title: Low-wind and other microclimatic factors in near-road black carbon variability: A case study and assessment implications.
abstract: Airborne black carbon from urban traffic is a climate forcing agent and has been associated with health risks to near-road populations. In this paper, we describe a case study of black carbon concentration and compositional variability at and near a traffic-laden multi-lane highway in Cincinnati, Ohio, using an onsite aethalometer and filter-based NIOSH Method 5040 measurements; the former measured 1-min average black carbon concentrations and the latter determined the levels of organic and elemental carbon (OC and EC) averaged over an approximately 2-h time interval. The results show significant wind and temperature effects on black carbon concentration and composition in a way more complex than predicted by Gaussian dispersion models. Under oblique low winds, namelyux[=u× sin(g=q)]~ (0,-0.5 m s-1), which mostly occurred during morning hours, black carbon concentrations per unit traffic flow were highest and had large variation. The variability did not always follow Gaussian dispersion but was characteristic of a uniform distribution at a near-road distance. Under all other wind conditions, the near-road black carbon variation met Gaussian dispersion characteristics. Significant differences in roadside dispersion are observed between OC and EC fractions, between PM2.5and PM10-2.5, and between the morning period and rest of the day. In a general case, the overall black carbon variability at the multi-lane highway can be stated as bimodal consisting of Gaussian dispersion and non-Gaussian uniform distribution. Transition between the two types depends on wind velocity and wind angle to the traffic flow. In the order of decreasing importance, the microclimatic controlling factors over the black carbon variability are: 1) wind velocity and the angle with traffic; 2) diurnal temperature variations due to thermal buoyancy; and 3) downwind Gaussian dispersion. Combinations of these factors may have created various traffic-microclimate interactions that have significant impact on near-road black carbon transport.
authors.affiliation: University of Cincinnati, Department of Biomedical, Chemical, and Environmental Engineering, College of Engineering and Applied Science, P.O. Box 210012, Cincinnati, OH 45221-0012, USA.
authors.forename: Marissa S
authors.initials: MS
authors.lastname: Liang
authors.affiliation: University of Cincinnati, Department of Biomedical, Chemical, and Environmental Engineering, College of Engineering and Applied Science, P.O. Box 210012, Cincinnati, OH 45221-0012, USA.
authors.forename: Timothy C
authors.initials: TC
authors.lastname: Keener
authors.affiliation: U.S. Center for Disease Control, National Institute of Occupational Safety and Health, 5555 Ridge Avenue, Cincinnati, OH 45213, USA.
authors.forename: M Eileen
authors.initials: ME
authors.lastname: Birch
authors.affiliation: US EPA Office of Research and Development, National Risk Management Research Laboratory, Research Triangle Park, NC 27711, USA.
authors.forename: Richard
authors.initials: R
authors.lastname: Baldauf
authors.affiliation: US EPA Office of Research and Development, National Risk Management Research Laboratory, 26W Martin Luther King Dr., Cincinnati, OH 45268, USA.
authors.forename: Jill
authors.initials: J
authors.lastname: Neal
authors.affiliation: US EPA Office of Research and Development, National Risk Management Research Laboratory, 26W Martin Luther King Dr., Cincinnati, OH 45268, USA.
authors.forename: Y Jeffrey
authors.initials: YJ
authors.lastname: Yang
keywords: Black carbon; Climate change; Gaussian dispersion; Near-road transport; On-road traffic emission