Five years of urban eddy covariance CO2 emissions correlated with dynamic shifts in urban structure and traffic regulations in the city center of Heraklion, Greece

Quantifying the contribution to CO2 emissions by the diverse sinks and sources that exist in the urban environment is still a work in progress, despite several studies attempting to estimate them. The majority of emissions come from anthropogenic sources, associated with e.g. fossil fuel combustion from vehicular traffic activity, space heating/cooling, household combustion or human breathing. An eddy covariance flux tower has been established in the center of Heraklion, monitoring urban CO2 fluxes since November 2016. CO2 fluxes are averaged over 30-min periods, filtered and gap filled using a moving look-up table technique mLUT. Since January 2018, the municipality of Heraklion is introducing significant traffic regulation and rebuilding initiatives. Some of these have the potential to change the urban form, and when they occur within the flux footprint area, the estimation of the local CO2 budget becomes more complicated. The investigation of the extent to which CO2 emissions are affected by these dynamic shifts is the subject of the present almost 5-year long study. New indicators are used to describe changes in traffic flow patterns of the major roads in the city center, and their effect on the observed CO2 fluxes. Results show that pedestrianization and road closures have a significant impact on daily patterns and the annual budget. Finally, government actions to prevent the spread of COVID-19 in 2020, resulted in a temporary reduction of CO2 emissions due to reduced vehicular traffic during lockdown periods.