On the spatial distribution and evolution of ultrafine aerosols in urban air
1Institute of Environmental Assessment and Water Research (IDǼA), Consejo Superior de Investigaciones Científicas (CSIC), C/ Jordi Girona 18-26 08034 Barcelona, Spain
2School of Physics, Centre for Climate & Air Pollution Studies, National University of Ireland Galway, University Road, Galway, Ireland
3National Centre for Atmospheric Science, Division of Environmental Health & Risk Management, School of Geography, Earth & Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
4Department of Chemistry and Environmental Research Institute, University College Cork, Ireland
5CIEMAT, Environment Department, Av. Complutense 40, 28040 Madrid, Spain
6Department of Environmental Sciences/Center of Excellence in Environmental Studies, King Abdulaziz University, Jeddah, 21589, Saudi Arabia
Abstract. Sources and evolution of ultrafine particles (<0.1 μ m diameter) were investigated both horizontally and vertically in the large urban agglomerate of Barcelona, Spain. Within the SAPUSS project (Solving Aerosol Problems by Using Synergistic Strategies), a large number of instruments was deployed simultaneously at different monitoring sites (road, two urban background, regional background, urban tower 150 μa.s.l., urban background tower site 80 m a.s.l.) during a 4 week period in September-October 2010. Particle number concentrations (N>5nm) are highly correlated with black carbon (BC) at all sites only under strong vehicular traffic influences. By contrast, under clean atmospheric conditions (low condensation sinks, CS) such correlation diverges towards much higher N/BC ratios at all sites, indicating additional sources of particles including secondary production of freshly nucleated particles. This is also evident in the urban background annual mean diurnal trend of N/BC, showing a midday peak in all seasons. Size-resolved aerosol distributions (N10-500) as well as particle number concentrations (N>5nm) allow us to identify two types of nucleation and growth events: a regional type event originating in the whole study region and impacting almost simultaneously the urban city of Barcelona and the surrounding background area; and an urban type which originates only within the city centre but whose growth continues while transported away from the city to the regional background. Furthermore, during these clean air days, higher N are found at tower level than at ground level only in the city centre whereas such a difference is not so pronounced at the remote urban background tower. In other words, this study suggests that the column of air above the city ground level possesses the best compromise between low CS and high vapour source, hence enhancing the concentrations of freshly nucleated particles. By contrast, within stagnant polluted atmospheric conditions, higher N and BC concentrations are always measured at ground level relative to tower level at all sites. Our study suggests that the city centre is a source of both non-volatile traffic primary (29–39%) and secondary freshly nucleated particles (up to 61–71%) at all sites. We suggest that organic compounds evaporating from freshly emitted traffic particles are a possible candidate for new particle formation within the city and urban plume.