Simultaneous measurements of near-surface CO ₂ and NO ₂ to monitor the fossil-fuel combustion-derived CO ₂ in the Tokyo megacity

Abstract. Year-round continuous measurements of near-surface carbon dioxide (CO2) concentrations using in-situ trace gas analyzers were conducted simultaneously with nitrogen dioxide (NO2) measurements by International Air Quality and SKY Research Remote Sensing Network (A-SKY) Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) at Chiba (35.625° N, 140.104° E, 60 m above sea level), located within the Tokyo megacity, Japan, during 2024. These simultaneous measurements revealed that CO2 concentrations were low on days when near-surface NO2 concentrations were markedly reduced. Furthermore, the CO2 enhancement relative to the baseline concentration determined based on such low-NO2-concentration days ([ΔCO2]N) was positively correlated with NO2 and black carbon concentrations. This finding indicates that [ΔCO2]N is useful in observing the increase in fossil-fuel combustion-derived CO2 within the Tokyo megacity. By employing this relatively simple method, CO2 concentrations in megacities can be monitored with high accuracy and precision, contributing to more effective emission mitigation strategies.