Yields of perfluorocarboxylic acids from the atmospheric oxidation of Montreal Protocol related gases

Abstract

We present here a systematic evaluation of the molar yields of trifluoroacetic acid (TFA), perfluoropropanoic acid (PFPrA), and perfluorobutanoic acid (PFBA) from the atmospheric degradation of gases relevant to the Montreal Protocol and its Amendments. The yields are dependent on the molecular structure of the parent compound and the primary degradation products and radical intermediates formed. We incorporate new data into the Tropospheric Ultraviolet Visible (TUV) model and discuss how recent studies improve our understanding of the relative importance of the photochemical pathways for perfluoroaldehydes, C_xF_2x+1C(O)H, which are key degradation products from some chlorofluorocarbon (CFC) replacement compounds. We identify areas for further research that could advance our understanding of the environmental fate of precursors to short-chain length perfluorocarboxylic acids.

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Environmental significance

There is substantial scientific, technical, and policy interest in the sources and environmental impacts of trifluoroacetic acid (TFA) and other short chain perfluorocarboxylic acids (PFCAs), which belong to the broader group of chemicals known as per- and polyfluoroalkyl substances (PFAS). Quantifying the anthropogenic contribution to the past, present, and future PFCA loading in the environment requires robust estimates of yields from the atmospheric degradation of volatile fluorinated organic compounds. In this work, we present an updated and expanded evaluation of the molar yields of TFA, perfluoropropionic acid (PFPrA), and perfluorobutanoic acid (PFBA) in the atmospheric degradation of gases under the purview of the Montreal Protocol and other related gases.

1 Introduction

Perfluorocarboxylic acids (PFCAs) are a group of chemicals with the general formula C_xF_2x+1C(O)OH. PFCAs are part of the larger group of chemicals known as per- and polyfluoroalkyl substances (PFAS).^1,2 The ionized form of PFCAs (salts) are ubiquitous contaminants present in precipitation and in river, lake, and ocean water. Concerns have been raised that PFCAs are toxic and threaten global ecosystems and human health. It is well established that long-chain members of the group such as perfluorooctanoic acid (C₇F₁₅C(O)OH, PFOA) bioaccumulate and are hazardous. PFOA is regulated as part of national and international agreements.^3,4 However, short-chain members of the group such as trifluoroacetic acid (CF₃C(O)OH, TFA) do not biomagnify and have been shown to have low toxicity in experimental studies.^5,6 There is current research and regulatory interest in the concentrations of PFAS in the environment, anthropogenic sources, and effects on human and ecosystem health. Robust estimates of yields of PFCAs from the atmospheric degradation of volatile fluorinated organic compounds are essential for quantifying the anthropogenic contribution to the PFCA loading in environment in the past, present, and future.

Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), halons, perfluorocarbons (PFCs), hydrofluoroolefins (HFOs), hydrochlorofluoroolefins (HCFOs), hydrobromofluoroolefins (HFBOs), hydrofluoroethers and hydrochlorofluoroethers (HFEs/HCFEs) are classes of commercial fluorinated organic compounds. Most of these gases are either controlled, or are substitutes for compounds controlled, by the Montreal Protocol on Substances that Deplete the Ozone Layer and its amendments. These compounds have been released into the atmosphere in significant amounts.⁷ Some of these compounds can degrade to yield PFCAs.⁸ To assess the anthropogenic contribution to the historical, current, and future PFCA loading in the environment, the yields from the atmospheric degradation of volatile fluorinated organic compounds need to be established.⁹ Molar yields of TFA from 21 Montreal Protocol-related gases were evaluated by Madronich et al.¹⁰ The yields of two other short-chain acids, pentafluoropropionic acid (perfluoropropanoic acid, PFPrA), and heptafluorobutanoic acid (perfluorobutanoic acid, PFBA) were briefly discussed by Madronich et al.¹¹ These are the only PFCAs that can be formed from gases under the purview of the Montreal Protocol and its amendments or from related gases that are in use, but not currently controlled under the Montreal Protocol.⁷

The atmospheric fate of perfluorinated aldehydes is a critical factor in assessments of the yield of PFCAs in the degradation of Montreal Protocol related gases. Since the publication of the previous two assessments,^10,11 new research has been published on the photolysis^12–14 and reaction with OH radicals¹⁵ of CF₃C(O)H. We incorporate the new data into the Tropospheric Ultraviolet Visible (TUV) model to provide estimates for the importance of photolysis in the atmospheric fate of C_xF_2x+1C(O)H (x = 1–3). We discuss how the new studies improve our understanding of the relative importance of the photochemical pathways for perfluorinated aldehydes. We present an updated and expanded assessment of the molar yields of TFA, PFPrA, and PFBA from the atmospheric degradation of Montreal Protocol related gases, including CFCs, HCFCs, HFCs, PFCs, HFOs, HCFOs, HBFOs, HFEs, halons, alcohols, and ketones.

