A proxy-based approach to predict spatially resolved emissions of macro- and microplastic to the environment

Large disparities on micro- and macroplastic concentrations are to be expected between residential, industrial, natural and agricultural areas, since specific uses of plastic will determine the magnitude of the corresponding emissions. The aim of this work was to develop a method to regionalize emissions of macroplastic and microplastic for soil, freshwater and air using geographical datasets on land-use statistics, traffic and population densities, wastewater treatment plants and combined sewer overflows as proxies. High resolution maps of the emissions were then generated for micro- and macroplastic using emission data available for Switzerland for seven commonly used polymers (low-density-polyethylene, high-density-polyethylene (HDPE), polypropylene (PP), polystyrene, expanded polystyrene, polyvinyl-chloride and polyethylene-terephthalate). Most of the emissions can be found in areas with high human activity, but the influence of the different proxies varies for each polymer. The median emission rate of macroplastic on soil varies from 0.0006 to 0.06 kg/ha/year, whereas no emission flows are predicted for more than 50% of the raster cells for microplastic regardless of the polymer, but the maxima can reach up to 12.7 kg/ha/a in the case of HDPE. The average emission rate of macroplastic along river segments ranges between 0.062 kg/km/a and 1.5 kg/km/a. For microplastic, the average emission rate varies from 0.0025 kg/km/a to 0.11 kg/km/a. The analysis reveals that a significant deviation is expected if the population density is used as only proxy. The correlation between the population density and the predicted emissions is only r = 0.16-0.23 for a cell size of 100 × 100 m and goes up to r = 0.86-0.88 for a resolution of 10 km, however an r of only 0.56-0.68 is observed for those polymers used a lot in agriculture such as HDPE and PP. The emission maps obtained in this work can serve as input to regionalized fate models for macro- and microplastics.