New and Improved Emissions Estimates of Ozone Depleting Substances and Their Replacement Compounds

The Montreal Protocol (MP) on Substances that Deplete the Ozone Layer controls the production and consumption of ozone depleting substances (ODSs) and their replacement compounds. Emissions of ODSs, e.g., chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), are key metrics of the evolution of the stratospheric ozone layer and its recovery. In light of the recent findings on MP-controlled substances (e.g., Liang et al., 2016; Lunt et al., 2018; Montzka et al., 2018; Rigby et al., 2019), there has been a growing need to monitor the transition of CFCs usage and the associated observationally derived emission estimates to the 1^stgeneration replacements (HCFCs), and subsequently the 2^ndgeneration replacements (HFCs). Understanding the temporal and spatial evolution of ODS emissions is key to determining whether the Montreal Protocol is effectively working. The comparison of these observationally derived emissions versus those estimated from the production and usage data reported to the MP provides valuable information to assess the compliance with the MP regulations.

We have developed a new technique using the NASA 3-D GEOSCCM model forced with surface mixing ratio observations from the Advanced Global Atmospheric Gases Experiment (AGAGE) ground-based network. This technique produces geographically and seasonally-resolved emission estimates of CFCs, HCFCs, and HFCs from the mid-1990s to present. The technique specifically exploits the observed surface mixing ratios to derive spatially and temporally-resolved emissions needed to balance each trace gas’ budget within the 3-D model simulation. This new technique eliminates a number of critical assumptions and parameterizations that have been traditionally used for emission estimates, while exploiting the model simulation of the chemical loss and transport processes of each gas. With this new technique, we will demonstrate the transition from CFCs to their replacement compounds in the last two decades (1995-2018) using AGAGE observations, and assess how our top-down emission results compare to the production and usage-based bottom-up emission estimates.