2 Mechanisms of formation of PFCAs

Fig. 1 shows several possible routes to the formation of PFCAs. The photochemical pathways depend on the molecular structure of the parent compound and the primary degradation products and radical intermediates formed. Compounds with a C_xF_2x+1CF-moiety (where for the purpose of this discussion of gases related to the Montreal Protocol x ≤ 3) can lead to the formation of C_xF_2x+1C(O)F as a degradation product.¹⁶ The fate of C_xF_2x+1C(O)F depends on whether it is formed in the troposphere or the stratosphere. In the troposphere, the sole fate of CF₃C(O)F is incorporation into water droplets followed by hydrolysis to give TFA (on a time scale of 5–15 days¹⁷). In the stratosphere, the main fate (≈90%) of CF₃C(O)F is photolysis¹⁸ with the remaining 10% transported to the troposphere where it is removed by hydrolysis.

Fig. 1 Key intermediates and degradation pathways leading to formation of TFA during atmospheric degradation of HCFCs, HFCs, HFOs, and HCFOs. Hydrolysis of trifluoroacetylfluoride (CF₃C(O)F) and trifluoroacetylchloride (CF₃C(O)Cl) leads to TFA in unity molar yield. Hydration of trifluoroacetaldehyde (CF₃C(O)H) and reactions of the trifluoroacylperoxyradical (CF₃C(O)O₂) can lead to formation of TFA. Approximate atmospheric lifetimes are indicated in parenthesis. In addition (not shown in figure), dissolution and hydrolysis of esters generated in the atmospheric oxidation of HFEs can lead to the formation of TFA. Similar mechanistic pathways lead to the formation of PFPrA and PFBA.

Compounds with a CF₃CCl-moiety can degrade to give CF₃C(O)Cl, which undergoes photolysis in competition with incorporation into water droplets. The tropospheric photolytic lifetime for CF₃C(O)Cl for an overhead sun is estimated to be 23 days.¹⁹ The atmospheric lifetime of CF₃C(O)Cl with respect to uptake and hydrolysis in cloud water is 5–30 days.¹⁷ On average, an estimated 60% of CF₃C(O)Cl in the troposphere is converted into TFA.²⁰ If CF₃C(O)Cl is formed as a degradation product in the stratosphere, its photolytic lifetime is approximately 16 days and conversion to TFA will be of little importance.⁸

Compounds with a perfluoroalkyl group can degrade to give perfluoroalkyl radicals. The sole atmospheric fate of alkyl radicals is addition of O₂ to give alkylperoxy radicals. Alkylperoxy radicals undergo self- and cross-reactions via the Russell mechanism.²¹ One channel of the Russell mechanism involves transfer of a hydrogen atom between peroxy radicals which gives alcohol and acyl products. For perfluoroalkylperoxy radicals this channel sets in motion a sequence of reactions leading to PFCAs. In sequence, C_xF_2x+1O₂ radicals react with α-hydrogen containing peroxy radicals (e.g., CH₃O₂) to give perfluoroalcohols (C_xF_2x+1OH) that eliminate HF to give perfluoroacylfluorides (C_x−1F_2x−1C(O)F) which then undergo hydrolysis to give PFCAs.²² Another channel of the Russell mechanism leads to the formation of alkoxy radicals. Linear perfluoroalkoxy radicals decompose rapidly by eliminating COF₂ resulting in a perfluoroalkyl radical with one less carbon atom than the parent, i.e., the perfluorinated chain “unzips”. The result in the atmosphere is the formation of a series of PFCAs in small yields (1–10%).²³

HFEs give fluorinated esters as intermediate degradation products. These are removed from the atmosphere by reaction with OH radicals and by dissolution in clouds and seawater. Hydrolysis of fluorinated esters can result in the formation of PFCAs.^24,25

Finally, compounds with a C_xF_2x+1CH-moiety (x ≤ 3) can give perfluoroaldehydes, C_xF_2x+1C(O)H, as a degradation product. These aldehydes are primarily formed from parent compounds whose atmospheric lifetimes are too short to allow transport to the stratosphere to any significant degree. Hence, the degradation of C_xF_2x+1C(O)H generated from Montreal Protocol related gases occurs mainly under tropospheric conditions. Recent studies have improved our understanding of the relative importance of the photochemical pathways leading to formation of PFCAs from C_xF_2x+1C(O)H. These are discussed in the following section.

Table 1 summarizes the estimated molar yields of PFCAs (TFA, PFPrA, and PFBA) from the primary degradation products and radical intermediates in the processes described above.

Table 1 Molar yields of PFCAs (TFA, PFPrA, and PFBA) from intermediates and radicals in the degradation of volatile fluorinated organic compounds relevant to the Montreal Protocol. An em dash (—) indicates that no formation (0% molar yield) of the acid is predicted

