Annual particle flux observations over a heterogeneous urban area 1Department of Physics, P.O. Box 48, 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
Received: 24 April 2009 – Published in Atmos. Chem. Phys. Discuss.: 17 June 2009 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.
Revised: 02 October 2009 – Accepted: 02 October 2009 – Published: 20 October 2009
Citation: Järvi, L., Rannik, Ü., Mammarella, I., Sogachev, A., Aalto, P. P., Keronen, P., Siivola, E., Kulmala, M., and Vesala, T.: Annual particle flux observations over a heterogeneous urban area, Atmos. Chem. Phys., 9, 7847-7856, doi:10.5194/acp-9-7847-2009, 2009.