Tall-tower urban eddy covariance flux ratios of CO2, CO, and NOx in three European cities

In-situ measurements of CO₂ emissions play a critical role in quantifying cities’ contributions to regional and global emissions and are a key tool in the validation of city emission inventories and models. Cities are also complex environments containing a multitude of anthropogenic CO₂ emission sources such as traffic, residential heating, and industrial production. Correct sectoral attribution of urban GHG emissions is necessary to monitor emission reduction efforts, compare against emission inventories, and separate anthropogenic from biogenic emissions.

As part of the ICOS-Cities (PAUL - Pilot Application in Urban Landscapes) project, tall-tower urban eddy covariance (EC) systems were installed in a small city (Zurich, Switzerland), a moderate-sized city (Munich, Germany) and a large city (Paris, France). Use of a high-frequency multi-species gas analyser (MGA-7, MIRO Analytical, Switzerland) together with an ultrasonic anemometer (CSAT3, Campbell Scientific, USA), enabled simultaneous flux measurements of CO₂, and co-emitted species CO and NO_x. EC measurements provide gas fluxes which integrate all emission sources and sinks within the measurement footprint. By examining the ratio of these gas fluxes in combination with a spatially-resolved emission inventory within the EC footprint, one may validate or improve the emission inventory.

We present six months of EC flux ratios of CO₂/CO, CO₂/NO_x, and NO_x/CO from each of the three cities covering both growth and dormant seasons, comparing diurnal and seasonal trends in the fluxes and flux ratios, as well as direct comparison against bottom-up emission inventory ratios for each species pair. We further demonstrate that a linear-mixing model is able to generally decompose CO₂ fluxes into major sectors and anthropogenic vs biogenic emissions and discuss its performance and limitations.