Reaction between linear perfluoroaldehydes and hydroperoxy radical in the atmosphere: reaction mechanisms, reaction kinetics modelling, and atmospheric implications

Linear perfluoroaldehydes are important products formed in the atmospheric oxidation of industrial fluorinated compounds. However, their atmospheric lifetimes are incompletely known. Here, we employ high level quantum chemistry methods and a dual-level strategy for kinetics to investigate the reactions of C₂F₅CHO and C₃F₇CHO with HO₂. Our calculated results unveil almost equal activation enthalpies at 0 K for linear perfluoroaldehydes reaction with HO₂, indicating that the carbon chain length negligibly influences reaction thermodynamics. The calculated kinetics reveal that vibrational anharmonicity enhances rate constants by a factor of 3–10, while torsional anharmonicity reduces rate constants by 34 %–55 %. Additionally, we also find that the reaction of C₃F₇CHO with HO₂ exhibits significant pressure dependence, with transition pressures ranging from 0.026 to 2.3 bar across a temperature range of 190–350 K. Furthermore, atmospheric lifetimes of C₂F₅CHO and C₃F₇CHO are discussed based on the homogenous and heterogeneous processes. Our findings also reveal that the reactions of C₂F₅CHO and C₃F₇CHO with HO₂ radicals dominate over those with OH radicals in Russia, Malaysia, and parts of Africa by the calculated results in combination with data based on global atmospheric chemical model simulations. Under nighttime conditions, HO₂-initiated degradation represents a major atmospheric sink, comparable in magnitude to photolysis and Cl-initiated oxidation in gas phase, whereas hydrolysis at the air-water interface plays a critical role in the sink of linear perfluoroaldehydes. These findings establish chain-length-dependent pressure effects and conformational sampling as critical, previously unrecognized factors in kinetics calculations, providing a framework for modelling complex fluorotelomer transformations and guiding emission mitigation strategies.

1 Introduction

Poly- and perfluoroalkyl substances (PFASs) are highly fluorinated compounds with long atmospheric lifetimes, which have important influences on global warming potential (GWP) and environmental health (Ackerman Grunfeld et al., 2024; Rupp et al., 2023; Sznajder-Katarzyńska et al., 2019; Wu et al., 2024). During their degradation in the atmosphere, PFASs undergo complex chemical transformations, leading to the formation of linear perfluoroaldehydes. (Alam et al.,2024; Burkholder et al., 2015; David et al., 2021; Wang et al., 2021, 2024). Linear perfluoroaldehydes (C_nF_2n+1CHO) are significant intermediate compounds, which belong to the aldehydes family of PFASs (Li et al., 2024; Thackray et al., 2020). Chlorofluorocarbons (CFCs) and their temporary replacements, hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and hydrofluoroolefins (HFOs) are the important source of the linear perfluoroaldehydes (Burkholder et al., 2015; Hurley et al., 2006; Martin et al., 2005; Rand and Mabury, 2017; Wang et al., 2023; Waterland and Dobbs, 2007). For example, under low NO_x conditions, the reaction of OH radicals with the potential foaming agent CF₃(CF₂)₂CH=CH₂ (HFC-1447fz) leads to the formation of C₃F₇CHO (Jiménez et al., 2016; Yu et al., 2024). Furthermore, the atmospheric chemical processes of linear perfluoroaldehydes are of key importance for determining the atmospheric oxidation of fluorotelomer alcohols (FTOHs) (Antiñolo et al., 2012; Hurley et al., 2004).

Linear perfluoroaldehydes were generally considered to be removed through photochemical reactions (Chiappero et al., 2006; Sellevåg et al., 2004) and free radical reactions initiated by OH and Cl radicals (Andersen et al., 2004; Chiappero et al., 2010; Wang et al., 2007). Additionally, NO₃ may also contribute to their atmospheric degradation. (Burkholder et al., 2015; Ziemann and Atkinson, 2012). During the daytime, photolysis of linear perfluoroaldehydes was considered to be the dominant removal process for C_nF_2n+1CHO, with estimated atmospheric lifetimes ranging from hours to several days (Antiñolo et al., 2014; Chiappero et al., 2006). Antiñolo et al. (2014) reported that the photolysis lifetime of C₂F₅CHO is expected to be 3.5 h at 273 K, with the main degradation products of CF₃CFO and COF₂. In addition, previous investigations have shown that the length of the carbon chain in C_nF_2n+1CHO significantly affects the quantum yield of photolysis (Chiappero et al., 2006; Sellevåg et al., 2004). During the nighttime, the reactions of free radicals with C_nF_2n+1CHO were considered to be the major degradation pathways. However, previous studies reported relatively slow rate constants for the reaction between OH and C_nF_2n+1CHO (n=1–4) with the values of $(\mathrm{6.5}\pm \mathrm{1.2})\times {\mathrm{10}}^{-\mathrm{13}}$, $(\mathrm{5.57}\pm \mathrm{0.07})\times {\mathrm{10}}^{-\mathrm{13}}$, $(\mathrm{5.8}\pm \mathrm{0.6})\times {\mathrm{10}}^{-\mathrm{13}}$, and $(\mathrm{6.1}\pm \mathrm{0.5})\times {\mathrm{10}}^{-\mathrm{13}}$ cm³ molecule⁻¹ s⁻¹, respectively, at 298 K (Andersen et al., 2004; Antiñolo et al., 2014; Solignac et al., 2007). This corresponds to a longer atmospheric lifetime >20 d for these linear perfluoroaldehydes. Moreover, the rate constant of Cl atoms with C_nF_2n+1CHO (n=1–4) is approximately 2×10¹² cm³ molecule⁻¹ s⁻¹. This value is slightly faster than that of the corresponding OH radical reactions under similar conditions (Andersen et al., 2004; Sulbaek Andersen et al., 2003). The long atmospheric lifetimes of C_nF_2n+1CHO provide an opportunity for other atmospheric oxidation processes of C_nF_2n+1CHO by other atmospheric oxidants.

