A literature search on traffic related metals in polluted urban snow revealed a significant volume of references representing a substantive knowledge base. The frequently studied metals in urban snow included Zn, Cu, Pb, Cd and Ni. However, comparing metal concentrations across studies proves to be a complex effort due to the variations in site-specific factors among studies, such as traffic intensity, pavement conditions, hydrometeorological conditions, and research method aspects, such as sampling equipment and frequency, and laboratory analytical methods. The literature review indicated that among the commonly studied metals, Zn and Cu indicated potential environmental concerns, and that there was a lack of data on the occurrence and accumulation in snow of antimony (Sb), tungsten (W), and platinum group elements (PGEs). To partly mitigate this knowledge gap, a field study of these elements was carried out by sampling urban roadside snow at six locations with various land use and traffic intensities, focusing on accumulation of these elements in snowbanks along roadways. The results indicated that traffic related activities are the sources of PGEs, W and Sb in roadside snowbanks, as the concentrations of these metals increased with increasing traffic intensity. The mean concentrations of the studied metals followed this descending order: W (0.4 (Reporting limit-RL)–987 μg/l) > Sb (0.1 RL–33.2 μg/l) > Pd (0.02 (RL)–0.506 μg/l) > Rh (0.02 (RL)–0.053 μg/l). In laboratory melted snow, both W and Sb were mostly in the particulate-bound phase, with <25 % in the dissolved phase. For sites with metal concentrations above the detection limit, the regression analysis indicated linear trends in unit area deposition rates of W with time (snow age), described by R2 = 0.94.

Rapid development of new materials and technologies in the automotive sector and urban growth cause changes in emissions of certain pollutants to the environment and introduction of new ones. This can cause changes in the pollutants signature and content in urban snow. Thus, it is important to revisit the reported data on urban snow quality periodically to ensure that snow management planning is based on up-to-date information. In this paper, the concentrations of TSS and ubiquitous urban metals (Zn, Cu, Cr and Pb) in urban snow, sampled at six sites in the City of Luleå during the winter 2020-21, were compared with previous data (from 1995 and 2002) to examine historical changes in snow quality in Luleå during the period 1995-2021. The results show that the pollutant concentrations in snow are influenced by measured air temperatures and precipitation, and such influences bring uncertainties into data comparisons.

Le développement rapide de nouveaux matériaux et de nouvelles technologies dans le secteur automobile et la croissance urbaine entraînent des changements dans les émissions de certains polluants dans l'environnement et l'introduction de nouveaux polluants. Cela peut entraîner des changements dans la signature et la teneur en polluants de la neige urbaine. Il est donc important de revoir périodiquement les données rapportées sur la qualité de la neige urbaine afin de s'assurer que la planification de la gestion de la neige est basée sur des informations actualisées. Dans cet article, les concentrations de MES et de métaux urbains omniprésents (Zn, Cu, Cr et Pb) dans la neige urbaine, échantillonnée sur six sites de la ville de Luleå pendant l'hiver 2020-21, ont été comparées aux données précédentes (de 1995 et 2002) pour examiner les changements historiques de la qualité de la neige à Luleå au cours de la période 1995-2021. Les résultats montrent que les concentrations de polluants dans la neige sont influencées par les températures de l'air et les précipitations mesurées, et que ces influences entraînent des incertitudes dans les comparaisons de données.

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.

Trafikrelaterade föroreningar som partiklar, metaller, klorid, organiska föroreningar och mikroplaster (MP) lagras tillfälligt i plogvallar längs gator och vägar i urbana områden. När snön smälter frigörs de ackumulerade föroreningarna och smältvattnet som innehåller olika föroreningar kommer infiltrera marken eller avledas via dagvattenledningar och så småningom släppas ut i, och påverka, recipienter. För att kunna hantera utsläppen till miljön som uppstår som en följd av detta är det viktigt att få en mer omfattande förståelse av (i) förekomst, distribution och tidsvariation av konventionella (TSS, Cu, Zn, Pb, Cd, klorid och PAH:er) samt ”nya” föroreningar (däck- och vägslitagepartiklar (T&RWP), platinagruppelement (PGE), volfram (W) och antimon (Sb)) i urban snö, (ii) storleksfraktionering av föroreningar i snö, (iii) spridning av föroreningarna vid snösmältning samt (iv) hur metoder och tillvägagångssätt för snöprovtagning i fält påverkar osäkerheten i mätdata. Därför utgör dessa fyra punkter fokus för denna doktorsavhandling.

