Measurement of the gross alpha activity of the fine fractions of road dust and near-roadway ambient particle matter

Abstract

ABSTRACT Traffic-related air pollution, including direct exhaust emissions and road dust (RD), impacts individuals living near busy roads. We recently conducted a study to investigate the sources and composition of tailpipe and non-tailpipe traffic emissions, where we collected and analyzed samples of ambient air fine particulate matter (PM2.5) and fine RD (RD2.5) at different distances from major roadways. We analyzed a subset of the samples, including those collected at the roadside and local background, for their alpha activity level. Subsequently, we investigated whether there is a distance-related decay in the alpha activity in RD2.5 or PM2.5 similar to those observed for traffic-related species in PM2.5 and RD2.5. We found that the alpha activity of ambient air PM2.5 (Bq/mg) was more than an order of magnitude higher than the activity level of the corresponding RD2.5 sample, suggesting that PM2.5 may be more toxic than RD2.5. Using mixed-effects regression models, we found that ambient PM2.5 alpha activity was significantly higher during the cold months than during warm months, and that the background was higher than the roadside (though not significantly). In contrast, the RD2.5 alpha activity was significantly higher at the background site compared to the roadside but was not significantly affected by season. In addition to sampling position, both Zn and elemental carbon (EC) were significant predictors of RD2.5 alpha activity. In addition, the roadside RD2.5 activity levels were found to be higher at highways as compared to secondary roads. While traffic-related emissions do not appear to be significant sources of either ambient PM2.5 or RD2.5 alpha activity, the RD2.5 results suggest that traffic-related particles may contribute to RD2.5 alpha-activity. Implications: Many studies have reported the effects of traffic-related particulate matter (PM) on human health, and there is growing interest in the health effects of exposure to environmental PM alpha activity. This is the first study to report on the alpha activity of road dust (RD) or near-roadway ambient PM. We found that the alpha activity of ambient PM is twenty times higher than RD, suggesting that ambient PM may be more toxic. In PM and RD, the alpha activities were higher at background sites than at the roadside, indicating that traffic-related emissions are not a significant source of particulate radioactivity.