HO₂ radicalsare of ubiquitous active species in the atmosphere with the concentration being two orders of magnitude higher than that of OH radicals (Bottorff et al., 2023; Gao et al., 2024; Albrecht et al., 2019; Zhang et al., 2019, 2024, 2022) . Moreover, previous investigations have shown that the reactions of aldehydes with HO₂ affect the degradation process of aldehydes (Hermans et al., 2005; Albrecht et al., 2019; Zhou et al., 2024a). Additionally, global three-dimensional chemistry-transport model calculations suggest that the oxidation reactions of formaldehyde and acetone initiated by hydroperoxyl radical contribute to 30 % loss of formaldehyde and acetone at the tropical troposphere (Hermans et al., 2005). Nevertheless, the importance of sink pathway by HO₂ is still unknown because there have not been kinetics data for linear perfluoroaldehydes with HO₂ in the literature. Moreover, chain elongation may have influences on reaction kinetics due to multiple conformers. Furthermore, it is unknown for the pressure-dependent effects of larger perfluoroaldehydes with HO₂. Although our previous investigations have revealed the importance of CF₃CHO + HO₂ in the atmosphere (Long et al., 2022), their kinetics of larger perfluoroaldehydes with HO₂ are further required to investigate due to the unique features that depend on the specific reaction systems such as multi-structural anharmonicity and pressure effects in these complex systems. Additionally, it is a big challenge for addressing the larger perfluoroaldehydes with HO₂ because the computational cost grows very rapidly with system size, making such calculations impractical for high-level quantum chemistry methods.

In this article, we have investigated the reactions of HO₂ with linear perfluoroaldehydes C_nF_2n+1CHO (n=2–5), specifically focusing on C₂F₅CHO and C₃F₇CHO, referred to as Reactions (R1) and (R2) respectively. To delve into these reactions, high-level quantum chemistry calculation close to CCSDT(Q) accuracy in conjunction with dual-level strategy were performed to obtain their quantitative kinetics. Simultaneously, to provide further insight into kinetics, we detailly evaluated the impact of various parameters, including torsional anharmonicity, anharmonicity on the reaction kinetics over atmosphere-related temperatures and pressures. In addition, the chemical transformation of the formed intermediate products has been discussed in Reactions (R1) and (R2). We further estimate the enthalpies of activation at 0 K for the larger-sized reactions of longer-chain perfluoroaldehyde with HO₂. Moreover, we also discuss the importance of these Reactions (R1) and (R2) by combining the calculated reaction kinetics with global atmospheric modelling. The current results not only provide a comparative analysis with the kinetics of analogous OH-initiated reactions and photolytic processes, but also extend our understanding of the role of HO₂ in modulating the atmospheric lifetime of linear perfluoroaldehydes. This study not only resolves the knowledge gap regarding HO₂-initiated oxidation of linear perfluoroaldehydes but also establishes a computational strategy for predicting the atmospheric fates of long-chain PFAS derivatives. Our findings provide critical insights for refining emission control strategies and mitigating the environmental persistence of these compounds.

$$\begin{array}{}\text{(R1)}& {\displaystyle }{\mathrm{C}}_{\mathrm{2}}{\mathrm{F}}_{\mathrm{5}}\mathrm{CHO}+{\mathrm{HO}}_{\mathrm{2}}\to {\mathrm{C}}_{\mathrm{2}}{\mathrm{F}}_{\mathrm{5}}\mathrm{CH}\left(\mathrm{OH}\right)\mathrm{OO}\text{(R2)}& {\displaystyle }{\mathrm{C}}_{\mathrm{3}}{\mathrm{F}}_{\mathrm{7}}\mathrm{CHO}+{\mathrm{HO}}_{\mathrm{2}}\to {\mathrm{C}}_{\mathrm{3}}{\mathrm{F}}_{\mathrm{7}}\mathrm{CH}\left(\mathrm{OH}\right)\mathrm{OO}\end{array}$$

