Characterization of Spatiotemporal Patterns in Air Pollutant Concentrations within Complex Urban Environments

JENNIFER RICHMOND-BRYANT (1), Sastry S. Isukapalli (2), Daniel A. Vallero (3)

(1) National Center for Environmental Assessment, U.S. Environmental Protection Agency, (2) Environmental and Occupational Health Sciences Institute, Rutgers University, (3) National Exposure Research Laboratory, U.S. Environmental Protection Agency

     Abstract Number: 88
     Last modified: October 27, 2009

Abstract
Substantial variability exists in air pollutant exposures at neighborhood scales. Factors driving exposure variability include pollutant source variability, chemical composition of the pollutant mixture, meteorology, layout of the built environment, building dimensions, and human time-activity patterns. States and cities do not typically have sufficient monetary and human resources to perform the saturation sampling necessary to obtain temporally and spatially resolved pollutant concentration data. Moreover, emergency responders may require guidance on safe procedures in the case of an intentional or accidental contaminant release. Thus, short-term high spatiotemporal resolution studies can be valuable for providing insight into the complex relationships between pollutant levels and relevant factors such as meteorology and building dimensions. Scaling relationships derived from such studies may help cities understand spatially resolved estimates of concentration decay using current meteorological and built environment data in the absence of dense contaminant sampling networks.

The URBAN 2000, Joint Urban 2003, and Urban Dispersion Program studies held respectively in Salt Lake City, Oklahoma City, and New York City (jointly referred to here as the Urban Dispersion Program, or UDP) were designed and implemented to collect spatiotemporally resolved ambient concentration, personal exposure, and meteorology data. Perfluorocarbon (PFT) tracers were released at several locations within the study sites and then sampled at gridded locations using time-programmable gas capture devices to attain a time-evolving concentration surface. Programmable capture devices were worn by subjects who walked on scripted paths during the campaign. Roof-top and ground-level mean and turbulent wind data were obtained using sonic anemometers and SODAR systems to profile the wind.

In this current work, UDP concentration and microscale turbulent wind data were reanalyzed to examine scaling relationships. Street-level PFT concentration time-series were reviewed to find time periods that included a peak and decay. Exponential decay curves were fit to each period, and a characteristic residence time was derived from each model slope. That residence time was nondimensionalized by the ratio of mean wind speed to height of the downwind building bounding the street canyon in which the concentration was measured. SODAR data were used to assess atmospheric turbulence conditions at times concurrent with the concentration decay measurements. Reynolds number and freestream turbulence intensity were calculated from the 15-minute average and standard deviation of velocity. An inverse relationship was observed between the nondimensionalized residence time and turbulence intensity computed from the wind data, although no apparent relationship could be discerned regarding Reynolds number. These data suggest that atmospheric turbulence plays a role in ventilating urban street canyons even within a dense urban environment and that contaminant residence time can be predicted with turbulence intensity data. In addition to providing information on contaminant exposure, these results can provide guidance for emergency response protocols. Validation work is needed at sites with concurrent ambient concentration monitors to determine if trends in concentration can be predicted using this approach and, if so, for which distribution of pollutants such an approach could be applicable in every-day or emergency response scenarios. (This presentation does not necessarily reflect the policy of the U.S. EPA.)