Extension of the Total Carbon Column Observing Network (TCCON) over the Eastern Mediterranean and Middle East: the Nicosia site in Cyprus

Long-term greenhouse gas (GHG) measurements are essential for understanding the carbon cycle, detecting trends in atmospheric composition, and assessing the efficiency of climate change mitigation strategies. However, observational gaps over large geographic areas such as the Eastern Mediterranean and Middle East (EMME), a well-known regional GHG hotspot, are likely to increase uncertainties in estimations of their sources and sinks. Here, we describe a new Total Carbon Column Observing Network (TCCON) observatory for solar absorption spectroscopy measurements that has been operating in Nicosia, Cyprus, since September 2019. The site helps bridge a regional observational gap in the EMME, a strategic location at the crossroads of air masses from Europe, Asia, and Africa. Using near-infrared (NIR, InGaAs detector) solar absorption spectra, TCCON-Nicosia measures total column average dry-air mole fractions (Xgas) of key trace gases, including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), carbon monoxide (CO), hydrogen fluoride (HF), water vapor (H2O), and semi-heavy water (HDO). These continuous observations, spanning more than 4 years, are presented along with a description of the quality control procedures, compliant with the TCCON standards, to ensure total column atmospheric data with minimal errors. In 2023, observations were extended into the mid-infrared (MIR) spectral region with the addition of a liquid-nitrogen-cooled InSb (LN2-InSb) detector enabling the retrieval of additional trace gases such as formaldehyde (HCHO), carbonyl sulfide (OCS), nitrogen monoxide (NO), nitrogen dioxide (NO2), and ethane (C2H6), herewith further contributing to the global Network for the Detection of Atmospheric Composition Change (NDACC). To tie the TCCON Nicosia with the WMO reference scale, an AirCore (AC) campaign conducted in June 2020 over Cyprus provided vertical in situ profiles, which were converted into total column quantities (AC.Xgas) and compared to TCCON observations (Xgas). The TCCON/in situ comparison showed agreement well within their respective uncertainty budget.