2025-03-11

Recipe for Cleaner Air: Add Salt!

In cold-climate areas, it is common to find vehicles with studded tyres during the winter season. This facilitates driving on icy roads. However, the use of such tyres causes wear of both the studs and the surface of the roads, and the result is high amounts of dust, notably in the shape of PM10 and finer fractions. Frequent sanding of the slippery roads also adds to the dust.

Initially, this may be less of a problem since the particles often become bound to the road surface by snow and ice, and eventually meltwater. However, in areas where the end of the winter season is characterized by low amounts of snow and rain, the particles are released from the roads by passing vehicles. This may be very notable along city streets with high traffic intensity and buildings forming street canyons preventing natural dispersion of the ambient air. The PM levels can then literally go sky-high.

A Detectable Issue

This type of pollution source can show a specific pattern with respect to PM levels. During nighttime when the traffic intensity is low, the particles may slowly settle on the ground. Condensing water can add to trapping the particles on the surface. However, when the morning traffic picks up, the turbulence starts, and the street surface dries. This causes re-release of the particles, with accompanying high PM levels and potential health issues. The levels can remain high throughout the day and into the beginning of the night, before the process repeats itself.

Not all days are necessarily like this. Precipitation can efficiently bind the particles to the ground and will also eventually transport them away from the streets. Windy days may effectively scatter the airborne particles. However, during calm and dry periods, the daily repeating pattern can be quite visible. The day-time peaks may also get higher and higher each day.

…and Also an Addressable Issue

We have already touched the remedy: water binds the particles. But how can we assure that there is water on the road surface also during dry days? The answer is salt and preferably calcium chloride (CaCl₂) which is notably hygroscopic, that is having the ability to attract and retain the water molecules.

Spreading salt for de-icing purposes is a common practice. A proper mix of different types of salt assures efficiency also in temperatures much below freezing. A mixture of 80 % NaCl and 20 % CaCl₂ has turned out to be favourable also in low temperatures, and the CaCl₂ share can then also prevent the salt solution from drying, thereby providing particle-binding effects also during regular salting. This can give added value at temperatures around freezing, with shifting icy, wet, and dry road surfaces.

Continued Salt Spreading

The key to preventing high PM levels in early spring is continued salt spreading, also at temperatures safely above freezing. This may seem odd, but experiments including road-side PM monitoring with high temporal resolution have shown very promising results. Since there is no need for any de-icing, this type of salting is best done with 100 % calcium chloride, having very good hygroscopic properties.

There are a few things to consider before sending out the salting brigade. First, the weather forecasts – it only makes sense to attempt binding the particles if the road surfaces are about to dry, and they will remain dry for some time. Rainfall will just wash away the salt and anyway binds the particles to the surface.

The next thing to consider is the risk for crystallisation. Too high calcium chloride concentration and crystals are formed. The surface then becomes slippery, and it can be just as bad as if it was icy. That is not what the drivers expect at temperatures well above freezing and must of course be avoided. To complicate things, the crystallisation process also depends on the ambient temperature – the higher temperature the higher risk for crystallisation. There can also be residual salt on the surface from preceding salt spreading, which also must be attended to in order not to get too high concentrations.

Practical Approach

If the weather forecast is favourable for salt spreading for PM reduction purposes, the first step is to establish how much salt there already is on the road surface. This is done by sampling the surface at strategic points. A refractometer is then used to determine the present salt concentration. That, and the predicted highest temperature of the day gives the concentration of the new salt solution and the spreading rate. This is fed into the controls of the spreading devices on the salt trucks, which then can be dispatched to deliver the salt batch of the day to the city streets. Allt this is carried out by the road authority in a well-trimmed sequence with minimal time from sampling to salt spreading. Properly done, the surface is barely perceived as wet, but the water binding effect is there, and the PM levels can be reduced notably.

There is of course a cost for salting, and it is only done when there is expected to be a clear effect on the air quality. Nevertheless, there are records of this kind of salting being carried out as late as in the month of June, long after the winter season’s last day of night freezing.

Work in Progress

Facts for this article originate from the city of Karlstad in Sweden where staff at the local road authority has developed the methods and by now become internationally renowned experts on salting for cleaner air. OPSIS contributes with PM monitoring services in continuing tests to further develop the methods.