1. BACKGROUND Aerosols influence many things in the world around us from obvious visibility issues to climate changes to human health. The mortality and morbidity rates of people living and working in cities causes a great need for the detection of urban aerosol levels. Though measurements of the concentrations of aerosols in the urban background and at street level have been performed for large cities in the past, this campaign was primarily concerned with the levels of aerosols rising above the city, which may be carried away from the city centre towards suburban residential or agricultural areas, and into the regional background. This type of analysis has been performed in the past by Dorsey et al. (2002) above the busy city of Edinburgh, Scotland by directly measuring particle number emission fluxes from the city centre. CityFlux is a project funded by NERC and conducted by the University of Manchester and Centre for Ecology and Hydrology in Edinburgh. This multi-site project measured aerosol and gas concentrations and physico-chemical characteristics and fluxes. A key element of this project is the direct measurement of particle fluxes using the same technique and similar instrumentation to that of Dorsey et al. (2002) to observe the processes of ventilation of the urban canopy in the City of Manchester over both summer and winter periods (‘summer’ June - August 2005 and ‘winter’ February - March 2006) and consequently compare and review the basic features of this flux data. CONCLUSIONS AND FUTURE WORK These campaigns have been a successful initial indication of the differences and similarities of summer and winter conditions on particle number flux and heat flux. Along with the Edinburgh campaign, they have assisted in suggesting the behaviour of particles within and around large cities. Further measurements in this area are planned for this year, extending the areas of research to accumulation mode and coarse particles, gas analysis and particle composition. Measurements from additional sites are also designed to include roadside, street level and rooftop height. One such field campaign recently completed by the CityFlux team has been around a road with high traffic density and an extremely busy bus route (“the busiest bus route in Europe”). This site – Oxford Road – is used by thousands of commuters each day to enter and exit the city centre. Therefore it is of great importance to investigate the impact of this route on south Manchester’s working and student population. PARTICLE NUMBER EMISSION FLUXES MEASURED ABOVE THE URBAN CANOPY OF CENTRAL MANCHESTER Claire Martin, Ian Longley, Martin Gallagher Centre for Atmospheric Science, University of Manchester, UK MEASUREMENT SITE Measurements were made using a slender mast on top of Portland Tower, Manchester’s sixth tallest building. The mast was at a height of 90m above street level with particles being drawn down a copper inlet pipe turbulently to a condensation particle counter (TSI 3010 in summer and TSI 3025a in winter). The pulse output was logged and combined with data from a co-located Solent R3 sonic anemometer to provide 20Hz fluxes using the eddy covariance technique. Portland Tower is in the heart of the city centre, surrounded on all sides by dense urban fabric and streets with busy traffic. There are four buildings taller than the Portland Tower that lie in the north or north-west fetch between 0.25 and 1 km distant, a zone which also contains a concentration of buildings in the range 40 – 60 m high. Three more tall buildings have been constructed between the summer and winter campaigns in the west-north-west and west-south-west fetches, all ~ 1 km distant. According to UK National Atmospheric Emissions Inventory (2002), traffic in Manchester city centre emits 5 t km -2 a -1 of PM 10, or 55 % of the total PM 10 found in the city. Portland Tower is surrounded by many busy roads; immediately north and west of the site are two main roads that are frequently congested. The city centre is completely surrounded by an Inner Ring Road that has the highest traffic flows in the central area. PARTICLE NUMBER FLUX The main consideration for this campaign was how particle number fluxes vary, both temporally and spatially. Figure 3 indicates the clear diurnal cycle of particle number flux for the two seasons. The summer and winter flux cycles follow a similar cycle, in particular rising rapidly in the morning with a gradual decline in the evening. On summer evenings the particle flux