Simultaneous measurements of near-surface CO₂ and NO₂ to monitor the fossil-fuel combustion-derived CO₂ 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.