics flowed out of the research area. The grid microplastic concentration obtained through linear interpolation characterizes the local water body microplastic background load, and the grid time-segmented microplastic transport volume. The actual total amount of microplastics exchanged across the boundary in a single time period of the grid is the core indicator for segment division. The spatial coordinates (longitude and latitude) of the center of each surface grid are extracted simultaneously as a unique spatial index for layer matching. Based on the spatial coordinate matching of the surface mesh to the corresponding boundary section, the residual flow distribution map and the microplastic abundance spatial distribution map of the surface mesh are obtained. The residual flow distribution map corresponds to the first normal direction of the boundary section. Surface mesh residual flow velocity during the period The spatial distribution map of microplastic abundance corresponds to the first boundary section after linear interpolation. Surface mesh Microplastic abundance over time period The grid-based time-segmented microplastic transport volume is The value is obtained by multiplying the first two grid parameters and the computation time, and corresponds one-to-one with the grids of the two distribution maps. It is used to draw the flux heat map. The specific matching process is as follows: Coupling of residual flow distribution map: Using grid space coordinates as the matching key, the residual flow velocity in the cross-sectional normal direction is coupled. Assign values ​​to the corresponding layer grid cells. The direction and length of the layer arrows indicate the direction and velocity of the residual flow. Figure 3 , Figure 4 As shown, this ensures that each spatial cell corresponds one-to-one with the computational grid, and that the flow velocity values ​​perfectly match the surface vectors. Coupling of microplastic abundance spatial distribution maps: Based on the same set of grid coordinates, the interpolated surface grid microplastic abundance maps are... Assign the value to the isovalue color stop layer, such as Figure 5 , Figure 6 As shown, the color level represents the concentration of microplastics in the grid, allowing for the simultaneous retrieval of two sets of data: residual flow and abundance, at the same spatial location. Superimposed to generate a grid transport thermodynamic auxiliary map: the microplastic transport volume of the grid is divided into time periods. Overlaying the previous two layers, red represents input and blue represents output, and the shades of the color blocks represent the size of the transport load, thus distinguishing high and low exchange sections.

[0035] Based on the abundance of microplastics in the surface mesh, microplastic input / output cross-sectional segments are divided, and spatial zoning results are output. The specific process is as follows: Batch extraction of all boundary surface mesh structured parameters, such as the latitude and longitude coordinates of the mesh center and the residual flow velocity of the surface mesh in the normal direction of the boundary section. Surface mesh microplastic abundance The grid-based time-segmented microplastic transport volume is ; First-level partitioning is completed using the sign of the normal residual flow velocity: If This indicates that the grid-like water body carrying microplastics flowed into the research sea area and was included in the input candidate segment. This indicates that the grid-like water body carrying microplastics has flowed out of the research area and has been included in the output candidate segment. This indicates that water exchange is weak and the area is classified as a low-exchange transition zone. Based on a secondary subdivision of the segment levels using abundance and transport capacity, the input and output candidate segments are divided into high / medium / low load segments using measured abundance interval grading thresholds, and the microplastic abundance of all surface meshes in the entire domain is analyzed. Sort by abundance from smallest to largest, divide into three equal parts to define low, medium, and high abundance threshold intervals, and combine with grid-based time-segmented microplastic transport volume. Merge adjacent grids of the same level and with the same transport direction to form continuous spatial segments: For example, the high input segment: and , This area, located in the high-threshold range, is a major external input channel for marine microplastics, such as the middle input segment: The abundance and transport volume are at a moderate level, indicating it is a secondary input channel, such as in low-input segments: The abundance and transport volume are low, indicating weak exogenous input, such as in high-output regions: and , This area is in the high-threshold range, which is the main channel for the export of microplastics from the sea to the open ocean, such as the mid-export section: Abundance and transport volume are moderate, such as in low-output areas: Microplastics have low external load output, such as in transition zones: Water exchange is weak, and the entry and exit of microplastics are negligible; By converting the above segmented results into vector surface layers and overlaying geographical elements such as the coastline, cross-sectional boundaries, and river estuaries of the study area, and labeling the total cumulative microplastic exchange, average abundance, and dominant transport periods for each segment, and exporting thematic maps, flux statistics tables, and source tracing reports, the pollution sources (river input, offshore exchange, port discharge, etc.) of each high-load segment can be clearly identified, forming a complete spatial zoning source tracing result.

