Plastics are ever prevalent and have been an indispensable part of our lives for the last 50 years. As a result, the production and over-consumption of this material has led to a significant increase in global plastic production and subsequent disposal over the last two decades. However, due to poor waste management practices all around the world, a substantial amount of plastics, ranging from macro (>25mm) to nano (<100nm) sizes, ends up in marine ecosystems. In particular, microplastics are particles of between 1 to 5mm that originate mainly from primary sources such as textile fibres, cosmetic microbeads, and manufacturing residues, or secondary sources such as the breakdown of larger plastic pieces . These plastic particles occur in varying chemical composition, and have different properties, buoyancies and spectral wavelengths. The most commonly found microplastic polymers include polyethene (PE), polypropylene (PP), polyvinylchloride (PVC), polystyrene (PS), and polyamide (PA) which all break down under the influence of UV radiation, sea salinity and abrasion due to, e.g. waves .
The unique spectral characteristics of these different plastic polymers in the near-infrared (NIR) to short-wavelength infrared (SWIR) spectra are nowadays widely used in optical sorting processes in the waste industry . Despite this, a robust and comprehensive analysis of the spatial extent of micro-sized polymers in the marine environment is still lacking due to the inability of current airborne tools to reliably assess global distribution patterns and accumulations . As a result, there is a clear need for advancements in this field as remote sensing imagery offers the high temporal, spectral and spatial resolutions of the ocean surface needed to quantify the amount of microplastic pollution floating in the sea surface layers. Due to a rise in global awareness and advancements in technology, researchers are now engaged in a variety of projects and studies. One of the most recent studies in this field examined the optical properties of microplastics in correlation with the enhanced reflectance in ocean color imagery to improve the assessment of its detection and quantification in the visible and infrared wavelengths. The study was conducted by Garaba and Dierssen and assessed the spectral reflectance of microplastic from the North Atlantic Ocean. They found that spectral reflectances “could be represented as a single bulk average spectrum with notable absorption features at ~931, 1215, 1417 and 1732 nm” , thereby having a similarity to the spectra of polymers known from the waste industry .
However, another crucial factor, namely ocean color, has to be taken into account for the successful assessment of microplastic abundance and distribution patterns in the seas and other bodies of water. Emberton and colleagues gave an overview of multispectral and hyperspectral remote sensors for ocean color on satellites  which also frequently carry other instruments that are useful for the detection of marine plastics. One example is with the Sentinel-3 satellite, which carries the ‘Ocean and Land Cover Instrument’ (OLCI) . This instrument has 21 bands in the range of 0.4–1.02 μm, but Sentinel-2 also carries the Sea and Land Surface Temperature Radiometer (SLSTR) that comprises nine bands in the spectrum of 0.55–12 μm . These instruments allow for the application of particle-tracking models as well as ocean surveys in general, hopefully leading to a reflectance model showing accurate distributions of marine microplastics. The European Space Agency (ESA) is responding to the problem of marine plastic pollution and the potential for earth observation techniques to determine accumulations and distribution of this gigantic problem. They are tackling the issue by the launch of their new program ‘OptiMAL’ – short for ‘Optical Method for Marine Litter Detection’ – that uses data derived from Sentinel-3’s OLCI and SLSTR instruments to detect not only macro debris but also microplastics which are suspended in the sea surface layers . When more mature, this monitoring approach will be able to provide accurate and near-time global measurements and thus be a powerful tool in the scientific context. The ‘power of the picture’ is then hopefully able to raise politic as well as public awareness and thereby increase the participation for mitigation and remediation of a significant global problem.
Another issue that needs to be addressed is the more detailed observation of ocean dynamics and sea surface currents in particular. This is a factor which is currently not directly measured by existing earth observation systems  but instead by tracking drifting buoys. In the seas, microplastics are, depending on their buoyancy, either floating in the sea surface layer or are suspended in the water column . Global ocean circulation, mainly driven by the five subtropical gyres, and coastal currents enable the long-distance transport of these particles in complex patterns due to seasonal and local variations. These circulation pathways account for the associated risks of marine microplastic pollution due to their spatial and temporal distribution.
Therefore, there is not only a need to further develop ocean color sensors as well as the confinement of spectral reflectance of different types of microplastics in the oceans, but also the need to fill the gap in our observational capabilities from space to determine sea surface currents. This is in particular valid for boundary habitats such as frontal zones, which are known to be hot spots of surface polymer aggregation . One of the two finalists of the proposal to be the ESA’s Earth Explorer 9, launching in 2025, is the Sea Surface Kinematics Multiscale Monitoring (SKIM) mission that has been developed by the ‘French Institute For Oceanic Research’ (Ifremer). SKIM is designed to explicitly detect and measure sea surface currents, ice drifts as well as waves by using an innovative Doppler Ka-band radar. It will estimate the ocean surface velocity vector by using satellite Doppler oceanography as well as ocean wave spectra. This new era of sea surface current assessment would make a significant improvement in the detection of microplastic carrying currents and ocean dynamics in general by covering not only global oceans but also regional seas .
In this context, the need for interdisciplinary science is more important than ever in order to gather the pieces together for the big picture. The good news is that accurate high resolution and temporal maps of marine microplastic distribution can be created in the near-future by using satellite remote sensing techniques based on multiple-source input data such as an updated spectral library for marine plastics in combination with an accurate understanding of detailed near-time sea surface current data. We hope that recent advancements within the field of Earth Observation and the growing institutional support for the development of applications are boosting the improvements of spaceborne remote sensing techniques to raise global awareness for marine microplastic pollution.
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