N2O as a tracer of mixing stratospheric and tropospheric air based on CARIBIC data with applications for CO2

Abstract Stratosphere–troposphere exchange (STE) affects distributions of trace gases, with ozone (O 3 ) and CO being commonly used to study mixing process in the UTLS (Upper Troposphere and Lower Stratosphere) region. Here we explore the application of N 2 O as a tracer of stratosphere to troposphere flux for CO 2 and its isotopes, using N 2 O and CO 2 mixing ratios including a set of CO 2 isotope data for CARIBIC aircraft samples and NOAA-Carbon Cycle flask samples. A frequency distribution of CARIBIC N 2 O data (mostly UTLS at mid- and high-latitudes) reveals a narrow distribution around a tropospheric maximum at nearly the same N 2 O mixing ratio as the distribution peak for the station Mauna Loa (MLO) however with a skewing due to STE. We demonstrate that upper-tropospheric and STE-affected air can be distinguished using a threshold value based on the N 2 O distribution width at MLO. A comparison with the use of O 3 is given. N 2 O is discussed to be a robust and linear (season and latitude independent) tracer of STE mixing proportions. We propose that CARIBIC data coupled with CO 2 isotopic data published for the stratosphere and data of NOAA stations can be used as a frame of reference for δ 18 O(CO 2 ) and Δ 17 O(CO 2 ) STE fluxes. The Δ 17 O(CO 2 ) flux is of particular interest as it is thought to constrain estimates of biosphere productivity, atmospheric oxygen cycle and global gross CO 2 fluxes. CARBIC data can also help tracing STE flux by deep stratospheric intrusions and a data analysis is given for the Walliguan (WLG) observatory on the Tibetan Plateau.