Transport processes in and above two-dimensional urban street canyons under different stratification conditions: results from numerical simulation

Thermal stratification (neutral, unstable and stable) plays an important role in determining the transport processes in and above urban street canyons. This paper summarizes the recent findings of the effect of thermal stratification on the transport of momentum, heat, and pollutants in the two-dimensional (2D) urban street canyons in the skimming flow regime. Special attention is paid to the results from large-eddy simulations (LESs), while other experimental and numerical results are referred to when necessary. With increasing Richardson number, Ri, the drag coefficient of the 2D street canyon as felt by the overlying atmosphere decreases in a linear manner. Under neutral and stable stratification, a nearly constant drag coefficient of 0.02 is predicted by the LESs. Under unstable stratification, the turbulent pollutant transport is dominated by organized turbulent motions (ejections and sweeps), while under stable stratification, the unorganized turbulent motion (inward interactions) plays a more important role and the sweeps are inhibited. The unstable stratification condition also enhances the ejections of turbulent pollutant flux, especially at the leeward roof-level corner, where the ejections dominate the turbulent pollutant flux, outweighing the sweeps. With increasing Ri, both the heat (area active scalar source) and pollutant (line passive scalar source) transfer coefficients decrease towards a state where the transfer coefficients become zero at Ri≈0.5. It should be noted that, due to the limit of the 2D street canyon configuration discussed in this paper, great caution should be taken when generalising the conclusions drawn here.Singapore. National Research FoundationSingapore-MIT Alliance for Research and Technology. Center for Environmental Sensing and Modelin

Flow and Pollutant Transport in Urban Street Canyons of Different Aspect Ratios with Ground Heating: Large-Eddy Simulation

A validated large-eddy simulation model was employed to study the effect of the aspect ratio and ground heating on the flow and pollutant dispersion in urban street canyons. Three ground-heating intensities (neutral, weak and strong) were imposed in street canyons of aspect ratio 1, 2, and 0.5. The detailed patterns of flow, turbulence, temperature and pollutant transport were analyzed and compared. Significant changes of flow and scalar patterns were caused by ground heating in the street canyon of aspect ratio 2 and 0.5, while only the street canyon of aspect ratio 0.5 showed a change in flow regime (from wake interference flow to skimming flow). The street canyon of aspect ratio 1 does not show any significant change in the flow field. Ground heating generated strong mixing of heat and pollutant; the normalized temperature inside street canyons was approximately spatially uniform and somewhat insensitive to the aspect ratio and heating intensity. This study helps elucidate the combined effects of urban geometry and thermal stratification on the urban canyon flow and pollutant dispersion.Singapore National Research Foundation (Singapore-MIT Alliance for Research and Technology (SMART)

Integrated Urban Sensing: A Geo-sensor Network for Public Health Monitoring and Beyond

Pervasive environmental monitoring implies a wide range of technical, but also socio-political challenges, and this applies especially to the sensitive context of the city. In this paper, we elucidate issues for bringing out pervasive urban sensor networks and associated concerns relating to fine-grained information provision. We present the Common Scents project, which is based on the Live Geography approach, and show how it can overcome these challenges. As opposed to hitherto sensing networks, which are mostly built up in monolithic and closed systems, the Common Scents approach aims to establish an open, standards based and modular infrastructure. This ensures interoperability, portability and flexibility, which are crucial prerequisites for pervasive urban sensing. The implementation – a real-time data integration and analysis system for air quality assessment – has been realised on top of the CitySense sensor network in the City of Cambridge, MA US together with the city’s Public Health Department responding to concrete needs of the city and its inhabitants. The second pilot using mobile sensors mounted on bicycles has been deployed in Copenhagen, Denmark. Preliminary results show highly fine-grained variability of pollutant dispersion in urban environments.Singapore-MIT Alliance. Center for Environmental Sensing and MonitoringSingapore-MIT Alliance for Research and Technology CenterEuropean Commission (FP7 GENESIS project)Bundesministerium für Wissenschaft und ForschungResearch Studio iSPAC

