Comment on acp-2022-570

Abstract

Continuous observations of the atmospheric O2 / N2 ratio and CO2 amount fractions have been carried out at Ryori (RYO), Japan since August 2017. In these observations, the O2 : CO2 exchange ratio (oxidative ratio (OR), −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 significant effect of CO2 emission 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 OR values for each of the incorporated CO2 fluxes, and then simulated OR values were calculated from the calculated O2 and CO2 amount fractions. 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 activity and fossil fuel combustion obtained by simultaneous analyses of observed O2 / N2 ratios and CO2 amount fractions and simulated ORs. We confirmed that the observed and simulated OR 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 O2 / N2 ratios 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.