Pollutant Concentrations Downwind of Freeway Sound Walls

NEELAKSHI HUDDA (1), Katharine F Moore (1), Winnie Kam (1), Nancy Daher (1), Constantinos Sioutas (1), Jorn Herner (2), Kathleen Kozawa (2), Steve Mara (2), Seong Park (2)

(1) University of Southern California (USC), (2) California Air Resources Board (CARB)

     Abstract Number: 178
     Last modified: November 7, 2009

Abstract
Motor vehicle emissions of both particle and gas phase species are of specific interest in urban environments. Not only is motor vehicle traffic responsible for a significant fraction of urban air pollution, but increased exposure to these pollutants due to time spent in proximity to freeways has been associated with adverse health effects, particularly among children. Studies investigating pollutant dispersion downwind of freeways have shown rapid decay monotonically downwind until “urban background” concentrations are reached (Zhang et al. (2004). Atmos. Environ. 38: 6655 - 6665). The distance required for dispersion depends strongly upon local meteorology, downwind obstructions, traffic volume and other parameters. For example, in Los Angeles, Zhu et al ((2002) Atmos. Environ. 36: 4323 – 4335) reported daytime downwind concentrations measured at distances greater than 300 meters conditions were similar to upwind background concentrations. These freeway studies did not specifically investigate the role of obstructions in downwind pollutant dispersion. Recently near roadway studies in North Carolina (Baldauf, R. et al. (2008). Atmos. Environ. 42: 7502 – 7507, Bowker, G. et al. (2007). Atmos. Environ. 41 8128–8139) suggested that the typical dispersion pattern reported by Zhu and co-workers can be significantly modified due to the presence of freeway noise barriers or sound walls. In these studies, the sound wall disrupted the cross flow wind field resulting in relatively lower and then higher pollutant concentrations as the plume moves downwind.

During June/July 2009, we conducted a sampling campaign, to assess the effects of sound walls on downwind pollutant dispersion in typical urban environments. The study sites included two sets of near roadway sites – soundwall and “non sound wall” or control – adjacent to Interstate-710 and Interstate–5. These two freeways were chosen due to their high traffic volume, the proximity of the sound wall and “non sound wall” sites to each other along the freeway, and prevailing meteorology. Experimental conditions (e.g. sound wall height) vary between the two freeways in order to explore how robust the changes in downwind pollutant dispersion are despite them. Simultaneous measurements of select aerosol and gas-phase species were made at a fixed near-freeway site and at varying distances downwind using a mobile measurement platform. The parameters measured include particle number concentrations and size distributions, nitrogen oxides, carbon dioxide and meteorology. Subsequent analysis indicates that meteorological conditions were reasonably consistent during the time period at each freeway. Preliminary results indicate that immediately adjacent to the sound wall, a recirculation zone, consistent with the analysis suggested in Bowker et al. At short distances downwind, observed concentrations were much lower than those observed further downwind. Relatively elevated concentrations were observed at distances ca. 50 meters -150 meters downwind of the sound walls than observed at comparable distances in the absence of the sound wall. As a consequence, larger distances were usually necessary downwind of the sound walls for background urban pollutant background concentrations to be reached. As additional data are processed, further results will be presented at the conference.