# Aerial Remote Sensing of Aquatic Microplastic Pollution:
## 4 Discussion and Recommendations  
Based on the literature, there has been interest in the potential to detect and quantify aquatic microplastics via aerial remote sensing for about a decade, but progress has been gradual, and no single approach has yet reached maturity or operational status. The main approaches that have emerged are the detection of MPs' spectral characteristics using passive optical sensors, the detection of water surface roughness suppression by MP-engendered surfactants using active radar sensors, and the detection of various indirect proxies for MPs. The latter approach has been shown to be viable in certain cases, although in all instances to date, the utility or reliability of proxies is limited to specific locations, systems, or phenomena. Notably, in most cases, there is no direct causal relationship between MPs and proxies. Thus, these indirect proxy-based approaches can be ruled out as ideal wide-scale, broadly applicable aquatic MP remote sensing solutions, although they may remain useful in specific situations, provided adequate independent validation measures are taken. The detection of aquatic MPs via satellite-borne radar remote sensing of water surface roughness reduction was reported as far back as 2018 [28], but relatively few studies have since followed up and built upon this approach, and three of the six published studiesare brief conference papers [28, 29, 30]. However, the most recent contributions to this avenue of investigation [31, 32] have taken a more promisingly rigorous and methodical turn, helping to clarify and elucidate some of the unknowns associated with this approach, namely whether the relationship between surfactant and MP concentrations is consistent or predictable, and the precise physics underlying the surface roughness suppression. Nevertheless, further unknowns would benefit from similarly methodical studies involving experiments under controlled conditions or rigorous comparisons between remote sensing and in situ observations. These unknowns may include questions surrounding (1) the mechanisms and conditions affecting microbial digestion of MPs and in turn the production of surfactants, which could impact the relationship between their respective concentrations; (2) other sources of ocean surfactants (including microbial digestion of larger plastic debris) or surface smoothening substances that might be mistaken for MP-engendered surfactants; and (3) precisely how wind conditions and small-scale ocean circulation patterns (e.g., Langmuir cells [28]) affect the manifestation, detectability, and interpretation of the water surface phenomenon. Direct detection of aquatic MPs would be ideal, and doing so via their spectral signature seemingly involves fewer potentially confounding factors compared to the other two approaches, but is particularly challenging due to several limitations. From the outset, the very small size of the particles, combined with their relatively low proportional coverage of the near-water surface area (estimated to be 0.005% at most based on reported ocean concentrations [23]), are bound to result in a weak signal in the context of spectral unmixing in much larger image pixels. Second, most environmental MPs are weathered, which can cause various changes in the particles, including fading or loss of their distinct visible colors [39]. As such, weathered MPs may not be discernible in the visible light region in addition to being minimally distinctive in the NIR region, the range within which the most readily accessible and highest spatial resolution airborne and spaceborne optical sensors operate. Third, although a number of multispectral and hyperspectral sensors possess SWIR bands where MPs are most distinctive, the severe attenuation of the SWIR signal when the particles are wet and submerged under even a thin layer of water considerably reduces their detectability in this region.