We sought to evaluate the air quality implications of rail traffic at two sites in Washington State. Our goals were to quantify the exposure to diesel particulate matter (DPM) and airborne coal dust from current trains for residents living near the rail lines and to measure the DPM and black carbon emission factors (EFs). We chose two sites in Washington State, one at a residence along the rail lines in the city of Seattle and one near the town of Lyle in the Columbia River Gorge (CRG). At each site, we made measurements of size-segregated particulate matter (PM 1 , PM 2.5 and PM 10 ), CO 2 and meteorology, and used a motion-activated camera to capture video of each train for identification. We measured an average DPM EF of 0.94 g/kg diesel fuel, with an uncertainty of 20%, based on PM 1 and CO 2 measurements from more than 450 diesel trains. We found no significant difference in the average DPM EFs measured at the two sites. Open coal trains have a significantly higher concentration of particles greater than 1 µm diameter, likely coal dust. Measurements of black carbon (BC) at the CRG site show a strong correlation with PM 1 and give an average BC/DPM ratio of 52% from diesel rail emissions. Our measurements of PM 2.5 show that living close to the rail lines significantly increases PM 2.5 exposure. For the one month of measurements at the Seattle site, the average PM 2.5 concentration was 6.8 µg/m 3 higher near the rail lines compared to the average from several background locations. Because the excess PM 2.5 exposure for residents living near the rail lines is likely to be linearly related to the diesel rail traffic density, a 50% increase in rail traffic may put these residents over the new U.S. National Ambient Air Quality Standards, an annual average of 12 µg/m 3 .

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