Measurement report: Method for evaluating CO2 emissions from a cement plant using atmospheric δ(O2 ∕ N2) and CO2 measurements and its implication for future detection of CO2 capture signals

Continuous observations of atmospheric δ(O2/N2) and CO2 amount fractions have been carried out at Ryori (RYO), Japan, since August 2017. In these observations, the O2 : CO2 exchange ratio (ER, -Δy(O2)Δy(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 CO2 emissions from a cement plant located about 6 km northwest of RYO. To evaluate this effect quantitatively, we simulated CO2 amount fractions in the area around RYO by using a fine-scale atmospheric transport model that incorporated CO2 fluxes from terrestrial biospheric activities, fossil fuel combustion, and cement production. The simulated CO2 amount fractions were converted to O2 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 O2 and CO2 amount fraction changes were used to derive a simulated ER for comparison with the observed ER. To extract the contribution of CO2 emissions from the cement plant, we used y(CO2∗) as an indicator variable, where y(CO2∗) is a conservative variable for terrestrial biospheric activities and fossil fuel combustion obtained by simultaneous analysis of observed δ(O2/N2) and CO2 amount fractions and simulated ERs. We confirmed that the observed and simulated ER values and also the y(CO2∗) values and simulated CO2 amount fractions due only to cement production were generally consistent. These results suggest that combined measurements of δ(O2/N2) and CO2 amount fractions will be useful for evaluating CO2 capture from flue gas at carbon capture and storage (CCS) plants, which, similar to a cement plant, change CO2 amount fractions without changing O2 values, although CCS plants differ from cement plants in the direction of CO2 exchange with the atmosphere.