Continuous observations of atmospheric ð³(O2/N2) and CO.sub.2 amount fractions have been carried out at Ryori (RYO), Japan, since August 2017. In these observations, the O.sub.2 Çë:ÇëCO.sub.2 exchange ratio (ER, -ðöy(O2)ðöy(CO2)-1) has frequently been lower than expected from short-term variations in emissions from terrestrial biospheric activities and combustion of liquid, gas, and solid fuels. This finding suggests a substantial effect of CO.sub.2 emissions from a cement plant located about 6Çëkm northwest of RYO. To evaluate this effect quantitatively, we simulated CO.sub.2 amount fractions in the area around RYO by using a fine-scale atmospheric transport model that incorporated CO.sub.2 fluxes from terrestrial biospheric activities, fossil fuel combustion, and cement production. The simulated CO.sub.2 amount fractions were converted to O.sub.2 amount fractions by using the respective ER values of 1.1, 1.4, and 0 for the terrestrial biospheric activities, fossil fuel combustion, and cement production. Thus obtained O.sub.2 and CO.sub.2 amount fraction changes were used to derive a simulated ER for comparison with the observed ER. To extract the contribution of CO.sub.2 emissions from the cement plant, we used y(CO2êù) as an indicator variable, where y(CO2êù) is a conservative variable for terrestrial biospheric activities and fossil fuel combustion obtained by simultaneous analysis of observed ð³(O2/N2) and CO.sub.2 amount fractions and simulated ERs. We confirmed that the observed and simulated ER values and also the y(CO2êù) values and simulated CO.sub.2 amount fractions due only to cement production were generally consistent. These results suggest that combined measurements of ð³(O2/N2) and CO.sub.2 amount fractions will be useful for evaluating CO.sub.2 capture from flue gas at carbon capture and storage (CCS) plants, which, similar to a cement plant, change CO.sub.2 amount fractions without changing O.sub.2 values, although CCS plants differ from cement plants in the direction of CO.sub.2 exchange with the atmosphere.