# Aerial Remote Sensing of Aquatic Microplastic Pollution:
## 3 Literature Survey
### Remote Sensing of Aquatic MPs Based on Their Spectral Characteristics
Building on these studies, others [23, 24] have aimed to theoretically assess the potential of spaceborne remote sensing of aquatic MPs (among other plastic pollution and debris) based on their known spectral characteristics in relation to the capabilities and specifications of current optical Earth observation satellites. Since MP particles are only a minute fraction of the size of a pixel in a typical satellite image (>1 m to >100 m; also called the'spatial resolution'), it is expected at minimum that a technique called'sub-pixel spectral unmixing' will be necessary to identify the signature of MPs among other features (e.g., open water) sharing the same pixel space. [PERSON] [23] in 2021 provided valuable detailed calculations taking into account factors such as measured marine MP concentrations and the sensitivity of satellite sensors, concluding that the particles are unlikely to be detectable. However, they only considered the visible-NIR range, whereas the most distinctive spectral features of MPs are in the SWIR region. [PERSON] et al. [24] in 2023--additionally considering the SWIR region--similarly suggested that satellite detection of aquatic MPs, given the coarse spatial resolution of the imagery, may not be possible unless they are present in very high concentrations. Nevertheless, their work facilitates the identification of satellites that could potentially achieve detection based on the positions and widths of their bands in relation to the most pronounced spectral features of environmental aquatic MPs (Figure 2). Among multispectral satellites, MODIS (launched in 2002; free imagery) and WorldView-3 (2014; imagery for purchase) stand out: both have NIR bands aligned with MPs' most pronounced NIR feature at the 931 nm wavelength, although WorldView-3's band may be too wide to resolve it; both have relatively narrow bands at or very near MPs' most pronounced SWIR feature at 1215 nm; and WorldView-3 additionally has a band aligned with MPs' second most pronounced SWIR feature at 1732 nm. Two more recently launched hyperspectral satellites--PRISMA (2019) and EnMAP (2022), both providing imagery at no cost--have contiguous narrow bands spanning the entire visible-SWIR range, also making them potentially capable of detecting MPs. Despite the skepticism expressed by the authors of these earlier studies, [PERSON] et al. [25] recently (2024) reported successful detection of MPs in the Bohai Sea, China, in MODIS imagery. Rather than directly detecting the spectral signature, they tested for correlations between reflectance levels in MODIS bands and lab-measured MP concentrations in water samples collected from a series of locations throughout the sea. Several correlations were found, most significantly in the two bands corresponding to the most pronounced spectral features of MPs in the SWIR and NIR regions. The correlations were used to create a predictive model of MP concentrations based on MODIS imagery that was then applied to the full extent of the sea, as well as across imagery captured on different dates. However, the authors caution that uncertainties remain in the model and require further investigation. Most recently, [PERSON] et al. [26] employed a similar correlative approach to detect and predict MP concentrations in coastal waters adjacent to Dubai, UAE. Interestingly, they did so solely using NIR bands from the Sentinel-2 multispectral satellite constellation (2015; free imagery), which may have been made possible by significantly higher MP concentrations along the coast of a major urban center than would be expected farther offshore. In contrast, in 2023, [PERSON] et al. [27] reported that neither Sentinel-2 nor PlanetScope multispectral satellite imagery was effective for detecting MPs in the Tisza River across Central and Eastern Europe.