Bayesian modeling of HFC production pipeline suggests growth in unreported CFC by-product and feedstock production

Abstract Observationally-derived emissions of regulated ozone depleting substances (ODSs) must be scrutinized to maintain the progress made by the Montreal Protocol in protecting the stratospheric ozone layer. Recent observations of three chlorofluorocarbons (CFCs), CFC-113, CFC-114, and CFC-115, suggest that emissions of these compounds have been higher than expected given global reporting. These emissions have been associated with hydrofluorocarbon (HFC) production, which can require CFCs as feedstocks or generate CFCs as by-products, yet emissions from these pathways have not been rigorously quantified. Here, we develop a Bayesian framework to jointly infer emissions of CFC-113, CFC-114, CFC-115, and hydrochlorofluorocarbon HCFC-133a during HFC-134a and HFC-125 production. We estimate that feedstock usage in HFC-134a production accounted for 86% (75--92%) and 62% (46--74%) of CFC-113 and CFC-114 emissions, respectively, from 2015--2019, while by-product generation during HFC-125 production accounted for 81% (68--92%) of CFC-115 emissions. Our results suggest that unreported feedstock production in Article 5 countries may explain the unexpected atmospheric growth rates of CFC-113 and CFC-114, although uncertainties within the chemical manufacturing processes call for further investigation and industry transparency. Nonetheless, this work demonstrates the environmental benefits of tightened ODS feedstock regulations and underscores the importance of the HFC production phasedowns scheduled by the Kigali Amendment.