STUDY OF THE EFFECTS OF BUILDING DENSITY AND OVERALL SHAPE OF A CITY ON POLLUTANT DISPERSION BY COMBINATION OF WIND TUNNEL EXPERIMENTS AND CFD SIMULATIONS

Despite the improvement made in controlling local air pollution, urban areas are undergoing increasing environmental pressures and poor air quality is one of the major concerns. Recently, much attention has focused on the relationship between urban form and sustainability. There are indications that the density and the overall shape of cities can have implications on street level ventilation and the “compact city” is by many regarded as the most sustainable urban form. In this framework, this paper is devoted to the study of flow and pollutant dispersion from a ground level line source at pedestrian level within different urban configurations. The urban-like configurations vary from the scenario of an urban sprawl to the opposite scenario of a compact city. Wind tunnel experiments and CFD simulations are performed to evaluate pollutant concentrations in each of the idealized city structures. The overall aim is that of assessing and clarifying the effect of city density on atmospheric flow patterns and pollutant dispersion

CFD modelling: The most useful tool for developing mesoscale urban canopy parameterizations

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Potential impacts of green infrastructure on NOx and PM10 in different local climate zones of Brindisi, Italy

This study delves into Green Infrastructure (GI) planning in Brindisi, Italy, evaluating its influence on urban air quality and thermal comfort. Employing an LCZ-centred Geographic Information System (GIS) based classification protocol, the prevalence of LCZ 6 (Open low-rise) and LCZ 2 (Compact mid-rise) is highlighted. Despite generally low PM10 levels in Brindisi, intermittent NOx spikes surpassing WHO and EU standards pose health risks. Within LCZ 2, diverse GI interventions (green walls, hedges, trees) were tested, with green walls emerging as the most effective, albeit falling short of expectations, while trees exhibited adverse air quality impacts. LCZ 6 demonstrated enhanced air quality attributed to wind patterns, GI, and urban canyon improvements. Thermal comfort analysis consistently revealed positive outcomes across various GI types, reducing discomfort by a minimum of 10%. The study emphasized GI's favourable com-fort impact on sidewalks but cautioned against trees in street canyons with aspect ratios exceeding 0.7, heightening pollutant levels and implying increased exposure risks. Conversely, street canyons with lower aspect ratios displayed variable conditions influenced by prevailing regional wind patterns. In conclusion, the integrated assessment of LCZ and GI holds promise for in-formed urban planning, guiding decisions that prioritize healthier, more sustainable cities. This underscores the crucial need to balance GI strategies for optimal urban development, aligning with the overarching goal of promoting urban well-being and sustainability

Recurrence Analysis of Vegetation Indices for Highlighting the Ecosystem Response to Drought Events: An Application to the Amazon Forest

Forests are important in sequestering CO2 and therefore play a significant role in climate change. However, the CO2 cycle is conditioned by drought events that alter the rate of photosynthesis, which is the principal physiological action of plants in transforming CO2 into biological energy. This study applied recurrence quantification analysis (RQA) to describe the evolution of photosynthesis-related indices to highlight disturbance alterations produced by the Atlantic Multidecadal Oscillation (AMO, years 2005 and 2010) and the El Niño-Southern Oscillation (ENSO, year 2015) in the Amazon forest. The analysis was carried out using Moderate Resolution Imaging Spectroradiometer (MODIS) images to build time series of the enhanced vegetation index (EVI), the normalized difference water index (NDWI), and the land surface temperature (LST) covering the period 2001–2018. The results did not show significant variations produced by AMO throughout the study area, while a disruption due to the global warming phase linked to the extreme ENSO event occurred, and the forest was able to recover. In addition, spatial differences in the response of the forest to the ENSO event were found. These findings show that the application of RQA to the time series of vegetation indices supports the evaluation of the forest ecosystem response to disruptive events. This approach provides information on the capacity of the forest to recover after a disruptive event and, therefore is useful to estimate the resilience of this particular ecosystem

