2015 News & Events

Some New Refrigerant Compounds Break Down to Form Potent Long-lasting Greenhouse Gas

2 November 2015

kinetics lab
Lead author Aaron Jubb in the kinetics laboratory at CSD. Photo: Will von Dauster, NOAA

New CSD study shows atmospheric breakdown of some refrigerants forms CF4 (lifetime: 50,000 years).

Depletion of the ozone layer by long-lived chlorofluorocarbons (CFCs) that were used in refrigeration and air conditioning led to the 1987 international Montreal Protocol agreement and the development of new, more ozone-friendly compounds to replace CFCs. A new study shows that some of the replacement compounds break down in the atmosphere to form carbon tetrafluoride (CF4), which is a potent and effectively permanent greenhouse gas that persists in the atmosphere for about 50,000 years, far outlasting the replacement compounds themselves. The lead author of the CSD study, published November 2 online in Geophysical Research Letters, is Aaron Jubb, a former CSD scientist with the Cooperative Institute for Research in Environmental Sciences at the University of Colorado.

Carbon tetrafluoride 3D

The CF4 is formed during the atmospheric degradation of several widely used refrigerant halocarbons, such as HFC-134a (commonly used in automobile air conditioners), HCFC-124, CFC-114a, HFC-227ea, and HFO-1234yf. These CFC replacements are less damaging to the ozone layer and in some cases are removed more quickly from the atmosphere. But they all form trifluoroacetyl fluoride (CF3C(O)F) during breakdown, which is then further broken apart by solar ultraviolet radiation to form the long-lasting and potent greenhouse gas, CF4.

The breakdown of the refrigerants isn't a large source of CF4 compared to direct anthropogenic emissions of CF4, which is primarily emitted during the manufacture of aluminum. But this paper is a first demonstration of this particular source, and the authors note that it is "rather unusual for first generation end products to be formed that are much longer lived and stable than the source compound." It's a good illustration of how thorough studies are needed to fully characterize what happens to compounds once they are released into the atmosphere.

CSD research chemist Jim Burkholder was a guest on CPR's Colorado Matters the morning of November 17, talking about this study that finds some chemicals less damaging to the ozone layer can degrade to form long-lived greenhouse gases. Listen to the broadcast

Aaron Jubb (formerly NOAA ESRL CSD and CIRES, now at Oak Ridge National Laboratory), Max R. McGillen (NOAA ESRL CSD and CIRES), Robert W. Portmann (NOAA ESRL CSD), John S. Daniel (NOAA ESRL CSD), James B. Burkholder (NOAA ESRL CSD), An atmospheric photochemical source of the persistent greenhouse gas CH4, Geophysical Research Letters, doi:10.1002/2015GL066193, 2015.

Abstract

A previously uncharacterized atmospheric source of the persistent greenhouse gas tetrafluoromethane, CF4, has been identified in the UV photolysis of trifluoroacetyl fluoride, CF3C(O)F, which is a degradation product of several halocarbons currently present in the atmosphere. CF4 quantum yields in the photolysis of CF3C(O)F were measured at 193, 214, 228, and 248 nm, wavelengths relevant to stratospheric photolysis, to be (75.3 ± 1) x 10-4, (23.7 ± 0.4) x 10-4, (6.6 ± 0.2) x 10-4, and ≤0.4 x 10-4, respectively. A 2-D atmospheric model was used to estimate the contribution of the photochemical source to the global CF4 budget. The atmospheric photochemical production of CF4 from CF3CH2F (HFC-134a), CF3CHFCl (HCFC-124), and CF3CCl2F (CFC-114a) per molecule emitted was calculated to be (1–2.5) x 10-5, 1.0 x 10-4, and 2.8 x 10-3, respectively. Although CF4 photochemical production was found to be relatively minor at the present time, the identified mechanism demonstrates that long-lived products with potential climate impacts can be formed from the atmospheric breakdown of shorter-lived source gases.