The Effects of Stratospheric Chemistry and Transport on the Isotopic Compositions of Long-Lived Gases Measured at Earth's Surface

Abstract:

The isotopic compositions of a number of long-lived gases in Earth's atmosphere, including those for carbon dioxide (δ¹⁸O, Δ¹⁷O, and Δ¹⁴C), nitrous oxide (δ¹⁵N, δ¹⁵N^α, and δ¹⁸O), methane (δ¹³C and δD), and molecular hydrogen (δD) undergo large changes in the stratosphere. These changes arise from the often unique photochemical isotope fractionation occurring there as well as the long residence times and mean ages of stratospheric air with respect to exchange with the troposphere of up to 5 years. Stratospheric air then returns to the troposphere and, in each case, can affect the isotopic composition of these gases measured at Earth's surface. In this work, we estimate the effect of stratospheric isotope fractionation on free tropospheric isotope compositions of CO₂, N₂O, CH₄, and H₂ on an annual and global mean basis. To do so, we calculate net isotope fluxes between the stratosphere and troposphere empirically from the correlation of the measured isotope compositions of these species with measured N₂O mixing ratios on whole air samples collected in the stratosphere from stratospheric aircraft and balloons coupled with independent information on the global, annually-averaged loss rate of N₂O. In each case, the effect is large enough to include in global models. In addition, we present arguments and evidence that deconvolving the stratospheric influence on surface measurements from source (or other) signals on higher spatial and temporal scales than ‘global’ and ‘annually-averaged’ is also necessary when using surface measurements of isotopic compositions to constrain the magnitudes and geographic distributions of the sources of these gases to the atmosphere.