peaks unexpectedly between 21:00 and 22:00 local time. This is an area under further investigation to determine the cause; possible explanations include excess particles released from nearby restaurants, boundary layer anomalies at this height or other local sources or movement of aerosol layers. HEAT FLUX As seen in figure 4, both summer and winter cycles rise in the morning and taper off towards the late evening. The winter data has its peak at midday with a rapid decline after sunset indicating its strong correlation with the cycle of the sun. The summer data appears to start rising later in the morning but this can be attributed to the storage of heat in the buildings and urban fabric. This storage mechanism also helps to explain the shift of the peak of the summer heat flux to later in the afternoon and the extension of this peak over much later in the night than for the winter data. It is worth noting that the summer heat flux can be as much as 3 times higher in the afternoon than the winter heat flux. ACKNOWLEDGEMENTS The University of Manchester has been supported in the CityFlux project by Natural Environment Research Council, UK. Claire Martin has a Natural Environment Research Council studentship. Image produced from the Ordnance Survey Get-a-map service. Image reproduced with kind permission of Ordnance Survey and Ordnance Survey of Northern Ireland. Dorsey, J.R., Nemitz, E., Gallagher, M.W., Fowler, D., Williams, P.I., Bower, K.N., Beswick, K.M., 2002. Direct measurements and parameterisation of aerosol flux, concentration and emission velocity above a city. Atmos. Environ. 36, 791-800. Hussein, T., Puustinen, A., Aalto, P.P., Mäkelä, J.M., Hämeri, K., Kulmala, M., 2004. Urban aerosol number size distributions. Atmos. Chem. Phys., 4, 391–411. Longley, I.D., Gallagher, M.W., Dorsey, J.R., Flynn, M., Allan, J.D., Alfarra, M.R., Inglis, D., 2003. A case-study of aerosol (4.6nm<D p <10μm) number and mass size distribution measurements in a busy street canyon in Manchester, U.K. Atmos. Environ. 37, 1563-1571. Longley, I.D., Gallagher, M.W., 2006. Particle and sensible heat fluxes measured by eddy covariance above and within an urban canopy. Sixth International Conference on Urban Climate. Stull R.B., 1988, An Introduction to Boundary Layer Meteorology, Kluwer Academic Publishers, Dordrecht, The Netherlands Wehner, B., Wiedensohler, A., 2003. Long term measurements of submicrometer urban aerosols: statistical analysis for correlations with meteorological conditions and trace gases. Atmos. Phys. Chem. 3, 867-879. CONTACT DETAILS for Claire Martin: Centre for Atmospheric Science School of Earth, Atmospheric & Environmental Science The University of Manchester PO Box 88 Manchester M60 1QD UK tel. +44 161 200 8771 claire.martin-2@postgrad.manchester.ac.uk FIGURE 3: Diurnal average particle number flux for winter and summer campaigns FIGURE 4: Diurnal average sensible heat flux for winter and summer campaigns FIGURE 5: Average particle number flux vs. wind direction for the summer campaign FIGURE 6: Average particle number flux vs. wind direction for the winter campaign FIGURE 1: OS Map of Manchester, UK, City Centre. (Purple area displays Portland Tower location) FIGURE 2: Portland Tower FIGURE 7: Model of Oxford Road site WIND DIRECTION The summer data initially demonstrated a large flux connected with west-north-westerly directions (Figure 5). However, this appeared in conjunction with the majority of the data coming from only this direction so it was difficult to conclusively relate the two. With the additional winter data again there is a dominance of data from this direction but there was also a significant amount from north-easterly winds. The winter data agrees with the summer data that the largest particle number fluxes are associated with wind from the west-north-westerly regions (Figure 6). Whether this is due to inhomogeneities in upwind sources of particles or turbulence, the effect of the tower itself on turbulence or variations in aerodynamic surface roughness of the area in this direction is under investigation. Roughness length (z 0 ) has been calculated for this data set, and initial analysis suggests it is greater in the west-north-west direction than others (~ 2 – 3 m), but this is also under further examination. Although the Portland Tower is centrally located the median building height in the west and north-west sectors is significantly higher (~ 18 m within a 1 km radius, compared to ~ 12 m for other directions).