2 Computational methods and atmospheric modelling

2.1 Options for electronic structure density functionals

Our goal is to establish a precise set of electronic structure and kinetic calculation methods for the XCHO + HO₂ reaction, delivering satisfactory quantitative results (Long et al., 2022). This previous study indicated that the CCSD(T)-F12a/cc-pVTZ-F12//M06-2X/MG3S theoretical methods can make good agreement with beyond-CCSD(T) results for the similar reaction of HCHO + HO₂ (Long et al., 2022). Furthermore, CCSD(T)-F12a/cc-pVTZ-F12 has been shown good performance for molecules containing fluorine atoms (Dong et al., 2021; Long et al., 2022; Xia et al., 2024a). Consequently, we intend to utilize the well-validated methods in the present investigations in Reactions (R1) and (R2). Specifically, the M06-2X (Zhao and Truhlar, 2008b, a) density functional with the MG3S (Lynch et al., 2003) basis set was employed to optimize the geometries, while CCSD(T)-F12a (Adler et al., 2007; Knizia et al., 2009)/cc-pVTZ-F12 for R1 and R2 and FNO-CCSD(T)-F12 (Gyevi-Nagy et al., 2021; Taube and Bartlett, 2008)/cc-pVDZ-F12 for other C_nF_2n+1CHO + HO₂ (n=1–5) were used to calculate single-point energies. The FNO-CCSD(T) approach that significantly improves computational efficiency with cost reduction of up to an order of magnitude was utilized to calculate larger systems. Furthermore, intrinsic reaction coordinate (IRC) calculation was done to determine the correct transition states by examining the connections of each saddle point to its corresponding minima (Hratchian and Schlegel, 2004, 2005; Kenyon, 1968).

2.2 Vibrational frequencies

We found that standard scale factor is actually not applicable for some transition states in previous investigation, so we used two scale factors (Zheng et al., 2014, 2015). The standard scale factor for M06-2X/MG3S is 0.970. Furthermore, we also calculated the specific reaction scale factors to assess the effects of anharmonicity. The reaction-specific scale factors were obtained by using the MPW1K/6-311+G(2df, 2p) electronic structure method based on the hybrid degeneracy-corrected second-order vibrational perturbation theory (HDCVPT) (Bloino et al., 2012; Kuhler et al., 1996). This is necessary and effective for eliminating the activation enthalpy error caused by the standard scale factors and the results were listed in Tables S1 and S2 in the Supplement. This was obtained by Eq. (1),

$$\begin{array}{}\text{(1)}& {\mathit{\lambda }}^{\mathrm{SRP},\mathrm{ZPE}}={\mathit{\lambda }}^{\mathrm{Anh}}{\mathit{\lambda }}^{\mathrm{H}}\end{array}$$

where λ^Anh is the ratio of anharmonic zero-point vibrational energies (ZPE) to harmonic ZPE at the MPW1K/6-311+G(2df, 2p) level. λ^H is 0.983 for M06-2X/MG3S to correct harmonic frequencies. The result shows that the specific reaction scale factors are 0.955 for TS1 (see Table S1) and 0.956 for TS2 (see Table S1), which is a large deviation from the standard value of 0.970; this results in a decrease in calculated enthalpies of activation of 0.72 and 0.78 for TS1 and TS2 at 0 K, respectively. In addition, multi-structural torsional anharmonicity involving reactant and transition state were all calculated using MS-T method (multi-structural method for torsional anharmonicity) (Yu et al., 2012; Zheng et al., 2011; Zheng and Truhlar, 2013).

2.3 Kinetics calculations

The dual-level strategy was utilized to compute the high-pressure limit rate constant (Long et al., 2016, 2019; Xia et al., 2024a). As shown in Eq. (2), we integrated a conventional transition-state theory rate constant $k_{\mathrm{TST}}^{\mathrm{HL}}$ predicated on higher-level (HL, CCSD(T)-F12a/cc-pVTZ-F12//M06-2X/MG3S) inputs with transmission coefficients derived from direct dynamics at a lower level (LL, M11-L/MG3S), employing a specific density functional that is chosen from the results of benchmark calculations (see Table S3). We have incorporated both a recrossing transmission coefficient $k_{\mathrm{CVT}/\mathrm{TST}}^{\mathrm{LL}}$ and a tunneling transmission coefficient $k_{\mathrm{SCT}}^{\mathrm{LL}}$, as calculated through reaction-path variational transition state theory, with a particular emphasis on the canonical variational theory coupled with small-curvature tunneling (CVT/SCT) (Garrett and Truhlar, 1979; Liu et al., 1993; Truhlar et al., 1982). Additionally, a multi-structural transmission coefficient (F^MS-T) was introduced to this framework to cancel the errors caused by the multi-structural anharmonicity, thereby advancing our approach to the DL-MS-CVT/SCT method, which provides a detailed and multifaceted treatment of the rate constant calculation, and can effectively obtain quantitative kinetics.