On the Impact of Trees on Ventilation in a Real Street in Pamplona, Spain

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

THE INFLUENCE OF BUOYANCY ON FLOW AND POLLUTANT DISPERSION IN STREET CANYONS

In this paper, the effect of buoyancy on flow and pollutant dispersion within street canyons is studied by means of computational fluid dynamics simulations. We consider a neutral boundary layer approaching a 3D street canyon assuming a wind direction perpendicular to the street canyon. The Boussinesq hypothesis for incompressible fluids is chosen for modelling buoyancy. We distinguish three cases: leeward, ground and windward wall heating. Thermal effects on both the flow and dispersion are investigated for several Richardson numbers. The analysis focuses on the influence of street canyon geometry on flow and temperature distribution, by considering different aspect ratios W/H canyon between 0.5 and 2, where W is the width and H the height of the street canyon. Three-dimensional effects are observed, depending on L/H, where L is the length of the canyon. Three dimensional effects become negligible for aspect ratio L/H larger than 20. Results obtained for the case with a large Richardson number show that dispersion patterns in a street canyon differ substantially the isothermal case. In case with windward heating large concentration values are found close to the windward wall. Our findings can be of interest for many urban environment applications in which natural ventilation and thermal comfort are being of concern

EVALUATION OF NUMERICAL FLOW AND DISPERSION SIMULATIONS FOR STREET CANYONS WITH AVENUE-LIKE TREE PLANTING BY COMPARISON WITH WIND TUNNEL DATA

Flow and traffic-originated pollutant dispersion in an urban street canyon with avenue-like tree planting have been studied by means of wind tunnel and CFD investigations. The study comprises tree planting of different crown porosity, planted in two rows within a canyon of street width to building height ratio W/H = 2 and street length to building height ratio L/H = 10 exposed to a perpendicular approaching boundary layer flow. Numerical simulations have been performed with the commercial CFD code FLUENT™ by employing the RSM turbulence model. In the presence of tree planting, both measurements and simulations show considerable larger pollutant concentrations in proximity of the leeward wall and slightly lower concentrations in proximity of the windward wall in comparison to the tree-less street canyon. In particular, FLUENT slightly underestimated pollutant concentrations in proximity of the leeward wall in all cases studied, while near the windward wall there is no general tendency towards underestimation or overestimation. Overall, numerical computations compare qualitatively well with experimental data. Results from commonly used statistical tests also suggest the CFD predictions to be satisfactory. Results obtained in this work by combining wind tunnel experiments and CFD based simulations in a novel aspect of research suggest ways to obtain quantitative information for planning and implementation of exposure mitigation using trees in urban street canyons

Analysis of olive grove destruction by xylella fastidiosa bacterium on the land surface temperature in Salento detected using satellite images

Agricultural activity replaces natural vegetation with cultivated land and it is a major cause of local and global climate change. Highly specialized agricultural production leads to extensive monoculture farming with a low biodiversity that may cause low landscape resilience. This is the case on the Salento peninsula, in the Apulia Region of Italy, where the Xylella fastidiosa bacterium has caused the mass destruction of olive trees, many of them in monumental groves. The historical land cover that characterized the landscape is currently in a transition phase and can strongly affect climate conditions. This study aims to analyze how the destruction of olive groves by X. fastidiosa affects local climate change. Land surface temperature (LST) data detected by Landsat 8 and MODIS satellites are used as a proxies for microclimate mitigation ecosystem services linked to the evolution of the land cover. Moreover, recurrence quantification analysis was applied to the study of LST evolution. The results showed that olive groves are the least capable forest type for mitigating LST, but they are more capable than farmland, above all in the summer when the air temperature is the highest. The differences in the average LST from 2014 to 2020 between olive groves and farmland ranges from 2.8 °C to 0.8 °C. Furthermore, the recurrence analysis showed that X. fastidiosa was rapidly changing the LST of the olive groves into values to those of farmland, with a difference in LST reduced to less than a third from the time when the bacterium was identified in Apulia six years ago. The change generated by X. fastidiosa started in 2009 and showed more or less constant behavior after 2010 without substantial variation; therefore, this can serve as the index of a static situation, which can indicate non-recovery or non-transformation of the dying olive groves. Failure to restore the initial environmental conditions can be connected with the slow progress of the uprooting and replacing infected plants, probably due to attempts to save the historic aspect of the landscape by looking for solutions that avoid uprooting the diseased plants. This suggests that social-ecological systems have to be more responsive to phytosanitary epidemics and adapt to ecological processes, which cannot always be easily controlled, to produce more resilient landscapes and avoid unwanted transformations