# Aerial Remote Sensing of Aquatic Microplastic Pollution:
## 3 Literature Survey
### Remote Sensing of Aquatic MPs Based on Physical Water Surface Effects  
Inferences can be made about substances and materials at or near the water surface via their effects on surface roughness/smoothness, viscosity, tension, etc. ; for example, an oil slick covering the water surface. These effects can be detected by measuring and analyzing the geometric reflective properties of the surface; for example, whether incident radiation scatters from the surface in many directions (indicative of a rough texture), reflects more unidirectionally (smoother texture), or experiences changes in wave polarization (the orientation of the waves in the plane perpendicular to their direction of travel) post-reflection. In remote sensing, such properties are most commonly measured using active sensors such as synthetic aperture radar (SAR) and bistatic radar, which emit pulses of radiation in the microwave region and measure the 'backscatter' that reflects back to the sensor. [PERSON] et al. [28] were the first, in 2018, to hypothesize that smoother-than-expected ocean surface conditions--assessed via satellite-borne SAR imaging--under certain wind speeds may result from the presence of microplastics (Figure 3). Further research by several of the same authors [29] provided evidence that surface roughness suppression is caused by surfactants--substances that alter water surface tension, and a known by-product of microbial colonization and digestion of MPs--and that the surfactants in their study areas were not associated with other known sources, such as algal blooms. [PERSON] and [PERSON] [30] reported using the same approach to detect MPs in the Yamuna River, India, while [PERSON] et al. [27] reported that it was ineffective in the Tisza River. Separately, [PERSON] and [PERSON] [31] presented a similar approach to detect and quantify concentrations of ocean MPs using satellite-borne bistatic radar. Notably, they showed that MP concentrations predicted by their approach were generally consistent with previous models based on large-scale ocean circulation patterns, but had a better ability to reveal short-term variations. They further noted that detection of the water surface effects required moderate wave-inducing sea surface winds in the range of 3-11 m/s. [PERSON] et al. [32] followed up with controlled laboratory experiments that elucidated the mechanisms and conditions through which surface roughness suppression is induced, demonstrating conclusively that surfactants and not MPs themselves are responsible for the satellite-observed phenomenon. Figure 3: When the water surface is calm, microwave radiation pulses emitted by satellite-borne synthetic aperture radar (SAR) experience mirror-like (i.e., ‘specular’) reflection from the surface. Since SAR pulses are emitted at an angle, they reflect away from the sensor, and the water surface therefore appears dark in the resulting image. Under moderate wind conditions, the formation of capillary waves results in a more uneven (or 'rough') water surface that reflects SAR pulses in multiple directions (i.e., 'diffuse' reflection), including back toward the sensor, causing the water surface to appear brighter. The presence of surfactants at the surface generated by microbial digestion of microplastics partially suppresses capillary waves, causing the water surface to be smoother than usual under moderate wind conditions, and therefore appear darker than expected in SAR imagery. In this Sentinel-1 SAR image of the North Atlantic Ocean from [PERSON] et al. [28]--acquired under moderate wind conditions in an area of low chlorophyll-a concentrations (precluding algal bloom-generated surfactants)--it is hypothesized that the patterns of varying water surface brightness result from concentrations of microplastic-related surfactants in the darker areas, where they have caused relative smoothening of the surface. Another physical water surface property, the refractive index of the surface microlayer, was shown by [PERSON] et al.