$$\begin{array}{}\text{(2)}& k_{\text{MS-CVT/SCT}}^{\mathrm{DL}}=F^{\text{MS-T}}\times k_{\mathrm{TST}}^{\mathrm{HL}}\times k_{\mathrm{SCT}}^{\mathrm{LL}}\times {\mathrm{\Gamma }}_{\mathrm{CVT}/\mathrm{TST}}^{\mathrm{LL}}\end{array}$$

The pressure-dependent rate constants were done by employing the system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory in the temperature range of 190–350 K (Bao et al., 2016a, b; Bao and Truhlar, 2017). This method relies only on the high-pressure limiting rate constant that was calculated by the dual-level strategy. The computational details of pressure-dependent rate constants are presented in the Supplement.

2.4 Atmospheric modelling

We used GEOS-Chem 14.4.2 with a horizontal resolution of 2.0° × 2.5° to simulate space distribution of HO₂ and OH at 47 vertical layers in the period from February 2018 to February 2019 (Bey et al., 2001). The time includes six months of spin-up and output per hour. GEOS-Chem is a global, three-dimensional chemical transport model associated with atmospheric composition (http://geos-chem.org, last access: 16 October 2025). Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) (Gelaro et al., 2017) was used as meteorological field data and Harmonized Emissions Component (HEMCO 3.9) was used as the source of emissions data (Lin et al., 2021). The emissions include biogenic emissions from Model of Emissions of Gases and Aerosols from Nature (MEGANv2.1) (Hu et al., 2015; McDuffie et al., 2020) and anthropogenic emissions from the global Community Emissions Data System (CEDS) (McDuffie et al., 2020) inventory. Simulation uses default full chemistry mechanism including HO_x-NO_x-VOC-O₃-halogen chemistry, which is done by our previous investigation (Bloss et al., 2007).

2.5 Software

All density function calculation, including Zero-point energy (ZPE) correction were carried out using Gaussian 16 software package (Zhao and Truhlar, 2008b) and the single point energy calculations for CCSD(T)-F12a/cc-pVTZ-F12 and FNO-CCSD(T)-F12a/cc-pVDZ-F12 were done using Molpro 2019 (Werner et al., 2019) and MRCC code (Kállay et al., 2020, 2022). MS-T method was executed through MSTor-2023 program package (Chen et al., 2023). The rate constants were done with the KiSThelP (Canneaux et al., 2014) Polyrate 2017-C (Zheng et al., 2017) and Gaussrate 2017-B (Zheng et al., 2018).

3 Results and discussion

3.1 The electronic structure of the ${\mathrm{C}}_{\mathrm{2}}{\mathrm{F}}_{\mathrm{5}}\mathrm{CHO}/{\mathrm{C}}_{\mathrm{3}}{\mathrm{F}}_{\mathrm{7}}\mathrm{CHO}+{\mathrm{HO}}_{\mathrm{2}}$ reaction

We considered the C₂F₅CHO/C₃F₇CHO + HO₂ reaction similar to the reactions of aldehydes with HO₂ (Long et al., 2022). The dominant mechanism is that the hydrogen atom of HO₂ is transferred to the terminal oxygen atom of ${\mathrm{C}}_{\mathrm{2}}{\mathrm{F}}_{\mathrm{5}}\mathrm{CHO}/{\mathrm{C}}_{\mathrm{3}}{\mathrm{F}}_{\mathrm{7}}\mathrm{CHO}$, and simultaneously, the oxygen atom of HO₂ is connected to the carbon atom of carbonyl group of ${\mathrm{C}}_{\mathrm{2}}{\mathrm{F}}_{\mathrm{5}}\mathrm{CHO}/{\mathrm{C}}_{\mathrm{3}}{\mathrm{F}}_{\mathrm{7}}\mathrm{CHO}$. Figure 1 depicts ZPE corrected potential energy profile of the reaction of ${\mathrm{C}}_{\mathrm{2}}{\mathrm{F}}_{\mathrm{5}}\mathrm{CHO}/{\mathrm{C}}_{\mathrm{3}}{\mathrm{F}}_{\mathrm{7}}\mathrm{CHO}$ + HO₂ at the CCSD(T)-F12a/cc-pVTZ-F12//M06-2X/MG3S level. ${\mathrm{C}}_{\mathrm{2}}{\mathrm{F}}_{\mathrm{5}}\mathrm{CHO}/{\mathrm{C}}_{\mathrm{3}}{\mathrm{F}}_{\mathrm{7}}\mathrm{CHO}$ and HO₂ form reaction complexes RC1/RC2, and then pass through transition states TS1 and TS2 to form the intermediate products of C₂F₅CH(OH)OO (M1) and C₃F₇CH(OH)OO (M2), respectively. TS1 (−2.7 kcal mol⁻¹) denotes the global minimum optimized structures of the stationary points of enthalpy of activation at 0 K; this is 0.3 and 1.6 kcal mol⁻¹ lower than that of the CF₃CHO + HO₂ and CH₃CHO + HO₂ reactions, respectively (Long et al., 2022). This shows that the reaction of perfluoroaldehydes with HO₂ may be kinetically feasible. As a comparison, the further energy profile of the C₃H₇CHO + HO₂ reaction shows an equal enthalpy of activation of −2.7 kcal mol⁻¹ for TS2; this is slightly lower than that of enthalpy of activation −2.4 kcal mol⁻¹ for the reaction of CF₃CHO + HO₂ (Gao et al., 2024).

