Performance of Air Quality Models in Urban Areas  Objectives and Motivation  St. Louis study and ISC urban  Model Improvements  Performance of Improved Model  Conclusions Akula Venkatram 1, Vlad Isakov 2, 1 University of California, Riverside, CA 2 NOAA, NERL, RTP, NC

Motivation and Objectives The most severe air pollution problems in Asia are caused by emissions in urban areas u Examine the performance of currently used urban air quality models-ISC u Suggest improvements based on recent tracer studies in urban areas

St. Louis Experiment  Conducted during resulting in 26 daytime and 16 nighttime experiments  Cadmium sulfide particles released from ground- level source and measured at distances ranging from 800 m to 16 km using 50 samplers  Winds, temperature, and horizontal velocity fluctuations measured using TV tower and tether sondes upto a height of 140 m. Briggs (1974) used data to derive McElroy- Pooler urban dispersion curves used in ISC

McElroy-Pooler Curves Briggs(1974)

Horizontal Plume Spread Comparison St. Louis Experiment

Vertical Plume Spread Comparison Inferred from ground-level concentrations

Model Results using stability classes from Lambert Air Field

Problems with McElroy-Pooler Curves  Using McElroy Pooler curves requires stability and wind speed information. Model results depend on location of measurements.  MP curves refer to St. Louis. Need not apply to other urban areas.  MP curves implicitly account for the effects of limited mixing. Thus, do not allow the use of mixed layer information in urban areas.

Improved Model

Model Results using measured meteorology

Barrio Logan Experiment  Conducted during summer of 2001 resulting in 50 hours of data  Sulfur hexafluoride released from ground- level source and measured at distances ranging from 200 m to 2 km using 50 samplers  Winds, temperature, and velocity fluctuations measured using sonic anemometers and minisodars

Model Results using Boundary Layer Information and Initial Spread

Near Source Modeling

Model Evaluation Results

Conclusions u The use of McElroy-Pooler curves in all urban areas cannot be justified u Turbulence above the canopy controls dispersion once the plume spread exceeds canopy height. Simple models for dispersion provide adequate concentration estimates provided above canopy meteorology is used u Dispersion models should account for magnification of horizontal spread near sources- channeling ? u Near source dispersion requires meteorological data close to source, and model needs to incorporate meandering.