1. D ispersion of A ir P ollutants & their P enetration into the L ocal E nvironment EPSRC Infrastructure & Environment Programme Dr Samantha Arnold (C.Geog.) DAPPLE Fieldwork Manager Dept. of Env. Sci. & Tech., Imperial College London & Dept. of Meteorology, University of Reading Thursday 26 th August 2004, Clean Air Congress, London. Marylebone Rd Gloucester Pl

2. Thursday 26 th August 2004, Clean Air Congress, London. To enhance understanding of dispersion processes, over short distances, at a street canyon intersection. To use this information to make improvements in predictive ability that will enable better planning and management of urban air quality, accidental and non-accidental releases, and the development of safer more sustainable cities. Aims:

3. Consortium: Supported by: London - ALG/APRIL/EA/Local Government/TfL/WCC DEFRA, AEQ Division, DSTL, HSE, Home Office, Met Office Thursday 26 th August 2004, Clean Air Congress, London. University of Surrey Wind Tunnel University of Cambridge Numerical Modelling Imperial College Personal Exposure University of Leeds Traffic, Pollution & Met. University of Reading Meteorology University of Bristol Tracer Release

4. Thursday 26 th August 2004, Clean Air Congress, London. Field Site:

5. Thursday 26 th August 2004, Clean Air Congress, London. During oblique rooftop winds (a) the flow in the constrained streets can be explained by the linear superposition of the parallel (b) and perpendicular (c) rooftop components (with relation to the streets); the parallel component giving the direction of channelling and the perpendicular component driving in-street recirculation/helical vortex. In-Street Flow: (a) (b) (c) (b) (c)

6. Thursday 26 th August 2004, Clean Air Congress, London. Pollution implied in-street vortex during oblique rooftop winds. Pollution:

7. Thursday 26 th August 2004, Clean Air Congress, London. At the intersection winds are relatively unconstrained, hence oblique rooftop winds decompose along the two streets. This is visible in the double peak of the Site 1 wind direction pdf below. Intersection Winds:

8. Thursday 26 th August 2004, Clean Air Congress, London. Relative Wind Directions: Rooftop SSW θ = + 50 Rooftop W θ = - 20 Marylebone Rd WNW θ = 0 S wind Gloucester Pl N wind Gloucester Pl θ = - 20 θ = + 50 θ = 0

9. Thursday 26 th August 2004, Clean Air Congress, London. Tracer - Temporal: Wind Direction Time of flight (tstart- t50%conc) = 4.5 mins Rapid vertical mixing (see WCC) Coherent trends in tracer concentrations WCC

10. Thursday 26 th August 2004, Clean Air Congress, London. The maximum concentration of the tracer gas decreases with increasing straight line fetch (R = 50-250 m) from the release. Tracer - Spatial:

11. Thursday 26 th August 2004, Clean Air Congress, London. Personal Exposure: Personal exposure is consistently greater than fixed monitoring stations would suggest Mean personal exposure is highly variable in both space and time – synchronised sampling and visualisation with HSL.

12. Thursday 26 th August 2004, Clean Air Congress, London. DAPPLE Summary: Chemical Release Where? Orientation of the Roads When? Wind direction above roof Relative angle of the approach flow Decompose flow into the streets for direction of transport of release – channelled, vortex etc. Define hazard area Based on transport direction, flow speed, decrease in release conc. with distance, etc Time & Space Dependent Check interpretation using pollution data and incorporate difference between fixed monitoring and actual personal exposure which may be greater

13. Thursday 26 th August 2004, Clean Air Congress, London. Despite the complexity of the urban intersection topology the main features of the mean wind field can be related to those described in idealised 2-D canyons. This information can be used to interpret in- street, time averaged, tracer and pollution concentrations. Further work is required to relate the these data to the non-static, personal exposure measurements. Analysis of the data from the two DAPPLE field campaigns and a road dust release experiment is on going. Additional tracer releases will be conducted in Oct 2004 at the request of the Home Office. For more details and publications please see http://www.dapple.org.uk Conclusions: