Contribution of Hydrofluorocarbons (HFCs) and Hydrofluoro-Olefins (HFOs) Atmospheric Breakdown Products to Acidification (“Acid Rain”) in the EU at Present and in the Future

While hydrogen fluoride (HF) and hydrogen chloride (HCl) are not considered main air-pollutants in the EU, they have the potential to contribute to acidification. Hydrofluorocarbons (HFCs), hydrofluoro-olefins (HFOs) and hydrochlorofluoro-olefins (HCFOs) are used as refrigerants and for other applications. They break down in the atmosphere to produce HF and HCl (for HCFOs) and some of these fluorocarbons also break down to produce trifluoroacetic acid (TFA). For the emissions of these fluorocarbons in the EU, a worst-case scenario estimates their theoretical potential contribution to acidification and compares it to the acidification potential for the main air pollutants contributing to acidification, which are nitrous oxides (NOx), sulphur oxides (mainly SO2), and ammonia (NH3). The Acidification Potential from these fluorocarbons in 2016 is estimated at <0.5% of the total due to SO2, NOx, NH3, and it can be concluded that this is insignificant in the context of the main acidification air-pollutants. Assuming that the EU targets for emissions of SO2, NOx and NH3 by 2030 are achieved, the Acidification Potential from HFCs, HFOs and HCFOs in 2030 is also estimated at <0.5% of the total due to SO2, NOx, NH3 and will remain insignificant.

ro-olefins (HCFOs) are used as refrigerants and for other applications. They break down in the atmosphere to produce hydrogen fluoride (HF) and hydrogen chloride (HCl, for HCFOs) and some of these fluorocarbons also break down to produce trifluoroacetic acid (TFA). While HF and HCl are not considered main air-pollutants in the EU, they are among the substances normally considered as potential contributors to acidification [1]: • sulfur dioxide (SO 2 ) • sulfur trioxide (SO 3  The main air pollutants contributing to acidification in the EU are nitrous oxides (NO x ), sulphur oxides (mainly SO 2 ), ammonia (NH 3 ) as they form acidic species in the atmosphere, which are then rained out ("acid rain"). SO 2 is used as basis for determination of the acidification potential (or the equivalency factor) for each substance. The method of establishing effect factors for acidifying substances is based on stoichiometric considerations and it is internationally accepted (see Table 1) [2].
For completeness, as TFA is an acidic breakdown product for some HFCs and HFOs, the equivalency factor for trifluoroacetic acid (TFA) is shown in Table 2 and, due to its high molecular weight, is significantly lower than those for HF and HCl.
The trend [3] in emissions of nitrous oxides (NOx), sulphur oxides (mainly SO 2 ), and ammonia (NH 3 ) in the EU and its targets [4] for 2030 are shown in   was developed to allow engineers and non-chemists to easily identify the refrigerant [6].
Acidification is regarded as a regional effect. Widely used HFCs have lifetimes in the range 1.6 years (HFC-152a,1,1-difluoroethane) to 51 years (HFC-143a, 1,1,1-trifluoroethane) for the most widely used HFCs and are well mixed in the atmosphere globally and their degradation does not occur on a regional basis.
However, for a worst-case scenario, it is assumed that their atmospheric breakdown is regional and that all the HFCs emitted in the EU breakdown within the EU boundaries, and their theoretical potential contribution to acidification estimated. The short atmospheric lifetimes of the HFOs and HCFOs means that their decomposition is mainly regional and the acid species, including HF [7], generated can be assumed to be deposited within the EU. For the HCFOs, HCl will also be produced as an atmospheric breakdown product.
The generation and fate of trifluoroacetic acid (TFA), although primarily a natural substance, from breakdown of fluorocarbons including HFCs and HFOs is the subject of a large number of studies [8]. This paper only considers TFA in the context of acidification. Carbon dioxide (CO 2 ) is also a breakdown product of some fluorocarbons, but the quantities are minute compared to the CO 2 from fossil fuels which contribute to the acidification of the sea [9] as well as global warming.

Other Sources of Atmospheric HCl and HF
Most of the HCl in the lower atmosphere (where acid rain is generated) is the result of acidification of sea salt aerosol [10]. This HCl therefore does not represent additional H + as it is simply a change of counter-ion from SO 4 to Cl.
HCl is also emitted to the atmosphere from volcanic sources. Fluorides are released into the environment naturally through the weathering and dissolution of minerals, in emissions from volcanoes and in marine aerosols.
Estimates of the annual global release of hydrogen fluoride from volcanic sources through passive degassing and eruptions range from 0.06 to 6 million tonnes/year. The wide distribution in soils means that there is significant natural movement of fluoride through the atmosphere on wind-borne dust particles (estimates vary from 1 to 10 million tonnes/year) [11] [12] [13].
Hydrogen fluoride (HF) and hydrogen chloride (HCl), are released to air from combustion of fuels [14] that contain trace amounts of fluoride or chloride.
Emissions of HF and HCl display a similar source pattern with significant emissions from the combustion of coal. As the importance of coal has decreased, the contribution of other sources has grown. Most significant of these is brickmaking as a source of HF. Solid fuels, and particularly coal, do contain chlorine and, as a result, the combustion of coal is responsible for HCl emissions [15], reduced by the installation of emission controls at coal-fired power stations. Of particular note, the burning of biomass and municipal solid waste (MSW) to generate heat and power contribute to emissions of HCl.