	Atmospheric fate	Lifetime^a	Yield estimates (%)			Effective PFCA yield^b (%)	Notes/references
			TFA	PFPrA	PFBA
a Stratospheric lifetime is a local not a global lifetime.b Effective PFCA yields include competitive direct (e.g. wet deposition/hydrolysis) and indirect sources (e.g., further reactions of radical intermediates) in the atmospheric oxidation of the individual species.c Loss process forms a product or radical intermediate that leads to the formation of TFA, PFPrA or PFBA.
Primary degradation products
CF3C(O)F	UV photolysis (strat. only)	≈100 days	—	—	—	TFA: 10 (strat.)–100 (trop.)	Jubb et al. (2015)^18
	Deposition/hydrolysis	5–15 days	100	—	—		Wallington et al. (1994)^17
CF3CF2C(O)F	UV photolysis (strat. only)^c	≈100 days	1–10	—	—	TFA: 0 (trop.)–<10 (strat.)	Jubb et al. (2015)^18
	Deposition/hydrolysis	5–15 days	—	100	—	PFPrA: 10 (strat.)–100 (trop.)	Wallington et al. (1994)^17
CF3CF2CF2C(O)F	UV photolysis (strat. only)^c	≈100 days	1–10	1–10	—	TFA: 0 (trop.)–<10 (strat.)	Jubb et al. (2015)^18
	Deposition/hydrolysis	5–15 days	—	—	100	PFPrA: 0 (trop.)–<10 (strat.)	Wallington et al. (1994)^17
						PFBA: 10 (strat.)–100 (trop.)
CF3C(O)Cl	UV photolysis^c	Strat: ∼16 days	≤10	—	—	TFA: < 10 (strat.)–60 (trop.)	Photolysis branching: CF3C(O) + Cl/CF3 + C(O)Cl
		Trop: 23 days	≤10	—	—		Tropospheric fate: Hayman et al. (1994)^20
	Deposition/hydrolysis	5–30 days	100	—	—
CF3C(O)H	OH reaction^c	23 days	20	—	—	TFA: <58	OH reaction: → CF3C(O)
	UV photolysis	3 days	—	—	—		∼2% CF3C(O) decomp
	Deposition/hydrolysis	≥2 days	≤ 100	—	—		Hurley et al. (2006)^26
							Photolysis: > 98% CF3 + HCO
C2F5C(O)H	OH reaction^c	23 days	1–10	10	—	TFA: <7, PFPrA: <28	OH reaction: → CF3CF2C(O)
	UV photolysis^c	0.8 days	1–10	—	—		∼52% CF3CF2C(O) decomp
	Deposition/hydrolysis	≥2 days	—	≤100	—		Hurley et al. (2006)^26
n-C3F7C(O)H	OH reaction^c	23 days	1–10	1–10	5	TFA: <8	OH reaction: → CF3CF2CF2C(O)
	UV photolysis^c	0.5 days	1–10	1–10	—	PFPrA: <8, PFBA:<20	∼81% CF3CF2CF2C(O) decomp
	Deposition/hydrolysis	≥2 days	—	—	≤ 100		Hurley et al. (2006)^26
CF3C(O)CF3	UV photolysis^c	27 days	20 ± 10	—	—	TFA: 20 ± 10	(→ CF3C(O) + CF3) Calvert et al. (2011)^19
CHF2C(O)CF3	UV photolysis^c	n/a	20 ± 10	—	—	TFA: 20 ± 10	(→ CF3C(O) + CF3) Calvert et al. (2011)^19
(CF3)2HCOC(O)F	Deposition/hydrolysis	n/a	<100	—	—	TFA: <100	Kutsuna et al. (2004)^25
C4F9OC(O)H	OH reaction^c	2 years	1–10	1–10	1–10	TFA: <5	Kutsuna et al. (2005).^24 OH reaction and hydrolysis assumed equally important
	Deposition/hydrolysis	n/a	—	—	100	PFPrA: <5, PFBA:<55
C2F5CH2OC(O)H	OH reaction^c	n/a	<7	<28	—	TFA: 4, PFPrA: 64	Kutsuna et al. (2005).^24 OH reaction and hydrolysis assumed equally important
	Deposition/hydrolysis	n/a	—	100	—
CF3CHFCF2OC(O)H	OH reaction^c	n/a	100	—	—	TFA: 100	Kutsuna et al. (2005).^24 OH reaction and hydrolysis assumed equally important
	Deposition/hydrolysis	n/a	100	—	—
n-C3F7OC(O)CH3	OH reaction^c	n/a	1–10	1–10	—	TFA: <5, PFPrA: <55	Kutsuna et al. (2005).^24 OH reaction and hydrolysis assumed equally important
	Deposition/hydrolysis^c	n/a	—	100	—
C4F9OC(O)CH3	OH reaction^c	n/a	1–10	1–10	1–10	TFA: <5	Kutsuna et al. (2005).^24 OH reaction and hydrolysis assumed equally important
	Deposition/hydrolysis	n/a	—	—	100	PFPrA: <5, PFBA:<55
CF3C(O)OCHF2	OH reaction^c	>1 year	<20	—	—	TFA: 100	Kutsuna et al. (2004, 2005).^24,25
	Deposition/hydrolysis^c	∼1 year	100	—	—		Hydrolysis assumed dominant
C2F5C(O)OCHF2	OH reaction^c	>1 year	<10	≈10	—	TFA: <10, PFPrA: <100	Sulbaek Andersen et al. (2024)^27
	Deposition/hydrolysis	∼1 year	—	100	—		Kutsuna et al. (2004, 2005).^24,25
CF3CHFCF2OC(O)CF2CF3	OH reaction^c	n/a	<110	—	—	TFA: 53, PFPrA: 50	Kutsuna et al. (2005).^24 OH reaction and hydrolysis assumed equally important
	Deposition/hydrolysis	n/a	<100	100	—
C3F7CF(OC(O)CH3)CF(CF3)2	OH reaction^c	n/a	100	—	100	TFA: <110	Kutsuna et al. (2005)^24
	Deposition/hydrolysis^c	n/a	<110	<10	<5	PFPrA: <10, PFBA: <5	Hydrolysis assumed dominant
C2F5CF(OC(O)H)CF(CF3)2	OH reaction^c	>2 years	100	100	—	TFA: 100, PFPrA: 50	Kutsuna et al. (2005).^24 OH reaction and hydrolysis assumed equally important
	Deposition/hydrolysis	n/a	100	—	—
C3F7CF(OC(O)H)CF(CF3)2	OH reaction^c	n/a	100	—	100	TFA: <110	Kutsuna et al. (2005)^24
	Deposition/hydrolysis^c	n/a	<110	<10	<5	PFPrA: <10, PFBA: <5	Hydrolysis assumed dominant
Radical intermediates
CF3CF2	O2/CH3O2 →→ TFA	µs	1–10	—	—	TFA: <10	Ellis et al. (2004)^22
	O2/NO →→ CF3 + CF2O + NO2		0	—	—
CF3CF2CF2	O2/CH3O2 →→ PFPrA, TFA	µs	1–10	1–10	—	TFA: <10, PFPrA: <10	Ellis et al. (2004)^22
	O2/NO →→ CF3CF2 + CF2O + NO2		—	—	—
CF3CF2CF2CF2	O2/CH3O2 →→PFBA, PFPrA, TFA	µs	1–10	1–10	1–10	TFA: <10	Ellis et al. (2004)^22
	O2/NO →→ CF3CF2CF2 + CF2O + NO2		—	—	—	PFPrA: <10, PFBA: <10
CF3C(O)	O2/HO2 →→ TFA	mins	39	—	—	TFA: 20 ± 10	∼2% CF3C(O) undergoes prompt decomp. Hurley et al. (2006)^26
	O2/NO →→ CF3 + CO2 + NO2		—	—	—		Sulbaek Andersen et al. (2018)^28
CF3CF2C(O)	O2/HO2 →→ PFPrA	mins	—	50	—	PFPrA: ≈10	∼52% CF3CF2C(O) undergoes prompt decomp. Hurley et al. (2006)^26
	O2/NO →→ CF3CF2 + CO2 + NO2^c		1–10	—	—
CF3CF2CF2C(O)	O2/HO2 →→ PFBA	mins	—	—	53	PFBA: ≈5	∼81% CF3CF2CF2C(O) undergoes prompt decomp. Hurley et al. (2006)^26
	O2/NO →→ CF3CF2CF2 + CO2 + NO2^c		1–10	1–10	—
CF3CHFO2	+ RO2 → CF3CHFO + RO + O2^c	sec–mins	25–81	—	—	TFA: 7–20	Wallington et al. (1996)^29
	+ NO → CF3CFHO + NO2^c → CF3 + HC(O)F + NO2		25–81	—	—		Wallington et al. (2017)^30
			0				- See also reaction below
CF3CHFO	+ O2 → CF3C(O)F + HO2^c	µs–ms	25–81	—	—	TFA: 25–81	Pressure/temperature dep
	+ Δ → CF3 + HC(O)F		0	—	—		Wallington et al. (2017)^30
CF3CF2HFO	+ O2 → CF3CF2C(O)F + HO2^c	µs–ms	—	100	—	TFA: <10, PFPrA: <1	Møgelberg et al. (1997)^31
	+ Δ → CF3CF2 + HC(O)F^c		1–10	—	—