Arbetet som presenteras i avhandlingen inkluderar en litteraturstudie av föroreningar såsom metaller i urban snö, fältprovtagning av plogvallar och snöhögar samt smältningsexperiment i laboratorieskala. Fältprovtagningen genomfördes på tre platser i Sverige – Frihamnen (en av Stockholms hamnar), samt Luleå och Umeå kommuner i norra Sverige, och användes till studier av variationer i snökvalitet med avseende på suspenderade partiklar, metaller, klorider, PAH:er och mikroplast partiklar. Ett urval av fältproverna användes också i laboratorieexperiment för att studera hur olika föroreningar påverkas under snösmältningsprocessen.

En litteraturöversikt identifierade Zn, Cu, Pb, Cd och Ni som de mest frekvent studerade metallerna i urban snö. Sb, W och PGEs har studerats mer sällan och benämns därför här som "nya föroreningar”. Koncentrationerna av dessa föroreningar i de analyserade snöproverna skiljde sig tydligt på grund av skillnader mellan provtagningsplatserna såsom meteorologiska förhållanden, trafikintensitet och -sammansättning, vintervägunderhåll samt snöns ålder (SCA). Undersökningar av mätnoggrannhet för bestämning av föroreningsmängder lagrade i snöhögar påverkades av hur proverna togs. Enstaka prover genom hela snödjupet hade en tendens att underskatta eller överskatta föroreningsbelastningen i snöhögarna, med variationer på upp till 400%, vilket observerades i prover som togs i Frihamnen. Det understryker vikten av att samla in och analysera flera prover för att kunna göra pålitliga bedömningar av den faktiska föroreningsbelastningen.

Jämförande analys av snökvalitet under tre vintersäsonger (1994-95, 2002-03 och 2020-21) visade en statistiskt signifikant minskning av Pb- och Cd-koncentrationerna i snöprover från 1995 till 2021. Denna minskning kan tillskrivas lagar och föreskrifter om minskad användning av dessa ämnen under årens gång.

I laboratorieexperiment med snösmältning följde endast 10% av totala metaller (Cu, Zn och Cd) och PAH:er med det avrinnande smältvatten, medan resten stannade kvar (immobiliserade) som sediment på platsen för snöhögen. De lösta (<0,45 µm) och sant lösta (<3000 MWCO) storleksfraktionerna av metallerna samt klorider frigjordes tidigt under smältförloppet (preferential elution), medan TSS och PAH:er uppvisade en fördröjd frisättning (delayed release).

Sammanfattningsvis bidrar avhandlingen till att utveckla en övergripande förståelse för dynamiken av föroreningar i urban snö och lyfter fram betydelsen, och behovet, av effektiv snöhantering för att minska miljöpåverkan av föroreningar som förekommer i urban snö.