[0036] Example 2 The difference from Example 1 is that, as in Example 2, ... Figure 2 As shown, the present invention also provides a system for calculating the transboundary flux of microplastics in marine surface waters, including a processor, a computer-readable storage medium, and the following functional modules: The 3D tidal current numerical simulation module 10 includes a built-in data input subunit, a model building and verification subunit, and a hydrodynamic data output subunit. The data input subunit is used to import measured observation data of coastline, water depth, and tidal level / tidal current. The model building and verification subunit integrates hydrodynamic software interfaces such as FVCOM and HEC-RAS, automatically generalizes the study area into a polygonal boundary section, sets the surface water depth to 0.5m, performs multi-station accuracy verification of tidal level and tidal current, and outputs a mesh densification prompt if the verification fails. The hydrodynamic data output subunit batch exports the surface mesh velocity vector of the boundary section, the x-axis velocity time series of each surface mesh, the y-axis velocity time series, the width of each surface mesh, the surface mesh water depth, and the mesh center coordinates, etc. The cross-sectional normal residual flow calculation module 20 includes a multi-cycle average residual flow solution sub-unit, a residual flow velocity-direction conversion sub-unit, and a cross-sectional normal projection sub-unit. The multi-cycle average residual flow solution sub-unit is used to retrieve the simulated time-series flow velocity and automatically calculate the residual flow velocity in the x and y directions for each surface grid of the boundary section. The residual flow velocity-direction conversion sub-unit is used to solve for the residual flow velocity and direction of the surface grid of the boundary section. The cross-sectional normal projection sub-unit is used to input the angle between the boundary section and the north direction, calculate the residual flow velocity of the surface grid in the normal direction of the boundary section, and mark the positive and negative signs of the velocity (inflow / outflow). The grid flow calculation module 30 is used to retrieve the residual flow velocity, water depth, and width of the surface grid in the normal direction of the boundary section, and to calculate the flow rate of each surface grid in the normal direction of the boundary section at each time period in batches. The microplastic abundance and flux calculation module 40 includes a built-in sampling data entry subunit, a one-dimensional linear interpolation subunit, and a single-section flux integration subunit. The sampling data entry subunit is used to input the microplastic abundance of discrete stations detected in the field and remove abnormal detection values. The one-dimensional linear interpolation subunit is used to convert the sampling stations and grids into linear distances by projecting them along the center line of the cross section. The interpolation is used to obtain the microplastic abundance of the surface grid of the target cross section in the target time period. Constant extrapolation constraints are used at both ends of the cross section. The single-section flux integration subunit calls the flow rate, microplastic abundance of the surface grid, and duration of each time period of each surface grid, and accumulates to calculate the cross-boundary flux of microplastics in the surface water of each boundary cross section, and marks the flux as positive or negative. The total flux summary module 50 is used to traverse all polygonal boundary sections, retrieve the cross-boundary flux of microplastics in the surface water of each boundary section, and accumulate them to obtain the total cross-boundary flux of microplastics in the surface water of the entire study area. It also simultaneously counts the total input and output of each boundary section. The Layer Coupling and Segment Partitioning Module 60 includes a built-in grid parameter matching sub-unit, an automatic segment division sub-unit, and a source tracing result export sub-unit. The grid parameter matching sub-unit is used to extract grid coordinates, normal residual flow velocity, grid abundance, and single grid transport volume. Using grid coordinates as a unique index, it matches and generates a boundary section surface grid residual flow distribution map, a microplastic abundance spatial distribution map, and a transport thermal overlay map. The automatic segment division sub-unit is used to execute the above input / output segment division process. It automatically merges adjacent grids and divides continuous partitions based on residual flow direction, abundance threshold, and transport load. The source tracing result export sub-unit is used to batch output vector surface layers, partition thematic maps, partition flux statistics tables, and source tracing text reports, and connects to environmental assessment and marine management business platforms. The storage output module 70 is used to store model parameters, hydrodynamic grid data, microplastic sampling data, time-segmented flux, and zoned spatial vector data. Computer-readable storage media include USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), optical discs, server cloud storage media, etc. The computer-readable storage media stores a computer program. When the computer program is executed by the processor, it implements the method for calculating the transboundary flux of microplastics in the surface water of the sea as shown in Embodiment 1 above, and drives the above seven modules to work together to complete the calculation of the transboundary flux of microplastics in the surface water of the sea.

