Comparative analysis of atmospheric CO₂, CH₄, and ¹⁴CO₂ between a Hungarian urban site and an ICOS regional background station

Urban areas are global hotspots of anthropogenic greenhouse gas emissions; however, distinguishing between fossil fuel combustion and biogenic fluxes remains challenging due to the complexity of the urban environment. High-precision atmospheric observations are essential for validating "bottom-up" emission inventories and guiding local green strategies. This study presents a comprehensive comparative analysis examining atmospheric CO₂ and CH₄ mole fractions, as well as atmospheric radiocarbon (¹⁴C) signals, from May 2025 in Debrecen (an urban environment) and two elevations at the regional background station in Hegyhátsál (ICOS HUN).

During the research campaign, Picarro Cavity Ring-Down Spectroscopy (CRDS) analyzers were employed at both sites for continuous, high-resolution measurement of CO₂ and CH₄ concentrations. These measurements were complemented by a two-week integrated ¹⁴CO₂ sampling, followed by Accelerator Mass Spectrometry (LEA-AMS) analysis. This dual-tracer approach enables the separation of the Debrecen CO₂excess into fossil and biogenic components.

Our results highlight that the urban-derived excess varies dynamically relative to the regional background. The continuous mole fraction data reveal characteristic diurnal and seasonal patterns, with wintertime enrichment of CO₂ and CH₄, driven by reduced boundary layer mixing and increased heating demand. Analysis of CH₄:CO₂ correlations provides further insight into sector-specific emissions, distinguishing between traffic-dominated and heating-dominated periods. By combining high-frequency concentration measurements with isotopic constraints, our study provides a more precise understanding of the urban carbon cycle in a mid-sized city in Hungary, highlighting the importance of parallel urban-rural monitoring networks in verifying climate protection measures.