Atmospheric Dry and Wet Deposition of Microplastics in an Urban Area and a Remote Island: Year-Round Consecutive Monthly Observations

Atmospheric transport and deposition are key processes in the global spread of microplastics (MPs). However, field-based observational data are limited, and the dynamics of MP transport and deposition are still insufficiently understood. This study aimed to identify the effects of long-range transport and urbanization by comparing identical time-series data on atmospheric MP deposition between an urban city (Seoul, SE; ~10 million residents) and a remote region (Baengnyeong Island, BI; located on the Yellow Sea). In 2023, atmospheric MP deposition samples were simultaneously collected from both regions during the same period every month. Each sample was continuously collected using selective air deposition samplers to separate dry (DD) and wet deposition (WD). A total of 48 samples, comprising 12 pairs of DD and WD from each site, were analyzed for MPs (≥20 µm). Both sites exhibited a lower trend in total deposition (TD; sum of DD and WD) flux of MPs during the summer compared to other seasons, suggesting that MP deposition was influenced by atmospheric stability and monsoon scouring effects. Despite similar monthly pattern, TD flux of MPs was 3.5 times higher at SE (271±155 n/m²/day) than at BI (77.1±76.4 n/m²/day), with significant differences between the two regions for all monthly samples (paired t-test; p<0.01) except for March, supporting the emission effect from local sources in urban area. In March, which was marked by the dominance of westerly winds from eastern China and the highest aerosol (PM10) concentrations at both BI and SE, MP-TD fluxes recorded the highest at BI (311 n/m²/day) and the second highest at SE (330 n/m²/day) among those observed at each site. The March-MP flux increased by 6.3-fold (BI) and 1.6-fold (SE) compared to other months, with a larger increase in BI compared to PM10 (2.4-fold vs. 2.3-fold), indicating stronger trans-boundary transport at the remote regions. Major five polymers accounted for 94.4% (BI) and 84.2% (SE) of MPs, with the most weathering-prone PP dominating (69.0% and 59.3%), indicating fugitive input from aged plastics. WD was 10 (BI) and 5 (SE) times more efficient in MP deposition than DD, but contributed only 47% (BI) and 36% (SE) to monthly TD due to limited precipitation. Although fragment-shaped MPs prevailed in TDs of both BI (88.6%) and SE (94.9%), higher fiber proportion in WD than DD (14.2% vs. 8.2% in BI and 5.8% vs. 3.7% in SE) represented susceptible precipitation-scavenging potential of fibers. However, higher proportion of fibers in TD of BI (10.7%) than of SE (4.5%), with significant differences (p<0.01) between the two regions, suggests that fibers are more likely to survive long-distance transport. Our findings help improve the uncertainties in global MP dispersion and budget.

Acknowledgement: This work was supported by 'Land/Sea-based input and fate of microplastics in the marine environment' of Korea Institute of Marine Science & Technology Promotion (KIMST) funded by the Ministry of Oceans and Fisheries, Republic of Korea (20220357), and was also partially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2024-00356940).