July 2008
Characterization of Near-Road Pollutant Gradients Using Path-Integrated Optical Remote Sensing
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Eben D. Thoma and Richard C. Shores,
Office of Research and Development, National Risk Management Research Laboratory, Air Pollution Prevention and Control Division, U.S. Environmental Protection Agency, Research Triangle Park, NC
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Vlad Isakov,
National Oceanic and Atmospheric Administration, Sciences Modeling Division (In Partnership with U.S. Environmental Protection Agency), Research Triangle Park, NC
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Richard W. Baldauf,
Office of Research and Development, NRMRL, APPCD, U.S. Environmental Protection Agency, Research Triangle Park, NC, and Office of Air and Radiation, Office of Transportation and Air Quality, National Vehicle and Fuel Emissions Laboratory, Ann Arbor, MI
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AbstractUnderstanding motor vehicle emissions, near-roadway pollutant dispersion, and their potential impact to nearroadway populations is an area of growing environmental interest. As part of ongoing U.S. Environmental Protection Agency research in this area, a field study was conducted near Interstate 440 (I-440) in Raleigh, NC, in July and August of 2006. This paper presents a subset of measurements from the study focusing on nitric oxide (NO) concentrations near the roadway. Measurements of NO in this study were facilitated by the use of a novel pathintegrated optical remote sensing technique called deep ultraviolet differential optical absorption spectroscopy (DUV-DOAS). This paper reviews the development and application of this measurement system. Time-resolved near-road NO concentrations are analyzed in conjunction with wind and traffic data to provide a picture of emissions and near-road dispersion for the study. Results show peak NO concentrations in the 150 ppb range during weekday morning rush hours with winds from the road accompanied by significantly lower afternoon and weekend concentrations. Traffic volume and wind direction are shown to be primary determinants of NO concentrations with turbulent diffusion and meandering accounting for significant near-road concentrations in off-wind conditions. The enhanced source capture performance of the open-path configuration allowed for robust comparisons of measured concentrations with a composite variable of traffic intensity coupled with wind transport (R2 0.84) as well as investigations on the influence of wind direction on NO dilution near the roadway. The benefits of path-integrated measurements for assessing line source impacts and evaluating models is presented. The advantages of NO as a tracer compound, compared with nitrogen dioxide, for investigations of mobile source emissions and initial dispersion under crosswind conditions are also discussed.
ImplicationsNO is shown to be a useful tracer for characterization of mobile source emissions and dispersion in close proximity to major roadways. The DUV-DOAS system has proved to be a useful instrument for time-resolved measurement of NO in these applications. Near-road measurements using coordinated deployments of open-path systems, coupled with wind and traffic characterization, are shown to be useful tools for near-road impact studies.
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