Neutral Boundary Layer Urban Dispersion in Scaled Uniform and Nonuniform Residential Building Arrays

Area of Science:

• Environmental Science
• Atmospheric Science
• Computational Fluid Dynamics

Background:

• Accurate atmospheric dispersion modeling is crucial for urban air quality assessments.
• The U.S. Environmental Protection Agency (EPA) relies on the AERMOD model for regulatory purposes.
• Existing models may require refinement for complex urban terrain effects on pollutant dispersion.

Purpose of the Study:

• To evaluate the performance of the AERMOD model's mechanical turbulence and concentration predictions in idealized urban environments.
• To establish a baseline of dispersion characteristics within uniform and nonuniform building arrays.
• To assess the impact of urban geometry on pollutant plume spread and concentration predictions.

Main Methods:

• Physical modeling in a 1:200 scale boundary layer wind tunnel simulating urban environments.
• Particle Image Velocimetry (PIV) for measuring velocity and shear stress profiles.
• Hydrocarbon Analyzers (HCAs) for measuring ethane concentrations at defined points.
• Comparison of experimental data with AERMOD predictions using Factor of 2 (FAC2) and Fractional Bias (FB) metrics.

Main Results:

• Urban configurations reduced AERMOD's FAC2 performance by 30.1%–34.1% compared to a no-building scenario.
• Modeled concentrations were significantly lower than observed, capturing only 48.1%–62.4% of the highest measured values.
• Simple first-order corrections improved AERMOD's performance in uniform urban cases, mitigating FB and enhancing FAC2.

Conclusions:

• Urban geometry significantly impacts the accuracy of Gaussian dispersion models like AERMOD.
• The study highlights the need for improved parameterizations in AERMOD to better represent dispersion in complex urban terrains.
• Further research is needed to develop robust corrections for nonuniform urban environments.