Development and Initial Application of the Micro-Calgrid Photochemical Model for High-Resolution Studies of Urban Environments

Impact of height and shape of building roof on air quality in urban street canyons

Improved parameterisation for the numerical modelling of air pollution within an urban street canyon

Urban air pollution is one of the unsettled issues in urban development and planning process. Urban street canyons are one of the main focused areas of investigation due to the heavy emission of the pollutants from vehicles. In the present study, the effects of elevated metro rail track (EMRT) on pollutant dispersion phenomena have been studied using computational fluid dynamics (CFD) technique. As a practical application of a CFD model, k-e turbulence model was applied for airflow modelling. Species transport model was applied for pollutant dispersion modelling. Geometrical configuration of urban street canyon of aspect ratio 'one', with elevated metro rail track, has been considered. Numerical simulations were carried out for perpendicular wind flow. Pollutant concentrations, at various locations of the street canyon, were largely affected by the introduction of elevated metro rail track. These results were compared with the street canyon without elevated metro rail track. It has been observed that the pollutant concentration increased at lower region and decreased at the upper region of the street canyon. Variations were not uniformly distributed in street canyon volume. Contours and pollutant concentrations have been compared and discussed for different sampled locations in the street canyon.

A wind tunnel study of organised and turbulent air motions in urban street canyons

<strong class="journal-contentHeaderColor">Abstract.</strong> The layout of urban buildings shows significant heterogeneity, which leads to the significant spatial inhomogeneity of the wind field in and over the canopy of urban street canyons. However, most of the current urban canopy models do not fully consider the heterogeneity of the urban canopy. Large discrepancies thus exist between the wind speeds simulated by the current urban canopy models and those observed in the street canyon. In this study, a parameterization scheme for wind field, Inhomogeneous Wind Scheme for Urban Street (IWSUS), is developed to better characterize the heterogeneity of the urban canopy. We use a Computational Fuild Dynamics method to generate the IWSUS scheme and compare with observations of wind profile and turbulent flux in and over the street canyon for validation. In IWSUS, the wind speed vertical profiles at six representative positions located in a typical street canyon (i.e., the windward or leeward side of a long straight street, or the inflow or outflow end) are parameterized separately. The wind profile by IWSUS thus can better describe the horizontal heterogeneity of the urban near-surface wind field, e.g., the dynamic drag effect of building in the lower atmosphere layer over the urbanized land use. The validation based on observations shows that the performance of simulation results by IWSUS is better than that by logarithmic-exponential (exp-log) law widely used in the current urban schemes. As we consider typical building arrangement and specific street orientations in IWSUS for wind field simulations, which can better match the distribution characteristics of street canyons around the observation point in the street canyon. The averaged wind profiles and turbulence energy fluxes in the model grids of urban area by IWSUS are also more approach to the observation than those by exp-log law. The normalized mean errors (NME) between the simulated and the observed vertical average wind speed are 49.0 % for IWSUS and 56.1 % for exp-log law in the range from the ground to four times the average height of the building, and 70 % for IWSUS and 285.8 % for exp-log law in the street canyon (range from the ground to building top). This study proves that the accuracy of land surface process and near-grounded meteorological process simulations over urban canopy can be improved by fully considering the heterogeneity of the urban canopy layout structures and the inhomogeneous of wind field distributions in and over the street canyon. The IWSUS is expected to be coupled with mesoscale atmospheric models to improve the accuracy of the wind field, land surface energy budget, meteorological, and atmospheric chemistry simulations.

Characteristics of flow and reactive pollutant dispersion in urban street canyons

This dataset contains the observations of air quality collected within two experimental campaigns conducted in summer 2017 (07 August 2017-26 September 2017) and winter 2018 (15 January 2018-15 February 2018) in two street canyons in the city of Bologna, to verify the effectiveness of vegetation in altering ventilation levels and pollutant concentration in real-world street canyons. The dataset also contains the meteorological data collected during non-synoptic periods, representing the ideal condition to assess the behavior of the local circulation and turbulence. The experimental field campaigns were setup under WP3 (“Planning and evaluation of PCS solutions”) and conducted under WP5 (“Monitoring and Evaluation of the Interventions”) of the iSCAPE project. Briefly, two parallel urban street canyons characterized by a different occurrence of vegetation, i.e. one almost free of vegetation (Marconi St.) and one with a tree line along both sides of the street (Laura Bassi St.), were instrumented for measurements of meteorology and turbulence variables at three height levels, while pollutant concentrations were monitored at ground level inside both canyons. This dataset is linked to the paper “Disentangling the effect of vegetation on ventilation and air quality in urban street canyons: the Bologna iSCAPE experimental campaigns” by Di Sabatino et al., which will be submitted for publication on “Science of the Total Environment” edited by Elsevier, and to the paper “Characteristic Scales for Turbulent Exchange Processes in a Real Urban Canopy” by Barbano et al., which is under review for publication on “Boundary-Layer Meteorology” edited by Springer. If you use this dataset for your research or work purposes, we ask that you acknowledge us in the use of your data, which can be done by including a proper citation in any documents or publications using these data.

In this study, the effects of building-roof cooling on scalar dispersion in three-dimensional street canyons are investigated using a computational fluid dynamics (CFD) model. For this, surface temperature of building roof is systematically changed and non-reactive pollutants are released from street bottom in urban street canyons with the aspect ratio of 1. The characteristics of flow, air temperature, and non-reactive pollutant dispersion in the control experiment are analyzed first. Then, the effects of building-roof cooling are investigated by comparing the results with those in the control experiment. In the control experiment, a portal vortex which is a secondary flow induced by ambient air flow is formed in each street canyon. Averaged air temperature is higher inside the street canyon than in both sides of the street canyon, because warmer air is coming into the street canyon from the roof level. However, air temperature near the street bottom is lower inside the street canyon due to the inflow of cooler air from both sides of the street canyon. As building-roof temperature decreases, wind speed at the roof level increases and portal vortex becomes intensified (that is, downdraft, reverse flow, and updraft becomes stronger). Building-roof cooling contributes to the reduction of average concentration of the non-reactive pollutants and average air temperature in the street canyon. The results imply that building-roof cooling has positive effects on improvement of thermal environment and air quality in urban areas.

Application of roadside air purifiers in urban street canyons: A pilot-scale study in Hong Kong

The trend of urbanization is increasing due to socioeconomic factors, leading to a decline in air quality and heightened exposure for individuals. Modeling dispersion of emissions within street canyons is crucial for understanding&amp;#160; local-scale air pollution exposure&amp;#160; indicators and devising effective strategies to enhance urban air quality. However, microscale urban air quality simulations are complex and involve a large number of interacting physical processes. Phenomena taken into consideration in simulations of urban pollution dispersion largely vary from one study to another. In this context, we aim at identifying the most influential parameters governing this phenomenon.&amp;#160;&amp;#160;First, we present a comprehensive database of pollution dispersion in street canyon by perturbing the most important parameters. In this study, these parameters include wind speed, background concentration, emission rates, background turbulence intensity, temperature disparity between building surfaces and the atmosphere, as well as model-specific parameters. We employ COMSOL Multiphysics for the computational fluid dynamic simulations. To keep feasible computational times and create a large database, turbulence is modeled by means of the Reynolds Averaged Navier-Stokes (RANS) k-&amp;#949; technique and the domain is considered two-dimensional.&amp;#160;&amp;#160;This simulation bank is used to develop a surrogate model based on Gradient Boosting Machine and the Principal Component Analysis to reduce system complexity. This surrogate model offers a solution to carry out a multivariate global sensitivity analysis. To this end, we compute the Sobol Indexes, providing insights into the global contributions of input factors to the resulting pollution concentrations at each point of the physical domain.&amp;#160;&amp;#160;Results show that, as expected, background concentration dominates the pollution field far from the emission point. The temperature of building surfaces plays a secondary role, acting mainly on the interface between the street canyon cavity and the flow above the buildings. Emission plays a fundamental role, especially in the vertical concentration profile. Other&amp;#160; model-specific parameters have a minor role when compared to the other physical variables. Wind speed plays a key role, along with temperature, in determining how pollution exits the cavity.&amp;#160;By integrating advanced simulation techniques with rigorous sensitivity analyses, this study aims to provide valuable insights into the factors influencing air quality in urban street canyons. Such insights are crucial to perform model calibration, and to understand the implications of typical approximations such as non-buoyancy flows.

Impact of trees on pollutant dispersion in street canyons: A numerical study of the annual average effects in Antwerp, Belgium

Influence of avenue trees on traffic pollutant dispersion in asymmetric street canyons: Numerical modeling with empirical analysis

The air flow and pollutant concentration fields in a street canyon affected by trees could affect the comfort and health of residents. At present, the description of the non-uniform/discontinuous distribution of leaves is difficult. In this study, the leaf distribution in the canopy was characterized by establishing non-continuous (uniform/random) algorithm based on a numerical simulation method, and the effects of canopy properties including, height, porosity and uniform/random leaf distribution, on the airflow and pollutant concentration fields in urban street canyons were investigated. The position of the tree canopy was found to directly affect the airflow field form and the air velocity distribution in the street canyon at low inflows. The average air velocity in the street canyon could be reduced significantly when the top of the tree canopy is near the top of the street canyon. The air velocity and pollutant concentration in the street canyon would vary only slightly due to the canopy porosity. Due to the increasing canopy porosity, the air velocity would increase, and the pollutant concentration would be reduced. The leaves are non-continuous and uniformly distributed at constant porosity, which does not significantly change the velocity distribution and pollutant concentration in the street canyon.

Pollutants dispersion emitted from road traffic is a dominant source of urban air pollution. Therefore, it is necessary to quantify emission levels in urban street canyon to evaluate their impacts on the public health and the environment. In this article, we study the effect of different street geometrical layouts on dispersion emissions of traffic exhausts for winds perpendicular to the roadway. A wind tunnel experiment modeling three different street geometrical layouts was executed to examine the flow fields and the concentration distributions in urban canyon. The flow and pollutants concentration were measured using a hot-wire anemometer with a split-fiber probe and a fast flame ionization detector under neutral atmospheric conditions. Flow and Dispersion characteristics were studied that included mean velocity, turbulence intensity, Reynolds stress, turbulent kinetic energy (TKE), mean concentrations, and fluctuation intensity of concentration in the street canyon. The results show that the vertical velocity and turbulent energy increases as the aspect ratio increases. The pollutant concentration increases as the aspect ratio decreases. The concentration fluctuation intensities of the aspect ratio W/H = 1 was found lower than that of the aspect ratios W/H = ½ and ¾. The pollutant concentration at the leeward side of the aspect ratios W/H = ¾ and 1 was higher than that in the aspect ratios W/H = ½. The pollutant concentration distributions measured in the experiment indicates that the variability of the street geometrical layouts in urban environment are important parameters for estimating air quality in the urban canyon.

Dispersion characteristics of vehicle emission in an urban street canyon