The microplastics (MP) pollution has been receiving high attention in recent years, because of the massive amounts of plastics it contributes to the environment. Tyre wear and road wear particles (TWP and RWPs) were identified as major sources of MPs, but the observed data on these particles in urban snow deposits and snowmelt is scarce. To contribute to remediation of this situation, a study designed to quantify TWPs and RWPs in urban roadside snowbanks, and assess the MP occurrence in three size fractions, was conducted in the Luleå and Umeå municipalities in Northern Sweden. TWPs and RWPs were determined in three size fractions: 50–100 μm, 100–300 μm, and ≥300 μm, and their release from melting snow was investigated in the laboratory under controlled conditions. Among the MPs identified in snow and the associated snowmelt samples, a majority consisted of both types of particles (T&RWPs) with an average of 20,000 ± 48,000 number/L, whereas other MPs (fibres, fragments, flakes, and films of plastic) were much less plentiful with an average concentration of 24 ± 16 number/L. The largest proportion of T&RWPs was detected in the size fraction 50–100 μm (around 80 %), and the smallest proportion was in the fraction ≥300 μm (about 2 %). Of the T&RWPs, about 85 % were black bitumen particles (RWPs), composed of bitumen, mineral material and polymer modifiers, and 15 % were tire wear particles (TWPs) composed of rubber. The laboratory snow melting experiments demonstrated that urban snow stored MPs, which were eventually released during snowmelt. The ultimate fate of released MPs would depend on snowmelt drainage; it may either drain away from the road pavement and infiltrate into the ground, or enter the road gutter and be conveyed to storm sewers discharging to the receiving waters.

Choosing the appropriate sampling strategy is significant while estimating the pollutant loads in a snow pile and assessing environmental impacts of dumping snow into water bodies. This paper compares different snow pile sampling strategies, looking for the most efficient way to estimate the pollutant loads in a snow pile. For this purpose, 177 snow samples were collected from nine snow piles (average pile area − 30 m², height − 2 m) during four sampling occasions at Frihamnen, Ports of Stockholm’s port area. The measured concentrations of TSS, LOI, pH, conductivity, and heavy metals (Zn, Cu, Cd, Cr, Pb, and V) in the collected samples indicated that pollutants are not uniformly distributed in the snow piles. Pollutant loads calculated from different sampling strategies were compared against the load calculated using all samples collected for each pile (best estimate of mass load, BEML). The results/study showed that systematic grid sampling is the best choice when the objective of sampling is to estimate the pollutant loads accurately. Estimating pollutant loads from single snow column samples (collected at a point from the snow pile through the entire depth of the pile) produced up to 400% variation from BEML, whereas samples composed by mixing volume-proportional subsamples from all samples (horizontal composite samples) produced only up to 50% variation. Around nine samples were required to estimate the pollutant loads within 50% deviation from BEML for the studied snow piles. Converting pollutant concentrations in snow to equivalent concentrations in snowmelt and comparing it with available guideline values for receiving water, Zn was identified as the critical pollutant.

I denna artikel sammanfattas resultat från projektet ”Hållbar snöhantering i urbana miljöer” och sätter resultaten i en nationell och global kontext. Det övergripande målet i projektet har varit att ge underlag till en miljömässigt förbättrad snöhantering. Därför har snö från olika områden provtagits, snö och dagvatten från samma avrinningsområde jämförts och föroreningstransporten från smältande snö har studerats. Utifrån erhållna data och tidigare publicerade undersökningar har en konceptuell beräkningsmodell för att bedöma föroreningsmängder i urban snö tagits fram och olika möjligheter till hantering av bortforslad snö har belysts.

Snow deposited in urban areas is exposed to pollutants originating from traffic, wet and dry atmospheric deposition, de-icing chemicals and traction materials. The conventional pollutants found in urban snow include solids (TSS), metals (Zn, Cu, Cd, Pb, etc.), Polycyclic Aromatic Hydrocarbons (PAHs), and chlorides. Microplastics (MPs) are emerging pollutants of interest and their presence in urban snow was reported only recently. The pollutants accumulate in snow deposits over time and may be released when snow melts, and the resulting snowmelt may carry pollutants to the receiving waters. Understanding the concentrations and mass loads of pollutants in snow and pollutant behaviour during snowmelt is helpful for planning and developing site-specific snow management practices. The overall aim of this thesis was to evaluate the quality of snow in urban areas, with respect to: (i) quantity of metals, PAHs and MPs in urban snow storage piles and roadside snowbanks, (ii) compare the quality of snow collected in the same catchment during repeated sampling campaigns and evaluate the effect of the sampling design on estimation of pollutant loads in snow piles, and (iii) investigate the pollutant release patterns and temporal variations in their concentrations in water leaving the melting snow piles (in laboratory). For such studies, snow samples were collected from snow storage piles in Frihamnen (a port facility in Stockholm, Sweden) and roadside snowbanks in Luleå and Umeå cities in Northern Sweden.