Figure 1 Enthalpy profile of the HO₂ addition reaction with C₂F₅CHO and C₃F₇CHO as calculated by CCSD(T)-F12a/cc-pVTZ-F12//M06-2X/MG3S level with the scale factor by the standard method at 0 K.

It is noteworthy that Fig. 1 only depicts the potential energy profile of the reaction featuring the global minimum structure. Nevertheless, the internal rotation of the C–C bond produces multiple conformers for reactants, transition states, and formed intermediate products. Their geometric configurations and energy distributions relative to the global minimum structure are presented in Fig. S1. Regarding the reactions of C₂H₅CHO and C₃H₇CHO with HO₂, we have observed that as the carbon chain lengthens, the number of conformers of both reactants and transition states increases, and the energy distribution broadens. For instance, TS1 has three isomers, with an energy distribution spanning from 0 to 1.7 kcal mol⁻¹, whereas TS2 has five isomers, and its energy distribution ranges from 0 to 1.9 kcal mol⁻¹. In terms of geometric configurations, the low-energy isomers tend to have more linear structures, while the high-energy conformations exhibit more pronounced curling.

NO is a highly reactive gas (Lee et al., 2024). Human activities, especially agriculture and industrial processes, have led to significant NO emissions (Andersen et al., 2024; Thomson et al., 2012). Industrial activities contribute to NO levels such as fossil fuel combustion in power plants and chemical manufacturing, along with vehicle emissions. Given its prevalence from human-induced emissions, we further explore the degradation pathways of intermediate products M1 and M2 in the presence of NO. As depicted in Fig. S2, M1 and M2 undergo initial reactions with NO to yield the products C₂F₅CH(O)OH, C₃F₇CH(O)OH, and NO₂, exhibiting activation enthalpies of −9.9 and −11.5 kcal mol⁻¹ at 0 K, respectively. These results are consistent with previous studies on similar reactions involving RO₂+ NO. (Berndt et al., 2015; King et al., 2001; Nie et al., 2023; Orlando et al., 2000; Vereecken and Peeters, 2009). These products then undergo unimolecular reactions to decompose into C₂F₅ and C₃F₇ radicals and formic acid in Fig. 2. Notably, the unimolecular decomposition of C₂F₅CH(O)OH and C₃F₇CH(O)OH represents the rate-determining step of the overall reaction, with corresponding activation enthalpies of 5.6 and 4.7 kcal mol⁻¹ (0 K), respectively; this indicates that formic acid may potentially be formed via ${\mathrm{C}}_{\mathrm{2}}{\mathrm{F}}_{\mathrm{5}}\mathrm{CHO}/{\mathrm{C}}_{\mathrm{3}}{\mathrm{F}}_{\mathrm{7}}\mathrm{CHO}$ + HO₂ in the presence of high concentration NO in the atmosphere. Additionally, the formed intermediate products (M1 and M2) are a typical class of RO₂ radicals. In the low NO_x levels, these RO₂ radicals can also participate in bimolecular reactions (Ding and Long, 2022). RO₂ can react with HO₂, resulting in the formation of the stable product ROOH. Moreover, RO₂ can react with other RO2 or R^′O₂ (where R^′ denotes a hydrocarbon fragment) (Bottorff et al., 2023). The reaction with R^′O₂ frequently yields alkoxy radicals, and both of these reactions are capable of producing stable products (Goldman et al., 2021). Due to the complexity of these bimolecular reactions of the formed RO₂ in the Reactions (R1) and (R2), we did not further investigate their reaction mechanisms and kinetics in the present work.

Figure 2 Relative enthalpies at 0 K for the decomposition of C₂F₅CH(O)OH (M3) and C₃F₇CH(O)OH (M4) calculated by CCSD(T)-F12a/cc-pVTZ-F12//M06-2X/MG3S.

