Urban sources, wastewater treatment plants (WWTPs), untreated wastewater (not connected to WWTPs), and especially combined sewer overflow systems (CSS) including stormwater are major pathways for microplastics in the aquatic environment. We compile microplastics emission data for the Baltic Sea region, calculate emissions for each pathway and develop emission scenarios for selected polymer types, namely polyethylene (PE)/polypropylene (PP) and the polyester polyethylene terephthalate (PET). PE/PP and PET differ with respect to their density and can be regarded as representative for large groups of polymers. We consider particles between 20–500 μm with varying shapes. The emission scenarios serve as input for 3D-model simulations, which allow us to estimate transport, behavior, and deposition in the Baltic Sea environment. According to our model results, the average residence time of PET and PE/PP in the Baltic Sea water body is about 14 days. Microplastics from urban sources cause average concentrations of 1.4 PE/PP (0.7 PET) particles/m2 sea surface (20–500 μm size range) in the Baltic Sea during summer. Average concentrations of PET, resulting from urban sources, at the sea floor are 4 particles/m2 sediment surface during summer. Our model approach suggests that accumulation at the shoreline is the major sink for microplastic with annual coastal PE/PP and PET accumulation rates of up to 108 particles/m each near emission hot-spots and in enclosed and semi-closed systems. All concentrations show strong spatial and temporal variability and are linked to high uncertainties. The seasonality of CSS (including stormwater) emissions is assessed in detail. In the south-eastern Baltic, emissions during July and August can be up to 50% of the annual CSS and above 1/3 of the total annual microplastic emissions. The practical consequences especially for monitoring, which should focus on beaches, are discussed. Further, it seems that PET, PE/PP can serve as indicators to assess the state of pollution.

中文翻译：

---

波罗的海城市源头微塑料排放的传输和行为

城市水源、废水处理厂 (WWTP)、未经处理的废水（未连接到 WWTP），尤其是包括雨水在内的联合下水道溢流系统 (CSS)，是水生环境中微塑料的主要途径。我们为波罗的海地区编译微塑料排放数据，计算每个途径的排放，并为选定的聚合物类型开发排放情景，即聚乙烯 (PE)/聚丙烯 (PP) 和聚酯聚对苯二甲酸乙二醇酯 (PET)。PE/PP 和 PET 的密度不同，可被视为一大类聚合物的代表。我们考虑 20-500 μm 之间具有不同形状的颗粒。排放情景作为 3D 模型模拟的输入，使我们能够估计波罗的海环境中的运输、行为和沉积。根据我们的模型结果，PET 和 PE/PP 在波罗的海水体中的平均停留时间约为 14 天。夏季，来自城市的微塑料导致波罗的海平均浓度为 1.4 PE/PP (0.7 PET) 颗粒/平方米海面（20-500 微米大小范围）。夏季，来自城市源头的 PET 平均浓度在海底是 4 颗粒/平方米沉积物表面。我们的模型方法表明，海岸线的积累是微塑料的主要汇，在排放热点附近以及封闭和半封闭系统中，每年沿海 PE/PP 和 PET 的积累率高达 108 个颗粒/米。所有浓度都显示出强烈的空间和时间可变性，并与高度不确定性有关。详细评估 CSS（包括雨水）排放的季节性。在波罗的海东南部，7 月和 8 月的排放量可高达年度 CSS 的 50%，占年度微塑料排放总量的 1/3 以上。讨论了特别是监测的实际后果，这应该集中在海滩上。此外，PET、PE/PP似乎可以作为评估污染状况的指标。