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UNDERSTANDING DIRECT GWPS AND INDIRECT EFFECTS FOR HYDROCARBON AND FLUOROCARBON REFRIGERANTS

08 October 2021

Recently there has been considerable focus on the potential breakdown products of HFCs and HFOs and if they have indirect GWPs. The IPPC AR6 Climate Change 2021 – The Physical Science Basis [1] includes GWPs for hydrocarbons, but what, if any, are the indirect effects for hydrocarbon and fluorocarbon refrigerants? Some indirect effects are included in the AR6 GWPs.

AR6 GWPs - what effects are included

The GWP of a refrigerant is its global warming impact relative to the impact of the same quantity of carbon dioxide over a 100-year period. The dominant contribution to GWP is the refrigerant’s direct radiative forcing.

For fluorinated refrigerants the magnitude of this effect depends on:

  • the number of C-F bonds in the molecule
  • its atmospheric lifetime which is determined by how rapidly it breaks down. HFOs and HCFOs breakdown very rapidly which means they have ultra-low GWPs.

The effect of a compound on climate is not limited to its direct radiative forcing. Ideally, emission metrics should include all indirect effects to be consistent, but limits to knowledge restrict how much can be included in practice [1]. AR6 GWPs include the direct radiative forcing effect of the compound and the indirect effect of the compound on the carbon cycle affecting atmospheric CO2 concentrations. Any agent that warms the surface perturbs the terrestrial and oceanic carbon fluxes, typically causing a net flux of CO2 into the atmosphere and hence further warming. See LEARN ABOUT…Climate change, global warming & HFCs Selecting and Using GWP values for Refrigerant 2021 September [2].

Indirect effects not included in AR6 GWPs: chemical reactions from emitted compounds can produce or destroy other greenhouse gases or aerosols. There could be indirect effects if the degradation products of the gas also behaved like greenhouse gases, or if the presence of the emitted gas and its degradation products affected the distribution of other greenhouse gases. Examples of these indirect effects include ozone formation or destruction, enhancement of stratospheric water vapour, changes in concentrations of the OH radical with the main effect of changing the lifetime of CH4, and secondary aerosol formation. These indirect effects are not included in the AR6 GWPs.

Hydrocarbon refrigerant indirect effects: hydrocarbon refrigerants influence the atmospheric radiative balance indirectly, as precursors for ozone (O3) and they can affect the oxidative capacity of the atmosphere and therefore alter the lifetime of methane (CH4). The magnitude of these effects has been quantified for some hydrocarbons [3]. The indirect GWP of propane is 9.5 due to indirect effects for CH4 lifetime (5.5 GWP) and as an O3 precursor (4 GWP). Similar effects occur for butane.

HFCs, HFOs and HCFOs degradation products: fluorinated refrigerants do not affect the oxidative capacity as much as hydrocarbons based on their lower Photochemical Ozone Creation Potential (POCP) values. The degradation of fluorocarbon refrigerants has extensively studied and the effects, if any, determined for the intermediate and final fluorinated degradation products. These are summarised in the table. From this data it can be concluded that the contribution to global warming for intermediate and final breakdown products is of no significance.

CO2 formed from breakdown of refrigerant: Degradation of refrigerants lead ultimately to the net production of atmospheric CO2. This yield may be less than 100% (on a molar basis) due in part to some of the reaction products being directly removed from the atmosphere or before being completely oxidised. Refrigerants produced from fossil fuel sources lead to additional fossil CO2 in the atmosphere from their degradation. This CO2 can already be included in carbon emission reporting totals so care needs to be taken if applying the fossil correction to avoid double counting [1].