Figure 3 The impacts of perfluorinated carbon length on the enthalpies of activation at 0 K in the C_nH_2n+1CHO/C_nF_2n+1CHO + HO₂ reactions. The values for C_nH_2n+1CHO (n=1–5) + HO₂ and C_nF_2n+1CHO + HO₂ are obtained from references (Ding and Long, 2022; Gao et al., 2024) and calculated by using FNO-CCSD(T)-F12a/cc-pVDZ-F12.

We further conducted an extended study on the reactions of C₄F₉CHO and C₅F₁₁CHO at the FNO-CCSD(T)-F12//cc-pVDZ-F12//M06-2X/MG3S level, aiming to investigate the effects of increasing carbon chain length on the enthalpy of activation at 0 K. The calculated results show a deviation of only 0.2 kcal mol⁻¹ in activation enthalpy at 0 K between FNO-CCSD(T)-F12//cc-pVDZ-F12 (−2.6 kcal mol⁻¹) and CCSD(T)-F12a/cc-pVTZ-F12 (−2.4 kcal mol⁻¹) in CF₃CHO + HO₂, validating the robustness of FNO-CCSD(T)-F12//cc-pVDZ-F12 for complex fluorinated systems. Data from Fig. 3 reveal an interesting phenomenon that the activation enthalpy at 0 K remains almost equal C₂ (C₂F₅CHO) to C₅ (C₅F₁₁CHO). This finding aligns with the similar trend for the reaction of C_nH_2n+1CHO with HO₂, suggesting that the impact of carbon chain length growth on the enthalpy of activation at 0 K is quite minor (Ding and Long, 2022; Gao et al., 2024). However, the introduction of CF₃ leads to a relatively lower enthalpy of activation at 0 K for the C_nH_2n+1CHO + HO₂ reactions, primarily due to the strong electron-withdrawing ability of fluorine atoms, which can stabilize the transition state and lower the enthalpy of activation at 0 K. As the size of perfluoroaldehyde increases, the multi-structure effects caused by torsion of C-C bonds become more pronounced. The relative energy values of reactants and transition states shown in Figs. S1 and S3 (relative to the global minimum energy value, without ZPE correction) indicate that with increasing molecular size, the number of possible isomers increases, leading to a broader energy distribution. For instance, C₂F₅CHO exhibits three transition state conformers with energy differences spanning 0–1.7 kcal mol⁻¹, while C₃F₇CHO has five conformers distributed over 0–1.8 kcal mol⁻¹. This trend amplifies for longer chains: C₅F₁₁CHO generates 36 distinct conformers in its transition state, with energy variations extending up to 4.8 kcal mol⁻¹. This broad energy distribution has significant implications for the thermodynamics and kinetics of the degradation process of perfluoroaldehydes, potentially increasing the diversity and complexity of reaction pathways.

3.2 Kinetics of C₂F₅CHO/C₃F₇CHO + HO₂

The high-pressure limiting rate constants were calculated for the temperature range of 190-350 K, covering a wide atmospheric temperature range. For the reactions C₂F₅CHO + HO₂ (Reaction R1) and C₃F₇CHO + HO₂ (Reaction R2), the rate constants incorporating multi-structure anharmonicity corrections are defined as k₁ andk₂, respectively. According to Zheng and Truhlar (2010), the rate constants at high pressure are fitted using Eq. (3).

$$\begin{array}{}\text{(3)}& k=A{\left({\displaystyle \frac{T+T_{\mathrm{0}}}{\mathrm{300}}}\right)}^n\exp \left[-{\displaystyle \frac{E\left(T+T_{\mathrm{0}}\right)}{R\left(T^{\mathrm{2}}+T_{\mathrm{0}}^{\mathrm{2}}\right)}}\right]\end{array}$$

Table S4 lists the fitting parameters A, n, E, and T₀. Here, T represents temperature in Kelvin, and R is the ideal gas constant (0.0019872 kcal mol⁻¹ K⁻¹). The temperature-dependent Arrhenius activation energies are determined from the fits using Eq. (4).

$$\begin{array}{}\text{(4)}& E_{\mathrm{0}}=-R{\displaystyle \frac{\mathrm{d}lnk}{\mathrm{d}(\mathrm{1}/T)}}\end{array}$$

The high-pressure limit rate constants, incorporating multiple-structure anharmonicity torsional corrections, are illustrated in Fig. 4, with more comprehensive data provided in Tables S5–S7. Regarding the C₂F₅CHO + HO₂ reaction, the rate constant k₁ exhibits a decrease from $\mathrm{3.35}\times {\mathrm{10}}^{-\mathrm{12}}$ cm³ molecule⁻¹ s⁻¹ at 190 K to $\mathrm{5.42}\times {\mathrm{10}}^{-\mathrm{14}}$ cm³ molecule⁻¹ s⁻¹ at 350 K in Fig. 4 and Tables S5–S7. Similarly, the rate constant k₂ for the C₃F₇CHO + HO₂ reaction also decreases with increasing temperature. These trends are consistent with theoretical studies of non-fluorinated aldehydes such as C₂H₅CHO and C₃H₇CHO, where rate constants for reactions with HO₂ were reported in the range of 10⁻¹⁴ to 10⁻¹³ cm³ molecule⁻¹ s⁻¹ at atmospheric temperatures, indicating similar reactivity between fluorinated and non-fluorinated aldehydes with HO₂ (Ding and Long, 2022; Gao et al., 2024).

