Session Overview
Session
UCP7 (cont): Air Quality in Urban Boundary Layer : processes
Time:
Tuesday, 21/Jul/2015:
11:00am - 12:30pm

Session Chair: Xuemei Wang, Sun Yat-sen University
Location: Spot Room

Presentations

Turbulence and pollutant transport in urban roughness sublayer under stable stratification: a large-eddy simulation

Xian-Xiang Li1, Rex Britter2, Leslie Norford3

1Singapore-MIT Alliance for Research and Technology, Singapore; 2Department of Urban Studies and Planning, Massachusetts Institute of Technology; 3Department of Architecture, Massachusetts Institute of Technology

Thermal stratification of the atmospheric surface layer has strong impact on the land-atmosphere exchange of turbulent, heat, and pollutant fluxes. Few studies have been carried out for the interaction of the weakly to moderately stable stratified atmosphere and the urban canopy. This study performs a large-eddy simulation of a modeled street canyon within a weakly to moderately stable atmosphere boundary layer. To better resolve the smaller eddy size resulted from the stable stratification, a higher spatial and temporal resolution is used. The detailed flow structure and turbulence in the street canyon and in the roughness sublayer are analyzed. The local scaling of turbulent quantities in the roughness sublayer is tested. The relationship of pollutant dispersion and Richardson number of the atmosphere is investigated. Differences between these characteristics and those under neutral and unstable atmosphere boundary layer are emphasized.


The ClearfLo project Ė Are sea breezes a mechanism to change the air in London?

Sylvia I Bohnenstengel1,2, Simone Kotthaus2, Janet F Barlow2, CSB Grimmond2, O Coceal2,4, CH Halios2, HW Lean1, A Tremper5, D Green5, L Crilley6, JF Hamilton7, S Visser8, ASH Prevot8, SE Belcher2,3

1Met Office@Reading, Reading, UK; 2Department of Meteorology, University of Reading; 3Met Office Hadley Centre, Exeter, UK; 4National Centre for Atmospheric Science, Department of Meteorology, University of Reading, UK; 5Environmental Research Group, Kings College London, London, UK; 6School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK; 7National Centre for Atmospheric Science, Department of Chemistry, University of York, UK; 8Paul Scherrer Institute, Villigen, Switzerland

The ClearfLo project aimed to understand the processes generating pollutants like O3, NOx and particulate matter and their interaction with the urban atmospheric boundary layer. ClearfLo (www.clearflo.ac.uk), a large multi-institution NERC-funded project in the UK established integrated measurements of the meteorology, composition and particulate loading of Londonís urban atmosphere accompanied by modeling of urban meteorology and air pollution.

The project established a new long-term measurement infrastructure in London encompassing measurement capabilities at street level, in the urban background, at elevated levels and in the rural surrounding to determine the urban increment in meteorology and air pollution. Two intensive observation periods (IOPs) in January/February 2012 and during the Olympics in summer 2012 measured Londonís atmosphere with higher level of detail.

We know that sea breezes reach London and previous studies, for example New York, have shown that urban areas change (delay) sea breezes. However, we do not know how sea breezes affect the urban boundary layer structure and the surface energy balance in London due to a lack of vertically distributed measurements until now. This talk presents high-resolution surface energy balance measurements taken during the ClearfLo and ACTUAL projects (www.actual.ac.uk) at different locations and heights in central London alongside lidar and ceilometers derived backscatter and turbulence measurements of the urban boundary layer during a sea breeze event on 25th July 2012. Observations are compared against numerical model simulations with the UK Met Office UKV at 1.5 km horizontal resolution and higher resolutions to explain accompanying peaks in NOX and black carbon concentrations during the passage of the sea breeze.


Simulations of Pollutant Dispersal over Nairobi City, Kenya

George OTIENO

IGAD Climate Prediction and Applications Centre

The current rapid deterioration of air quality in urban centres can be attributed to urbanization. Poor air quality has been associated with several negative effects on human health, climate and ecosystems. Most cities in developing countries, especially in Africa have poor or in some cases no air quality management systems in place despite having the fastest growing urban populations. City populations have high vulnerability to the impacts air pollution following high density of residents and economic activities as well.

Air pollution is evident in most cities; the case of Nairobi is an illustrative of this. The common air pollutants include carbon monoxide and total suspended particulates among others, the latter being the most widespread and the most serious for human health.

