Trends in Atmospheric Abundance and Inferred Emissions of Ozone-Depleting Substances (Invited Presentation)

Ozone-depleting substances, such as chlorofluorocarbons (CFCs) and their replacements, are regulated under the Montreal Protocol (MP). This agreement has led to a decline in total atmospheric chlorine and bromine since the mid-1990s. Periodic assessments are carried out to inform the parties of recent scientific findings and provide an update on the effectiveness of the MP. The most recent assessment was completed in 2018. We will present recent trends in atmospheric abundances and inferred emissions of ozone-depleting substances from the latest assessment. Highlights include, 1) the MP has been effective in reducing emissions of ozone-depleting substances, 2) the rate of increase of the major hydrofluororocarbons (HCFCs) was slower than anticipated based on what was allowed under the MP, 3) emissions of CFC-11 are not declining as fast as anticipated, and have actually increased since 2013, 4) the global budget of CCl₄ is better understood owing to a better understanding of its global lifetime (related to revised ocean and soil sinks) and the discovery of previously unknown by-product and fugitive emission sources, and 5) although very-short-lived (VSL) chlorinated gases make up a small fraction of total atmospheric chlorine, the abundance of some VSL species is increasing, partially offsetting reductions in controlled species. For bromine, natural sources represent more than half the total bromine reaching the stratosphere.