Road Salt Influences Urban Winter Air Quality
Road salt is the primary source of airborne chloride particles that react in the wintertime atmosphere to produce a molecule known to alter air quality.
In the atmosphere, particulate chloride can react with dinitrogen pentoxide to produce nitryl chloride (ClHNO2–). Upon sunrise, this molecule breaks apart into chlorine atoms and nitrogen dioxide that contribute to poor air quality. This atmospheric chlorine chemistry is known to happen in coastal urban areas, where sea spray aerosol provides a chloride source. Based on models constrained to particulate chloride across the United States, the majority of atmospheric nitryl chloride is suggested to be produced inland, particularly in wintertime, but the inland chloride sources are largely unknown.
During winter in a Michigan city, scientists measured the amount of nitryl chloride as an atmospheric gas using mass spectrometry. At the same time, they measured the chemical composition of individual aerosol particles in that same city using a different type of mass spectrometry along with X-ray spectroscopy. The researchers then developed a new modeling method to calculate the amount of nitryl chloride produced from four different types of aerosol particles that they found in their samples. They determined that 80% to 100%of the atmospheric nitryl chloride came from only one of the types, road salt aerosols.
While road salt has long been hypothesized to contribute to inland nitryl chloride production, inland sources of particulate chloride in the atmosphere are largely untracked and unquantified. The lack of data hinders assessments of air quality that could link specific pollutants to their sources. This study connects the common practice of deicing roads with salt to air quality in inland cities during the winter.
In the winter, road crews spread tens of millions of tons of salt to deice roads around the world. Traffic grinds the salt into tiny particles that become airborne. Scientists measured atmospheric trace gases and aerosol particles in Ann Arbor, Michigan, in the winter of 2016. They used chemical ionization mass spectrometry to measure gaseous nitryl chloride, and single-particle mass spectrometry and X-ray spectroscopy to measure the chemical composition of thousands of individual aerosol particles. This work was part of the Isotope and Chemical Analysis Integrated Research Platform at the Environmental Molecular Sciences Laboratory (EMSL), where the researchers used computer-controlled scanning electron microscopy (CCSEM) with energy dispersive X-ray (EDX) spectroscopy.
Once they knew the distribution of chloride among individual particles of road salt, aged road salt, residential wood burning, and soot, the scientists used a new single-particle parametrization to identify which particle type was the primary source of atmospheric nitryl chloride. They found that road salt was the source for 80% to 100% of nitryl chloride. These findings could be used to quantify road salt emissions and generate inventories for air quality models.
Kerri A. Pratt
University of Michigan
Funding was provided by the University of Michigan, Alfred P. Sloan Foundation, National Science Foundation, U.S. Department of Education, U.S. Department of Energy (DOE) Office of Science, including support of the Environmental Molecular Sciences Laboratory (EMSL), a scientific user facility, and DOE Office of Biological and Environmental Research.
McNamara, S.M., Kolesar, K.R., Wang, S. et al. “Observation of road salt aerosol driving inland wintertime atmospheric chlorine chemistry.” ACS Central Science 6(5), 684–694 (2020). [DOI:10.1021/acscentsci.9b00994]