This study simulated air pollutant dispersal over the city using Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT), considering a case for emission of total suspended particles into the environment. The predominant wind speed over the city is 4-6 knots and the wind direction is easterly. The forward trajectory of a pollutant released in the city is generally observed to flow to the western side of the city. The pollutant is observed to be dispersed beyond 100 km from the city reducing the concentration of the same in the city.

The study thus recommends for a consultative planning process of the city that factors in the wind characteristics over the city; most industrial activities should be located to the extreme western side of the city to minimize concentration of pollutants over the city. The study further recommends studies that studies be carried out to ascertain the quality of rain water during the long rain season.

Key words: Pollution, Environment, HYSPLIT, Planning, Nairobi


On the Transport of Chemically Reactive Pollutants over Urban Roughness in the Atmospheric Boundary Layer

Zhangquan WU

The University of Hong Kong, Hong Kong S.A.R. (China)

Air quality in urban areas is a crucial topic nowadays because of its substantial health impact on the general public. Transport and pollution chemistry over urban rough surfaces are complicated by many factors such as atmospheric turbulence, building geometry/orientation and thermal stratification. In attempt to elucidate their tight coupling, dispersion of chemically reactive pollutants in the atmospheric boundary layer (ABL) over hypothetical urban areas is numerically investigated using the large-eddy simulation (LES). In this paper, idealized street canyons of unity aspect ratio are used fabricating the LES model of urban roughness in order to study the transport processes in isothermal conditions. The worst scenario is considered in which the prevailing flow is perpendicular to the street axes. After arriving the pseudo steady-state flows, nitric oxide (NO) is emitted from the ground level in one of the street canyons into the urban ABL doped with ozone (O3). As a preliminary study to advance our understanding of flows and chemistry in urban areas, an irreversible bi-molecular chemical reaction (NO + O3 ? NO2 + O2, where NO2 is nitrogen dioxide and O2 is oxygen) is used. Air exchange rate (ACH) and pollutant exchange rate (PCH) are used to analyze the ventilation and the pollutant removal of street canyons, respectively. Turbulence plays an important role in both the processes of pollutant removal and mixing that in turn influences significantly the chemical reactions and pollutant distribution. We specifically focus on the effect of urban roughness on the mixing of the chemically reactive pollutants. Detailed analytical approach will be reported in the conference.

UCP7 (cont)-4-1811162_a.pdf

MICROSCALE MODELLING OF EFFECTS OF REALISTIC SURFACE HEAT FLUXES ON POLLUTANT DISTRIBUTION WITHIN A SIMPLIFIED URBAN CONFIGURATION

Jose Luis Santiago, Beatriz Sanchez, Alberto Martilli

Air Pollution Division, Environmental Department, CIEMAT, Madrid, Spain

Modelling micrometeorology and pollutant dispersion within urban areas is very important for both urban climate and air quality applications. The interaction between atmosphere and urban surfaces induces complex flow fields and large heterogeneities in temperature and pollutant distribution within urban canopy layer. These surface-atmosphere interactions may be classified as mechanical (blocking and deviation of the wind by obstacles) and thermal (buoyancy effects due to heat exchange between the atmosphere and buildings). Urban surface heat fluxes are responsible for temperature distribution within street inducing, in some scenarios, modifications of flow properties respect to a neutral case that can affect to pollutant dispersion. Thermal processes are usually neglected, or modeled in a simple way (one facet with different (but constant) temperature with respect to the other urban surfaces) in computational fluid dynamic (CFD) simulations. The main objective of this work is to analyse the effects of realistic surface thermal forcing on pollutant dispersion within a simple urban configuration. CFD-RANS simulations are carried out over a periodic array of cubes with a packing density of 0.25 using realistic surface heat fluxes as boundaries conditions at street and building walls. The microscale heat flux distributions are computed by the TUF3D model (Krayenhoff and Voogt, Boundary-Layer Meteorology 2007). Forty four scenarios with different solar position and ratios of buoyancy to dynamical forces are simulated (Santiago et al., Urban Climate 2014). In this work, one passive tracer is emitted for each scenario at the bottom part of the domain representing traffic emissions. Concentration maps within street and the changes with respect to neutral case are analysed. In addition, spatial average concentrations are also studied and related with properties of the flow. These results are focused on providing useful information to parametrize processes by urban canopy models. These processes are resolved by CFD but are subgrid scale with respect to typical mesoscale model grid resolutions.

UCP7 (cont)-5-4801452_a.pdf