The Effect of Microplastic on the Radiative Properties of Sand and Water

Plastic is ubiquitous throughout the natural environment, but we are currently unsure how plastic affects the thermal radiation of its surroundings, making it difficult to ascertain environmental effects and detect it remotely. To determine the thermal characteristics of plastic in two substrates on Earth where plastic has been accumulating, two different forms of white high-density polyethylene (HDPE) microplastics have been mixed with dry sandy beach sediment and separately with freshwater in different proportions. The temperatures of these mixtures were recorded by both a contact and a remote sensor as they tended to room temperature after being initially heated or cooled. The temperature comparisons between the contact thermocouples and the remote infrared (IR) thermal camera gave strong evidence for the presence of white HDPE generally increasing the emissivity and decreasing the thermal inertia of both the sand and water mediums, especially when heat energy left the system. However, when heat entered the system and the sand mixture warmed up towards room temperature, as the pellet form of HDPE increased in proportion, the thermal emissivity declined, increasing thermal inertia. This indicated that initially cooled sand and pellet mixtures were more resistant to absorbing heat energy, having implications for marine life in beach environments. The water and plastic mixtures only experienced an increase in emissivity and a drop in thermal inertia when the HDPE microplastic covered 50% or more of the water’s surface. As polyolefins such as HDPE are commonly found afloat within the Great Pacific Garbage Patch gyre circulation, the potential decrease in thermal inertia caused here has consequences for marine life. Plastic is likely to be altering the absorption and emission of electromagnetic radiation in the Earth’s surface energy balance.