[0037] To further demonstrate the application value of the method for calculating the transboundary flux of microplastics in the surface waters of the sea in this invention, the following examples are used: Example 1: Calculation of the transboundary flux of microplastics in the Weifang sea area across multiple seasons; Example 2: Calculation of the single-section river flow into the sea at the mouth of the Dagu Jiahe River. The entire process of calculating the transboundary flux of microplastics in the surface waters of the sea in this invention is described in detail below.

[0038] Example 1: Calculation of Microplastic Transboundary Flux in Weifang Sea Area across Seasons The study area is a closed polygonal sea area near the coast of Weifang, divided into 4 boundary sections (section 1 is the mouth of the Jiaolai River, section 2 is the northernmost turning point of the bay, section 3 is the northeastern boundary, and section 4 is the mouth of the Xiaoqing River). The simulation time is May and October 2020, and the calculation period is 31 days. Based on the three-dimensional tidal current numerical simulation results of the Weifang sea area, the number of surface grids at boundary sections 1, 2, 3, and 4 in the Weifang sea area are 52, 1, 40, and 34, respectively. The widths of boundary sections 1, 2, 3, and 4 are 50120m, 783m, 32583m, and 26198m, respectively. The water depth at the surface grids of the boundary sections is 0.5m. The residual current distribution at the surface grids of the boundary sections in May and October 2020 is as follows. Figure 3 and Figure 4 As shown, the spatial distribution of microplastic abundance on the surface of the boundary section is as follows: Figure 5 and Figure 6 As shown, the calculation time for out-of-bounds flux is... It lasts for 31 days; The calculated results of the transboundary flux of microplastics in the surface waters of Weifang sea area are shown in Table 1. According to Table 1, in May 2020, a total of 1409 × 10⁻⁶ microplastics were exported from the surface waters to areas outside Weifang sea area. 8 One section, of which section 1 outputs 140×10⁻⁶ tons of wastewater to the sea. 8 One, cross section 2 outputs 14×10 to the sea area. 8 One, cross section 3 input 128×10 into the sea area 8 One, cross section 4 output 1383×10 to the sea area. 8 In October 2020, a total of 625 × 10⁶ microplastics were exported from surface waters to areas outside the Weifang sea area. 8 One section, of which section 1 inputs 37 × 10⁻⁶ ppm into the sea area. 8One, with cross-section 2 outputting 5×10 to the sea area. 8 One, cross section 3 output 2142×10 to the sea area. 8 One, cross section 4 input 1485×10 into the sea area 8 indivual; Table 1 - Schematic diagram of microplastic transboundary flux in surface water (unit: 10) 8 indivual)

