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Photochemical Ozone Creation Potential

What is photochemical smog

Some organic compounds, in the presence of sunlight and NOx, take part in ground-level ozone formation and thereby contribute to the deterioration in regional and urban air quality, with adverse effects on human health and environment. 

The atmospheric degradation of HFCs can contribute to tropospheric ozone formation, but their photochemical ozone creation potentials are very small, and the impact of HFCs on tropospheric ozone formation are still estimated to be negligible. A recent modelling analysis assessed the impact on ozone formation of commercially relevant ODSs and HFCs replacing HFOs and HCFOs, including HCFO-1233zd, and concluded that they too will have a small impact.[1]

The atmospheric chemistry of refrigerants (HFCs, HFOs, HCFOs and hydrocarbons) has been assessed for their impact on air quality through formation of tropospheric ozone (O3) on urban or regional scales. In contrast, to these refrigerants, the primary environmental effects of ammonia are its role in the formation of fine particles in the atmosphere and its deleterious impacts on aquatic ecosystems. The UNEP 2022 Assessment Report of the Environmental Effects Assessment Panel (EEAP 2022) [1] provides updated Photochemical Ozone Creation Potential (POCP) values for HFCs, HFOs, HCFOs and hydrocarbon, based on the most recent reactivity evaluations and estimated for both north-west European and United States urban reference conditions. It has been shown that POCP values for volatile organic chemicals (VOCs) can be rationalized in terms of their molecular structure and OH reactivity. These are used to provide index values on the same comparable scale and can be used as an approximate indicator of the potential impact on tropospheric ozone production from these substances.

In Europe, POCP is used to address long-range transboundary formation and transport of ozone.

The alkanes (ethane, propane and isobutane) and alkenes (ethene and propene) exhibit steadily increasing reactivities from ethane, which is unreactive, to propylene, which is highly reactive. Alkanes generate ozone efficiently on a multi-day scale.
In the USA, a compound may be excluded as a VOC as a result of scientific data that demonstrates its negligible effect on the formation of ground-level ozone. The EPA uses the reactivity of ethane as the threshold for determining whether a compound has negligible reactivity. Compounds that are less reactive than, or equally reactive to, ethane under certain assumed conditions may be deemed negligibly reactive and, therefore, suitable for exemption from the regulatory definition of VOC.

POCP values are also available for hydrocarbon refrigerants, with some values using the most recent evaluation methods. Widely used HFOs and HCFOs have POCP values that lie between those for methane and ethane. The POCP values for HFOs are generally larger than those for the analogous HFCs, but much smaller than those for the parent alkenes e.g. propene (R-1270 refrigerant). The POCP estimation method employed in EEAP 2022 [1] is based on structure and reactivity for the species and different parameters for each geographical region (Northwest European or urban USA), and is the method used for alkenes. Estimated photochemical ozone creation potentials (POCPE) calculated for North-West European conditions and USA urban conditions are listed in the Table.

ChemicalDesignationTotal Atmospheric LifetimePOCPE, North-west European conditionsPOCPE, USA urban conditions
Reference hydrocarbons
CH4Methane11.8 years0.60.2
CH3CH3Ethane58 days10.94.5
Hydrocarbon Refrigerants
CH3CH=CH2Propene R-12700.4 days110.6134.5
CH3CH2CH3Propane R-29015 days [a]13.6 [b]9 [c]
CH3CH(CH3)CH3Iso-butane R600a7 days [a]28 [c]20 [c]
HFCs
CH2F2HFC-325.4 years0.300.12
CF3CHF2HFC-12530 years0.020.01
CF3CH2FHFC-134a14 years0.060.02
CHF2CH3HFC-152a1.6 years0.700.27
CF3CHFCF3HFC-227ea36 years0.01<0.01
HFOs/HCFOs
CF3CF=CH2HFO-1234yf12 days7.324.23
E-CF3CH=CHFHFO-1234ze(E)19 days5.602.88
E-CF3CH=CHCF3HFO-1336mzz(E)122 days0.980.39
Z-CF3CH=CHCF3HFO-1336mzz(Z)27 days2.901.35
E-CF3CH=CHClHCFO-1233zd(E)42 days0.550.21

POCPE Table Explanatory notes:

In the United States, the EPA uses the reactivity of ethane as the threshold for determining whether a compound has negligible reactivity.
All data is from Montreal Protocol On Substances that Deplete the Ozone Layer UNEP 2022 Assessment Report of the Environmental Effects Assessment Panel (EEAP 2022) Chapter 6 SI Table 3, except where indicated. Unless otherwise noted, estimated total atmospheric lifetimes are those quoted in IPCC AR6

  • a. World Meteorological Organization (WMO). Scientific Assessment of Ozone Depletion: 2022, GAW Report No. 278, Annex Table A5. Available at https://ozone.unep.org/science/assessment/sap
  • b. Jenkin, M. E., Derwent, R.G., Wallington, T.J. (2017). Photochemical ozone creation potentials for volatile organic compounds: Rationalization and estimation. Atmospheric Environment 163, 128–137. https://doi.org/10.1016/j.atmosenv.2017.05.024 
  • c. Jenkin, M. E., Derwent, R.G., Wallington, T.J. (2017). Photochemical ozone creation potentials for volatile organic compounds: Rationalization and estimation. Atmospheric Environment 163, 128–137. https://doi.org/10.1016/j.atmosenv.2017.05.024  supplementary information Table S3

Switch to HFO-1234yf from HFC-134a has negligible impact on formation of tropospheric ozone

Atmospheric modelling studies of HFO-1234yf have shown that ozone production from HFO-1234yf is indistinguishable from that from ethane, and that replacing HFC-134a in vehicle air conditioning units with HFO-1234yf across the United States has a negligible impact (< 0.01 %) on the formation of tropospheric ozone. It is clear from the above, that the small increases in tropospheric ozone formation generated from a transition from HFC emissions to emissions of HFOs would not be of concern (EEAP 2022) [1]