Direct measurements of net CO2 emissions using a tall-tower eddy-covariance system and a rooftop CO2 sensor network in the city of Zurich

Tall-tower eddy-covariance (EC) measurements are useful to quantify the dynamics of net CO₂ emissions of large urban areas. With increasing measurement height, however, the change in CO₂ storage below the sensors (storage flux) and the divergence of horizontal advection may affect the accuracy of EC-based emission estimates more strongly. Such problems are expected if atmospheric layers become decoupled at night or if thermally induced circulations such as land-lake breezes develop over complex land cover or topography. For consistent comparisons with models and emission inventories, it is important to assess these effects. In the ICOS-Cities project, a tall-tower EC system has collected data in the city of Zurich, Switzerland, for more than two years. Furthermore, a network of mid-cost CO₂ concentration sensors was installed in and around the city – mainly on rooftops in combination with wind and temperature sensors. This study investigates the combined use of the tall-tower EC and the rooftop CO₂ and weather observations for an improved direct estimation of the net CO₂ emissions. The storage flux is estimated by dividing the control volume into two layers characterized by a CO₂ concentration change derived from the rooftop network and the tall-tower observation, respectively. The rooftop network is also used in a proof of concept for estimating the along-wind CO₂ concentration gradient and horizontal advection effects. Median diurnal cycles of the turbulent CO₂ flux show a pronounced morning peak but surprisingly low values during the evening rush hour if the wind originates from the direction of the city center and, at greater distance, lake Zurich. Considering the wind field and differences in potential temperature, CO₂ concentration, and specific humidity between the tall-tower and low-altitude sites, we investigate circulation patterns and vertical decoupling. The significance of the estimated storage flux and horizontal advection effects will also be discussed.