Emissions of HFCs, HFOs and HCFOs
Emissions years which means that the formation of HCl and HF is gradual and not regional and will fall over time as the "bank" is declining. The HCFCs were replaced by the HFCs which have been used in refrigeration, stationary and mobile air-conditioning, heat pumps, MDIs (metered dose inhalers), insulation foams and some fire-fighting systems since the early 1990s.

Emissions of HFCs
The emission data for HFCs is available as CO 2 e (calculated from the emitted tonnes and the global warming potential (GWP) of each HFC) and in 2016 HFC emissions were reported as 107,067 ktonnes CO 2 e. The emissions of HFCs (as CO 2 e) in the EU are shown in Figure 2. In 2017 about 90% of HFC emissions, as CO 2 e, were from Refrigeration and Air-Conditioning (RAC) [16], as shown in

Switch to Short Lived Hydrofluoro-Olefins (HFOs) and Hydrochlorofluoro-Olefins (HCFOs)
Under the EU F-Gas regulation (517/2014), the European Union is gradually reducing the supply of HFCs, weighted for their Global Warming Potential (GWP).
As the use of HFCs decreases due to the F-Gas Regulation, the use of HFOs and HCFOs is expected to continue to increase. Figure 4 shows the HCFCs, HFCs  potential. In 2025, the placing on the market quota for HFCs reduces to 31%, which will have a major effect on the refrigerants that can be made available under the phase-down cap. The reduction in the HFC cap is resulting in a switch to refrigerants with lower GWPs. Several of the lower GWP refrigerants that are being adopted as replacements for HFCs contain HFO-1234yf as a component (e.g. R-448A, R-449A, R-452A, R-452B, R-454C, R-455C, R-457C and R-513A).
Therefore there will be some emissions of HFO-1234yf from stationary refrigeration, air-conditioning and heat pumps (RACHP) as well as mobile air-conditioning (MAC). In addition, there will be increased installed equipment using HFC-32 (difluoromethane) instead of R-410A, and refrigerants containing HFC-143a (mainly R-404A) will have substantially been replaced. The naming convention for series 400 and series 500 refrigerants allow engineers and non-chemists to easily identify the refrigerant as they contain two or more components.
The composition of these refrigerants is available [19].

Atmospheric Formation of HF, HCl and TFA from HFCs/HFOs/HCFOs Emitted in the EU
The atmospheric chemistry of degradation of fluorocarbons is well documented [20]. The table shows the weighted average HF generation assuming that the HFCs breakdown to form HF exclusively rather than some minor quantities of trifluoroacetic acid (TFA) (from HFC-134a). This is a worst-case scenario for acidification potential as it generates the maximum moles of acid per mole of HFC. Table 3 is derived from available 2014 HFC placing on the market data [21] and shows the average maximum HF generation per tonne of HFC as 0.78 tonnes per tonne.
The actual total HFCs placed on the EU market is higher than shown in Table   3
The refrigerants adopted more widely in the period up to 2030 will be lower GWP single components and blends of some of the substances in Table 6. Some of these substances will also be used as propellants and foam blowing agents (and may also be blended with non-fluorocarbons). The trend is to use lower GWP refrigerants as replacements for R-404A, R-410A and R-134a (the most widely installed refrigerants). Table 7 shows the range of refrigerants that might be used as their replacements and these have lower acid generation.

Potential Contributors to Acidification/Acidification Potential
Acidification Potential (AP) is based on the contributions of SO 2 , NO x , HCl, NH 3 , HF and other acids (or acid precursors) make to the potential acid deposition, i.e. on their potential to form H + ions [26] and is calculated from the Equivalency Factor and the emissions of each substance.

Acidification Potential of Emissions of SO2, NOx, NH3 in the EU
The EU has published 2016 emissions [27] of SO 2 , NO x , NH 3 and targets for 2030 [4]. The Acidification Potentials of these substances are shown in Table 9.

Conclusions: Relative Acidification Potential of HFCs, HFOs and HCFOs
Assuming that the EU targets for 2030 are achieved allows a comparison of the Acidification Potential of HFCs, HFOs, and HCFOs emissions with the 2030 target emissions in addition to a comparison for emissions in 2016. Table 10 shows   Furthermore, the "2018 Progress towards the achievement of the EU's air quality and emissions objectives" [4] estimated that the further reduction in SO 2 emissions in order to comply with the NEC Directive requirements will resolve most of the threat of acidification of forest soils, and full implementation of additional reduction potentials would allow meeting the critical loads for acidification at 99.8% of all European forest areas. This supports the important conclusion that HFCs, HFOs and HCFOs emissions will make an insignificant contribution to acidification.

Disclaimer
The work and opinions expressed in this paper are those of the authors in a personal capacity