Temporal dynamics of CO₂ fluxes and profiles over a Central European city

Summary

In Summer 2002 eddy covariance flux measurements of CO₂ were performed over a dense urban surface. The month-long measurements were carried out in the framework of the Basel Urban Boundary Layer Experiment (BUBBLE). Two Li7500 open path analysers were installed at z/z _H = 1.0 and 2.2 above a street canyon with z _H the average building height of 14.6 m and z the height above street level. Additionally, profiles of CO₂ concentration were sampled at 10 heights from street level up to 2 z _H . The minimum and maximum of the average diurnal course of CO₂ concentration at 2 z _H were 362 and 423 ppmv in late afternoon and early morning, respectively. Daytime CO₂ concentrations were not correlated to local sources, e.g. the minimum occurred together with the maximum in traffic load. During night-time CO₂ is in general accumulated, except when inversion development is suppressed by frontal passages. CO₂ concentrations were always decreasing with height and correspondingly, the fluxes – on average – always directed upward. At z/z _H = 2.2 low values of about 3 µmol m⁻² s⁻¹ were measured during the second half of the night. During daytime average values reached up to 14 µmol m⁻² s⁻¹. The CO₂ fluxes are well correlated with the traffic load, with their maxima occurring together in late afternoon. Daytime minimum CO₂ concentrations fell below regional background values. Besides vertical mixing and entrainment, it is suggested that this is also due to advection of rural air with reduced CO₂ concentration. Comparison with other urban observations shows a large range of differences among urban sites in terms of both CO₂ fluxes and concentrations.