Oxygen anomaly in near surface carbon dioxide reveals deep stratospheric intrusion

Stratosphere-troposphere exchange could be enhanced by tropopause folding, linked to variability in the subtropical jet stream. Relevant to tropospheric biogeochemistry is irreversible transport from the stratosphere, associated with deep intrusions. Here, oxygen anomalies in near surface air CO₂ are used to study the irreversible transport from the stratosphere, where the triple oxygen isotopes of CO₂ are distinct from those originating from the Earth’s surface. We show that the oxygen anomaly in CO₂ is observable at sea level and the magnitude of the signal increases during the course of our sampling period (September 2013-February 2014), concordant with the strengthening of the subtropical jet system and the East Asia winter monsoon. The trend of the anomaly is found to be 0.1‰/month (R² = 0.6) during the jet development period in October. Implications for utilizing the oxygen anomaly in CO₂ for CO₂ biogeochemical cycle study and stratospheric intrusion flux at the surface are discussed.