particle fluxes measured by eddy covariance above and within a downtown urban canopy i.d. longley,...

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  • PARTICLE FLUXES MEASURED BY EDDY COVARIANCE ABOVE AND WITHIN A DOWNTOWN URBAN CANOPY I.D. Longley, M.W. Gallagher School of Earth, Atmospheric & Environmental Science, University of Manchester, UK The CityFlux project is funded by the UK Natural Environment Research Council (NERC). Its key aims are to directly measure the vertical turbulent flux of particles within and above an urban canopy. Phase One was a pilot study conducted in the city of Manchester (UK) over six weeks in the summer of 2005 at 3 central sites. [1] Dorsey et al., 2002. Atmos. Environ. 36, 791-800. [2] Longley, I.D. et al., 2004. Atmos. Environ. 38, 3595-3603. Access to Portland Tower by kind permission of Bruntwood Properties Access to Maybrook House by kind permission of GVA Grimley Forthcoming analysis: To predict urban canopy particle ventilation exchange velocity as a function of emission and meteorological data, To use the predicted flux to predict concentrations within the canopy as a function of particle size, To parameterise the emission of particles from the urban centre as a function of size and composition, To further investigate spatial inhomogeneity of urban canopy ventilation and the role of tall buildings in that ventilation. above: time series of particle and heat fluxes measured at 90 m height Above: diurnal mean particle and sensible heat fluxes at 90 m. Above: diurnal mean friction velocity at 90 m Above: mean particle flux as a function of wind direction, plus view from tower to WNW (inset) Vertical turbulent particle number fluxes were measured at a height of 90 m in the centre of the city. A CPC flux system was deployed based upon a TSI 3010 Condensation Particle Counter, as previously deployed above the city of Edinburgh [1]. The sonic anemometer (Gill Instruments Inc R3 Solent) and particle inlet were situated atop a slender mast mounted upon the roof of one of Manchesters tallest buildings (Portland Tower). Measured fluxes followed a distinct diurnal cycle strongly resembling the sensible heat flux cycle (left). Enhanced fluxes were consistently observed in WNW winds, which is responsible for the late evening rise in the diurnal cycle shown (middle left). Current work is investigating whether this is related to a strong source, enhanced turbulence or effect of the tower. Above: mast at tower site Left: TSI 3010 CPC The Tower Site Manchester Manchester is 50 km inland and sits at the centre of a roughly circular conurbation of 10 - 15 km radius. The city centre sits on flat terrain and is compact (~2 km diameter) with a mean building height of 20 m. Ultrafine particle number concentrations were measured at roof level and at street level at a third nearby site using two DMPS instruments. Above: diurnal mean DMPS particle concentrations at 2 heights note particle size ranges are 7 71 nm at 2 m and 3 69 nm at 25 m Above: diurnal mean traffic volume outside the 2 m sampling point patterns are typical for Manchester Furthermore, the composition of the semi-volatile fraction of the aerosol was analysed using an Aerodyne Aerosol Mass Spectrometer (image above, data shown right). This clearly illustrated that the aerosol was dominated by an organic fraction. Peak organic mass loadings were observed in the evening, especially in W, NW and N winds. Above: diurnal mean mass loadings The Street Canyon Site Particle fluxes were measured on the rooftop of a six-floor building overlooking a street canyon less than 1 km from the tower site. At this site an OPC (DMT ASASP-X) providing particle counts in the range 0.1 3.0 m was deployed, as previously used within a street canyon in Manchester [2]. Above: diurnal mean particle flux (>0.1 mm) at 6 th floor roof The In-Canopy Flux Site