First modeling of recent stratospheric radiocarbon levels
Creators: name: University Of California Office Of The President
Publisher: name: University of California Office of the President
publisherIdentifier: https://ror.org/00dmfq477
publisherIdentifierScheme: ROR
Year: 2024

description: Carbon-14 is produced is produced in the stratosphere and upper troposphere by nuclear reactions of atmospheric nitrogen with neutrons produced naturally by cosmic rays and by atmospheric nuclear bomb testing in the 1950s and early 1960s. This cosmogenic carbon is rapidly oxidized first to CO and then CO2, an inert gas which circulates throughout the stratosphere and troposphere; in the troposphere it can enter the oceans or terrestrial biosphere carbon reservoirs. As a result of its production and redistribution in the Earth system, particularly after the global-scale bomb "spike" in the atmosphere, 14CO2 is a unique geophysical and biogeochemical tracer. For example, the 14CO2 distribution in the stratosphere and the decay of the bomb 14CO2 signal yield insight into the stratospheric circulation and the rate of stratosphere-troposphere exchange that is independent of chemistry occurring there, unlike most other tracers. Likewise, its partitioning between the atmosphere, oceans, soils, plants and other carbon reservoirs, as well as its absence in fossil fuel-derived CO2, has been used to quantify the inventories, residence times, and gross fluxes of carbon in and between these reservoirs. Until recently, however, there were no observations of 14CO2 in the stratosphere after it purged itself of the bomb 14CO2, and, further, there are no observations using modern sampling and analysis techniques. This lack of information has greatly limited our ability to use 14CO2 as a stratospheric tracer and to validate models of the 14C production rate and transport to the troposphere needed for carbon cycle studies, particularly now that the natural cosmogenic production rate is playing and will continue to play an increasingly important role relative to the bomb radiocarbon input. We propose to collaborate with LLNL scientists to use their 3D atmospheric chemistry and transport model IMPACT (Integrated Massively Parallel Atmospheric Chemical Transport), along with recent measurements of 14CO2 made at LLNL's Center for Accelerator Mass Spectrometry on air samples we collected from high altitude balloon flights, to directly constrain the natural 14C production rate and stratospheric transport and residence times. Preliminary analysis of these new measurements on air samples collected in 2003, 2004, and 2005 suggests that 14C production rates change significantly enough due to natural changes in solar activity that this must be accounted for in order to accurately simulate current stratospheric 14CO2 levels and to accurately predict the transport rates to the troposphere. These new constraints and insights into stratospheric dynamics and natural 14C production rates will ultimately feed into both stratospheric ozone studies, that aim to predict the timing of the recovery of the ozone hole and the effect of climate change on ozone levels and the stratospheric circulation, and carbon cycle studies, that seek to quantify gross carbon fluxes between the atmosphere, oceans, and terrestrial biosphere. Thus, along with additional future measurements on already archived samples and samples that may be collected over the next few years in NASA field campaigns (under separate funding), this proposed collaboration between UC and LLNL will provide a significant, ongoing contribution to understanding and monitoring climate change and the impact of human activities on the atmosphere.