“Exposure Track”—The Impact of Mobile-Device-Based Mobility Patterns on Quantifying Population Exposure to Air Pollution

Air pollution is now recognized as the world’s single largest environmental and human health threat. Indeed, a large number of environmental epidemiological studies have quantified the health impacts of population exposure to pollution. In previous studies, exposure estimates at the population level have not considered spatially- and temporally varying populations present in study regions. Therefore, in the first study of it is kind, we use measured population activity patterns representing several million people to evaluate population-weighted exposure to air pollution on a city-wide scale. Mobile and wireless devices yield information about where and when people are present, thus collective activity patterns were determined using counts of connections to the cellular network. Population-weighted exposure to PM2.5 in New York City (NYC), herein termed “Active Population Exposure” was evaluated using population activity patterns and spatiotemporal PM2.5 concentration levels, and compared to “Home Population Exposure”, which assumed a static population distribution as per Census data. Areas of relatively higher population-weighted exposures were concentrated in different districts within NYC in both scenarios. These were more centralized for the “Active Population Exposure” scenario. Population-weighted exposure computed in each district of NYC for the “Active” scenario were found to be statistically significantly (p < 0.05) different to the “Home” scenario for most districts. In investigating the temporal variability of the “Active” population-weighted exposures determined in districts, these were found to be significantly different (p < 0.05) during the daytime and the nighttime. Evaluating population exposure to air pollution using spatiotemporal population mobility patterns warrants consideration in future environmental epidemiological studies linking air quality and human health

Large-Eddy Simulation of Flow and Pollutant Transport in Urban Street Canyons with Ground Heating

Our study employed large-eddy simulation (LES) based on a one-equation subgrid-scale model to investigate the flow field and pollutant dispersion characteristics inside urban street canyons. Unstable thermal stratification was produced by heating the ground of the street canyon. Using the Boussinesq approximation, thermal buoyancy forces were taken into account in both the Navier–Stokes equations and the transport equation for subgrid-scale turbulent kinetic energy (TKE). The LESs were validated against experimental data obtained in wind-tunnel studies before the model was applied to study the detailed turbulence, temperature, and pollutant dispersion characteristics in the street canyon of aspect ratio 1. The effects of different Richardson numbers (Ri) were investigated. The ground heating significantly enhanced mean flow, turbulence, and pollutant flux inside the street canyon, but weakened the shear at the roof level. The mean flow was observed to be no longer isolated from the free stream and fresh air could be entrained into the street canyon at the roof-level leeward corner. Weighed against higher temperature, the ground heating facilitated pollutant removal from the street canyon.Singapore-MIT Alliance for Research and Technology. Center for Environmental Sensing and Monitorin

Urban Energy Fluxes in Built-Up Downtown Areas and Variations across the Urban Area, for Use in Dispersion Models

Surface energy fluxes, at averaging times from 10 min to 1 h, are needed as inputs to most state-of-the-art dispersion models. The sensible heat flux is a major priority, because it is combined with the momentum flux to estimate the stability, the wind profile, and the turbulence intensities. Because of recent concerns about dispersion in built-up downtown areas of large cities, there is a need to estimate sensible heat flux in the midst of tall buildings. In this paper, the authors work with some high-quality and relevant but arguably underutilized data. The results of analysis of urban heat flux components from 10 locations in suburban and built-up downtown areas in Oklahoma City, Oklahoma, during the Joint Urban 2003 (JU2003) field experiment are presented here. At street level in the downtown area, in the midst of tall skyscrapers, the ground heat flux and the sensible heat flux are relatively large and the latent heat flux is relatively small when compared with concurrent fluxes observed in the upwind suburban areas. In confirmation of measurements in other cities, the sensible heat flux in the downtown area is observed to be slightly positive (10–20 W m[superscript −2]) at night, indicating nearly neutral or slightly unstable conditions. Also in agreement with observations in other cities is that the ground heat flux in the downtown area has a magnitude that is 3 or 4 times that in suburban or rural areas. These results should permit improved parameterizations of sensible heat fluxes in the urban downtown area with tall buildings.National Science Foundation (U.S.) (Award 0750878)Singapore. National Research FoundationSingapore-MIT Alliance for Research and TechnologySingapore-MIT Alliance. Center for Environmental Sensing and Monitorin

