# Aerial Remote Sensing of Aquatic Microplastic Pollution:
## 1 Introduction  
Microplastics (MPs) are ubiquitous environmental contaminants globally, including in marine and freshwater systems [1]. They are plastic particles less than 5 mm in size that can enter the environment as primary MPs or through the breakdown of larger plastic objects, and can be considered a complex contaminant class due to their high variability (e.g., plastic type, particle shape and size, weathering; [2]). MPs are small enough to be ingested by aquatic biota of all sizes, including microscopic plankton [3], and the implications of exposure are not fully known. However, current research efforts are multitudinous, and evidence is mounting that these pollutants can have negative impacts [4]. Effects on aquatic biota have been recorded at all levels of biological organization and include oxidative stress, genetic effects, inflammation, digestive disruption, and reduced growth [4, 5], which may be influenced by MPs' chemical additives and/or contaminants or pathogens that can adhere to their surface [1]. One of the most important elements in understanding the risks microplastics pose to aquatic biota is to characterize current real-world exposures and consistently monitor them over time. For example, in Canada alone, an estimated 5000 kilotons of plastic waste is produced per year and is projected to increase [6]; therefore, we may expect MP concentrations to continue rising as these products break down over their long lifespans. Developing methodsto efficiently monitor MPs on a wide scale would be highly beneficial and could help fill existing data gaps in monitoring [78]. Currently established methods for determining aquatic MP concentrations and characterizing plastics involve collecting water samples followed by specialized laboratory microscopy techniques [9], which are time-consuming, costly, and require sophisticated instruments and high levels of expertise [10], effectively precluding frequent or wide-scale monitoring. While in situ sampling and detailed analyses of this nature cannot be replaced with higher-level monitoring, the benefits of wide-scale techniques could be used to advance various goals, including mapping aquatic MP spatial distributions, examining broad temporal trends, informing modeling of MP fate and behavior in aquatic systems, and identifying sources and hotspots where detailed research can be focused. Aerial (i.e., satellite-, crewed aircraft-, or drone-based) remote sensing techniques have the potential to fill this gap, but as of yet have been an underdeveloped resource in aquatic microplastic research. Remote sensing relies on the measurement of electromagnetic radiation, i.e., pure energy traveling in the form of waves, with varying wavelengths giving rise to a spectrum that includes, from shortest to longest wavelength: radioactive gamma rays, X-rays, ultraviolet rays, visible light, infrared waves, microwaves, and radio waves. The Sun is a significant source of electromagnetic radiation, and so-called 'passive' sensors measure solar radiation that travels down through the Earth's atmosphere and reflects back up off features and objects on the ground or on or below water surfaces. Alternatively, 'active' sensors emit their own radiation toward surfaces and targets and measure the reflected radiation to gain information about the nature and characteristics of the targets. Many studies over the past two decades have employed aerial remote sensing to detect and survey larger floating and beached plastic litter [11], suggesting potential applicability to aquatic microplastics, although publications related to the latter seem to be comparatively sparse. Our objectives were to assess and summarize the state of the science of aerial remote sensing of aquatic microplastics by means of a literature survey, and to identify potential next steps that are likely to help advance the science.