Rock salt and its use as a winter ice management tool continues to receive a bad rap.
The controversy started more than a year ago with scientific reports linking winter rock salt use with unsafe salinity levels in freshwater bodies throughout the snow states.
Now, recent research points to rock salt use as a potential source for chemical components that have a negative impact on winter air quality.
There has been some question about road salt’s connect with urban air pollution, says Kerri Pratt, the research’s corresponding author and an assistant professor of chemistry and earth and environmental science at the University of Michigan. A 2010 University of Washington paper showed significant amounts of chlorine containing compounds – similar to coastal urban areas — in Colorado air, surprising given the Rocky Mountain State’s distance from the ocean.
Living in the Midwest, Pratt was familiar with road salt being used to mitigate ice on roads and highways. In fact, some sources estimate as much as 10 million tons is spread on US pavement each year. Pratt hypothesized the same chemistry observed around urban coastal areas – where sodium and chloride salts are naturally present in the air due to ocean wave action — could be replicated in other non-coastal areas due to rock salt use.
Pratt’s previous study using Environmental Protection Agency (EPA) data indicated the amount of sodium and chloride particles in the air away from the coast was correlated with the presence of snow. “This brought more confidence to our hypothesis that road salt is a major inland source of chloride,” Pratt says.
In addition, research out of Norway – a major road salt user – discovered when vehicles drove over road salt, particles – as small as one hundredth of the diameter of a human hair – are lofted into the air. “Since a lot of road salt was being used, and we knew if it exists as very small particles (in the air), then (rock salt) had to be the source of the chloride chemistry (being observed),” Pratt says.
The researchers made measurements that identified the sources of chloride particles on a particle-by-particle basis in and around Ann Arbor and the University of Michigan campus, and developed a model that was able to determine that road salt was responsible for contributing to chemical reactions in the atmosphere that lead to the production of air pollutants.
So, what role does road salt play in air quality? Well, this is where the science comes in.
According to Pratt, vehicles burn fossil fuels which produce nitrogen oxides. At night, the nitrogen oxides react and produce the compound dinitrogen pentoxide. The dinitrogen pentoxide compound reacts with chloride particles – which enter the air when rock salt is spread onto the pavement or driven over — to form nitryl chloride, which is a gas. When the sun comes up, sunlight breaks apart nitryl chloride into chlorine atoms and nitrogen dioxide.
“Chlorine is a very strong oxidant – it is part of why bleach is used as a cleaner,” Pratt says. “In the atmosphere, that causes other reactions to happen that can lead to ozone production, which is regulated by the Clean Air Act. It can also make more particulate matter due to other (chemical) reactions in the atmosphere.
“So, it takes something that is inert on the ground and, once it’s lofted into the air, all of sudden you’re connecting to these compounds that are air pollutants,” she adds.
While the research establishes a link between rock salt use and urban air quality, Pratt cautions additional research in needed to determine the extent of the production of air pollutants.
“We know this reaction happens. We know road salt is responsible,” she says. “So, how big of an impact is this having on our (winter) air quality? … There’s more work to be done.”