Signatures of cross-tropopause exchange seen in the oxygen anomaly in CO2

Stratospheric signals, such as oxygen anomaly in CO2, are distinct from those originated from the Earth's surface. The stratospheric anomaly in CO2 was explained as a result of the exchange reaction between CO2 and O3 via the excited state oxygen atom O(1D), produced by the UV photolysis of O3. The subsequent large scale circulation (Brewer-Dobson circulation) and synoptic eddy mixing bring the anomalous CO2 to the surface, providing another opportunity for studying biogeochemical cycles involving CO2. Here, we show that the oxygen anomaly in CO2 increases during the course of our sampling period (September 2013-February 2014), concordant with the strengthening of winter monsoon and subtropical jet system. The interaction between winter monsoon and jet system could efficiently enhance the vertical mixing between upper troposphere/lower stratosphere and the lower troposphere. Moreover, an attempt to the mid-troposphere CO2 anomaly is also made. We report for the first time the triple oxygen isotope composition of CO2 in the middle troposphere collected by the CARRIBIC phase one mission. A clear correlation of D17O in CO2 with N2O mixing ratios is observed which confirms the incursion of stratospheric air to the mid-tropospheric region. This is also supported by the compact correlation with O3 mixing ratios.