CFD modelling of the aerodynamic effect of trees on urban air pollution dispersion.

The current work evaluates the impact of urban trees over the dispersion of carbon monoxide (CO) emitted by road traffic, due to the induced modification of the wind flow characteristics. With this purpose, the standard flow equations with a kε closure for turbulence were extended with the capability to account for the aerodynamic effect of trees over the wind field. Two CFD models were used for testing this numerical approach. Air quality simulations were conducted for two periods of 31h in selected areas of Lisbon and Aveiro, in Portugal, for distinct relative wind directions: approximately 45° and nearly parallel to the main avenue, respectively. The statistical evaluation of modelling performance and uncertainty revealed a significant improvement of results with trees, as shown by the reduction of the NMSE from 0.14 to 0.10 in Lisbon, and from 0.14 to 0.04 in Aveiro, which is independent from the CFD model applied. The consideration of the plant canopy allowed to fulfil the data quality objectives for ambient air quality modelling established by the Directive 2008/50/EC, with an important decrease of the maximum deviation between site measurements and CFD results. In the non-aligned wind situation an average 12% increase of the CO concentrations in the domain was observed as a response to the aerodynamic action of trees over the vertical exchange rates of polluted air with the above roof-level atmosphere; while for the aligned configuration an average 16% decrease was registered due to the enhanced ventilation of the street canyon. These results show that urban air quality can be optimised based on knowledge-based planning of green spaces.

[1]  H. Upmanis,et al.  Case Studies of the Spatial Variation of Benzene and Toluene Concentrations in Parks and Adjacent Built-up Areas , 2001 .

[2]  J. Crowther,et al.  Three-Dimensional Numerical Simulation of Air Pollutant Dispersion in Street Canyons , 2002 .

[3]  Hiroshi Yoshino,et al.  Examining tree canopy models for CFD prediction of wind environment at pedestrian level , 2008 .

[4]  Steven R. Hanna,et al.  Technical Descriptions and User's Guide for the BOOT Statistical Model Evaluation Software Package, Version 2.0 , 2005 .

[5]  Arthur M. Winer,et al.  Low-emitting urban forests: A taxonomic methodology for assigning isoprene and monoterpene emission rates , 1996 .

[6]  J. Finnigan,et al.  Atmospheric Boundary Layer Flows: Their Structure and Measurement , 1994 .

[7]  Dragutin T. Mihailović,et al.  An Empirical Relation Describing Leaf-Area Density inside the Forest for Environmental Modeling , 2004 .

[8]  Riccardo Buccolieri,et al.  Dispersion study in a street canyon with tree planting by means of wind tunnel and numerical investigations – Evaluation of CFD data with experimental data , 2008 .

[9]  E. Pehkonen,et al.  Variations in airborne pollen antigenic particles caused by meteorologic factors , 1994, Allergy.

[10]  M. Raupach,et al.  Averaging procedures for flow within vegetation canopies , 1982 .

[11]  D. Baldocchi,et al.  Spatial and temporal variation in respiration in a young ponderosa pine forest during a summer drought , 2001 .

[12]  Jörg Franke,et al.  COST 732 in practice: the MUST model evaluation exercise , 2011 .

[13]  A. Winer,et al.  Estimating the ozone-forming potential of urban trees and shrubs , 1998 .

[14]  W. Kuttler,et al.  On the reduction of urban particle concentration by vegetation : a review , 2008 .

[15]  Jeremy Colls,et al.  Air Pollution: Measurement, Modelling and Mitigation , 1997 .

[16]  F. Baret,et al.  Review of methods for in situ leaf area index (LAI) determination: Part II. Estimation of LAI, errors and sampling , 2004 .

[17]  J. Eichhorn,et al.  Simulation of effects of vegetation on the dispersion of pollutants in street canyons , 2001 .

[18]  Gail Taylor,et al.  Estimating the removal of atmospheric particulate pollution by the urban tree canopy of London, under current and future environments , 2011 .

[19]  Riccardo Buccolieri,et al.  Aerodynamic effects of trees on pollutant concentration in street canyons. , 2009, The Science of the total environment.

[20]  P. Richards,et al.  Appropriate boundary conditions for computational wind engineering models using the k-ε turbulence model , 1993 .