Effect of stable stratification on dispersion within urban street canyons: A large-eddy simulation

This study employs a validated large-eddy simulation (LES) code with high tempo-spatial resolution to investigate the effect of a stably stratified roughness sublayer (RSL) on scalar transport within an urban street canyon. The major effect of stable stratification on the flow and turbulence inside the street canyon is that the flow slows down in both streamwise and vertical directions, a stagnant area near the street level emerges, and the vertical transport of momentum is weakened. Consequently, the transfer of heat between the street canyon and overlying atmosphere also gets weaker. The pollutant emitted from the street level ‘pools’ within the lower street canyon, and more pollutant accumulates within the street canyon with increasing stability. Under stable stratification, the dominant mechanism for pollutant transport within the street canyon has changed from ejections (flow carries high-concentration pollutant upward) to unorganized motions (flow carries high-concentration pollutant downward), which is responsible for the much lower dispersion efficiency under stable stratifications. Keywords: Large-eddy simulation (LES); Urban street canyon; Pollutant dispersion; Stable stratificationNational Research Foundation Singapor

Global Mortality Attributable to Aircraft Cruise Emissions

Aircraft emissions impact human health though degradation of air quality. The majority of previous analyses of air quality impacts from aviation have considered only landing and takeoff emissions. We show that aircraft cruise emissions impact human health over a hemispheric scale and provide the first estimate of premature mortalities attributable to aircraft emissions globally. We estimate ~8000 premature mortalities per year are attributable to aircraft cruise emissions. This represents ~80% of the total impact of aviation (where the total includes the effects of landing and takeoff emissions), and ~1% of air quality-related premature mortalities from all sources. However, we note that the impact of landing and takeoff emissions is likely to be under-resolved. Secondary H[subscript 2]SO[subscript 4]−HNO[subscript 3]−NH[subscript 3] aerosols are found to dominate mortality impacts. Due to the altitude and region of the atmosphere at which aircraft emissions are deposited, the extent of transboundary air pollution is particularly strong. For example, we describe how strong zonal westerly winds aloft, the mean meridional circulation around 30−60°N, interaction of aircraft-attributable aerosol precursors with background ammonia, and high population densities in combination give rise to an estimated ~3500 premature mortalities per year in China and India combined, despite their relatively small current share of aircraft emissions. Subsidence of aviation-attributable aerosol and aerosol precursors occurs predominantly around the dry subtropical ridge, which results in reduced wet removal of aviation-attributable aerosol. It is also found that aircraft NO[subscript x] emissions serve to increase oxidation of nonaviation SO[subscript 2], thereby further increasing the air quality impacts of aviation. We recommend that cruise emissions be explicitly considered in the development of policies, technologies and operational procedures designed to mitigate the air quality impacts of air transportation

Live Geography -- Embedded Sensing for Standarised Urban Environmental Monitoring

Environmental monitoring faces a variety of complex technical and socio-political challenges, particularly in the urban context. Data sources may be available, but mostly not combinable because of lacking interoperability and deficient coordination due to monolithic and closed data infrastructures. In this work we present the Live Geography approach that seeks to tackle these challenges with an open sensing infrastructure for monitoring applications. Our system makes extensive use of open (geospatial) standards throughout the entire process chain – from sensor data integration to analysis, Complex Event Processing (CEP), alerting, and finally visualisation. We discuss the implemented modules as well as the overall created infrastructure as a whole. Finally, we show how the methodology can influence the city and its inhabitants by „making the abstract real“, in other words how pervasive environmental monitoring systems can change urban social interactions, and which issues are related to establishing such systems