From streets to seas: New greener ways to analyse urban snow pollution

Arctic cities experience long winters with heavy snowfalls. Every year, tonnes of urban snow contaminated with microplastics from tire wear and other traffic-related environmental pollutants are dumped into the sea.

Despite snow dumping being a yearly phenomenon, we have limited knowledge about the impacts of pollutants that are transported in this way from roadways to the sea. 

In the project “DUMP—Direct analysis of Urban snow particulates for traffic-related Microplastic additives and Polycyclic aromatic hydrocarbons”, a team of researchers in Tromsø set out to develop a more environmentally friendly method for the analysis of traffic-related pollutants in snow. 

Green analysis of particles in snow

“There is a paradox when it comes to analysing traffic-related pollutants: that is, traditional laboratory methods require extractions with toxic solvents and extensive clean-up procedures to remove the sample from whatever it is bound to or mixed with [e.g., organic material] before the sample can be analysed. Thus, there is a need to develop more environmentally friendly laboratory methods, as well as increasing our knowledge about the pollutants we dump into the sea,” says project leader, Cleo Davie-Martin from NILU. 

She goes on to explain: “The new ‘green chemistry’ method is based on thermal desorption technology that uses rapid heat to directly desorb—that is, release—the compounds of interest from the surface of a sample matrix (in our case, particles in snow) into the gas phase for analysis.” 

The method involves specially designed thermal desorption tubes made of metal to withstand high temperatures and hollow to allow gas to pass through. This enables direct analysis of solid samples, without the need for extra preparation steps.

Contextual novelty of the method

The method centres on the direct measurement of particles from melted snow collected on glass fibre filters (GFFs). This approach is more environmentally friendly, less time consuming, and more cost effective than existing methods. The method was first developed for polycyclic aromatic hydrocarbons (PAHs). PAHs are a group of chemicals that occur naturally in coal, crude oil, and petrol, but are also formed via incomplete combustion, for example, when you drive a petrol-powered car. 

The development of the method involved testing numerous stages of the snowmelt filtration and thermal desorption procedure. Examples were the volume of snowmelt required to collect measurable levels of PAHs, the drying time necessary to allow water to evaporate from the filters before analysis, and sample carryover during analysis. In the end, the researchers had developed a method that could document concentrations of PAHs in particles from urban snow.

Sampling of urban snow

Snow was sampled in April 2024 in the Arctic city of Tromsø in northern Norway. Kristine Bondo Pedersen, Senior Environmental Consultant at Akvaplan-niva, explains that the snow was sampled at three different sites where snow cleared from the streets is collected prior to dumping: Kløvervegen, a residential area with light traffic; the Fram Centre, a mixed commercial and residential area with heavy traffic; and the airport, a busy area near an arterial road with heavy traffic. The snow from these three sites was compared with fresh snow collected at night during a snowfall, in a residential area with little traffic. 

Snow samples were slowly melted, coarsely sieved to remove larger grit and debris, and then filtered onto GFFs to collect the particles in the snow. The GFFs were then packed in metal sample tubes that were heated to 300∞C to directly desorb the PAHs. The PAHs were then analysed using gas chromatography coupled with mass spectrometry. 

Green chemistry needs further work

The DUMP team found higher concentrations of PAHs in snow from the FRAM Centre and Airport sites than the residential site at Kløvervegen. As Cleo Davie-Martin explains, “These findings were expected, because these sites receive snow from more heavily trafficked roads and had more particles.” 

Concentrations in fresh snow were generally below detection limits. At sites where PAHs could be detected, there were higher concentrations of the heavier PAHs than many of the lighter PAHs. This was also expected, because the heavier PAHs tend to bind more strongly to particles, whereas the smaller PAHs can preferentially dissolve in the snowmelt water and pass through the filter.

While the findings are promising, the method needs further refinement and optimisation to be implemented more universally. One of the main challenges of the direct thermal desorption approach is interference from the sample matrix, which leads to poorer analyses and necessitates more frequent instrument maintenance. Future testing should focus on adjusting the amount of GFF introduced to the metal tubes and the flow rates within the instrument, to minimise interference from substances in the sample other than the pollutant(s) of interest. So far, the results have provided us with knowledge that pollutants, such as PAHs, can be found in urban traffic-related snow. 

In future, the method has the potential for expanded applications across different contaminant groups and a variety of disciplines—GFFs are inexpensive, widely available, and used for collecting particles from air and water. For example, thermal desorption can be used to assess indoor air quality, to investigate house dust, pollen, or suspended sediments in marine and freshwater ecosystems as vectors for environmental pollutants, and particles from road run-off related to new infrastructure.