Isotopic (Re)ordering Signatures of Stratospheric and Tropospheric O2

We present new clumped-isotope measurements of atmospheric O2 from the surface up to 32 km. The resulting Δ36 values, representing the abundance of 18O18O relative to a random distribution of isotopes, show that stratospheric and tropospheric air masses are easily discernable: Stratospheric air has higher Δ36 values than tropospheric air (i.e., more 18O18O), reflecting colder temperatures and higher ozone concentrations. The lower Δ36 values in tropospheric air reflect warmer temperatures, lower ozone concentrations, and the transport of high-Δ36 air from the stratosphere. These variations in atmospheric Δ36 values agree with predictions derived from a global 3-D chemical-transport model, which indicates that the spatial distribution of ozone and stratosphere-to-troposphere exchange govern observed variations in atmospheric Δ36 values on annual timescales.