Caltech Young Investigators Lecture

Chlorofluorocarbons (CFCs) are harmful ozone depleting substances and greenhouse gases. Under the Montreal Protocol, their production has been phased out, preventing a world with catastrophic ozone loss. However, recent studies identified an unexpected emission of CFC-11 after 2012, in violation of the Protocol. Quantifying this emission has been limited by large uncertainties in atmospheric lifetimes of these gases as well as uncertainties of ongoing emissions from old equipment (i.e. "banks"). Published bank estimates have relied on one of two methods. The "top-down" method makes use of reported CFC production and estimated emissions derived from observed concentrations along with atmospheric lifetime assumptions. The "bottom-up" method relies on a careful tallying of bank equipment and their leakage rates over time. These different approaches are sensitive to underlying assumptions (such as lifetimes and production) and have produced wide ranging bank size estimates.

In this talk, I will present a new Bayesian probabilistic model that incorporates the widest range of constraints to date to jointly infer the size of CFC-11, 12 and 113 banks, their atmospheric lifetimes, and ongoing emissions. Results from this approach indicate that bank sizes of CFC-11 and CFC-12 are larger than recent international scientific assessments suggest, and can account for much of current estimated CFC-11 and 12 emissions (with the exception of increased emissions after 2012). Left unrecovered, these CFC banks could delay Antarctic ozone hole recovery by about six years and contribute 9 billion metric tonnes of equivalent CO2 emission. Results indicate that lifetimes of all three gases are likely shorter than current recommended values, suggesting that previous studies underestimate the magnitude of ongoing emissions. Our results indicate that bank emissions are decreasing faster than total emissions for all three gases. We estimate new, unexpected emissions (not from banks) during 2014-2016 were 19.6, 16.2, and 7.7 Gg/yr for CFC-11, 12 and 113, respectively. While recent studies have focused on unexpected CFC-11 emissions, our results call for additional investigation into potential sources of CFC-12 and CFC-113.

This talk is part of the Caltech Young Investigators Lecture Series, sponsored by the Division of Engineering and Applied Science.