Airborne Road Deicing Salt at Suburban Chicago Sites

Gary J. Stensland
Dakota Science, 311 E Holmes, Urbana, IL 61801

Allen L. Williams
Center for Atmospheric Science, Illinois State Water Survey, Champaign, IL


No comprehensive airborne road salt project had ever been done prior to the Illinois Department of Transportation (IDOT) funding for the Illinois State Water Survey (ISWS) to undertake such a project. IDOT funded ISWS to conduct a multi-year project which included measurements of airborne road salt at five permanent sites in the area of Lemont IL, just west of Argonne National Laboratory (ANL), which is about 25 miles southwest of downtown Chicago. I55, a SW-NE oriented 4 lane interstate highway (2 lanes in each direction) with large traffic flows, is expected to be the dominant source or airborne road salt in the Lemont area. The five Lemont sites were at perpendicular distances of .9 to 2.8 miles southeast of I55 while the NADP site IL19, at ANL, was 1.2 miles southeast of I55. Monthly measurements at the five Lemont sites included sodium, chloride, and other major ions from both the wet-side and dry-side of Aerochem Metric samplers, as well as from air quality filters using high volume aerosol samplers and dichotomous aerosol samplers. Road salt amounts applied by IDOT to the major highways in the many sub-areas of the Chicago metropolitan area, for all salting events from 1987 to 2004, were assembled by ISWS. For four road salting cases, snowfall grab samples were collected at ten sites on either side of I57, at distances from 200 to 2000 feet from interstate, and analyzed for the major ions.

An overview of results from this project will be presented. The air quality measurements at the five Lemont sites show that most of the sodium aerosol mass is in particles greater than 10 microns in diameter. Current air quality networks (IMPROVE and CASTNET) do not include measurements of particles greater than 10 microns. From the literature it can be suggested that even the high volume sampler may not efficiently capture the larger deicing salt particles present at distances greater than one mile from the emitting source road. Thus the dry-side bucket seems to be a reasonable choice to capture the deposition of these large aerosols.

The monthly data record for dry-side deposition at the five Lemont sites extends from 1997 to 2004 while the data record for the IL19 Argonne site was 1980-1998. The application of road deicing salt for state roads in the area was found to have a high linear correlation (r2=.75) with salt dry deposited into the bimonthly dry-side samples at the IL19 site, for the period 1987-1997. It was estimated that at the NADP site, IL19, total sodium deposition due to road salt was about 103 mg/m2 /6 winter months for dry-side deposition and 14 mg/m 2 /6 winter months for wet-side deposition.