1. 13th EMS Annual Meeting & 11th European Conference on Applications of Meteorology (ECAM)
September 9 to 13, 2013; Reading, United Kingdom
Parameterization of Pollutant Plume Dispersion in Neutral Stratification over Hypothetical Urban Areas
Chi-To Ng and Chun-Ho Liu*
Department of Mechanical Engineering
The University of Hong Kong
*Corresponding Author: Chun-Ho LIU
Department of Mechanical Engineering, 7/F Haking Wong Building, The University of Hong Kong, Pokfulam Road, Hong Kong
Tel: (852) ; Fax: (852) ;

2. Introduction
Ground-level pollutants from human activities and vehicles are the primary pollutant sources, which adversely affect the health and living quality of urban inhabitants.
Large-scale roughness elements such as buildings and mountains can significantly modify the pollutant dispersion behaviour.
The Gaussian model of plume dispersion is commonly used to estimate the downwind pollutant concentration distribution. However, one of its major parameters, the dispersion coefficient (σz), often underestimates the importance of buildings in urban areas.

3. Objectives
The aerodynamic effects of idealized urban roughness were parameterized with a single variable – friction factor (f), which is believed to be sufficient to measure the resistance of urban roughness at constant UBL height for comparison in our previous studies (Wong and Liu, 2013).
Parameterize pollutant dispersion behaviour (σz) with idealized urban roughness (f) to enrich the understanding of pollutant dispersion and removal mechanism in urban areas.

4. Methodology
Renormalization Group (RNG) of kappa-epsilon (k-ϵ) turbulence model
2-dimensional computational domains consisting of idealized street canyons were used to represent the hypothetical urban areas
A total of 144 models with eight different building shapes and eighteen different building-height-to-street-width (aspect) ratios (ARs)
Covering the characteristic skimming flow, wake interference and isolated roughness regimes

5. Computational Domain and Boundary Conditions

6. Idealized Building Shapes

7. Aspect Ratios and Setups

8. Friction Factor ( f ) 𝑓= ∆𝑃 x 𝐻 𝐿 𝑈 2 2
A dimensionless number represents the surface roughness caused by different building morphology (building shapes and ARs)
∆𝑃, L
𝑓= ∆𝑃 x 𝐻 𝐿 𝑈 2 2 H

9. Friction Factor ( f ) vs ARs
Friction Factor decreases when the street is extremely wide, it is because the wind flows and street configurations reached isolated roughness regime

10. z0 vs Friction Factor ( f )
z0 is obtained from ensemble average of velocity profile in horizontal direction and log profile 𝑈 𝑧 = 𝑈 ∗ κ ln 𝑍−𝑑 𝑍

11. Gaussian Plume Model and Dispersion Coefficients (σz)
ϕ = 𝑄 2 π 𝑢 σ 𝑧 exp − 𝑧− 𝑧 𝑐 σ 𝑧 exp − 𝑧+ 𝑧 𝑐 σ 𝑧 2

12. Dispersion Coefficients (σz) and Length Scale
σ 𝑧 = 2 𝑘 𝑧 𝑥 𝑈 𝑘 𝑧 = 1 2 𝑈 𝑥 σ 𝑧 (1)
𝑘 ℎ = 𝑙 𝑚 𝑙 ℎ 𝑑 𝑢 𝑑𝑧
𝑘= 𝑙 𝑑𝑢 𝑑𝑧 𝑑𝑣 𝑑𝑧
𝑘 ℎ = 𝑐 ℎ 𝑙 ℎ σ 𝑤
𝑘 𝑚 = − 𝑢 ′′ 𝑤 ′′ 𝑑 𝑢 𝑑𝑧 =κ 𝑢 ∗ 𝑧 (2)
Sub (1) into (2) σ 𝑧 = κ 𝑥 𝑧 𝑓 1/4
Mixing length hypothesis
Near surface in neutral stratification

13. Dispersion Coefficients (σz) vs Friction Factor ( f )

14. Conclusions
The mean value of σz / x1/2 was found to be direct proportional to f1/4 with the coefficient of determination equal to
The mean value of σz / x1/2 is close to the upper limit.
Friction factor (f) is believed to be able to estimate the range of dispersion coefficients (σz) for urban areas.

15. Thank you