PAOC Colloquium: Kristie A. Boering (UC Berkeley)

A Tale of Two Tracers: Radiocarbon and nitrous oxide isotopologues
and their sources and sinks from recent observations

Both CO2 and N2O are long-lived atmospheric trace gases that, coincidentally, become enriched in rare, heavy isotopes in the stratosphere followed by return to the troposphere, where the stratospheric enrichments are diluted by emissions of isotopically light CO2 (e.g., fossil fuel combustion) and N2O (e.g., microbial production). In this talk, I will focus on measurements of 14CO2 and of the oxygen and intramolecular nitrogen isotopic composition of N2O in the stratosphere and troposphere. For CO2, I will show how we use new observations in the stratosphere (e.g., Ref 1) to estimate empirically the global annual mean production rate of 14C by cosmic rays and the net 14C flux from the stratosphere to the troposphere useful for carbon cycle studies, as well as to monitor stratospheric residence times to see if they are changing in response to a predicted acceleration of the Brewer-Dobson Circulation as the climate warms. For N2O, I will show how stratospheric and tropospheric isotope observations reveal a ‘smoking gun’ for increases in agricultural emissions (similar to the Suess Effect for 14CO2) and how N2O isotope measurements can be used to distinguish between the influence of the stratospheric sink and oceanic and biospheric sources, demonstrating that isotope measurements can help in the attribution and quantification of surface sources in general (e.g., Ref. 2).
1. Kanu, A. M., L. L. Comfort, T. P. Guilderson, P. J. Cameron-Smith,D. J. Bergmann, E. L. Atlas, S. Schauffler, K. A. Boering, “Measurements and modeling of contemporary radiocarbon in the stratosphere,” Geophys. Res. Lett. 43, 1399–1406, doi:10.1002/2015GL066921, 2016
2. S. Park, P. Croteau, K. A. Boering, D.M. Etheridge, D. Ferretti, P. J. Fraser, K.-R. Kim, P.B. Krummel, R.L. Langenfelds, T.D. van Ommen, L.P. Steele, and C.M. Trudinger, “Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940,” Nature Geoscience 5, 261-265, doi:10/1038/NGEO1421, 2012.