Figure 4 The high-pressure limit rate constants of the reactions of C₂F₅CHO and C₃F₇CHO with HO₂ at the temperature range of 190–350 K.

In addition, the effects of recrossing and multi-structural anharmonicity are quite limited, approximately ranging between 0.4 and 0.7 times. This results in the rate constants for Reactions (R1) and (R2) being 2–3 times slower than that of CF₃CHO + HO₂. For instance, the rate constants of C₂F₅CHO + HO₂ and C₃F₇CHO + HO₂ are estimated to be $\mathrm{1.19}\times {\mathrm{10}}^{-\mathrm{13}}$ and $\mathrm{7.92}\times {\mathrm{10}}^{-\mathrm{14}}$ cm³ molecule⁻¹ s⁻¹ at 298 K, respectively, which is slow by compared to $\mathrm{2.48}\times {\mathrm{10}}^{-\mathrm{13}}$ cm³ molecule⁻¹ s⁻¹ of CF₃CHO + HO₂ (Long et al., 2022). Moreover, the effect of anharmonicity in vibrational-frequency scale factors on high pressure limited rate constants were further discussed. We define “f” as the ratio between the rate constant calculated using the reaction-specific vibrational-frequency scale factors and that calculated using the standard vibrational-frequency scale factors. As depicted in Fig. 5, the rate constants obtained using the reaction-specific scale factors are 3–7 and 4–10 times faster compared to those calculated using the standard scale factors. Consequently, employing reaction-specific scale factors is crucial for accurate rate calculations.

Figure 5 The ratio between the rate constant calculated using the reaction-specific vibrational-frequency scale factors and the rate constant calculated using the standard vibrational-frequency scale factors, within the temperature range of 190–350 K. f₁ and f₂ represent the ratios for the reactions of C₂F₅CHO + HO₂ and C₃F₇CHO + HO₂, respectively.

The pressure-dependent rate constants of the HO₂ reaction with C₂F₅CHO and C₃F₇CHO were further calculated by using SS-QRRK method. As shown in Figs. 6–7 and Tables S8–S9, it can be observed that variations of the calculated rate constant with respect to pressure have a minimal impact on the rate constant of C₂F₅CHO + HO₂, indicating the absence of significant pressure effects. However, significant pressure effects are observed in the C₃F₇CHO + HO₂ reaction, particularly at temperatures above 300 K. To provide a clearer perspective, we define the transition pressure $p_{\mathrm{1}/\mathrm{2}}$ to quantify the pressure dependence. Specifically, the transition pressure $p_{\mathrm{1}/\mathrm{2}}$ is the pressure at which the pressure-dependent rate constant reaches half of its high-pressure limit. Figure 8 and Table S10 show that the transition pressure $p_{\mathrm{1}/\mathrm{2}}$ for the HO₂+ C₂F₅CHO reaction ranging from $\mathrm{2.6}\times {\mathrm{10}}^{-\mathrm{5}}$ to $\mathrm{7.4}\times {\mathrm{10}}^{-\mathrm{3}}$ bar at 190–350 K, while the transition pressure $p_{\mathrm{1}/\mathrm{2}}$ ranges from $\mathrm{2.6}\times {\mathrm{10}}^{-\mathrm{2}}$ to 2.3 bar at 190–350 K for the HO₂+ C₃F₇CHO reaction. This indicates that the HO₂+ C₃F₇CHO reaction exhibits a significant pressure dependence, and the increase in carbon chain length has a significantly affect pressure-dependent rate constants.

Figure 6 Pressure-dependent rate constants of C₂F₅CHO + HO₂ as functions of temperature obtained via the SS-QRRK method.

Figure 7 Pressure-dependent rate constants of C₃F₇CHO + HO₂ as functions of temperature obtained via the SS-QRRK method.

Figure 8 Transition pressure p${}_{\mathrm{1}/\mathrm{2}}$ calculated by the SS-QRRK method for the HO₂+ C₂F₅CHO and HO₂+ C₃F₇CHO reactions as functions of temperature.

3.3 Atmospheric Implications

To provide a further insight into the atmospheric degradation pathways of linear perfluoroaldehydes, we compare the HO₂-initiated linear perfluoroaldehyde reactions with the corresponding reactions with OH and Cl atom, their photolysis and hydrolysis.

