Inter-annual variability of Eddy Covariance CO₂ flux measurements in the city center of Heraklion, Greece

Understanding the interactions between urban CO₂ emissions with urban form and function and establishing city-scale emission inventories to account cities' contribution to climate change, are current challenges for the global scientific community. The Eddy Covariance (EC) method can provide in-situ measurements of energy and CO₂ fluxes (Fc) between a surface source area (local scale) and the atmosphere, proving to be an auspicious approach for quantifying CO₂ budget of urban areas. The center of Heraklion is an interesting study area in the global network of urban EC stations due to the complex urban morphology, the Mediterranean climate and the mixture between residential neighborhoods and busy commercial zone. A tower-based EC system is active for a three-year period over the city center of Heraklion. Fc was calculated for this period at 30-min time step and the time-series were quality-controlled and gap-filled using a moving look-up table (mLUT) technique. Gap-filled time-series were then temporally aggregated to monthly and yearly emission totals. Furthermore, the annual flux source area was estimated using the Flux Footprint Prediction (FFP) model, parameterized using urban morphological parameters extracted from a Digital Surface Model. To examine the directionality of the observed fluxes, the annual Fc has been additionally estimated by dividing the source into four wind direction sectors. The diurnal patterns per sector showed significant differences, especially the ones coming from the source area that characterize the commercial zone of the city center. The latter present considerably higher Fc than the sectors related to the residential zones. The inter-annual Fc variability per sector reflects the changes in the traffic patterns in the commercial area and the residential heating contribution during winter in the residential area which is related to winter temperature. Finally, the CO₂ fluxes during the government measures for COVID-19, in March and April of 2020 are presenting an important reduction in Heraklion case study that reflects the total lockdown of the commercial and traffic activities in the city center.