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3 Fate of C_xF_2x+1C(O)H, x = 1–3 and the impact on formation of PFCAs

The formation of PFCAs from C_xF_2x+1C(O)H can occur through several different pathways. Reaction with OH radicals gives C_xF_2x+1CO radicals. This reaction is thought to be a minor sink for C_xF_2x+1C(O)H, with an estimated lifetime of approximately 23 days.¹⁵ At 298 K and one atmosphere of air 1%, 50%, and 79% (x = 1, 2, and 3) of C_xF_2x+1CO radicals decompose to give perfluoroalkyl radicals and CO. The remaining C_xF_2x+1CO radicals add O₂ to form acylperoxy radicals, C_xF_2x+1C(O)O₂.²⁶ These acylperoxy radicals then react with NO, NO₂, and HO₂, radicals. Reaction of C_xF_2x+1C(O)O₂ with NO gives perfluorinated acyloxy radicals which decompose by eliminating CO₂ to give C_xF_2x+1 radicals. Reaction of C_xF_2x+1C(O)O₂ with NO₂ gives perfluorinated peroxyacyl nitrates. Reaction of C_xF_2x+1C(O)O₂ with HO₂ radicals leads to the formation of C_xF_2x+1C(O)OH, e.g., (39 ± 4% TFA from CF₃C(O)O₂, at room temperature).^26,32 This reaction is likely temperature dependent. In the analogous reaction of CH₃C(O)O₂ radicals with HO₂, the channel giving CH₃C(O)OH increases in importance as the temperature decreases.³³ Hence, the reaction of C_xF_2x+1C(O)O₂ with HO₂ radicals may produce increased yields of PFCA at lower temperatures, but until experimental studies have confirmed this, we chose to use the reported room temperature data.