The quality of snow collected in the three study areas varied considerably, because of differences in such area characteristics as the annual precipitation and snowfall, the population, average daily traffic, land use activities, and snow management activities. The average values of major parameters in analysed snow samples were as follows: TSS - 1500 mg/L, conductivity- 2.1 mS/cm, Zn – 870 µg/L, Cu – 240 µg/L, Cd – 0.48 µg/L, Cr – 120 µg/L, Pb – 50 µg/L and the sum of 16 PAHs – 3.5 µg/L. Microplastics were abundant in urban snow samples, with the following descending order of concentrations: black road wear particles, consisting of bitumen and tire wear particles,  mean = 19300 ± 47400 particles/L; road marking paints with the mean of 430 ± 998 particles /L; and, plastics particles, mean 33 ± 34 particles /L. No correlations were found between the numbers of MP particles and the site-specific parameters.

Comparison of snow pile sampling designs revealed that systematic 1-m square grid sampling yielded the best estimates of mass loads (BEML) of pollutants, compared to single snow cores, or horizontally composed core samples. The mass loads estimated from composite or single snow column sampling deviated up to 50 and 400%, respectively, from BEML.

Results of the laboratory snow melting indicated that PAHs in the snow samples were mostly attached to the particles; only 10% of the total PAHs burden was contributed by the meltwater and the rest stayed on the ground with the sediment residue. The dissolved concentrations of PAHs were below the detection limit (0.010 µg/L) in all the analysed samples except for Fluoranthene and Pyrene with concentrations ranging between 0.01 and 0.02 µg/L. PAHs displayed a delayed release from snow piles, which was similar to that of TSS. Truly dissolved fractions (<3000 MWCO, Molecular Weight Cutoff) of Zn, Cu and Cd represented 71-90% of dissolved fractions in the snow samples collected in Luleå (snow without road salt) and 74-98% in those from Umeå (snow with added road salt). Both dissolved and truly dissolved metals showed advanced releases from all the snow piles. The influence of road salt on releases of metals and PAHs from laboratory snow piles was hard to discern, because of great differences in snow quality characteristics at both locations.

Laboratory snow melting experiments were conducted with actual late-winter snow samples, collected just before the final snowmelt, in two similar northern Swedish cities, Luleå and Umeå, to investigate releases of the selected heavy metals (HM) (Cu, Pb, Zn, and Cd) and 16 USEPA PAHs from melting snow. Metal concentrations were determined in three fractions: total, dissolved, and truly dissolved (defined as the fraction passing through a 3-kMWCO ultrafilter). Total HM concentrations in snowmelt were rather high at both sites and reflected the accumulation of pollutants in the roadside snowbanks over a period of about 5 months: Cd = 0.43, Cu = 303, Pb = 41.9, Zn = 817 (μg/l), and TSS = 2000 (mg/l) in Luleå samples and Cd = 1.87, Cu = 905, Pb = 165, Zn = 3150 (μg/l), and TSS = 4800 (mg/l) in Umeå samples. The difference between metal and TSS concentrations at the two sites of similar characteristics was attributed to a smaller volume snowbank in Umeå. The dissolved HM concentrations represented relatively small fractions of the total concentrations (0.3–6.9% in Luleå and 0.01–3.1% in Umeå). The truly dissolved fraction represented 71–90% of the dissolved fraction in Luleå and 74–98% in Umeå. At both sites, the dissolved fractions exhibited preferential elution from the laboratory snow piles. The PAHs studied (16 US EPA PAHs) were mostly particulate bound, with only 5–12% of the total burden contributed by the meltwater, and most dissolved concentrations below the reporting limits. PAH concentrations in the Luleå samples were about one-third to one-fourth of those in Umeå. In general, the releases of PAHs from the snowbank were delayed, compared with releases of meltwater, and showed similar release patterns as TSS.