Impact of transition of heating types and traffic on urban CO2 emissions – long-term interannual and seasonal analysis of two flux towers in the city of Basel, Switzerland

In this study, long-term data from two flux eddy-covariance flux towers (since 2004 and 2009, respectively) in the city centre of Basel, Switzerland, which are only 1.6 km apart from each other, are analysed with focus on the source strengths of CO₂ emissions in the flux footprints of the towers. The wind field in the city of Basel is predominated by a valley wind system due to its location in the Rhine valley with a clear diurnal pattern of the wind flow, if not superimposed by a macroscale synoptic weather situation. For a detailed interpretation of the mean seasonal diurnal courses of CO₂ fluxes, a distinct flux footprint analysis, combined with a detailed land cover map of the city, is applied. Land cover classes like “buildings” and “roads” are extended with attributes characterizing the source strength of CO₂, i.e. the type of heating (oil, gas, district heating, etc.) and the traffic volumes, respectively, and these source strengths are weighted by the flux footprint. This framework allows a distinct interpretation of the observed seasonal and annual trends of the two flux towers. The long-term time series show different but, in both cases, declining trends in CO₂ emissions. These trends can be mainly attributed to the extension of the district heating network by the city administration during the last 15 years. Traffic volume changes have also strong impact on the total amount of CO₂ emissions, but, despite great efforts of the city administration to reduce the traffic in the city center, these measures have only minor impacts on the CO₂ flux.