[0039] exist Figure 3 and Figure 4 As shown, the red arrows represent the surface grid normal residual flow, and the arrow length corresponds to the flow velocity (1.5 cm / s in the legend is the reference scale). The arrows point in the direction of seawater flow. The blue area is the Weifang sea area, and the yellow area is land. When the water flow arrows point towards the interior of the blue Weifang sea area, it indicates that water is flowing into the sea (the normal residual flow velocity is positive, and microplastics are input). When the water flow arrows move away from the blue sea area and point towards the outer nearshore area, it indicates that water is flowing out of the sea area (the normal residual flow velocity is negative, and microplastics are output outward). according to Figure 3 , Figure 4 As shown in Table 1, the overall net water flow trend in May is that water and microplastics are input from Weifang Bay to the open sea and outside the estuary. The overall net water flow trend in October is that water and microplastics are input from Weifang Bay to the open sea and outside the estuary. The seasonal differences are caused by changes in monsoon and tidal residual flow, which change the total amount of microplastics entering and leaving. exist Figure 5 and Figure 6 In the diagram, the color scale represents the abundance of microplastics (numbers / L), with red indicating high abundance and blue indicating low abundance. Figure 5 It can be seen that section 4, Xiaoqing River Estuary, shows a high red-orange value, confirming the highest concentration of microplastics input into the river. Sections 2 and 3 are mainly blue with low values, indicating a low background concentration in the open sea. This matches the results of the large output at section 4 in May. Figure 6 It can be seen that: a red high abundance area appeared at the end of the Jiaolai River estuary in section 1, and the abundance increased throughout section 2. This corresponds to the flux input in section 1 and the large outflow in section 3 in October. The high-concentration water was transported to the open sea with the residual flow. Therefore, the total flux in May indicates that microplastics are being exported to the open sea, with section 4 at the Xiaoqing River Estuary being the core source of discharge. The total flux in October indicates that there is still a net export, but the dominant channel has shifted to section 3, while section 1 at the Jiaolai River Estuary and section 4 at the Xiaoqing River Estuary are receiving reverse inputs, reflecting the regulatory role of seasonal tides on microplastic exchange.

[0040] Example 2: Calculation of River Flux into the Sea at a Single Section of the Dagu Jiahe Estuary The study area is a single cross-section of the Dagu Jiahe River estuary. The simulation period was August 2020, with a calculation period of 31 days. Based on the three-dimensional tidal current numerical simulation results of the Dagu Jiahe River, the number of surface grids in the estuary cross-section was 20, the estuary width was 880m, and the water depth in the surface grids was 0.5m. The residual current in the surface grids of the estuary cross-section in August 2020 was observed... Figure 7 The abundance of microplastics in the surface mesh of the estuary section is shown in Figure 8 Calculation time for flux into the sea Based on a time period of 31 days, the calculated flux of microplastics entering the sea from the surface water of the Dagu Jiahe River in August 2020 was 1554 × 10⁻⁶. 8 indivual; exist Figure 7 In the diagram, the black horizontal line represents the calculated cross-section of the river estuary, and the red arrows all point towards the ocean (with the cross-section normal pointing outwards). The residual flow flows unidirectionally into the sea, consistent with the characteristics of a steady river flow. The evenly distributed arrows indicate minimal spatial variation in the surface velocity of the river channel. Figure 8 In the middle section, the left and right sides are red and orange high abundance areas, while the middle part is blue low value, indicating that sewage discharge along the riverbank has led to the enrichment of microplastics in the middle section of the estuary.

[0041] As can be seen from the above examples, the calculation method of the present invention has wide applicability. It can handle the calculation of net microplastic flux in the entire area of ​​bays with complex tidal currents and multiple boundaries, and can also be used for the calculation of river mouth sections with unidirectional flow, breaking through the traditional limitation of calculation that is only applicable to rivers.

[0042] In summary, this invention relies on three-dimensional tidal numerical simulation to separate the net transport residual current of the tidal cycle as the driving force for microplastic migration. It performs hierarchical calculations of grid normal flow, microplastic cross-boundary flux at each cross section, and total cross-boundary flux across the entire domain. It solves the spatiotemporal matching problem between discrete sampling and numerical grid through time-series and cross-sectional dual linear interpolation. With the addition of layer coupling and partitioned source tracing processes, it can divide high / medium / low input and output segments of microplastics and output visualized spatial source tracing results. It provides technical support for the source analysis, total quantity control, and ecological environment governance of microplastics, a new marine pollutant, and has good application value.

