Chemical and Physical Characterization of Non-Tailpipe and Tailpipe Emissions at 100 Locations near Major Roads in the Greater Boston Area
Petros Koutrakis, Brent Coull, Marco Martins, Joy Lawrence, and Stephen Ferguson
Harvard TH Chen School of Public Health, Boston MA, USA
Background. The purpose of the proposed research is to develop sampling and statistical analysis methods to improve assessment of exposure to traffic particles and pollutants for health effects studies, characterize direct (tailpipe and non-tailpipe) and indirect contributions, and identify parameters which influence them.
Methods. Using a mobile sampling platform equipped with coarse and fine particulate matter (PM) concentrators we are collecting ambient ultrafine, fine minus ultrafine (accumulation mode), and coarse particle samples near roads in the Greater Boston. In addition, we are collecting PM2.5 and PM10 samples of surface road dust aerosolized in situ, using our Road Dust Aerosolization sampler. We are collecting these road dust (RD) and ambient PM samples at three distance ranges (including background) from each of the 100 major (A1, A2, and busy A3) road sites included in this study.
Results. We analyzed particle samples collected at 26 sites (nine A1, five A2, and twelve A3-type roads) using XRF and reflectometry and conducted preliminary data analysis.
For both coarse and fine ambient PM and RD samples, we observed significant differences in composition between those collected at the roadside, local background, and intermediate distances. Ambient PM mass concentrations were generally highest at the roadside and lowest at the local background site. Comparison of ambient fine and coarse PM sample composition between near major roads and background locations indicated that vehicular sources were significant contributors to the concentrations of certain elements (e.g., Cu, Ti, Ba, Zn, Zr, Mn and Fe). Concentrations of these elements were higher at the roadside as compared to those observed at locations 50 to 200 m from the roadway.
For RD samples, we normalized elemental composition of coarse and fine fractions relative to Al. While Si is the most abundant element in the earth crust and has been used as a soil tracer, we selected Al because ceramic brake pads, which have been used extensively in recent years, contain Si. As with ambient PM, comparison of specific elements in fine and coarse RD samples between near major roads and background locations indicates that vehicular sources are significant contributors. Also, concentrations of many elements were elevated at the roadside as compared to those observed at locations 50 to 200 m from the roadway.
While distance from road had a significant impact on composition of both RD and ambient PM, road type did not. There was little difference in composition of RD or ambient PM among A1, A2, and A3 roads.
Conclusions. Composition of ambient and road dust samples near major roads compared to background indicated that vehicular sources are significant contributors for certain elements. Differences in composition between ambient PM and RD collected at roadside, intermediate, and local background sites were significant despite the small number of observations, and did not depend on road type.