1Department of Physics, University of Helsinki, P.O. Box. 48, Helsinki, Finland
2Department of Physics, University of Helsinki, P.O. Box. 64, Helsinki, Finland
3Department of Forest Sciences, University of Helsinki, P.O. Box 27, Helsinki, Finland
Abstract. Five years of carbon dioxide exchange measured with the eddy covariance technique at the world's northernmost urban flux station SMEAR III located in Helsinki, Finland, were analyzed. The long-term measurements and high-latitude location enabled us to examine the seasonal and annual variations of CO2 exchange, and to identify different factors controlling the measured exchange. Furthermore, the advantage of the station is that the complex surrounding area enables us to distinguish three different surface cover areas than can be evaluated separately. We also tested different methods (artificial neural networks and median diurnal cycles) to fill gaps in CO2 flux time series and examined their effect on annual emission estimates.
The measured fluxes were highly dependent on the prevailing wind direction with the highest fluxes downwind from a large road and lowest downwind from the area of high fraction of vegetation cover. On an annual level, the difference in CO2 emission of the two areas was 75% showing the impact of complex measurement surroundings in the flux measurements. Seasonal differences in the CO2 exchange downwind from the road were mainly caused by reduced traffic rates in summer, whereas in other directions seasonality was more determined by vegetation activity. Differences between the gap filling methods were small, but slightly better (0.6 μmol m−2 s−1 smaller RMSE) results were obtained when the artificial neural network with traffic counts was used instead of the without traffic network and method based on median diurnal cycles. The measurement site was a net carbon source with an average annual emission of 1760 g C m−2, with a biased error of 6.1 g C m−2 caused by the gap filling. The annual value varied 16% between the different years.