[21]  B. Launder,et al.  The numerical computation of turbulent flows , 1990 .

[22]  T. A. Black,et al.  E-ε modelling of turbulent air flow downwind of a model forest edge , 1996 .

[23]  W. Kuttler,et al.  Air quality measurements in urban green areas } a case study , 1999 .

[24]  David R. Miller,et al.  Spatial variability of canopy foliage in an oak forest estimated with fisheye sensors , 1992 .

[25]  N. Breda.  Ground-based measurements of leaf area index: a review of methods, instruments and current controversies. , 2003, Journal of experimental botany.

[26]  Bodo Ruck,et al.  Influence of trees on the dispersion of pollutants in an urban street canyon - experimental investigation of the flow and concentration field , 2007 .

[27]  Jorge Humberto Amorim,et al.  LISBON AIR QUALITY - EVALUATING TRAFFIC HOT-SPOTS , 2009 .

[28]  Ana Isabel Miranda,et al.  Determination of background concentrations for air quality models using spectral analysis and filtering of monitoring data , 2010 .

[29]  Luca Ridolfi,et al.  The Effect of Vegetation Density on Canopy Sub-Layer Turbulence , 2004 .

[30]  P. Meir,et al.  Photographic method to measure the vertical distribution of leaf area density in forests , 2000 .

[31]  H. Hiraoka,et al.  A (κ-ε) turbulence closure model for plant canopy flows , 2006 .

[32]  Erich J. Plate,et al.  Modeling of Velocity Distributions Inside and Above Tall Crops. , 1965 .

[33]  John D. Wilson,et al.  A second-order closure model for flow through vegetation , 1988 .

[34]  Hang Li,et al.  Responses of the urban roadside trees to traffic environment , 2010 .

[35]  Nicolas G. Wright,et al.  On the use of the k–ε model in commercial CFD software to model the neutral atmospheric boundary layer , 2007 .

[36]  David R. Miller,et al.  Air flow over and through a forest edge: A steady-state numerical simulation , 1990 .

[37]  M. Bruse,et al.  Simulating surface–plant–air interactions inside urban environments with a three dimensional numerical model , 1998 .

[38]  C. Borrego,et al.  Emission and dispersion modelling of Lisbon air quality at local scale , 2003 .

[39]  C. N. Hewitt,et al.  BIOGENIC VOLATILE ORGANIC COMPOUND (VOC) EMISSION ESTIMATES FROM AN URBAN TREE CANOPY , 2003 .

[40]  Bodo Ruck,et al.  On the Impact of Trees on Dispersion Processes of Traffic Emissions in Street Canyons , 2009 .

[41]  D. Nowak,et al.  The Urban Forest Effects (UFORE) model: quantifying urban forest structure and functions , 2000 .

[42]  J. C. Stevens,et al.  Air pollution removal by urban trees and shrubs in the United States , 2006 .

[43]  M. Oliveira,et al.  Pollen allergenic potential nature of some trees species: a multidisciplinary approach using aerobiological, immunochemical and hospital admissions data. , 2009, Environmental research.

[44]  A. Thom,et al.  Turbulence in and above Plant Canopies , 1981 .

[45]  Z. Yingxin,et al.  Improved k–ε two-equation turbulence model for canopy flow , 2006 .

[46]  Christophe Sanz,et al.  ONE- and TWO-Equation Models for Canopy Turbulence , 2004 .

[47]  Márton Balczó,et al.  Numerical modeling of flow and pollutant dispersion in street canyons with tree planting , 2009 .

[48]  T. Stathopoulos,et al.  CFD simulation of the atmospheric boundary layer: wall function problems , 2007 .

[49]  Timothy R. Oke,et al.  Aerodynamic Properties of Urban Areas Derived from Analysis of Surface Form , 1999 .

[50]  D. Groleau,et al.  Modeling the influence of vegetation and water pond on urban microclimate , 2006 .

[51]  O. Tchepel,et al.  INTEGRATED MODELING OF ROAD TRAFFIC EMISSIONS: APPLICATION TO LISBON AIR QUALITY MANAGEMENT , 2004, Cybern. Syst..

[52]  J. Finnigan.  Turbulence in plant canopies , 2000 .