Atmospheric deposition of anthropogenic particles and microplastics in south-central Ontario, Canada

Microplastics are ubiquitous in the environment; however, few studies have examined their abundance in atmospheric deposition in pristine environments, remote from anthropogenic emission sources. In the current study, atmospheric deposition samples were collected for 13 months (February 2019-March 2020) from four precipitation chemistry monitoring stations located in Muskoka-Haliburton, south-central Ontario, Canada. Anthropogenic particles (i.e., synthetic particles but not necessarily plastic) were observed at each station with an average deposition rate of 57 particles/m²/day (range from 32 to 73 particles/m²/day). Of the anthropogenic particles identified, 12% were plastic resulting in an average microplastic (mp) deposition rate of 7 mp/m²/day (range 4-9 mp/m²/day). Approximately 85% of the particles were fibres with fragments comprising only 15%. The most common particle colours were blue and red with 50% of the fragments and 84% of fibres being one of these two colours. Raman spectroscopy determined that polyamide and polyethylene terephthalate were the two most abundant polymers at 24% and 19%, respectively. Across the four stations anthropogenic particle concentrations were significantly related to wind speed (r_s = 0.32 to 0.62) and temperature (r_s = -0.53 to -0.84), with a noticeable increase in particle concentration when wind shifted from the west (average of 7.2 mp/L) to the south-east (average of 11.5 mp/L). Faster wind speed resulted in a larger airshed source area, and the seasonal effect associated with changes in temperature and wind direction led to changes in potential source regions that were contributing microplastics, such as the Greater Toronto Area (>200 km away).