The Madrid-CIEMAT GHG Station:  the first monitoring site for surveilling the influence of urban activities on GHG background levels in Spain

The imperative reduction of anthropogenic greenhouse gas (GHG) emissions, coupled with the implementation of adaptation and mitigation strategies against climate change, stand as paramount objectives for developed societies and particularly for the big urban areas within the European Union. Conducting a comprehensive quantitative analysis of urban GHG emissions beyond the official inventories remains as challenging issue. The absence of exhaustive, quantitative, contrasted and methodologically consistent GHG emission data from some urban activities and zones prevent a reliable documentation of actual emissions, avoiding the scientific understanding of their consequences at urban level and, eventually, the definition and application of effective mitigation policies.. Madrid City and its metropolitan area is the biggest and most densely populated zone in Spain. It is located in the center of the Iberian Peninsula enough far from other cities and it should play a pivotal role as a reference site for meticulous greenhouse gas monitoring in urban areas. To achieve this objective, a GHG monitoring station was launched in CIEMAT facilities in May 2023 equipped with a Picarro G2301 cavity ring-down spectrometer (CRDS), enabling real-time monitoring of CO2, CH4 and H2O with a 1-second time resolution. The ICOS protocol for its installation was strictly followed. This GHG station is located at the CIEMAT campus in Madrid  (coordinates: 40.456582, -3.725690) away from direct pollutant sources and very close to the Madrid-CIEMAT ACTRIS atmospheric monitoring station equipped with advanced instrumentation for aerosol monitoring, gaseous pollutants and meteorology. During the initial phase of operation of this station, significant variations in GHG concentrations have been documented, as expected, including clear cyclical changes (daily, weekly and monthly). These evolutions are associated to local and regional air mases flows and further detailed investigation will provide valuable information about the contribution of urban GHG sources and the role of natural areas. Ongoing database in the coming months will reveal important details for understanding the evolution of these concentration levels, providing essential information for comparative assessments with other urban centers, and to quantify the contribution of primary urban sources to background concentrations. Another objective for the middle term is to integrate Madrid-CIEMAT GHG station as an active part of ICOS Cities network.