In general, towns and cities are net sources for CO2 because of anthropogenic emissions. Although biogenic patterns in both, diurnal and seasonal variation of CO2 flux (FCO2) between urban surface and atmosphere has been observed due to the CO2 uptake by photosynthetically active urban vegetation.
Next to an overview on literature about urban eddy-covariance measurements, results of an EC study should be presented and discussed, which was carried out in the city of Essen, Germany within the densely built-up metropolitan Ruhr area between September 2006 and October 2007. The measurements took place at the border of a wide park area where different urban land-use types were influencing the station depending on the approaching flow: either the park with ambient less densely residential districts or the urban sector with heavy road traffic.
The time series shows the varying activity of the park vegetation: The flux of CO2 which is affected by the alternation of night time respiration and carbon dioxide assimilation during the day however is shifted into the positive direction (i.e. into the atmosphere) due to anthropogenic emissions. The mean diurnal variation of FCO2 over the whole period and all land-use sectors is between -0.9 (at noon) and 14.1 µmol m-2 s-1 (at night) with a mean of 7.4 µmol m-2 s-1. The dominant factor influencing the flux of CO2 is the approaching flow with very high values out of the urban sector, especially during autumn and winter situations with stable atmospheric conditions. On the other side the park area becomes a small net sink for CO2 within the summer months. The range of the data set seems to be very high and different reasons for the highly variable flux rates will be discussed within this contribution.