Temporal Variations and Influencing Factors on Atmospheric CO2 in Urban Environments: A Stable Isotope Perspective

Volcanoes are primary geological sources of carbon dioxide (CO₂), while the combustion of fossil fuels significantly contributes to raise the CO₂ concentration in the atmosphere, particularly within densely populated urban areas. Previous investigations have identified distinct sources of CO₂ at the district scale in urban environments and that the short term evolutions in atmospheric CO₂ concentration are influenced by meteorological parameters.

This study presents continuous monitoring of stable isotope compositions and CO₂ concentrations in the urban environment of Palermo over a yearly period from 2023 to 2024. A laser-based isotope mass spectrophotometer was employed for measurements, detecting various isotopologues of CO₂ (e.g., COO, ¹³COO, and C¹⁸OO isotopologues) through mid-infrared range laser absorption. The instrument calculated the ¹³C/¹²C ratio, ¹⁸O/¹⁶O ratio, and overall CO₂ concentration. Measurements were conducted outside the Istituto Nazionale di Geofisica e Vulcanologia (INGV) laboratory at an elevation of 16.30 meters above the ground floor, referenced hourly, and calibrated daily using a known stable isotope composition standard of pure CO₂.

Environmental parameters, including air temperature, atmospheric pressure, relative humidity, solar radiation and wind speed and direction, were recorded at a 5-minute sampling frequency. These data were utilized for processing the atmospheric CO₂ dataset. The correlation between stable isotopic ratios and CO₂ concentration, analyzed through the "Keeling plot" approach, enabled the determination of the isotopic signature of the predominant source of atmospheric CO₂ in the Palermo urban zone.

The results indicated that wind speed and atmospheric pressure exerted opposing effects on atmospheric CO₂ concentration. Elevated CO₂ levels coincided with periods of high atmospheric pressure and low wind speed, while reduced CO₂ concentrations were associated with increased air turbulence during windy periods. However, meteorological variables partly explain the variability in atmospheric CO₂, considering contributions from various CO₂ sources. The δ¹³C-CO₂ measurements aligned with CO₂ derived from fossil fuel combustion, attributed to urban vehicular mobility and residential heating, particularly during winter periods.

Analysis indicates that CO₂ levels in medium-sized urban areas like Palermo exhibit distinct seasonal and daily variations. Seasonal shifts primarily reflects CO₂ emissions from hydrocarbon combustion during winter which was unbalanced by CO₂ uptake during productivity season (spring and summer). On the weekly timescale, CO₂ variations reflect population behaviors. CO₂ concentrations are lowest during weekends and holiday periods contrasting raises of CO₂ concentration on weekdays and in periods of atmospheric stability, especially in winter. Urban-scale observations, where the majority of greenhouse gases are emitted, allow for tracking high-frequency variations driven by environmental conditions and changes in human activities. Monitoring CO₂ in urban areas offers crucial insights for assessing the effectiveness of climate change mitigation measures.