Abstract
De-icing of road surfaces is necessary in many countries during winter to improve vehicle traction. Large amounts of salt, most often sodium chloride, are applied every year. Most of this salt is removed through drainage or traffic spray processes but a certain amount may be suspended, after drying of the road surface, into the air and will contribute to the concentration of particulate matter. Though some measurements of salt concentrations are available near roads, the link between road maintenance salting activities and observed concentrations of salt in ambient air is yet to be quantified. In this study the NORTRIP road dust emission model, which estimates the emissions of both dust and salt from the road surface, is applied at five sites in four Nordic countries for ten separate winter periods where daily mean ambient air measurements of salt concentrations are available. The model is capable of reproducing many of the salt emission episodes, both in time and intensity, but also fails on other occasions. The observed mean concentration of salt in PM10, over all ten datasets, is 4.2 μg/m3 and the modelled mean is 2.8 μg/m3, giving a fractional bias of −0.38. The RMSE of the mean concentrations, over all 10 datasets, is 2.9 μg/m3 with an average R2 of 0.28. The mean concentration of salt is similar to the mean exhaust contribution during the winter periods of 2.6 μg/m3. The contribution of salt to the kerbside winter mean PM10 concentration is estimated to increase by 4.1 ± 3.4 μg/m3 for every kg/m2 of salt applied on the road surface during the winter season. Additional sensitivity studies showed that the accurate logging of salt applications is a prerequisite for predicting salt emissions, as well as good quality data on precipitation. It also highlights the need for more simultaneous measurements of salt loading together with ambient air concentrations to help improve model parameterisations of salt and moisture removal processes.
Highlights
Non-exhaust PM10 emissions are composed of a number of wear sources and may contain emissions from traction sanding, from deposited material from construction work, from road side soil sources or from deposition of atmospheric PM
Hornsgatan (Stockholm) Two years of data are shown for Hornsgatan (Figs. 1 and 2)
Levels of normal de-icing salt (NaCl) from 1 to 2 mg/m3 can be seen in May, long after the last salting operation in both years and this is assumed to represent background levels, presumably due to sea salt contributions
Summary
Amato et al (2014), Mathissen et al (2012), Bukowiecki et al (2010), Gustafsson et al (2008), Norman and Johansson (2006) and Boulter (2005) This is true in Nordic countries where studded tyres are used during winter. Non-exhaust PM10 emissions are composed of a number of wear sources (road, tyre and brake) and may contain emissions from traction sanding, from deposited material from construction work, from road side soil sources or from deposition of atmospheric PM. Another source of traffic induced PM is road salt, applied as a de-icing agent during winter. Regional background measurements estimate available of ambient air salt Filter sample Ionic analysis Only Na
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