An uncertainty estimate of a purely observational approach to derive hourly regional fossil fuel CO2 offsets (ΔCO2(foss)) at continental CO2 monitoring sites is presented. Weekly mean 14C-based fossil fuel CO2 mixing ratios and corresponding regional CO offsets (ΔCO) are proposed to determine weekly mean ΔCO/ΔCO2(foss) ratios in order to derive hourly ΔCO2(foss) mixing ratios from hourly ΔCO measurements. Respective regional model estimates of CO and CO2(foss) are applied to test this approach and obtain root mean square errors of the correspondingly determined regional hourly fossil fuel CO2 component. The method is further validated with campaign-based observations in Heidelberg. The uncertainty of the proposed method turns out to increase with decreasing fossil fuel CO2 fraction ranging from about 15% up to 40% for continental Europe. Together with the uncertainty of the ΔCO/ΔCO2(foss) ratio, which is determined by the precision of the 14CO2 measurement, this method is still more accurate and precise than any model-based approach.