Widespread road salt use could affect wintertime air quality

Each year during winter weather, crews spread more than 60 million tons of salt on roads worldwide to melt ice. Although there’s mounting evidence that this salt taints the water supply, until now no one has examined its impact on air quality. A new study demonstrates that road salt lofted into the air releases chloride-containing gases, which contribute to air pollution (ACS Cent. Sci. 2020, DOI: 10.1021/acscentsci.9b00994).

Kerri A. Pratt of the University of Michigan has been researching wintertime atmospheres in the Arctic for years. This time, she says, “instead of going to the Arctic, we just stayed at home.”

Ozone, particulate matter, and nitrogen dioxide (NO₂) are key pollutants regulated under the Clean Air Act that can cause respiratory issues and damage to ecosystems when they are produced at ground level. In coastal areas, sea spray aerosols impact the pollutants’ formation. These particles provide a chloride-containing surface on which nighttime dinitrogen pentoxide can react and form nitryl chloride (ClNO₂) and particulate sodium nitrate. Once the sun rises, photons break the ClNO₂ into chlorine atoms and NO₂, which interact with other atmospheric compounds and produce ozone and particulate matter.

Inland, ClNO₂ wasn’t officially documented until 2010, and chloride origins there have remained elusive—although research several decades ago hypothesized that aerosolized road salt could contribute to atmospheric chlorine chemistry.

Back in Ann Arbor, Michigan, Pratt is used to seeing ever-growing piles of road salt accumulating in the streets each winter. Her team decided to test whether lofted road salt could be a local chloride source for ClNO₂ production. The researchers employed chemical ionization mass spectrometry to analyze trace gases in the air outside their lab between February and March 2016. Using single-particle mass spectrometry and electron microscopy, they examined individual particles in the samples and identified five different particle sources: road salt, aged road salt, biomass burning, soot, and road dust. But they determined that only the fresh road salt particles contained significant amounts of chloride.

Pratt’s group also developed a method to simulate ClNO₂ formation. Using the data from their measurements, the researchers were able to model how much chloride each road salt particle contributed to ClNO₂ production.

“We were able to actually quantify the contribution of individual particles,” Pratt says, “and show that 80 to 100% of the ClNO₂ was from road salt aerosol.”

According to Cassandra J. Gaston, an expert in atmospheric chemistry and aerosols at the University of Miami, these experiments are among the first to combine gas-phase measurements and single-particle techniques to study ClNO₂ production in field samples.

“This study helps us understand the cause of all of this chlorine chemistry that we can’t explain by sea salts,” she says. “It gives us a new appreciation for the diversity of particle sources that can facilitate this chemistry.”

Road salt emissions, though, are not currently included in regional air-quality models. Researchers still need additional laboratory studies of these particles and others, as well as data from other inland locations.