Long-term observations and bottom-up modelling of carbon dioxide fluxes in Berlin, Germany

Urban environments play a critical role for greenhouse gas (GHG) monitoring as they are responsible for more than 70% of global carbon emissions and their role is expected to further increase with continued urbanization. At the same time, many political decisions are implemented on a city-level and many cities have their own targets for carbon neutrality. Yet, tracking changes in urban emissions is challenging because GHG are a complex mix of natural and anthropogenic fluxes with strong spatio-temporal variability.

Eddy covariance (EC) measurements of carbon dioxide (CO2) fluxes have become a widely applied method in urban areas in recent years. However, multi-annual studies capable of tracking long-term trends and inter-annual variabilities remain scarce. Here, we present a decade of CO2-flux measurements from the Urban Climate Observatory (UCO) Berlin site TU Campus Charlottenburg and a seven-year time-series from the ICOS-site Rothenburgstrasse (DE-BeR). We use footprint modeling, meteorological and high-resolution data on urban form and function to attribute the observed fluxes to local sources and sinks of CO2. The flux observations are used to validate first results from a bottom-up modelling study to quantify surface exchanges of CO2 for the whole city of Berlin at high spatial resolution. The model includes the main anthropogenic and biological components of the carbon cycle in the urban system, such as road transportation, building energy consumption, human respiration, and Net Ecosystem Exchange of CO2 for an holistic approach.

The observations show a slight gradual reduction of CO2 emissions over the analysed time. Further, observational and modelling results show a significant reduction of CO2 emissions during the COVID-19 pandemic in comparison to previous years, with a maximum decrease of up to 25% in traffic emissions at the city scale. Our data and findings support city-scale quantification of GHG emissions, as well as future studies focussing on, e.g., inter-city comparisons.