Quantifying the flux of carbon dioxide over an urban park area by means of eddy-covariance measurements

While eddy-covariance (EC) has become an established method in forest and agricultural meteorology to quantify the exchange of carbon dioxide (CO₂) between vegetated surfaces and the atmosphere, the use of this method in urban ecosystems has been accomplished only in recent years with a comparatively little number of publications. Thereby the heterogeneous surface structures in urban areas lead to methodical challenges and the comparability between different studies is weakened for various reasons.

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 (F_CO2) 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 F_CO2 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.