Transition metals in coarse, fine, very fine and ultra-fine particles from an interstate highway transect near Detroit
Cahill, TA; Barnes, DE; Lawton, JA; Miller, R; Spada, N; Willis, RD; Kimbrough, S
As one component of a study investigating the impact of vehicle emissions on near-road air quality, human exposures, and potential health effects, particles were measured from September 21 to October 30, 2010 on both sides of a major roadway (Interstate-96) in Detroit. Traffic moved freely on this 12 lane freeway with a mean velocity of 69 mi/hr. with little braking and acceleration. The UC Davis DELTA Group rotating drum (DRUM) impactors were used to collect particles in 8 size ranges at sites nominally 100 m south, 10 m north, 100 m north, and 300 m north of the highway. Ultra-fine particles were continuously collected at the 10 m north and 100 m north sites. Samples were analyzed every 3 h for mass (soft beta ray transmission), 42 elements (synchrotron-induced x-ray fluorescence) and optical attenuation (350 -800 nm spectroscopy). A three day period of steady southerly winds along the array allowed direct measurement of freeway emission rates for coarse (10 > D-p > 1.0 mu m), PM2.5, very fine (0.26 > D-p > 0.09 mu m), and ultra-fine (D-p < 0.09 mu m) particles. The PM2.5 mass concentrations were modeled using literature emission rates during the south to north wind periods, and averaged 1.6 +/- 0.5 mu g/m(3), versus the measured value of 2.0 +/- 0.7 mu g/m(3). Using European freeway emission rates from 2010, and modeling them at the I-96 site, we would predict roughly 3.1 mu g/m(3) of PM2.5 particles, corrected from the 4.9 PM10 value by their measured road dust contributions. Using California car and truck emission rates of 1973, this value would have been about 16 mu g/m(3), corrected down from the 19 mu g/m(3) PM5.0 using measured roadway dust contributions. This would have included 2.7 mu g/m(3) of lead, versus the 0.0033 mu g/m(3) measured. Very fine particles were distributed across the array with a relatively weak falloff versus distance. For the ultra-fine particles, emissions of soot and metals seen in vehicular braking studies correlated with traffic at the 10 m site, but only the soot was statistically significant at the 100 m north site. Otherwise, the 10 m north and 100 m north sites were essentially identical in mean concentration and highly correlated in time for most of the 5 week study. This result supports earlier publications showing the ability of very fine and ultra-fine particles to transport to sites well removed from the freeway sources. The concentrations of very fine and ultra-fine metals from brake wear and zinc in motor oil observed in Detroit have the potential of being a significant component in statistically established PM2.5 mortality rates.