Photochemical Ozone Creation Potentials (POCPs) for ultra-low GWP refrigerants
The photochemistry of ultra-low GWP (<30) [1] refrigerants (HFOs, HCFOs and hydrocarbons) could impact air quality through formation of tropospheric ozone (O3) on urban or regional scales. In contrast, 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. 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. EEAP 2022 [2] provides updated POCP values for HFOs and HCFOs, 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 values now 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 by EEAP 2022 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.
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).
Table: Estimated Photochemical Ozone Creation Potentials (POCPE)
| Chemical | Designation | Total Atmospheric Lifetime | POCPE, North-west European conditions (relative units) | POCPE, USA urban conditions (relative units) | |
| Reference hydrocarbons | |||||
| CH4 | Methane | 11.8 years | 0.6 | 0.2 | |
| CH3CH3 | Ethane | 58 days | 10.9 | 4.5 | |
| Hydrocarbon Refrigerants | |||||
| CH3CH=CH2 | Propene R-1270 | 0.4 days | 110.6 | 134.5 | |
| CH3CH2CH3 | Propane R-290 | 15 days [a] | 13.6 [b] | 9 [c] | |
| CH3CH(CH3)CH3 | Iso-butane R600a | 7 days [a] | 28 [c] | 20 [c] | |
| HFCs | |||||
| CH2F2 | HFC-32 | 5.4 years | 0.30 | 0.12 | |
| CF3CHF2 | HFC-125 | 30 years | 0.02 | 0.01 | |
| CF3CH2F | HFC-134a | 14 years | 0.06 | 0.02 | |
| CHF2CH3 | HFC-152a | 1.6 years | 0.70 | 0.27 | |
| CF3CHFCF3 | HFC-227ea | 36 years | 0.01 | <0.01 | |
| HFOs/HCFOs | |||||
| CF3CF=CH2 | HFO-1234yf | 12 days | 7.32 | 4.23 | |
| E-CF3CH=CHF | HFO-1234ze(E) | 19 days | 5.60 | 2.88 | |
| E-CF3CH=CHCF3 | HFO-1336mzz(E) | 122 days | 0.98 | 0.39 | |
| Z-CF3CH=CHCF3 | HFO-1336mzz(Z) | 27 days | 2.90 | 1.35 | |
| E-CF3CH=CHCl | HCFO-1233zd(E) | 42 days | 0.55 | 0.21 | |
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) SI Table 3, except where indicated.
- EEAP 2022 Annex Table A5
- 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
- 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
Atmospheric chemistry of hydrocarbons contributing to photochemical smog formation
The atmospheric chemistry of hydrocarbons such as propane and propene have been extensively investigated, with their degradation products contributing to the formation of photochemical smog in the presence of other pollutants (NOx) and depending on atmospheric conditions. The atmospheric degradation of propane is complex [3] and various intermediate carbonyl compounds are 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 [4].
Peroxyacetyl nitrate (PAN, CH3C(O)OONO2) is the principal member of a family of nitrogenous compounds produced by action of sunlight on NOx and reactive hydrocarbons. PAN is phytotoxic and shows an increasing role in human health effects due to ambient air exposure, especially in the presence of high ozone concentrations. Because of the similarity of the conditions required for their photochemical production PAN is observed in conjunction with elevated ozone concentrations. PAN has very low natural background concentrations so it is the very specific indicator of anthropogenic photochemical air pollution. Precursors of PAN in polluted areas are specific nonmethane hydrocarbons (NMHCs) (particularly propene, 1-butene, 2-butene, 2-pentene, etc.), aldehydes (formaldehyde, acetaldehyde) and NO2 [5]. Propane can contribute to the formation of PAN as acetaldehyde is an intermediate degradation product.
Environmental effects of ammonia
Ammonia plays a primary role in the formation of secondary particulate matter by reacting with acidic species such as nitric acid (HNO3) and sulfuric acid (H2SO4), to form ammonium (NH4+) containing aerosols, which constitute the major fraction of PM2.5 aerosols (particulate matter with a diameter < 2.5 μm) in the atmosphere. [6 - 8]. Ammonia emissions can lead to increased acid depositions and excessive levels of nutrients in soil, rivers or lakes, which can have negative impacts on aquatic ecosystems and cause damage to forests, crops and other vegetation. Eutrophication (excess nitrogen present in water bodies) can lead to severe reductions in water quality with subsequent impacts including decreased biodiversity, and toxicity effects [9].
References
[1] Montreal Protocol on Substances that Deplete the Ozone Layer, UNEP 2022 Report of the Refrigeration, Air-conditioning and Heat Pumps Technical Options Committee 2022 Assessment Table 3-1: Classification of 100-year GWP levels
[2] Montreal Protocol On Substances that Deplete the Ozone Layer UNEP 2022 Assessment Report of the Environmental Effects Assessment Panel (EEAP 2022).
[3] 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
[4] see IPCC AR6 in - Climate Change 2021, The Physical Science Basis- Chapter 7 section7.6.1.3
[5] Movassaghi et al. Chemistry Central Journal 2012, 6(Suppl 2):S8 http://journal.chemistrycentral.com/content/6/S2/S8
[6] Wang, S., et. al., Atmospheric ammonia and its impacts on regional air quality over the megacity of Shanghai, China, Sci Rep 5, 15842 (2015). https://doi.org/10.1038/srep15842
[7] Behera, S. N. & Sharma, M. Investigating the potential role of ammonia in ion chemistry of fine particulate matter formation for an urban environment. Sci. Total Environ. 408, 3569–3575 (2010).
[8] Updyke, K. M., Nguyen, T. B. & Nizkorodov, S. A. Formation of brown carbon via reactions of ammonia with secondary organic aerosols from biogenic and anthropogenic precursors. Atmos. Environ. 63, 22–31 (2012).
[9] Ammonia (NH3) emissions, European Environment Agency (EEA), Ammonia (NH3) emissions — European Environment Agency (europa.eu)