Traffic Particles in Ambient Air of a Major US Urban Area: Has Anything Changed over a Decade?

Few studies have addressed the potential changes in particulate matter (PM) exposure occurring in major metropolitan areas over substantial time periods, e.g., 5 or 10 years. The present study examined changes in the PM2.5 concentration and elemental composition between two monitoring campaigns carried out in 2002–2005 and 2010–2011 within the Greater Cincinnati area (USA). This area is recognized for high volume of diesel truck traffic (about 10 million trucks annually on regional freeways). The 24-hour filter samples were collected at four sites. General linear models were used to examine differences between the two data sets for elemental carbon (EC), organic carbon (OC), and EC/OC. The comparison was extended to the concentrations of PM2.5 and its relevant elemental constituents. At one site, which was previously identified as a particularly hot spot for traffic/diesel air pollution, the concentrations of most traffic related elements as well as EC and EC/OC ratio significantly decreased (p < 0.05) between the two campaigns. No significant differences between carbon data generated in the two campaigns were observed at the other three monitoring stations. These findings did not depend on whether the comparison model accounted for wind speed and direction. The EC/OC determined in the recent campaign across all sites showed no significant differences between the Summer and Fall data but Winter values were significantly lower. The site with the highest traffic influence revealed no significant seasonal difference in PM2.5 but essentially all relevant elements showed significant seasonal variations between the Fall (higher) and Winter (lower). The findings suggest that air quality and engine exhaust control policies implemented between 2005 and 2010 have not produced significant changes in metropolitan traffic air pollution levels. However, the decreasing trends in PM2.5, Ti, V, Mn, Fe, Zn, Br, and Pb, EC, OC, and EC/OC may become sustainable over a longer time.