Citation:

BROWN, M., S. POL, W. COIRIER, S. KIM, A. H. HUBER, M. NELSON, P. KLEIN, M. FREEMAN, AND A. GOWARDHAN. EXPERIMENTAL AND MODEL-COMPUTED AREA AVERAGED VERTICAL PROFILES OF WIND SPEED FOR EVALUATION OF MESOSCALE URBAN CANOPY SCHEMES. Presented at Sixth Symposium on the Urban Environment, Atlanta, GA, January 30 - February 02, 2006.

Impact/Purpose:

The objective of this task is to improve EPA's ability to accurately predict the concentrations and deposition of air pollutants in the atmosphere that are known or suspected to cause cancer or other serious health effects to humans, or adverse environmental effects. It is an essential component of EPA's National Air Toxics Assessment (NATA), which seeks to identify and quantify the concentrations and sources of those hazardous air pollutants which are of greatest potential concern, in terms of contribution to population risk. It is a major contributor to NERL's Air Toxics Research Program.

"Air toxics" or "hazardous air pollutants" (HAPs) is a category that covers a large variety of chemicals, which range from relatively non reactive to extremely reactive; can exist in the gas, aqueous, and/or particle phases; display a large range of volatilities; experience varying deposition velocities, including in some cases revolatilization; and are emitted from a wide variety of sources at a large variety of different scales. In addition, concentrations of air toxics are needed by regulators for both short (days) as well as long (up to a year) time scales. These requirements challenge our current capabilities in air quality models far beyond the needs for other pollutants, such as ozone. The specific work being done under this task involves 1.) developing and testing chemical mechanisms which are appropriate for describing the chemistry of air toxics; 2.) incorporating these chemical and physical mechanisms into EPA's CMAQ modeling system and applying the model at a variety of scales; and 3.) developing the methods for using models to predict HAPs concentrations at subgrid or neighborhood scales; and 4.) using these tools to assess the magnitude and variability of concentrations to which urban populations are exposed.

Description:

Numerous urban canopy schemes have recently been developed for mesoscale models in order to approximate the drag and turbulent production effects of a city on the air flow. However, little data exists by which to evaluate the efficacy of the schemes since "area-averaged" wind measurements in cities are difficult to obtain owing to the large number of wind sensors required to obtain a reasonable statistical sample. Quasi-area-averaged vertical profiles of wind speed are presented for several recent field and wind-tunnel experiments studies where a relatively large number of wind measurements were taken. In addition, area-averaged profiles were derived using numerical model-computed mean wind fields from numerical simulations performed in several cities. These computational fluid dynamics codes have the advantage of producing a dense array of "measurements" which can be used to obtain a more statistically-significant area average. The use of numerical models for creating "synthetic" data is discussed and demonstrated for several cities.

Record Details: