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In high-density cities, optimization of their compact urban forms is important for the enhancement of pollution dispersion, improvement of the air quality, and healthy urban living. This study aims to identify critical building morphological design factors and provide a scientific basis for urban planning optimization. Through a long-term mobile monitoring campaign, a four-month (spanning across summer and winter seasons) spatiotemporal street-level PM 2.5 dataset was acquired. On top of that, the small-scale spatial variability of PM 2.5 in the high-density downtown area of Hong Kong was mapped. Seventeen building morphological factors were also calculated for the monitoring area using geographical information system (GIS). Multivariate statistical analysis was then conducted to correlate the PM 2.5 data and morphological data. The results indicate that the building morphology of the high-density environment of Hong Kong explains up to 37% of the spatial variability in the mobile monitored PM 2.5 . The building morphological factors with the highest correlation to PM 2.5 concentration are building volume density, building coverage ratio, podium layer frontal area index and building height variability. The quantitative correlation between PM 2.5 and morphological factors can be adopted to develop scientifically robust and straightforward optimization strategies for planners. This will allow considerations of pollution dispersion to be incorporated in planning practices at an early stage. [ABSTRACT FROM AUTHOR]

This paper investigates air pollution dispersion and exposure time around a building in a turbulent flow via tracer gas techniques. In the wind tunnel experiments, the tracer gas as the air pollutant model was trapped around the building model and exposed to a boundary layer wind flow. First, flow visualisation was performed, illustrating the highly transient features of wind flow and air pollution dispersion process dominated by underlying wind-structure interaction mechanisms. Next, the concentration field was normalised, analysed and presented as a function of time around the building. Evidently, the stagnant air pollution shields from the wind flow in the building's wake, causing a much longer air pollutant exposure time compared to the regions near the windward and side faces. The air pollutant exposure time in the side region was longer than in the windward region, influenced by the separation-reattachment and bifurcating flow near the side and windward faces, respectively. These results are compiled in order to provide a validation database for similar CFD studies on exposure time. Also, the normalised air pollutant exposure time was scaled up and integrated with air pollutant concentration in the form of two new indices to establish a methodology for the interpretation of air pollutant exposure time in full-scale built-environment scenarios. • Novel experiments performed on air pollutant exposure time around a building model. • Flow visualisation shows the transient features of flow and pollution dispersion. • Air pollutant exposure time in the windward, leeward and side regions were measured. • Relationship of air pollutant exposure time at different scales was established. • Two new indices, APEX and t e , are proposed for evaluating air pollution dispersion. [ABSTRACT FROM AUTHOR]

Abstract: A recently developed building-averaged urban surface scheme as coupled to an operational mesoscale model, TAPM, is evaluated for both flow and tracer dispersion using data from the 2002 Basel UrBan Boundary Layer Experiment (BUBBLE) conducted in the city of Basel, Switzerland. This scheme is based on the so-called town energy balance (TEB) approach and simulates turbulent fluxes using a generic canyon geometry to resolve energy balances for walls, roads and roofs. Air conditioning to close the building energy budget, in-canyon vegetation, and the effects of recirculation and venting of air within the canyon on turbulent fluxes are included. Comparison is also made with the original urban surface scheme of TAPM based on a simple slab approach with separate urban and vegetation–soil tiles and a specified anthropogenic heat flux. The results show that the new scheme leads to an overall improvement in the prediction of surface fluxes, and is able to reproduce the observed near-neutral to weakly unstable conditions at night, which is a feature of urban meteorology. In contrast, the slab scheme predicts stable conditions at night. The observed concentration fields from the tracer experiments are better simulated using the new scheme, but because there were no nighttime tracer releases, the capability of the new scheme under full diurnal conditions could not be demonstrated. For the applications considered here, the computational efficiency of the new scheme in TAPM is on par with the slab scheme. [Copyright &y& Elsevier]

Abstract: In high-density megacities, air pollution has a higher impact on public health than cities of lower population density. Apart from higher pollution emissions due to human activities in densely populated street canyons, stagnated air flow due to closely packed tall buildings means lower dispersion potential. The coupled result leads to frequent reports of high air pollution indexes at street-side stations in Hong Kong. High-density urban morphologies need to be carefully designed to lessen the ill effects of high density urban living. This study addresses the knowledge-gap between planning and design principles and air pollution dispersion potentials in high density cities. The air ventilation assessment for projects in high-density Hong Kong is advanced to include air pollutant dispersion issues. The methods in this study are CFD simulation and parametric study. The SST κ–ω model is adopted after balancing the accuracy and computational cost in the comparative study. Urban-scale parametric studies are conducted to clarify the effects of urban permeability and building geometries on air pollution dispersion, for both the outdoor pedestrian environment and the indoor environment in the roadside buildings. Given the finite land resources in high-density cities and the numerous planning and design restrictions for development projects, the effectiveness of mitigation strategies is evaluated to optimize the benefits. A real urban case study is finally conducted to demonstrate that the suggested design principles from the parametric study are feasible in the practical high density urban design. [Copyright &y& Elsevier]

Abstract: Environmental impact assessment (EIA) of air pollution diffusion of different kinds of hazardous sources can provide support information for decision making of emergency plan preparation and approval of new construction project. A GIS-based software was developed as a computer aided tool for EIA of atmospheric pollution dispersion. The industrial source complex (ISC3) dispersion models developed by US Environmental Protection Agency were used for calculation of the ground-level concentrations for point, area, flare and volume sources. The Fortran code named SCREEN3 was integrated to implement ISC3 models. Hybrid programing technology of C# and Fortran was used to develop the software. ArcGIS was used as GIS platform to represent the pollutant concentration with contour visualization on a map. The developed software can be used as a tool for air pollution assessment with preliminary estimation scheme (screening scheme). [Copyright &y& Elsevier]

Abstract: This paper examines the utility of The Air Pollution Model (TAPM; version 2) in simulating meteorology and dispersion of PM10 for 1999 over the coastal city of Christchurch, New Zealand. Christchurch usually experiences severe degradation in air quality during austral winter. The formation of a nocturnal inversion layer and the emissions of particulate matter (PM10) mainly from solid fuel home heating appliances lead to severe smog episodes on an average of 30 nights during winter. The complex local topography surrounding the city in combination with influences from the urban areas can produce complicated boundary layer winds during quiescent weather. Simulated PM10 data are used for construction of annual exposure maps for the urban areas in order to assess the health impact of air pollutants due to chronic exposure (presented in an accompanying paper). Meteorology and PM10 dispersion results are statistically compared with the only permanent air pollution monitoring station available in order to evaluate the model''s performance. Statistical measures such as the Index of Agreement (IOA) between modelled and measured data indicate that the model performs well. IOA is greater than 0.6 for meteorological variables, and various calculated skill scores place confidence in the model''s performance. However, TAPM has a tendency to overestimate surface wind speed over urban areas during stagnant nocturnal conditions, resulting in quick flushing of pollutants. [Copyright &y& Elsevier]

The operational meteorological air quality model (OML) was used to calculate the urban dispersion of air pollutants originating from ships in three Danish ports: Copenhagen, Elsinore and Køge.Oxides of nitrogen (NOx=NO+NO2) emitted by ships in the port of Copenhagen contributed substantially to the overall NOx pollution in central Copenhagen. This would have an impact on human health if most of the NO were occasionally transformed into NO2, for which the European Community''s legal limit is defined by the 19th-highest annual hourly value being 200 μg m−3 NO2. Emissions by the ships caused a maximum 19th hourly NOx concentration of 615 μg m−3. In a small area of housing estates and office blocks near the harbour, it exceeded 200 μg m−3 NOx, and over several square kilometres of central Copenhagen, the emission by ships in harbour similarly caused values of 50–200 μg m−3 NOx. In the port of Elsinore, ferries contributed significantly to the NOx pollution in the neighbourhood around the harbour.Emissions of particulate matter (PM) by ships in Copenhagen harbour caused neighbourhood concentrations equivalent to only 0.2–0.4% of the European Community''s legal annual mass-based limit value for the protection of human health. However, ship emissions are dominated by ultrafine particles, as are diesel vehicle emissions, which adversely affect human health. Particulate emissions from ships contribute 8–15% of that of all urban road traffic to the background PM10 levels in the harbour neighbourhoods. In Elsinore, the PM problem was less significant than in Copenhagen.Ships in both harbours will contribute insignificantly to urban pollution with sulphur dioxide (SO2), once the anticipated regulations on sulphur content in marine fuels are implemented.In Køge, the low activity in the harbour meant that ships did not significantly affect urban air quality. [Copyright &y& Elsevier]

• The differences in ventilation between 3-D and 2-D street canyons generally disappear with the increase of street length. • A most unfavorable street length is found with maximum pollutant accumulation. • The air exchange rates at rooftop and street ends show opposite correlation with street length. • Pollutants are transferred directedly from ground level to rooftop by upward flow at the center part of street. • The corner vortices at street ends could interact with canyon vortices, resulting with local pollutant accumulation. Urban ventilation is important for building a healthy urban living environment. 2-D CFD simulation has been used widely for street canyon ventilation due to its high computational efficiency, but its applicability for a 3-D simulation has never been studied. This paper tried to answer the question: if and under what conditions, the widely-adopted 2-D CFD simulations on street canyon ventilation can represent a 3-D scenarios? 3-D simulations on street canyons with various street lengths and corresponding 2-D simulations are carried out with RNG k - ε model. Our study identified two important ventilation mechanism for controlling ventilation and dispersion in a 3-D street canyon, i.e., canyon vortex on the canyon top and the corner vortices at the street ends. The relative importance of these two driving forces will change with the street length/street width ratio (B/W). For isolated street canyon, when B/W is higher than 20 (for H/W = 1) and 70 (H/W = 2), the street canyon ventilation will be dominated by canyon vortex, and 3-D street canyon ventilation could be simplified as a 2-D case. For multiple street canyon, the threshold of B/W will become 20 when H/W = 1, and 50 when H/W = 2. The findings in this study could improve our approaches for simulating urban ventilation. [ABSTRACT FROM AUTHOR]