Long-term surface-atmosphere interaction measurements in Helsinki, Finland

An increasing fraction of the human population is exposed to urban climate in the future, and therefore there is a growing need to understand the exchange processes of momentum, heat and mass between the urban surface and the atmosphere as they affect the quality of living. Urban areas are a major source for many of the air pollutants, which affect human health, local visibility and global climate. Despite their various effects on environment, there is still uncertainty in the source/sink strengths, spatial distribution of sources and dispersion and mixing mechanisms of these air pollutants. This work aims to improve our understanding of these surface-atmosphere interactions with different temporal scales, and as a function of different land use covers with the aid of measurements carried out at the urban measurement station SMEAR III (Station for Measuring Ecosystem-Atmosphere Relationships) in Helsinki, Finland, since December 2005.

The turbulent exchanges of momentum, heat, water vapour, carbon dioxide (CO2) and aerosol particle number were measured with the eddy covariance (EC) technique in a 31 meter high measurement tower located on a rocky hill around 4 km north-east from the Helsinki City Centre. The surroundings of the tower are complex consisting of buildings, paved areas and green spaces, and area was divided into urban, road and vegetation sectors according to the typical land cover on each area. The urban sector represents the most built-up land cover with high surface fraction of buildings (42%), roads and parking lots (51%). One of the main roads with 45 000 vehicles workday^-1 leading to the Helsinki city centre passes the measurement tower at a distance of 150 m in the road sector. The space between is covered with deciduous forest. The university botanical garden and allotment garden are situated in the vegetation sector, where the vegetation covers 85% of the area.

Most of the turbulent fluxes exhibited a distinguishable seasonal and diurnal behavior. The sensible and latent heat fluxes followed closely the intensity of solar radiation, and sensible heat flux always exceeded latent heat flux due to anthropogenic heat emissions and efficient conversion of solar radiation to direct heat. This urban heat island effect was most distinguishable during winter nights, as the measurement surroundings acted as a source for sensible heat resulting in unstable atmospheric stratification. The effect of land use cover was seen as increased sensible heat fluxes in more built-up areas than in areas with high vegetation cover. During growing season, latent heat flux increased due to the solar heat consumed in transpiration. Both aerosol particle and CO2 exchange showed similarity in their sources and both of them were largely affected by road traffic. Their highest average fluxes reaching 10^9 m^-2 s^-1 and 20 µmol m^-2 s^-1, respectively, were observed in the direction of the largest road. Median emission factors from a mixed fleet traffic for particles and CO2 were estimated to be 3.0•10^14 # km^-1 and 370 g km^-1, respectively, corresponding well with previous studies. The complex urban surface was found to be a source for both particles and CO2 except in summer, when vegetation uptake of CO2 exceeded the anthropogenic sources in the vegetation sector in daytime, and a downward flux of 8 µmol m^-2 s^-1 was observed.