The atmospheric degradation of propane is complex [4] and various intermediate carbonyl compounds formed, with acetone, acetaldehyde, and propionaldehyde among the most prominent. Final breakdown products from the main degradation pathways include CO2 but its yield from propane is less than 100% (i.e. less than 3 molecules of CO2 from each molecule of propane) as some of the reaction products will be directly removed from the atmosphere before being completely oxidised. As a reference point, methane generates a yield of CO2 estimated at 75%, due in part to the some of the reaction products (mainly formaldehyde) being directly removed from the atmosphere [5].

The CO2 yield from HFCs, HFOs and HCFOs depends mainly on the yield, if any, of TFA, which is removed from the atmosphere by wet and dry deposition, and it depends to a lesser extent on the formation of formic acid which is also removed from the atmosphere partly by the same processes.

 

References

[1] IPCC AR6 in - Climate Change 2021, The Physical Science Basis- Chapter 7.SM

[2] EFCTC LEARN ABOUT…Climate change, global warming & HFCs Selecting and Using GWP values for Refrigerant 2021 September. This includes AR6 GWP values and explains how GWPs are used and what has changed from AR5

[3] Lifetimes, direct and indirect radiative forcing, and global warming potentials of ethane (C2H6), propane (C3H8), and butane (C4H10), Atmos. Sci. Lett. 2018;19:e804. wileyonlinelibrary.com/journal/asl2 https://doi.org/10.1002/asl.804  Øivind Hodnebrog, Stig B. Dalsøren, Gunnar Myhre, Center for International Climate and Environmental Research-Oslo (CICERO), Oslo, Norway

[4] Claudette M. Rosado-Reyes and Joseph S. Francisco, Atmospheric oxidation pathways of propane and its by-products: Acetone, acetaldehyde, and propionaldehyde, Journal of Geophysical Research, Vol. 112, D14310, doi:10.1029/2006JD007566, 2007

[5] see IPCC AR6 in - Climate Change 2021, The Physical Science Basis- Chapter 7 section7.6.1.3

[6] WMO (World Meteorological Organization), Scientific Assessment of Ozone Depletion: 2018, Global Ozone Research and Monitoring Project–Report No. 58, 2018, Appendix A.

[7] Symonds RB, Rose WJ, Reed MH. 1988. Contribution of Cl- and F-bearing gases to the atmosphere by volcanoes. Nature 334:415-418.

[8] UNEP Environmental Effects Assessment Panel Summary Update 2020 for Policymakers, available at Environmental Effects Assessment Panel (EEAP) | Ozone Secretariat (unep.org)

[9] Jubb, A.M., McGillen, M. R., Portmann, R. W., Daniel, J. S., Burkholder, J. B.: An atmospheric photochemical source of the persistent greenhouse gas CF4, Geophys. Res. Lett., Volume 42, 2015, Pages 9505-9511, DOI: 10.1002/2015GL066193.

[10] Published evidence supports very low yields of TFA from most HFOs and HCFOs - Fluorocarbons

[11] Malisa S. Chiappero, Fabio E. Malanca, Gustavo A. Arguello, Steven T. Wooldridge, Michael D. Hurley, James C. Ball, Timothy J. Wallington, Robert L. Waterland, and Robert C. Buck J. Phys. Chem. A 2006, 110, 11944-11953, Atmospheric Chemistry of Perfluoroaldehydes (CxF2x+1CHO) and Fluorotelomer Aldehydes (CxF2x+1CH2CHO): Quantification of the Important Role of Photolysis.

[12] Refrigerant Degradation: Is HFC-23 (CF3H) formed due to the decomposition of HFOs and HCFOs in the troposphere? - Fluorocarbons

[13] WMO (World Meteorological Organization), Scientific Assessment of Ozone Depletion: 2010, Global Ozone Research and Monitoring Project–Report No. 52, 2010. Section 5.4.3.2

[14] D. B. Millet et al, A large and ubiquitous source of atmospheric formic acid, Atmos. Chem. Phys., 15, 6283–6304, 2015 www.atmos-chem-phys.net/15/6283/2015/ doi:10.5194/acp-15-6283-2015

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