NCAS/APRIL Meeting on Urban Air Quality Modelling Dispersion modelling at Imperial College London Professor Helen ApSimon and Dr Roy Colvile Page 1/N © Imperial College London

Key questions Page no./ref What kind of modelling approach is fit for purpose? - What are the processes and mechanisms that need to be included? What purposes will be required of Urban Air Quality Modelling in future? - Personal exposure? Urban scale integrated assessment? Assessment of pollution control measures at an airport? Local air quality management during air pollution episodes in a mega-city? © Imperial College London

Modelling aircraft and road traffic sources at Heathrow Airport Page no./ref Model is sensitive to assumptions about variability in take-off trajectory in three dimensions © Imperial College London Work by Fernando Farias and Helen ApSimon, to be presented at 4th Urban Air Quality Conference, Prague

Modelling aircraft and road traffic sources at Heathrow Airport Page no./ref© Imperial College London Usual approach, modelling aircraft emissions as volume sources

Modelling aircraft and road traffic sources at Heathrow Airport Page no./ref© Imperial College London More detailed representation, resolving three-dimensional line- sources of trajectories

Modelling aircraft and road traffic sources at Heathrow Airport Page no./ref Model is sensitive to assumptions about variability in take-off trajectory in three dimensions Exposure of people living near airport, to aircraft emissions, is dominated by a very small region within the model output © Imperial College London

Page no./ref© Imperial College London Relative contribution of aircraft and traffic at a hospital 5 km north-east of the airport (1) Modelling aircraft and road traffic sources at Heathrow Airport

Page no./ref© Imperial College London Relative contribution of aircraft and traffic at a school 0.5 km south of the airport (2)

Page no./ref© Imperial College London Relative contribution of aircraft and traffic at a sports ground 1 km north of the airport (3) Modelling aircraft and road traffic sources at Heathrow Airport

Page no./ref© Imperial College London Relative contribution of aircraft and traffic at a young offenders’ institute 3 km south of the airport (4) Modelling aircraft and road traffic sources at Heathrow Airport

Page no./ref© Imperial College London More detailed study of traffic sources in street canyons How much detail is needed? Work by Kiki Asimakopoulos and Helen ApSimon, contribution to EUROTRAC-2 SATURN Sub-project

Page no./ref© Imperial College London More detailed study of traffic sources in street canyons How much detail is needed? Can we assume canyons are symmetrical?

Page no./ref© Imperial College London More detailed study of traffic sources in street canyons How much detail is needed? Asymmetry has an important influence on concentration

Page no./ref© Imperial College London More detailed study of traffic sources in street canyons Secondary pollutant: nitrogen dioxide (assuming 95% of NOx emissions are NO 2 )

Page no./ref© Imperial College London Mechanisms of ventilation of street canyon systems Quantification of fluxes at street canyon intersections by Athena Scaperdas and Alan Robins

Page no./ref© Imperial College London Mechanisms of ventilation of street canyon systems Wind tunnel measurements by Tom Bentham and Alan Robins

Page no./ref© Imperial College London Mechanisms of ventilation of street canyon systems Operational models assume all canyon ventilation is vertical from two-dimensional geometry Wind tunnel measurements and steady state computational fluid dynamics shows fluxes between streets are important Wind tunnel measurements show flow is unsteady at intersection and intermittent processes may be important in determining average pollutant concentrations

Application of adaptive mesh LES to street canyon intersections Use of Fluidity, a general purpose CFD code which solves the Navier-Stokes and accompanying field equations on arbitrary unstructured meshes, previously used to solve a wide range of problems from flow past ocean seamounts to fluidised beds The anisotropic adaptive mesh facility in FLUIDITY is an example of the state of the art in finite element methods. Work is currently under way to combine this with parallel domain decomposition work in progress by Chris Pain, Tom Bentham et al Page no./ref© Imperial College London

Application of adaptive mesh LES to street canyon intersections In order to simulate turbulent flows, FLUIDITY has been adapted to perform large-eddy simulations (LES) The sub-filter scale model is a ‘dynamic’ variant of the Smagorinsky model, in which the anisotropic filter length is linked to the local element size and shape The combination of adaptive mesh refinement and LES optimises resolution, increasing efficiency of computation especially in complex geometries where the flow is not known a priori. It also allows easy identification of flow structures see Page no./ref© Imperial College London

Application of adaptive mesh LES to street canyon intersections Application of Fluidity to Street Canyon Intersections, first model runs to be completed during 2003 Computational modelling carried out in conjunction with wind tunnel measurement Page no./ref© Imperial College London

Conclusions Page no./ref UK urban atmospheric science community capability to apply operational and advanced models has grown significantly in recent years Effort remains largely fragmented, and it is not clear exactly what future applications of models will be required, nor the extent to which operational and advanced approaches currently used or under development will be fit for purpose © Imperial College London

Thank you Page no./ref