Perfluorinated peroxyacyl nitrates are relatively stable and can undergo long range transport, e.g., trifluoro peroxyacetylnitrate (CF₃C(O)O₂NO₂, FPAN) has a lifetime >1 month above altitudes of about 3 km.³⁴ Two recent studies have investigated the reaction of FPAN with water vapor to establish the importance of this reaction and the potential molar yield of TFA and/or FPAN hydrate:^35,36

CF₃C(O)O₂NO₂ + H₂O → CF₃C(O)OH + HOONO₂ (1)

CF₃C(O)O₂NO₂ + H₂O + M → CF₃C(O)O₂NO₂H₂O + M (2)

The formation of TFA was observed in both studies. An upper limit for the rate of the gas-phase reaction of FPAN with H₂O was established and it was concluded that this reaction is unlikely to be an important source of TFA (<1%).³⁶ Still, the importance of heterogeneous reactions of FPAN with water to the formation of TFA in the atmosphere are unclear. Similar uncertainty applies to analogous reactions of the larger perfluoro acyl peroxynitrates C_xF_2x+1C(O)O₂NO₂, x = 2 and 3. Thermal decomposition of FPAN would reform the CF₃C(O)O₂ radical. Thus, FPAN could represent a temporary reservoir for CF₃C(O)O₂ and could impact the distribution of CF₃C(O)O₂ in the atmosphere as a precursor for the formation of TFA (see further discussion below). Until further studies become available, we do not include hydrolysis of acyl peroxynitrates in our evaluation of the yield of PFCAs from C_xF_2x+1C(O)O₂.

The formation of PFCAs from C_xF_2x+1C(O)O₂ depends on the local environment and the atmospheric abundance ratio of NO to HO₂. The effective yield of TFA from CF₃C(O) in the troposphere was indirectly accessed by Sulbaek Andersen et al.²⁸ in a global modelling study of emissions of HCFO-1233(zd). In their work, the atmospheric lifetimes for CF₃C(O)H were 2 days and 20 days with respect to photolysis and reaction with OH radicals, respectively. With an overall TFA yield of 2%, the model results of Sulbaek Andersen et al. suggest that the molar yield of TFA in the reaction of OH radicals with CF₃C(O)H (equivalent to considering the yield of TFA from CF₃C(O)), is on the order of ∼20%. For CF₃CF₂C(O), Sulbaek Andersen et al.²⁷ evaluated the importance of the NO/HO₂ reactions and estimated a ∼10% yield of PFPrA from the reaction of HO₂ with CF₃CF₂C(O). Using the approach of Sulbaek Andersen et al.²⁷ the yield of PFBA from atmospheric processing of C₃F₇C(O) can be estimated as approximately 5%.

Long et al.³⁷ recently conducted a computational study and suggested that reaction with HO₂ radicals is an important sink for aldehydes in the atmosphere, incl. for CF₃C(O)H. For CF₃C(O)H, this reaction would produce a α-hydroxy substituted peroxy radical, which may undergo further reactions to yield TFA. Presently, there are no experimental data for this reaction, and its atmospheric importance remains unclear. Until experimental data becomes available, we have not included the reaction of HO₂ reaction with C_xF_2x+1C(O)H in our evaluation of the yield of PFCAs from C_xF_2x+1C(O)H.

Photolysis is a major tropospheric sink for C_xF_2x+1C(O)H and dominates in the stratosphere. While there have been few studies of the quantum yields and photolysis lifetimes of C₂F₅C(O)H and n-C₃F₇C(O)H, the photolysis of CF₃C(O)H has been extensively studied recently, largely due to concerns about photochemical production of HFC-23 (CHF₃).³⁸ The photolysis of CF₃C(O)H in the troposphere proceeds mainly by two channels:

CF₃C(O)H + hv → CF₃ + HCO (3a)

CF₃C(O)H + hv → CHF₃ + CO (3b)

Photolysis of CF₃C(O)H is not a source of TFA. Since the evaluation of TFA yields by Madronich et al.¹⁰ three papers have been published that provide new measurements of the quantum yields for CF₃C(O)H.^12–14 The results of these studies have been evaluated by the International Union of Pure and Applied Chemistry (IUPAC) Task Group on Atmospheric Chemical Kinetic Data Evaluation.³⁹ Estimates in the literature for the atmospheric lifetime for photolysis are in the range ∼2.5–13 days.^12,40 Using the updated photochemical parameters, we have reevaluated the photolysis lifetime of CF₃C(O)H. Atmospheric photolysis coefficients were computed with the Tropospheric Ultraviolet Visible (TUV) model version 5.4, using the pseudo-spherical 4-stream discrete ordinates radiative solver option. Wavelength-dependent data were gridded according to the WMO2 scheme,⁴¹ spanning the range 121–750 nm with enhanced resolution (1 nm) over 220–420 nm to resolve for the strong spectral gradient in tropospheric actinic flux due to stratospheric ozone absorption. The pressure dependence was estimated from the total quantum yields at atmospheric pressure (taken as NTP, normal temperature and pressure), with air number density M = 2.45 × 10¹⁹ molecule cm⁻³ and the assumption that the reciprocal of the yield decreases linearly to unity at zero pressure (Stern–Volmer) at all wavelengths for which the yield is non-zero. Calculations were made for the Equator and 40° N, 21st March, in 15 min steps and averaged over 24 hours. Atmospheric conditions included cloud-free, aerosol-free skies, 0.1 ground albedo, with an ozone column of 250 Dobson Units at the Equator and 350 Dobson Units for 40° N. For input in the TUV model (see insert in Fig. 2 and SI 1.1), two new data sets were compiled:

