Deciphering carbon dioxide signals in urban environments: integrated stable isotope and concentration monitoring for evaluating the impact of human activities on the atmospheric composition

Urban environments function as major hotspots for anthropogenic greenhouse gas emissions, yet quantifying the contribution of specific sources is still a challenge due to the complex interplay between emissions, meteorology, and biospheric activity. Continuous high-frequency monitoring of atmospheric CO 2 concentration and stable isotope composition provides critical insight into the sources and processes governing urban carbon dynamics. In this study, airborne CO 2 concentration, δ 13 C-CO 2 , and δ 18 O-CO 2 were measured at 5-min intervals at the ACO-Pa1 station in Palermo (Italy) from November 2023 to September 2025, together with a comprehensive set of meteorological variables. The dataset reveals that CO 2 concentrations exceeded the global atmospheric background by approximately 4.5 % . Probability plot analysis showed four distinct CO 2 populations, highlighting urban CO 2 enhancement and revealing some pollution episodes. Carbon isotope data show systematic 13 C-depletion during elevated CO 2 conditions, and Keeling plot analysis shows that fossil fuel and natural gas combustion represent the dominant forcing sources, with episodic contributions from landfill gas under specific atmospheric conditions. Principal component analysis demonstrated that while δ 13 C-CO 2 and CO 2 concentration are strongly coupled to anthropogenic emissions and seasonal house-heating demands (explaining 30.5 % of the dataset variance), the δ 18 O-CO 2 signal behaves as a rather independent tracer linked to biospheric exchange and hydrological processes in the urban area of Palermo. Seasonal patterns are marked by higher CO 2 concentrations and stronger 13 C -depletion during fall and winter, which is consistent with enhanced residential heating and reduced biospheric uptake, while the spring–summer periods show lower concentrations and isotopic signature driven by photosynthetic carbon fixation. Oxygen isotope variability displays a pronounced seasonal cycle that is partly decoupled from δ 13 C-CO 2 , reflecting biosphere–atmosphere oxygen exchange. These results prove that integrated monitoring of the concentration and stable isotopes is a powerful tool for resolving urban CO 2 sources and processes, providing a robust observational framework for evaluating emission mitigation strategies in Mediterranean urban environments.