The pilot study to investigate the abundance of microplastics in atmospheric deposition within the Metropolitan Area of Hamburg was conducted at a total of 6 sites, each with 3 bulk deposition samplers, which were tested every 14 days over 12 weeks. Three of the sites were located in a rural area south of Hamburg comprising 1 open field site and 2 throughfall sites under beech and oak and Douglas fir forest canopy, respectively. Three further sites were selected within the city following a transect from north to south representing sites of varying degrees of urbanization in terms of population, traffic, and industrial pressures.
The results show that microplastic particles are ubiquitous at all sites. A median abundance between 136.5 and 512.0 microplastic particles per m²/day was found over the sampling period (a mean microplastic abundance of 275 particles per m²/day). In contrast to other studies, fragments significantly dominated compared to fibers. μRaman spectroscopy showed that at 48.8% polyethylenes dominate significantly as compared to other types of polymers.
The spatial distribution comparing the urban sites concentrations followed in the order from high to low: north (Henstedt-Ulzburg, low population density, suburb)—center (University; high population density)—south (Wilhelmsburg, medium population density, and with port and industrial facilities) with highly varying concentrations within the time series. Surprisingly, the rural sites in the southern part of Hamburg showed the highest concentrations (Douglas fir > open field > beech and oak woodland). This finding is most likely a result of factors such as the comb-out capacity of the different forest types and/or direct input pathways from the agricultural areas and the nearby highway.
