# Aerial Remote Sensing of Aquatic Microplastic Pollution:
## 3 Literature Survey
### Remote Sensing of Aquatic MPs Based on Their Spectral Characteristics  
Different materials and substances absorb and reflect different wavelengths of incident solar radiation (including visible light) in different proportions (proportion reflected ='reflectance'; proportion absorbed = 'absorbance'), resulting in unique'spectral signatures' that can be detected by passive 'optical' sensors that measure reflected radiation. For example, plastics have a distinct spectral signature compared to other materials, and accordingly, the presence of microplastics at or near the water surface should produce a theoretically detectable unique signature. However, it can be expected that the signal is faint due to a combination of the very small size of the particles, their limited concentration relative to the water surface area, and the rapid attenuation (i.e., weakening) of the reflectance signal of submerged features with increasing depth as a result of water's absorption of radiation. As a potential precursor to aerial optical remote sensing of aquatic MPs, several studies have analyzed the spectral reflectance of marine-harvested MPs in laboratory or outdoor  
Figure 1: Simplified visual summaries of the two primary approaches to aerial remote sensing of aquatic microplastics: (**left**) based on their spectral reflectance of solar radiation measured with passive optical sensors and (**right**) based on their reduction of water surface roughness measured with active radar sensors. controlled conditions with stationary sensors placed directly above samples, in some cases in a wet state or water suspension to approximate the natural conditions encountered by remote sensing [16, 17, 18, 19, 20]. These studies have notably employed 'hyperspectral' sensors capable of precisely measuring spectral signatures by recording numerous (up to hundreds of) narrow and typically contiguous spectral bands (i.e., discrete intervals of the electromagnetic spectrum) that may collectively span the near-ultraviolet (UV) to shortwave infrared (SWIR) regions (~350-2500 nm wavelengths), passing through the visible (~400-700 nm) and near-infrared (NIR;~700-1000 nm) regions [21]. In contrast, conventional optical cameras record three wide and overlapping bands centered in the visible red, green, and blue (i.e., 'RGB') regions, respectively; while'multispectral' sensors typically record a larger number of narrower bands that may extend into the NIR and SWIR regions, although the bands are significantly wider and less numerous than those of hyperspectral sensors. The key findings of these studies are as follows. First, the reflectance of a weathered, marine-harvested mixture of MPs of various plastic types is relatively flat (i.e., indistinctive) in the visible region (meaning RGB sensors will have a limited capacity to detect them) while presenting more pronounced (i.e., distinctive) absorption and reflection peaks in the NIR and particularly the SWIR region (best detected by multispectral or hyperspectral sensors). Second, wet MPs present significantly attenuated reflectance compared to dry MPs, with attenuation increasing with wavelength from ~10% in the near-UV region to ~90% in the SWIR region [17,20]. The spectral characteristics of this environmentally representative MP mixture are illustrated in Figure 2, including a comparison between dry and wet samples. Such a spectral signature poses a challenge for detecting aquatic MPs via aerial remote sensing because water's absorbance exponentially increases with wavelength across the visible-SWIR range, making particles below ~15 cm in depth likely impossible to detect [22]. It should also be noted that different plastic types (e.g., polyethylene, polypropylene, polystyrene, etc. ), when examined individually, present moderately varying spectral signatures [20].