Atmosphere-derived National Emissions of Ozone Depleting Substances and Substitutes for the United States

Abstract:

Chlorofluorocarbons (CFCs), halons, carbon tetrachloride (CCl₄), and methyl chloroform (CH₃CCl₃) are strong ozone-depleting substances (ODSs). Their production and consumption have been controlled by the Montreal Protocol since 1989 in developed countries and 1999 in developing countries. Although global atmospheric burdens of some of these gases have been declining for the last decade, their emissions continue due to releases from their existing reservoirs. Hydrochlorofluorocarbons (HCFCs) are transitional substitutes for CFCs; because they also deplete stratospheric ozone, they are also controlled by the Montreal Protocol. Hydrofluorocarbons (HFCs) are replacements for CFCs and HCFCs. Due to incomplete understanding of the reservoir size and emission rates for ODSs and their substitutes, uncertainty of their national emissions from inventory-based “bottom-up” estimates is undetermined.

In this study, we use our atmospheric observations from multiple surface sites and aircraft profiles across the continental US from 2008 to 2012, along with data from remote sites over the Pacific basin, to derive national emissions of ODSs and their substitutes using inverse modeling. The performance of our modeling framework and the sensitivity of derived emissions to prior fluxes and model-data mismatch errors were investigated by conducting a suite of synthetic-data experiments. Sensitivity of derived fluxes to boundary values and transport was explored in real-data inversions. Our preliminary results suggest that (1) US emissions of HCFC-22 and HCFC-142b are currently declining at faster rates than those reported by US EPA; (2) our emission estimate of HFC-134a, the most abundant HFCs in the atmosphere, is consistent with the estimate reported by US EPA, whereas our estimates for some currently minor HFCs (i.e. HFC-125 and HFC-143a) show no significant emission trends during 2008 - 2012, which is inconsistent with a 70 – 120 % increase over this period reported by US EPA; and (3) gases that are mainly used in refrigeration and air conditioning (i.e. HCFC-22 and HFC-134a) have significant seasonal variation in their national emissions whereas emissions of gases whose main applications are foam blowing and aerosol sprays (i.e. HCFC-142b, HFC-152a) do not vary seasonally.