•The impact of trees on street ventilation is assessed using the OpenFOAM CFD software.•Worst and best scenarios of trees on road emissions are evaluated.•The impact of trees changes depending on ...heights and locations within the street.•For perpendicular wind, trees decrease wind speed which leads to increased air pollution concentration.•For parallel wind, trees are found to increase the exchange rate at the roof leading to lower air pollution concentration.
This paper assesses the effects of trees (Platanus x hispanica) of different leaf area density on ventilation, NOx and PM2.5 concentrations across heights in Marylebone Rd street canyon in London (UK). Computational Fluid Dynamics steady state simulations are performed with OpenFOAM. The ventilation is evaluated through flow patterns and the analysis of the impact of trees on wind speed, turbulence kinetic energy, flow rates, mean and turbulent pollutant exchanges. Results show that the effects of trees are local. For parallel winds planting new trees is positive since flow channelling and turbulence distribute the pollutant over the height which is removed by both mean flow and turbulent fluctuations through the roof. Both areas close and far from the trees within the road have a beneficial effect, with pedestrian average concentration reductions up to 18% due to aerodynamic effects. For perpendicular winds recirculation zones diminish the dispersion of pollutants and the introduction of trees has an additional negative effect with local average concentration increase up to 108% close to trees. Overall, the positive deposition effects are larger for increased LAD and for perpendicular winds may counterbalance the negative aerodynamic effects at locations close to trees.
This paper is devoted to the application of the modelling approach, as one of the methods for the evaluation of thermal comfort, to neighborhoods located in two cities characterized by a different ...climate, i.e., a Mediterranean city in southern Italy (Lecce) and a northern European city in southern Finland (Lahti). The impact of the presence of vegetation in both places is evaluated and compared, further considering alternative scenarios for thermal comfort improvement. The thermal comfort condition is expressed in terms of indices (mean radiant temperature (MRT) and predicted mean vote (PMV)). Results show that at pedestrian level the presence of vegetation lead to an improvement of thermal comfort in summer of about 2 points in both neighborhoods. This improvement is also evident observing the spatial distribution of MRT with a difference of 7 °C in the Lecce neighborhood and 3 °C in Lahti. In winter, thermal discomfort is observed in the presence of vegetation with a difference of 1.3 °C in the Lecce neighborhood and 1.5 °C in Lahti in terms of MRT. However, trees and green cover have the important potential to offset climate change impact and to make urban environments less thermally stressful. This study aims to guide urban planners towards a motivated and necessary transaction towards new green infrastructure whose effect should, however, be analyzed and investigated case by case.
This study carries out a quantitative analysis of the impact on microclimate (air temperature and thermal comfort) of a row of 165 historical Pinus pinea L. located in a central neighbourhood of Rome ...(Italy). The analysis starts from a qualitative general analysis on the stressful conditions leading to tree decline in the urban environment especially during extreme climate change phenomena. Subsequently, the effects of planting new types of trees are assessed using ENVI-met, a 3D prognostic non-hydrostatic model for the simulation of surface-plant-air interactions. Results, obtained by simulating three different scenarios in which the trees are first removed and then modified, show that a gradual renewal of the existing trees, based on priority criteria of maturity or senescence, vegetative and phytosanitary conditions, efficiency of ecosystem services and safety for citizens, has positive effects on thermal comfort. By integrating current results and scientific literature, the final aim of this work is to provide stakeholders with a strategic and systemic planning methodology, which, based on the innovative integrated use of tree management and modelling tools, may (i) enhance the benefits of greening in a scenario of climate change and (ii) lead to intervention strategies based on complementarity between conservation of existing trees and tree renewal.
