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 (O3) and CO being commonly used to study mixing process in the UTLS (Upper Troposphere and Lower Stratosphere) region. Here we explore the application of N2O as a tracer of stratosphere to troposphere flux for CO2 and its isotopes, using N2O and CO2 mixing ratios including a set of CO2 isotope data for CARIBIC aircraft samples and NOAA-Carbon Cycle flask samples. A frequency distribution of CARIBIC N2O data (mostly UTLS at mid- and high-latitudes) reveals a narrow distribution around a tropospheric maximum at nearly the same N2O 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 N2O distribution width at MLO. A comparison with the use of O3 is given. N2O is discussed to be a robust and linear (season and latitude independent) tracer of STE mixing proportions. We propose that CARIBIC data coupled with CO2 isotopic data published for the stratosphere and data of NOAA stations can be used as a frame of reference for δ18O(CO2) and Δ17O(CO2) STE fluxes. The Δ17O(CO2) flux is of particular interest as it is thought to constrain estimates of biosphere productivity, atmospheric oxygen cycle and global gross CO2 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.