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Low-Wind and Other Microclimatic Factors in Near-road Black Carbon Variability: A Case Study and Assessment Implications
Citation:
Liang, M. S., T. C. Keener, M. Birch, R. Baldauf, J. Neal, AND Y. J. Yang. Low-Wind and Other Microclimatic Factors in Near-road Black Carbon Variability: A Case Study and Assessment Implications. ATMOSPHERIC ENVIRONMENT. Elsevier Science Ltd, New York, NY, 80:204-215, (2013).
Impact/Purpose:
Communicate to scientific community and urban management agencies on black carbon particulates and its roadside variability, and relationship with microclimate conditions in an urban environment.
Description:
Airborne black carbon from urban traffic is a climate forcing agent and has been associated with health risk 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-minute average black carbon concentrations and the latter determined the levels of organic and elemental carbon (OC and EC) averaged over an approximately 2-hour time interval. The results show significant wind and temperature effects on black carbon concentration and composition in a way more complex than those afforded in the Gaussian dispersion models. Under oblique low winds, namely u_x [=u×sin(θ) ]~ (0, -0.5 m/s) which mostly occurred during morning hours, black carbon concentrations per unit traffic flow were the highest with a large variation. The variability did not always follow Gaussian dispersion, but was characteristic of a uniform distribution in 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 PM2.5 and PM10, and between the morning period and the rest of a day. In a general case, the overall black carbon variability at the multi-lane highway can be stated as bimodal consisting of the Gaussian dispersion and non-Gaussian uniform distribution. Transition between the two types depends on wind speed 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 speed and the angle with traffic; 2) diurnal temperature variations due to thermal buoyancy; and 3) the downwind Gaussian dispersion. Combinations of these factors may have created various scenarios in traffic-microclimate interactions of significant impacts on near-road black carbon transport.
