CO₂ and CO temporal variability over Mexico City from ground-based total column and surface measurements

Abstract. Precise estimates of greenhouse gas (GHG) emissions and sinks are critical for understanding the carbon cycle and identifying key factors in the human-induced climate change feedback. Recent efforts were focused on reconciling bottom-up and top-down GHG emissions estimates, in particular on the city scale, using both space-based and ground-based atmospheric composition measurements that still show serious discrepancies. In this study, we explore the variability of the CO and CO₂ emissions from the Mexico City Metropolitan Area (MCMA) from long-term time-resolved total column measurements using solar absorption Fourier Transform Infrared Spectroscopy (FTIR). Measurements were performed at three stations, two of them located in the urban area at two opposite sides of Mexico City center and the third in a mountainous background site. Using a simple model and the mixed layer height from a ceilometer, the GHG concentration in the mixed layer and the CO/CO₂ ratio were determined from the total column observations and compared to surface measurements using Cavity ring-down spectroscopy (CRDS). Finally, combining the ground-based total column and space-based TROPOMI CO measurements, we estimate the annual CO and CO₂ MCMA emissions based on a simple model, i.e.: without recourse to complex transport models. By this way, we study the inter-annual variability of the CO and CO₂ MCMA anthropogenic emissions, and relate it to the main natural or anthropogenic changes occurring during the last decade, such as the 2015–2016 El Niño period or the COVID-19 lock-down event.