Annual particle flux observations over a heterogeneous urban area
1Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland
2Risø National Laboratory for Sustainable Energy, Technical University of Denmark, P.O. Box 49, 4000 Roskilde, Denmark
3Department of Forest Ecology, P.O. Box 27, 00014 University of Helsinki, Finland
Abstract. Long-term eddy covariance (EC) particle number flux measurements for the size range 6 nm to 5 μm were performed at the SMEAR III station over urban area in Helsinki, Finland. Heterogeneous urban environment allowed us to study the effect of different land-use classes in different wind directions on the measured fluxes. The particle fluxes were observed to be the highest from the road direction during weekdays with day-time median flux 0.8×109 m−2 s−1. Particle fluxes showed a~clear dependence on traffic rates and mixing conditions of the boundary layer. In the direction of road, the larger particle fluxes were dominated by smaller sizes. Footprint analysis was performed by using numerical modeling and emission rate of particles from road was estimated to be 0.8×1012 s−1 m−1 during day-time. With typical traffic rate of 2500 vehicles per hour this corresponds to average emission rate of 1.2×1015 vehicles−1 km−1. The particle fluxes from 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 annual scale, the highest fluxes were measured in winter when emissions from stationary combustion sources are higher.