Influence of deep stratosphere-to-troposphere transport on atmospheric carbon dioxide and methane at the Mt. Cimone WMO/GAW global station (2165 m a.s.l., Italy): A multi-year (2015–2022) investigation

Abstract

This work was dedicated to provide a first systematic assessment of the role played by deep stratosphere-to-troposphere transport events (denoted as “stratospheric intrusions–SIs”) in influencing the CO2 and CH4 variability at a high-elevation site in Italy (Monte Cimone, CMN, 2165 m a.s.l.). To select SI events, we used a methodology based on the analysis of the temporal variability of in-situ observed stratospheric tracers (O3, CO and RH), modeled PV along 5-day back-trajectories starting at the measurement site, and satellite measurements of total column ozone (TCO). The analysis of two case studies revealed clear fingerprints of the stratospheric air masses on the atmospheric composition at CMN: CO2 and CH4 decreases were observed in winter, while CO2CH4 increased (decreased) in summer. For 2015–2022, 9.8% of the hourly data were selected as influenced by SI events. The systematic analysis of the SI effect on CO2 confirmed differences between vegetative and non-vegetative seasons: during May–September, air masses from the stratosphere were richer in CO2 (+1.2 ± 0.1 ppm, average ±σ/N). On the other hand, during winter, air masses from higher altitudes were characterised by a lower amount of CO2 (−1.5 ± 0.1 ppm). Furthermore, SI events were usually characterised by an overall decrease in CH4 compared to the remaining data throughout the solar year (with average values ranging from −7.6 to −16.1 ppb as a function of the seasonal period). However, based on the analysis of seasonal anomalies of residuals, it looks that SI events did not play a major role in determining the interannual variability of CO2 and CH4 at CMN.