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FINE SCALE AIR QUALITY MODELING USING DISPERSION AND CMAQ MODELING APPROACHES: AN EXAMPLE APPLICATION IN WILMINGTON, DE
Citation:
CHING, J. K. FINE SCALE AIR QUALITY MODELING USING DISPERSION AND CMAQ MODELING APPROACHES: AN EXAMPLE APPLICATION IN WILMINGTON, DE. Presented at 5th Annual CMAS Model 3 User's Conference, Chapel Hill, NC, October 16 - 18, 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:
Characterization of spatial variability of air pollutants in an urban setting at fine scales is critical for improved air toxics exposure assessments, for model evaluation studies and also for air quality regulatory applications. For this study, we investigate an approach that supplement results of outputs from a local scale dispersion model (AERMOD) to CMAQ at regional scales for fine particulates and formaldehyde. The CMAQ runs were performed at 4 and 12 km grid sizes. Both modeling systems were run for the calendar year 2001. (A limited set of CMAQ runs were also made at 1 km grid size for July, 2001) Model results for formaldehyde, ozone and ultrafine PM are compared with observational data from a recent field study in Wilmington, DE. ). Our attention will be focused on an area covered by a 12 km CMAQ grid cell (and its nine 4 km cells) centered over Wilmington DE. The ultrafine particle and formaldehyde data were collected in situ using an instrumented van deployed in a mobile sampling mode for approximately one week periods during each of four seasons in 2005-2006. (During the summer of 2005, an ozone monitor was installed and operated in the van.). The sampling consisted of mobile transects over a set course covering much of the street blocks over an areas about 1 km by 2 km of downtown Wilmington. Each set of transects took approximately an hour to complete. The concentration distributions derived from each of the time series of the individual transects and constituted a 'sample' which is then compared against distributions based on the modeled results.