Excess heat in urban environments is an increasing threat to human health and well-being. Furthermore, the increasingly important phenomenon of the Urban Heat Island (UHI) is exacerbating problems of ...livability in urban centers. Hence, there should be an increasing effort to assess the impact of heat mitigation strategies (HMSs) on outdoor thermal comfort in cities. This research has investigated how urban areas in steppe climate zones can be more thermally comfortable due to the effects of water bodies and trees, and how this might help to mitigate heat waves. Numerical simulations using the ENVI-met microclimate model have been performed for an urban park in Tabriz, Iran. In-situ measurements of air temperature (Ta) and mean radiant temperature (MRT) have been carried out in the study site and the collected data was used to validate the model (RMSE value 0.98 °C for Ta and 5.85 °C for MRT). Results show that water body evaporation without trees may decrease the air temperature, but on the other hand also increases the humidity, which reduces the positive impact on thermal comfort. However, the combination of water body with trees represents a better performance in the regulation of urban microclimate and thermal comfort.
This paper is devoted to the quantification of changes in ventilation of a real neighborhood located in Pamplona, Spain, due to the presence of street trees Pollutant dispersion in this urban zone ...was previously studied by means of computational fluid dynamic (CFD) simulations. In the present work, that research is extended to analyze the ventilation in the whole neighborhood and in a tree-free street. Several scenarios are investigated including new trees in the tree-free street, and different leaf area density (LAD) in the whole neighborhood. Changes between the scenarios are evaluated through changes in average concentration, wind speed, flow rates and total pollutant fluxes. Additionally, wind flow patterns and the vertical profiles of flow properties (e.g., wind velocity, turbulent kinetic energy) and concentration, horizontally-averaged over one particular street, are analyzed. The approach-flow direction is almost perpendicular to the street under study (prevailing wind direction is only deviated 4º from the perpendicular direction). For these conditions, as LAD increases, average concentration in the whole neighborhood increases due to the decrease of wind speed. On the other hand, the inclusion of trees in the street produces an increase of averaged pollutant concentration only within this street, in particular for the scenario with the highest LAD value. In fact, the new trees in the street analyzed with the highest LAD value notably change the ventilation producing an increase of total pollutant fluxes inward the street. Additionally, pollutant dispersion within the street is also influenced by the reduction of the wind velocity along the street axis and the decrease of turbulent kinetic energy within the vegetation canopy caused by the new trees. Therefore, the inclusion of new trees in a tree-free street should be done by considering ventilation changes and traffic emissions should be consequently controlled in order to keep pollutant concentration within healthy levels.
This paper is devoted to the assessment of urban greening effects on two important ecosystem services, i.e., air quality and CO2 storage, including the corresponding economic impacts in a real urban ...area, i.e., a district located in the Mediterranean city of Lecce (southern Italy). Two tools were employed, i-Tree Canopy and the computational fluid dynamics (CFD) microclimate model ENVI-met. i-Tree Canopy allowed fully determining the land-cover percentage on the basis of different ground cover classes and obtaining an estimate of annual values of CO2 storage, air pollutant removal, and economic benefits in the presence of urban greening. The estimate in i-Tree Canopy considered only the amount of greening; therefore, air pollutant removal estimates were only potential. As the vegetation was located in street canyons, its interaction with local meteorology and urban geometry strictly affected the dispersion of nitrogen oxides (NOx) (taken here as an example) as obtained from ENVI-met simulations. In ENVI-met, both deposition/absorption and aerodynamic effects were considered, and local increases in concentration were found in the district. The analysis of results obtained from different tools (one complex (CFD model) and the other simple (i-Tree model)) showed the error associated with the simple model in the computation of impacts if the interaction among the vegetation characteristics, the meteorological conditions, and the urban geometry was neglected; however, it also uncovers a novel approach for comprehensively characterizing a given area in terms of its vegetation cover, CO2 storage, and economic benefits, as well as local effects on air quality. This study is set in a broader context aimed at assessing the air quality in urban canopies of Mediterranean areas characterized by the presence of narrow street canyons where pollutants can accumulate due to ineffective air exchange with the above atmosphere.
