The urban measurement station SMEAR III : Continuous monitoring of air pollution and surface–atmosphere interactions in Helsinki, Finland

We present results from the air pollution and turbulent exchange measurements made at the urban measurement station SMEAR III in Helsinki, Finland. First measurements at the station started in August 2004 and since then more measurements have gradually been added. We analyze data until June 2007. Temporal variations and dependencies between the size-fractionated particle number concentrations (both fine and coarse particle concentrations), gas concentrations (O 3 , NO x , CO and SO 2 ), turbulent fluxes of momentum, sensible and latent heat and CO 2 , and meteorological variables were studied. Most of the air pollutants and turbulent fluxes showed distinct annual and diurnal variation closely related to the local combustion sources (especially traffic) and the amount of available solar radiation. Ultrafine particles showed the most explicit dependence on traffic and traffic-related pollutants, while larger particles were more affected by the meteorological conditions. The surface fluxes were strongly affected by the specific conditions in urban environment.