Can we see differences between the C activities of urban (Zagreb) and rural (Cvetković) sites (central Croatia)?

Radiocarbon (ΔC) was measured for four years (2019-2022) in Zagreb (Croatia) and in Cvetković village near Jastrebarsko (Zagreb County, Croatia) to see whether there are differences between the city site and the rural one because of the fossil fuel combustion. The δC was measured as grab samples once in a month in period December 2020-November 2022. The bomb-produced C has been globally distributed across the planet, but the combustion of fossil fuels that do not contain C causes a local effect of lowering ΔC. Zagreb is considered to be a location with heavy fossil fuel combustion as compared to the Cvetković (rural site). Monthly C activity at Zagreb is constantly below the C activity at Cvetković. Mean C activity at Zagreb (ΔC = -18.4 ± 2.6 ‰) is lower than that in Cvetković (ΔC = -2.9 ± 2.1 ‰) due to fossil fuel combustion in the city of Zagreb. This is especially pronounced during winter when the mean value in Zagreb is ΔC = -26.0 ± 4.3 ‰ and in Cvetković ΔC = -5.9 ± 3.4 ‰. Natural gas consumption was used as the proxy for fossil fuel combustion, and it shows better correlation with ΔC in Zagreb than in Cvetković. The ΔC difference, ΔC ‒ ΔC, becomes statistically negligible when natural gas consumption is small. No difference is observed on δC; in Zagreb mean δC is -11.0 ± 1.3 ‰, and in Cvetković -11.4 ± 1.4 ‰. Lower δC values are observed in winter (Zagreb -11.9 ± 1.1 ‰, -12.2 ± 1.5 ‰ Cvetković) than in summer (-10.1 ± 0.8 ‰ vs. -10.4 ± 1.0 ‰) at both locations. Together with higher ΔC in Cvetković, it indicates that at the area of Cvetković biogenic samples of modern origin (biomass, wood) as energy source for heating is more pronounced.