Microplastic aging and adsorption in the atmosphere, and their associated impacts on various spheres of the earth: A review.

Microplastic (MPs, size <5 mm) is an emerging category of contaminants with detrimental effects on human health, climate, and ecology. The atmospheric pathway is a crucial transport route for the migration of MPs from source to receptor locations. This long-range transport leads to the ubiquitous presence of MPs across all environmental matrices and constrains the source-transport pathway-sink interaction. During atmospheric transport, MPs experience aging and adsorption as a result of interactions with winds, solar radiation, moisture, pH, and atmospheric pollutants, which alters their hydrophilicity, structure, surface area, size, color, and the capacity for adsorption, often resulting in elevated toxicity and associated risks. However, the multifaceted dynamics of atmospheric aging of MPs and consequent impacts are poorly understood. This review presents a critical assessment of three major factors that determine the nature and degree of MP aging and adsorption in the atmosphere, namely: intrinsic MP properties such as the degree of unsaturation, crystallinity, presence of functional groups, charge, specific surface area, and structural defects; environmental factors such as temperature, pH, moisture, and the presence of chemical species; and pollutant characteristics such as charge and hydrophilicity/hydrophobicity that influence adsorption, with an emphasis on potential mechanisms. Additionally, the review presents a comparative assessment of the critical factors and mechanisms responsible for aging and adsorption in atmosphere with those in other environmental media. Further, the potential impacts of atmospherically aged MPs on climate, the biosphere, cryosphere, pedosphere, and hydrosphere are summarized. The review finally identifies key knowledge gaps and outlines perspectives for future research.