Atmos. Chem. Phys. Atmospheric Chemistry and Physics, vol. 12 (2012) pp. 8475-8489
Five years of carbon dioxide exchange measured with the eddy covariance technique at the world's north-ernmost 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 CO 2 exchange, and to identify differ-ent factors controlling the measured exchange. Online traf-fic counts and soil respiration measurements were utilized in the study. Furthermore, the advantage of the station is that the complex surrounding area enables us to distinguish three different surface cover areas that can be evaluated separately. We also tested different methods (artificial neural networks and median diurnal cycles) to fill gaps in CO 2 flux time se-ries and examined their effect on annual emission estimates. The measured fluxes were highly dependent on the prevail-ing wind direction with the highest fluxes downwind from a large road and lowest downwind from the area of high frac-tion of vegetation cover. On an annual level, the area of the road emitted 3500 g C m −2 whereas the area of high fraction of vegetation cover emitted only 870 g C m −2 showing the ef-fect of surface cover to be large in urban areas. Seasonal dif-ferences in the CO 2 exchange downwind from the road were mainly caused by reduced traffic rates in summer, whereas in other directions seasonality was more determined by veg-etation activity. Differences between the gap filling meth-ods were small, but slightly better (0.6 µmol m −2 s −1 smaller RMSE) results were obtained when the artificial neural net-work with traffic counts was used instead of the one without traffic network and method based on median diurnal cycles. The measurement site was a net carbon source with aver-age annual emissions 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.
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