Abstract
Abstract. Long-term eddy covariance particle number flux measurements for the diameter range 6 nm to 5 μm were performed at the SMEAR III station over an urban area in Helsinki, Finland. The heterogeneity of the urban measurement location allowed us to study the effect of different land-use classes in different wind directions on the measured fluxes. The particle number fluxes were highest in the direction of a local road on weekdays, with a daytime median flux of 0.8×109 m−2 s−1. The particle fluxes showed a clear dependence on traffic rates and on the mixing conditions of the boundary layer. The measurement footprint was estimated by the use of both numerical and analytical models. Using the crosswind integrated form of the footprint function, we estimated the emission factor for the mixed vehicle fleet, yielding a median particle number emission factor per vehicle of 3.0×1014 # km−1. Particle fluxes from the vegetated area were the lowest with daytime median fluxes below 0.2×109 m−2 s−1. During weekends and nights, the particle fluxes were low from all land use sectors being in the order of 0.02–0.1×109 m−2 s−1. On an annual scale the highest fluxes were measured in winter, when emissions from stationary combustion sources are also highest. Particle number fluxes were compared with the simultaneously measured CO2 fluxes and similarity in their sources was distinguishable. For CO2, the median emission factor of vehicles was estimated to be 370 g km−1.
Highlights
Atmospheric aerosol particles are known to have adverse health effects especially in urban areas (Curtis et al, 2006; Keeler et al, 2005), where many pollution sources are lo-cated, and where a large number of people are exposed to these pollutants
A few studies have concentrated on modelling the particle emissions in urban areas using different methods (e.g. Palmgren et al, 1999; Kumar et al, 2008), while direct particle flux measurements have been made in a few cities during shorter campaigns (Nemitz et al, 2000, 2008; Dorsey et al, 2002; Longley et al, 2004a, b; Donateo et al, 2006; Martensson et al, 2006; Martin et al, 2008; Schmidt and Klemm, 2008)
The fluxes depend on wind direction due to the different surface types, while seasonal and diurnal variation is likely to exist in the measured fluxes
Summary
Atmospheric aerosol particles are known to have adverse health effects especially in urban areas (Curtis et al, 2006; Keeler et al, 2005), where many pollution sources are lo-cated, and where a large number of people are exposed to these pollutants. Atmospheric aerosol particles are known to have adverse health effects especially in urban areas (Curtis et al, 2006; Keeler et al, 2005), where many pollution sources are lo-. Road traffic has been identified as the main source especially for ultra-fine particles, which can either be directly emitted or formed in secondary reactions from exhaust gases A few studies have concentrated on modelling the particle emissions in urban areas using different methods Schmidt and Klemm (2008) were the first to report size-resolved particle number fluxes from an urban area (from Munster, Germany), whereas Nemitz et Published by Copernicus Publications on behalf of the European Geosciences Union
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