Microplastics, road dust, and biological particles drive hazardous urban ice fog formation

Ice fog is a common form of air pollution that forms at low temperatures and poses extreme hazards for aircraft and automobile travel, particularly in high-latitude environments. Despite its severe dangers, urban ice fog formation is not well understood, mainly because of the paucity of measurements of the particles that form ice fog. Here, we investigate the chemical composition and sources of atmospheric particles forming ice fog in Fairbanks, Alaska. Ice fog crystals were collected and sublimed, leaving behind residual particles, likely corresponding to the ice nucleating particles (INPs), which were examined in detail through single-particle analytical methods. Non-exhaust vehicular emissions were the dominant source of ice fog residual particles, in the forms of road dust (66–69%, by number) and microplastics (13–16%), such as tire wear. Biological particles (e.g., lichens) comprised 6–10%, highlighting their importance as INPs in wintertime urban and boreal environments. While mineral dust and primary biological particles are well-known INPs, their importance in wintertime cities was not known until recently. The observation of road dust reveals an unintended air quality impact of gravel application for traction on roads. These observations demonstrate microplastics acting as INPs in the urban atmosphere, corroborating recent laboratory experiments. Since most ice fog nuclei are from anthropogenic sources, their emissions, and subsequently, their impacts on ice fog formation, are controllable and addressable by technical and policy interventions.