Fig. 2 Estimated average photolytic lifetime using TUV model of CF₃C(O)H (x = 1, blue trace/circles), C₂F₅C(O)H (x = 2, green trace/squares) and n-C₃F₇C(O)H (x = 3, red trace/triangles) from 0 to 20 km at equinox. The hatched horizontal area denotes the density-weighted average altitude of the troposphere of 4.5 km. The insert shows the quantum yields used (at one atmosphere of pressure). For CF₃C(O)H, linear interpolation is used between the IUPAC recommended pressure dependent quantum yields at 248, 266, and 281 nm (blue solid trace), with an exponential decrease to a long-wavelength limit set to 360 nm. The quantum yields used for C₂F₅C(O)H and C₃F₇C(O)H follow those of CF₃C(O)H, with a shift of 10 nm towards the red (red/green dashed trace). A Stern–Volmer pressure dependency was applied in the TUV for all three compounds. The insert also shows the UV absorbance spectra of CF₃C(O)H, C₂F₅C(O)H and C₃F₇C(O)H. See text for details.

(A) A linear interpolation in wavelength between the quantum yield datapoints provided by IUPAC (248, 266, 281 and 308 nm) with a Stern–Volmer pressure dependence.³⁹ Following the approach of Sulbaek Andersen et al. (2023), in this dataset we assume 335 nm for the long-wavelength limit,¹² consistent with the long-wavelength zero point for the quantum yield of CH₃C(O)H, which has a similar absorption spectrum as CF₃C(O)H.⁴²

(B) A linear interpolation between the quantum yield datapoints provided by IUPAC (248, 266 nm, QY = 1) to 281 nm, with an exponential decrease to a long-wavelength limit set to 360 nm. This approach assigns a more realistic wavelength dependence to the quantum yields at the longer wavelength region, which is particularly important for species that primarily undergo decomposition in the troposphere. The long-wavelength limit reflects the measured CF₃C(O)H UV spectrum (see insert in Fig. 2).

For models A and B, inclusion of a Stern–Volmer pressure dependence increases the modeled photolysis frequencies resulting in a 20–40% decrease in estimated photolytic lifetime, on average. The actinic flux in the troposphere limits the importance of photolysis at wavelengths below 300 nm. However, CF₃C(O)H has an absorption maximum at 300 nm, with significant absorption between 300 and 360 nm and modeled atmospheric lifetimes are highly sensitive to the assumed quantum yield fall-off and long-wavelength limit in the region between 308 and 360 nm.

Fig. 2 includes the trace of the resulting atmospheric photolysis lifetime for CF₃C(O)H. The troposphere extends to approximately 10 km altitude with a density weighted average altitude of approximately 4.5 km.⁴³ At this altitude the average lifetime due to photolysis of CF₃C(O)H in the lower atmosphere is approximately 3 days. The modeled photolysis frequencies using either model A or B produce near identical lifetime estimates (see SI Fig. 1.2). This can be explained by the fact that the derivative for the photolysis frequency with respect to wavelength (dJ/dλ, s⁻¹ nm⁻¹) is significantly larger for model A than model B in the range 308–330 nm, while an approximate equal, but opposite difference is observed for the range 335–360 nm, i.e., the long-wavelength limit region (see SI Fig. 1.3). The region below ∼300 nm, contributes very little to the photolytic lifetime of CF₃C(O)H. Notably, the slightly lower 308 nm quantum yield value used in the quantum yield fit by Sulbaek Andersen et al.¹² (QY = 0.16 vs. 0.19 (IUPAC³⁹)) has an amplified impact on the predicted atmospheric lifetime, as this is the regional peak for dJ/dλ.

Relatively few studies have been conducted on the atmospheric lifetime of the longer chain-length perfluoro aldehydes, C₂F₅C(O)H and n-C₃F₇C(O)H.^44,45 For C₂F₅C(O)H, Chiappero et al.⁴⁰ give annual averages for photolysis lifetimes of approximately 0.9 days at 11 km altitude and 2.5 days at 0 km, while Antinolo et al.⁴⁶ estimate a photolysis lifetime of 3.5 hours at 3.5 km altitude and solar zenith angle of 16° (summertime, noon, Spain). For n-C₃F₇C(O)H, Chiappero et al.⁴⁰ averaged their measurements for CF₃C(O)H and n-C₄F₉C(O)H and used the resulting wavelength independent photolysis quantum yield (0.11) to estimate annual averages for the atmospheric lifetimes of approximately 0.75 days at 11 km altitude and 2 days at 0 km. Solignac et al.⁴⁷ studied the photolysis of n-C₃F₇C(O)H in the EUPHORE chamber in Valencia, Spain and estimated a photolytic lifetime of close to one day.

It is likely that the photolysis mechanism for C₂F₅C(O)H and n-C₃F₇C(O)H will be similar to that of CF₃C(O)H. The peaks of the UV absorbance for C₂F₅C(O)H and n-C₃F₇C(O)H are nearly identical, and both shifted approximately 10 nm to the red in comparison to that of CF₃C(O)H (see insert in Fig. 2). Here, we use a self-consistent approach to model the photolytic lifetimes C₂F₅C(O)H and n-C₃F₇C(O)H, assuming that the wavelength-dependent quantum yields of C₂F₅C(O)H and n-C₃F₇C(O)H are equal to that of CF₃C(O)H, red-shifted by 10 nm, and following a Stern–Volmer pressure dependency as discussed above. As seen from Fig. 2, the atmospheric lifetimes with respect to photolysis for C₂F₅C(O)H and n-C₃F₇C(O)H are shorter than for CF₃C(O)H, and both less than 1 day (0.8 and 0.5 days at 4.5 km altitude, respectively).

