Sustained observations are required to determine the marine plastic debris mass balance and to support effective policy for planning remedial action. However, observations currently remain scarce at the global scale. A satellite remote sensing system could make a substantial contribution to tackling this problem. Here, we make initial steps towards the potential design of such a remote sensing system by: (1) identifying the properties of marine plastic debris amenable to remote sensing methods and (2) highlighting the oceanic processes relevant to scientific questions about marine plastic debris. Remote sensing approaches are reviewed and matched to the optical properties of marine plastic debris and the relevant spatio-temporal scales of observation to identify challenges and opportunities in the field. Finally, steps needed to develop marine plastic debris detection by remote sensing platforms are proposed in terms of fundamental science as well as linkages to ongoing planning for satellite systems with similar observation requirements.

Most advances in the remote sensing of floating marine plastic litter have been made using passive remote-sensing techniques in the visible (VIS) to short-wave-infrared (SWIR) parts of the electromagnetic spectrum based on the spectral absorption features of plastic surfaces. In this paper, we present developments of new and emerging remote-sensing technologies of marine plastic litter such as passive techniques: fluid lensing, multi-angle polarimetry, and thermal infrared sensing (TIS); and active techniques: light detection and ranging (LiDAR), multispectral imaging detection and active reflectance (MiDAR), and radio detection and ranging (RADAR). Our review of the detection capabilities and limitations of the different sensing technologies shows that each has their own weaknesses and strengths, and that there is not one single sensing technique that applies to all kinds of marine litter under every different condition in the aquatic environment. Rather, we should focus on the synergy between different technologies to detect marine plastic litter and potentially the use of proxies to estimate its presence. Therefore, in addition to further developing remote-sensing techniques, more research is needed in the composition of marine litter and the relationships between marine plastic litter and their proxies. In this paper, we propose a common vocabulary to help the community to translate concepts among different disciplines and techniques.

Marine plastic litter has become a global challenge, affecting all regions of the planet, with massive plastic input to the marine environment every year. Novel remote sensing methods can greatly contribute to face this complex issue with their ability to provide large-scale data. Here we present experimental tests exploring the potential of the hyperspectral fluorescence LIDAR technique for the detection and characterization of plastics when plunged into a layer of natural water. The experiments were carried out in the laboratory by using an in-house developed fluorescence hyperspectral LIDAR with 355 nm excitation from a distance of 11 m on weathered commercial plastic samples plunged into natural water. Results showed the capability of the technique to detect the fluorescence features of several types of plastics, also when plunged into water, and to decouple it from the fluorescence due to colored dissolved organic matter and from Raman scattering due to water molecules. Discrimination of plastics against other marine debris, e.g., vegetation and wood, has also been discussed. The study lays a basis for fluorescence LIDAR remote sensing of plastics in marine environment and paves the way to the detection of MPL also in conditions (e.g., submerged or transparent plastics) that are likely to be challenging by using other passive remote sensing techniques.

Fluorescence lidar techniques offer considerable potential for remote, non-invasive diagnostics of stone cultural heritage in the outdoor environment. Here we present the results of a joint Italian-Swedish experiment, deploying two hyperspectral fluorescence lidar imaging systems, for the documentation of past conservation interventions on the Colosseum, Rome. Several portions of the monument were scanned and we show that it was possible to discriminate among masonry materials, reinforcement structures and protective coatings inserted during past conservation interventions, on the basis of their fluorescence signatures, providing useful information for a first quick, large-scale in situ screening of the monument.