[0043] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for calculating the transboundary flux of microplastics in marine surface water, characterized in that, Specifically, the following steps are included: Step S1: Construct a three-dimensional tidal current numerical model of the study sea area and complete the model accuracy verification. Output hydrodynamic data of the boundary section of the study sea area. The hydrodynamic data includes at least the surface grid velocity vector of the boundary section. The study sea area is generalized into a polygonal region. Each edge of the polygonal region is defined as the boundary section of the corresponding study sea area. Step S2: Based on the output results of the three-dimensional tidal current numerical model of the research sea area, calculate the residual flow velocity of each surface grid of the boundary section in the x and y directions. By solving the residual flow velocity in the x and y directions, the residual flow velocity and residual flow direction of the surface grid of the boundary section are obtained. Combined with the angle between the boundary section and the north direction, calculate the residual flow velocity of the surface grid in the normal direction of the boundary section. Step S3: Based on the residual flow velocity of the surface grid in the normal direction of the boundary section, and combined with the water depth and width of the surface grid in the boundary section, calculate the flow rate of each surface grid in the normal direction of the boundary section at each time period. Step S4: Collect surface seawater samples from the research area and detect the abundance data of microplastics in the surface seawater. Obtain the abundance of microplastics in the surface grid of the target section during the target time period through linear interpolation. Combine the flow rate, microplastic abundance of the surface grid, and duration of a single time period of each surface grid, and solve the cross-boundary flux of microplastics in the surface water of each boundary section. Step S5: Traverse all boundary sections of the study area, sum up the cross-boundary flux of microplastics in the surface water of each boundary section, and obtain the total cross-boundary flux of microplastics in the surface water of the entire study area, thus realizing the calculation effect of cross-boundary flux of microplastics in the surface water of the sea area. Step S6: Extract the residual flow velocity, microplastic abundance, and time-segmented microplastic transport of the surface grid in the normal direction of the boundary section. Based on the spatial coordinates of the surface grid, match the corresponding boundary section surface grid residual flow distribution map and microplastic abundance spatial distribution map. Divide the microplastic input / output section into microplastic input / output sections according to the microplastic abundance of the surface grid, and output the spatial partitioning source tracing results.

2. The method for calculating the transboundary flux of microplastics in marine surface waters according to claim 1, characterized in that, The construction of the three-dimensional tidal current numerical model for the research area in step S1 includes: Coastline data, water depth data, and boundary tidal level data are acquired. FVCOM or HEC-RAS hydrodynamic software is used for modeling. Hydrodynamic data of all boundary sections of the study area are extracted and output. The hydrodynamic data includes surface grid velocity vectors of the boundary sections, x-axis velocity time series of each surface grid, y-axis velocity time series, width of each surface grid, and water depth of the surface grid. The coastline data is obtained through field measurements or interpretation of remote sensing images, and the water depth data is obtained through field measurements or digital nautical charts. The measurement years of the coastline data and water depth data are matched. The model accuracy verification includes at least the verification of tidal current observation data from no less than two microplastic sampling survey stations and the verification of tidal level observation data from no less than three microplastic sampling survey stations at each boundary section of the study sea area. The verification duration of the tidal current observation data is no less than 25 hours, and the continuous verification duration of the tidal level observation data is no less than one month. The number of surface grids at each boundary section is no less than the number of microplastic sampling survey stations at the boundary section.

3. The method for calculating the transboundary flux of microplastics in marine surface waters according to claim 1, characterized in that, In step S2, the calculation process for the residual flow velocity in the x and y directions of each surface grid at the boundary section includes: Construct an orthogonal Cartesian coordinate system and decompose the velocity vector of the surface mesh at the boundary section along the x and y axes. In the numerical simulation, the formulas for calculating the residual velocity of the surface mesh at the boundary section in the x and y directions after decomposition include: ， ； in, For the boundary section The residual flow velocity in the x-direction of each surface grid. The number of time steps in the numerical simulation and satisfying the formula , This represents the number of tidal periods in the numerical simulation. For the duration of the tidal cycle, This represents the time step of the numerical simulation. For the boundary section The first surface mesh The flow velocity in the x-direction at each time step. For the boundary section The residual flow velocity in the y direction of each surface grid. For the boundary section The first surface mesh The flow velocity in the y-direction at each time step.