Finally, in addition to photolysis (major sink) and reaction with OH radicals (minor sink), C_xF_2x+1C(O)H can undergo dry and wet deposition. On contact with liquid water, CF₃C(O)H reacts to give an aldehyde hydrate (gem-diol, CF₃CH(OH)₂).⁴⁸ The gem-diol, at least in the gas-phase, reacts with OH radicals (lifetime of approximately 90 days) and generates TFA.⁴⁸ Unfortunately, key physical parameters needed to estimate dry and wet deposition velocities for CF₃C(O)H are presently unavailable. Measurements have not been reported for the hydration equilibrium constant, the Henry's Law Constant (H), or the effective HLC (H*) for CF₃C(O)H. Nielsen et al.⁴⁹ empirically estimated H* > 10⁴ M atm⁻¹, which suggests a wet-deposition lifetime of a few days (4–8 days). Dry deposition may be competitive with wet deposition but is highly dependent on season and geographical location. This, and the absence of any experimental data for CF₃C(O)H, renders estimation of dry deposition velocities for CF₃C(O)H at least as uncertain as those estimated for wet deposition. The atmospheric lifetime of species such as HNO₃, which deposit without surface resistance, provides an upper limit for the importance of dry and wet deposition for CF₃C(O)H.⁵⁰ Dry deposition of HNO₃ occurs on a timescale of 2–3 days, wet deposition in precipitation is more rapid but episodic.⁵¹ We calculate the overall yield of TFA from CF₃C(O)H by assuming a 2-day lower limit for the deposition lifetime (i.e., upper limit for the importance of dry and wet deposition) and that hydration is efficient, converting CF₃C(O)H into TFA in a yield of unity (100%). We make the same assumption for the longer chain-length aldehydes, C₂F₅C(O)H and n-C₃F₇C(O)H.

Table 1 includes the estimated molar yields of PFCAs (TFA, PFPrA, and PFBA) from C_xF_2x+1C(O)H based on the assumptions and caveats for the evaluated atmospheric sinks described above. For CF₃C(O)H, based on the preceding analysis, we estimate an upper limit of <58% molar yield of TFA. The overall yield of TFA would be reduced to 27%, if the upper limit for the estimated wet-deposition lifetime (8 days) was used instead of a 2-day deposition lifetime. The parameterization of wet/dry deposition for C_xF_2x+1C(O)H remains a significant source of uncertainty in quantifying an upper limit for PFCA yields.

4 Conclusion

A systematic evaluation of the yields of TFA, PFPrA, and PFBA from the atmospheric degradation of gases relevant to the Montreal Protocol and its Amendments is presented. Table 2 lists the estimated overall yields of PFCAs from these compounds. These overall yields are obtained as the sum of the individual contributions from the yields of primary degradation products multiplied by their evaluated yields of PFCAs (see Table 1). Details for the yields of PFCAs for each compound are given in the SI Section 2. A reevaluation of the atmospheric photolytic lifetime for CF₃C(O)H, C₂F₅C(O)H, and n-C₃F₇C(O)H leads to revisions in the PFCA yields of many of the species listed in Table 2 from those reported previously.^10,11

Table 2 Molar yields of PFCAs (TFA, PFPrA, and PFBA) from volatile fluorinated organic compounds relevant to the Montreal Protocol, see SI for details^a

