Trafikrelaterade föroreningar i urban snö : Koncentrationer, storleksfördelning och spridning vid snösmältning

In urban areas with seasonal snow, traffic-related pollutants such as solid particles, metals, chloride, organic pollutants, and microplastics (MPs) can be temporarily stored in snowbanks along roads and streets. When the snow melts, it releases the accumulated pollutants and the resulting snowmelt with diverse pollutants may partly infiltrate into the ground, or enter storm sewers and eventually be discharged into, and impact on, the receiving waters. To address the resulting environmental concerns, it is important to gain a more comprehensive understanding of (i) occurrence, distribution, and temporal variation of conventional (TSS, Cu, Zn, Pb, Cd, chloride and PAHs) as well as emerging pollutants (Tire and Road Wear Particles (T&RWPs), Platinum Group Elements (PGEs), tungsten (W) and antimony (Sb)) in urban snow, (ii) size fractionation of pollutants in snow, (iii) behaviour of pollutants during snow melting, and (iv) influence of various snow sampling strategies on estimating pollutant loads in snow. Therefore, these four points form the focus of this doctoral thesis. The work presented in the thesis includes a literature review of metal pollution in urban snow, field sampling of urban roadside snowbanks and snow storage piles, and laboratory-scale snow melting experiments. The field sampling included snow sampling surveys at three locations in Sweden – Frihamnen (one of the ports of Stockholm), and Luleå and Umeå municipalities in Northern Sweden, and served for studies of variations in snow quality in terms of solids, metals, chloride, PAHs and MPs. Some of the field samples were also used in laboratory-scale snow melting experiments to advance the understanding of the fate of pollutants during the snow melting process. A literature survey identified Zn, Cu, Pb, Cd and Ni as the metals most frequently studied in urban snow, while Sb, W and PGEs in urban snow were seldom studied, and consequently were designated here as ‘emerging pollutants’ in urban snow. These pollutant concentrations in the analysed snow samples differed distinctly because of differences in study area characteristics such as meteorological conditions, traffic intensity and composition, and winter road maintenance as well as snow cover age (SCA). Investigation of estimation accuracies for pollutant loads in temporary snow storage piles highlighted the significant role of the sampling design. Single-column samples were prone to underestimating or overestimating the pollutant loads in snow piles, with variations of up to 400%, observed in the samples collected at Frihamnen. This underscores the importance of collecting and analysing multiple samples for reliable pollutant load assessments. Comparison of snow quality in three winter seasons (1994-95, 2002-03, and 2020-21) showed a statistically significant decrease in Pb and Cd concentrations in snow samples from 1995 to 2021. This decline may be associated with the regulations limiting these pollutants in car manufacturing industry and the phasing out of leaded gasoline. In the laboratory snow melting experiments, only 10% of both total metals (Cu, Zn, and Cd) and PAHs, and 20% of T&RWPs, were carried away by the meltwater, while the rest stayed in the (immobilised) sediment residue. The dissolved (<0.45 µm) and truly dissolved (<3000 MWCO) metals and chloride exhibited a preferential elution during melting, whereas TSS and PAHs displayed a delayed release. In summary, the thesis contributes to developing a comprehensive understanding of urban snow pollution dynamics and underscores the significance of, and need for, effective snow management for mitigating environmental impacts of urban snow pollution.