4. The method for calculating the transboundary flux of microplastics in marine surface waters according to claim 3, characterized in that, The formulas for calculating the residual flow velocity and direction of the surface mesh at the boundary section in step S2 are as follows: include: ， ； in, For the boundary section The residual flow velocity of each surface grid, For the boundary section The direction of residual flow in each surface grid. For the boundary section The residual flow velocity in the x-direction of each surface grid. For the boundary section The residual flow velocity in the y direction of each surface grid.

5. The method for calculating the transboundary flux of microplastics in marine surface waters according to claim 4, characterized in that, The formula for calculating the residual flow velocity of the surface mesh in the normal direction of the boundary section in step S2 is as follows: ； in, The first normal direction of the boundary section Surface mesh Residual flow velocity during the time period For the boundary section The residual flow velocity of each surface grid, For the boundary section The direction of residual flow in each surface grid. The angle between the boundary section and the northward direction.

6. The method for calculating the transboundary flux of microplastics in marine surface waters according to claim 5, characterized in that, The formula for calculating the flow rate of each surface grid in the normal direction of the boundary section in step S3 is as follows: ； in, The first normal direction of the boundary section Surface mesh Traffic during a specific time period The surface water depth of the boundary section grid. The first normal direction of the boundary section Surface mesh Residual flow velocity during the time period For the first section on the boundary surface The width of each surface grid.

7. The method for calculating the transboundary flux of microplastics in marine surface waters according to claim 6, characterized in that, The formula for calculating the transboundary flux of microplastics in the surface water of each boundary section in step S4 is as follows: ； in, For the first Transboundary flux of microplastics in the surface water of a boundary section Calculate the quantity for the total time period. This represents the number of surface meshes at the boundary section. The first normal direction of the boundary section Surface mesh Traffic during a specific time period For the boundary section after linear interpolation Surface mesh Microplastic abundance over time period for The calculation duration of the time period.

8. The method for calculating the transboundary flux of microplastics in marine surface waters according to claim 7, characterized in that, The formula for calculating the total transboundary flux of microplastics in the surface water of the entire study area in step S5 is as follows: ； in, To study the total transboundary flux of microplastics in the surface waters of the sea, To study the number of marine boundary sections, For the first Transboundary flux of microplastics in the surface water of a boundary section.

9. The method for calculating the transboundary flux of microplastics in marine surface waters according to claim 1, characterized in that, In step S4, surface seawater samples from the study area are collected and surface seawater microplastic abundance data are detected. The surface grid microplastic abundance of the target section during the target time period is obtained through linear interpolation. Specifically, this includes: Several seawater surface sampling stations were set up at intervals along the boundary sections of the study sea area. Seawater samples were collected from the seawater surface. The samples were pre-processed and tested in accordance with the seawater sample processing specifications. Each sampling station was bound with the sampling station coordinates, sampling time period and measured microplastic abundance value to obtain a discrete station dataset. The seawater surface was defined as the water area from 0.1m to 1m below the sea surface. The measured microplastic abundance values ​​were subjected to time-series linear matching. The time-series linear interpolation method was used to uniformly interpolate the measured microplastic abundance values ​​to each calculation period of the three-dimensional tidal current numerical model of the study area, so as to obtain the time-series abundance of each sampling station in any calculation period. All sampling points and surface grids of the boundary section are projected along the direction of the section to the center line of the section and converted into the linear distance of the section. The microplastic abundance of each surface grid in the corresponding time period is obtained by using a one-dimensional two-point linear interpolation formula. After interpolation, the abundance range of all surface grids is verified, and gridded microplastic abundance data corresponding one-to-one with the hydrodynamic grid is generated, i.e., the surface grid microplastic abundance of the target section in the target time period.

10. The method for calculating the transboundary flux of microplastics in marine surface waters according to claim 1, characterized in that, The numerical sign of the total transboundary flux of microplastics in the surface water of the study area represents the transport direction. A positive value indicates that the water flows into the interior of the study area, while a negative value indicates that the water flows out of the study area.

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