Compound	Formula	PFCAs molar yields (%)
		TFA	PFPrA	PFBA
a (—) indicates that no formation (0% molar yield) of the acid is predicted.b Compounds regulated under the Montreal Protocol and its amendments. Note that Halothane (a Halon) is exempt from regulation under the Montreal Protocol, but a switch by the healthcare industry to non-ozone-depleting alternatives means that emissions are declining.^52
CFCs^b
CFC-113a	CCl3CF3	<10	—	—
CFC-114a	CCl2FCF3	≈10	—	—
CFC-216ba	CClF2CClFCF3	≈10	—	—
HCFCs/halons^b
HCFC-124	CHClFCF3	100	—	—
HCFC-133a	CH2ClCF3	<59	—	—
HCFC-225ca	CF3CF2CHCl2	<10	—	—
HCFC-233fb	CCl2FCH2CF3	<58	—	—
HCFC-243fa	CHCl2CH2CF3	<58	—	—
HCFC-244fa	CHFClCH2CF3	<58	—	—
HCFC-253 fb	CH2ClCH2CF3	<58	—	—
Halon-2311 (Halothane)^b	CHBrClCF3	60 ± 10	—	—
HFCs^b
HFC-125	CHF2CF3	<10	—	—
HFC-134a	CH2FCF3	7–20	—	—
HFC-143a	CH3CF3	<58	—	—
HFC-227ea	CF3CHFCF3	100	—	—
HFC-236cb	CF3CF2CH2F	<10	<1	—
HFC-236ea	CHF2CHFCF3	≈100	—	—
HFC-236fa	CF3CH2CF3	20 ± 10	—	—
HFC-245fa	CHF2CH2CF3	<33	—	—
HFC-329p	CF3CF2CF2CHF2	<10	<10	<10
HFC-365mfc	CH3CF2CH2CF3	<53	—	—
HFC-43-10mee	CF3CHFCHFCF2CF3	54–60	54–60	—
HFEs/HCFEs
HFE-236ea2 (Desflurane)	CHF2OCHFCF3	<20	—	—
HFE-347mcf	C2F5CH2OCHF2	<10	≈10	—
HFE-347mmz1 (Sevoflurane)	(CF3)2C(O)HCH2F	<95	—	—
HFE-365mcf3	C2F5CH2OCH3	<6	<36	—
HFE-54-11mecf	CF3CHFCF2OCH2C2F5	<103	25	—
HFE-7100	C4F9OCH3	<5	<5	<55
HFE-7200	C4F9OC2H5	<5	<5	<55
HFE-7300	C2F5CF(OCH3)CF(CF3)2	100	50	—
HFE-7500	C3F7CF(OC2H5)CF(CF3)2	<110	<10	<5
1-Ethoxy-1,1,2,2,-3,3,3-hepta-fluoro-propane	C3F7OCH2CH3	<5	<55	—
HCFE-235da2 (Isoflurane)	CHF2OCHClCF3	≈98	—	—
HFOs/HCFOs/HBFOs
HFO-1225zc	CF2=CHCF3	<58	—	—
HFO-1234yf	CF3CF=CH2	100	—	—
HFO-1234ze(Z/E)	(E)-CHF=CHCF3	<58	—	—
HFO-1243zf	CH2=CHCF3	<58	—	—
HFO-1336mzz(E/Z)	(E)-CF3CH=CHCF3	<116	—	—
HFO-1345zfc	CH2=CHCF2CF3	<7	<28	—
HFO-1438mzz(E)	(E)-CF3CH=CHCF2CF3	<65	<28	—
HFO-1447fz	CH2=CHCF2CF2CF3	<8	<8	<20
HCFO-1233zd(E)	(E)-CF3CH=CHCl	<58	—	—
HCFO-1233zd(Z)	(Z)-CF3CH=CHCl	<58	—	—
2-Bromo-3,3,3-trifluoro-propene (2-BTP)	CF3CBr=CH2	<58	—	—
Alcohols/ketones/nitriles
2,2,2-Trifluoroethanol (TFE)	CF3CH2OH	<58	—	—
Perfluoro(2-methyl)-3-pentanone	CF3CF2C(O)CF(CF3)2	101–110	—	—
Heptafluorobutyro-nitrile	(CF3)2CFCN	≈100	—	—

---

As seen from inspection of the values in Tables 1 and 2, there are substantial uncertainties in many of the estimated yields. For many compounds, it is only possible to provide upper limits for PFCA yields. The uncertainties associated with the yield estimates have two origins. First, inherent uncertainties in the measurements of physical parameters e.g. rate coefficients, absorption cross sections, and quantum yields, which are typically 5–20% of the measured values and are relatively straightforward to quantify. Second, for PFCA formation pathways with few, or no, direct measurements available, e.g., wet deposition of CF₃C(O)H, the yield estimates are based on semi-empirical extrapolations. The uncertainties associated with such extrapolations are significant, difficult to estimate and, in many cases, we can only provide upper limits for PFCA yields.

Without knowledge of the fate of liquid phase CF₃C(O)H, or its hydrate, after uptake into liquid water and deposition to surface waters, it is difficult to properly evaluate this contribution to the yields of TFA. However, the likely importance of wet and dry deposition of CF₃C(O)H, and the possibility that aqueous chemistry of CF₃C(O)H/hydrates would be an effective source of TFA require that this sink be assessed. Further work to better understand the aqueous chemistry of CF₃C(O)H/hydrates; hydrolysis of FPAN; quantum yields for photolysis of C₂F₅C(O)H, and n-C₃F₇C(O)H at wavelengths and pressures relevant to the troposphere; and the atmospheric fate of fluorinated esters is also needed. Closing these knowledge gaps will significantly advance our understanding of the environmental fate of precursors to PFCAs such as TFA, PrPFA, and PFBA.

Author contributions

M. P. Sulbaek Andersen: conceptualization, formal analysis, investigation, methodology visualization, writing – original draft; T. J. Wallington: investigation, writing – review & editing; J. B. Burkholder: investigation, writing – review & editing; S. Madronich: investigation, writing – review & editing; M. L. Hanson: writing – review & editing; D. Van Hoomissen: writing – review & editing; K. R. Solomon: writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data supporting this article have been included as part of the supplementary information (SI). Supplementary information: SI 1 CF₃C(O)H, C₂F₅C(O)H and n-C₃F₇C(O)H quantum yields, UV spectra and lifetimes with respect to photolysis. SI 1.1 CF₃C(O)H quantum yields used for TUV models. SI 1.2 CF₃C(O)H lifetimes. SI 1.3 Spectral contributions to the photolysis coefficient of CF₃C(O)H using the two different quantum yield models (Models A and B). SI 1.4 TUV model data input (tabulated). SI 2 Estimated perfluorocarboxylic acid yields from compounds listed in Table 2. See DOI: https://doi.org/10.1039/d5ea00179j.

Acknowledgements

We thank Stefan Reimann for helpful discussions.

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