This study analyses the interactions and impacts between multiple factors i.e., urban greening, building layout, and meteorological conditions that characterise the urban microclimate and thermal ...comfort in the urban environment. The focus was on two neighbourhoods of Lecce city (southern Italy) characterised through field campaigns and modelling simulations on a typical hot summer day. Field campaigns were performed to collect greening, building geometry, and microclimate data, which were employed in numerical simulations of several greening scenarios using the Computational Fluid Dynamics-based and microclimate model ENVI-met. Results show that, on a typical summer day, trees may lead to an average daily decrease of air temperature by up to 1.00 °C and an improvement of thermal comfort in terms of Mean Radiant Temperature (MRT) by up to 5.53 °C and Predicted Mean Vote (PMV) by up to 0.53. This decrease is more evident when the urban greening (in terms of green surfaces and trees) is increased by 1266 m2 in the first neighbourhood and 1988 m2 in the second one, with respect to the current scenario, proving that shading effect mainly contributes to improving the urban microclimate during daytime. On the contrary, the trapping effect of heat, stored by the surfaces during the day and released during the evening, induces an increase of the spatially averaged MRT by up to 2 °C during the evenings and a slight deterioration of thermal comfort, but only locally where the concentration of high LAD trees is higher. This study contributes to a better understanding of the ecosystem services provided by greening with regard to microclimate and thermal comfort within an urban environment for several hours of the day. It adds knowledge about the role of green areas in a Mediterranean city, an important hot spot of climate change, and thus it can be a guide for important urban regeneration plans.
This paper presents a conceptual framework that looks at photovoltaic systems in synergy with ecosystem services. The focus is to connect business success with social and ecological progress based on ...the operative concept of multifunctional land use. Such an approach attempts to harmonise the needs of the industrial processes of photovoltaic systems and the ecological and social needs of the landscape context. Different from the usual design of ground photovoltaic systems in farmlands or brownfields, a new framework is proposed, combining photovoltaic panels and vegetation. A case study is considered, applying the framework to existing photovoltaic systems in the Apulia region (southern Italy). The analysis shows how the framework has, among others, the major functions of increasing solar energy production, recycling wastewater, creating raw material for biofuel, as well as providing animal habitat and mitigating air temperature. The latter is preliminarily evaluated by means of modelling simulations performed with a computational fluid dynamics and microclimate model, ENVI-met. This approach opens up a new vision of the infrastructure design of photovoltaic systems which can produce new social and economic income.
This paper discusses current state-of-the-art features of the “plant evaluation model”, a framework which, starting from the representation of vegetation and its effects in microclimate models, ...defines a number of indices which can be employed for the evaluation of outdoor microclimate in terms of thermal environment and comfort in the urban environment. The key point of taking into account the impact of vegetation on microclimate is to implement appropriate parameterizations of such impact in a microclimate model. The paper, based on a review of literature studies, thus illustrates the basic principle and technical path of the impact assessment model of vegetation on microclimate and introduces related software. The aim is to provide the scientific community with a summary of (i) the current definition of vegetation in models employed for the evaluation of the impact of vegetation on urban outdoor microclimate, (ii) main models and evaluation indices and (iii) main input, output, vegetation-related processes implemented, strengths and weaknesses of those models, with suggested measures for output improvement. This review is not exhaustive but may help the user to select the proper model, which takes into account the effects of vegetation on outdoor urban microclimate, depending on the specific objective.
This study quantifies the influence of green spaces on microclimate and PM10 concentration in a typical residential district of Nanjing (China) by employing the CFD-based and microclimate model ...ENVI-met 4. Five green indices, related to quantity and structure of vegetation, are employed to investigate the impact of different types (grass, shrub and tree) and layouts of a green space located in the center of the residential district under an average Nanjing summer day. Results show that the thermal comfort (expressed by the mean radiant temperature MRT and the predicted mean vote PMV) is slightly enhanced with increasing green quantity, especially trees, even though more trees may increase the wind blocking effect with a consequent slight increase of pollutant concentration. In this regard, a single patch of trees located in the central part of the green space is preferable. The green indices are shown to be useful for studying the relationship between green space morphology, microclimate and air quality in cities.
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