Tracking spatial variations of CO2 isotopologues, NH3 and H2O in urban air: a case study from the Campi Flegrei Caldera

Anthropogenic CO 2 emissions are a major driver of greenhouse gas accumulation in the atmosphere, whereas natural sources, including volcanic degassing, can locally modify atmospheric composition. Since the onset of renewed bradyseismic activity at Campi Flegrei in 2005, the Solfatara area has released ~ 5,000 tons of CO₂ per day. Recent reports of elevated indoor CO₂ concentration in Pozzuoli prompted an in situ survey using a portable laser-based spectrophotometer. Measurements of stable isotope compositions (δ¹³C-CO₂, δ¹⁸O-CO₂, δ¹⁷O-CO₂) combined with NH 3 and H 2 O concentrations, enabled the identification of distinct atmospheric contributions despite the limited spatial variability of CO 2 concentrations. Atmospheric CO 2 in Pozzuoli exhibited distinct isotopic signatures consistent with contributions from soil degassing, dispersal of volcanic CO 2 from the Solfatara area, and fossil fuel combustion. In particular, atmospheric CO 2 showed a measurable 13 C-enrichment relative to the atmospheric background. This isotopic shift is incompatible with a simple dilution process and instead indicates admixture with an isotopically heavier geogenic CO 2 endmember, characterized by a δ 13 C-CO 2 value of approximately − 1.8‰ vs. VPDB. Spatial variations in H 2 O were related to distance from the marine shoreline, whereas NH 3 concentrations provided evidence of an anthropogenic influence on the atmosphere within the densely inhabited area. Concentrations of air CO 2 remained well below short-term exposure limits along the entire survey transect, although these thresholds are primarily intended for indoor environments. This study provides a combined characterization of carbon and triple oxygen isotope composition in atmospheric CO 2 for volcanic source attribution, and can establish a basis for the development of future high-resolution monitoring strategies. Continuous surveillance integrating CO 2 concentration and isotope compositions could therefore complement existing volcano monitoring approaches and strengthen environmental monitoring in densely populated volcanic areas.