We quantitatively evaluate the relative importance of OH- versus HO₂-initiated degradation for C₂F₅CHO and C₃F₇CHO through rate ratios defined in Eqs. (5) and (6).

$$\begin{array}{}\text{(5)}& {\displaystyle }{v}_{\mathrm{1}}={\displaystyle \frac{k_{\mathrm{1}}\left[{\mathrm{C}}_{\mathrm{2}}{\mathrm{F}}_{\mathrm{5}}\mathrm{CHO}\right]\left[{\mathrm{HO}}_{\mathrm{2}}\right]}{k_{\mathrm{OH}}\left[{\mathrm{C}}_{\mathrm{2}}{\mathrm{F}}_{\mathrm{5}}\mathrm{CHO}\right]\left[\mathrm{OH}\right]}}\text{(6)}& {\displaystyle }{v}_{\mathrm{2}}={\displaystyle \frac{k_{\mathrm{2}}\left[{\mathrm{C}}_{\mathrm{3}}{\mathrm{F}}_{\mathrm{7}}\mathrm{CHO}\right]\left[{\mathrm{HO}}_{\mathrm{2}}\right]}{k_{\mathrm{OH}}{}^{\prime }\left[{\mathrm{C}}_{\mathrm{3}}{\mathrm{F}}_{\mathrm{7}}\mathrm{CHO}\right]\left[\mathrm{OH}\right]}}\end{array}$$

Here, k₁ and k₂ are the rate constants of HO₂+ C₂F₅CHO and HO₂+ C₃F₇CHO calculated in the present work, respectively, while k_OH and k_OH^′ are the corresponding rate constants of OH+ C₂F₅CHO and OH + C₃F₇CHO obtained in the literature (Solignac et al., 2007; Wang et al., 2007). We calculate the rate ratios using a high OH concentration of 5×10⁶ molecules cm⁻³ (Lew et al., 2020) and a typical HO₂ concentration of 1.4×10⁸ molecules cm⁻³ (Brasseur and Solomon, 2006). The calculated results reveal that within the temperature range of 220–320 K, the rate ratios for v₁ and v₂ are in the range of 79.2 to 3.76 and 45.7 to 2.43, respectively (Table 1). Therefore, the present findings indicate that HO₂ initiated reactions dominate over OH initiated reactions for the degradation of C₂F₅CHO and C₃F₇CHO. We further consider the atmospheric lifetimes of C₂F₅CHO and C₃F₇CHO with respect to HO₂ at 0–50 km altitude in Table 2. Rapid HO₂-initiated degradation leads to short atmospheric lifetimes of ∼14.4–31.3 h for C₂F₅CHO and 21.6–51.8 h for C₃F₇CHO (Table 2), which are significantly shorter than the ∼20 d atmospheric lifetime driven by OH oxidation at below 10 km (Antiñolo et al., 2014).

Table 1 Rate ratios of HO₂+ C₂F₅CHO to OH + C₂F₅CHO and HO₂+ C₃F₇CHO to OH + C₃F₇CHO within the Temperature Range of 240 to 350 K.

^a k₁ and k₂^′ are the rate constants of the HO₂ reactions with C₂F₅CHO and C₃F₇CHO, from the literature respectively. ^b v₁ and v₂ denote the rate ratios of HO₂ with C₂F₅CHO and C₃F₇CHO to OH with C₂F₅CHO and C₃F₇CHO, respectively.

Table 2 Hydroperoxyl radical concentration (in molecules cm⁻³), rate constants (in cm³ molecule⁻¹ s⁻¹), and atmospheric lifetimes (in hours) with respect to bimolecular reactions as functions of altitude.

^a H denotes altitude (atmospheric scale height); T denotes temperature; p denotes pressure. ^b Data are from Brasseur and Solomon (2006). ^c k₁, k₂ are the rate constants of the HO₂ reactions with C₂F₅CHO and C₃F₇CHO, respectively. ^d ${\mathit{\tau }}_{\mathrm{1}}=\mathrm{1}/\left(k\right[{\mathrm{HO}}_{\mathrm{2}}\left]\right)$ and ${\mathit{\tau }}_{\mathrm{2}}=\mathrm{1}/\left(k_{\mathrm{2}}\right[{\mathrm{HO}}_{\mathrm{2}}\left]\right)$ define the atmospheric lifetimes for HO₂ reactions with C₂F₅CHO and C₃F₇CHO, respectively.

To provide further insight into the degradation of C₂F₅CHO and C₃F₇CHO under atmospheric conditions, further analysis has been done based on Geos-Chem data. GEOS-Chem simulations indicate that HO₂ concentrations reach a maximum of 4.99×10⁸ molecules cm⁻³ in the Amazon region, with a mean value of 9.93×10⁷ molecules cm⁻³ (Long et al., 2024). In contrast, the maximum OH concentration over the Atlantic and Pacific oceans is found to be 8.03×10⁶ molecules cm⁻³, with an average value of 1.06×10⁶ molecules cm⁻³. (Lelieveld et al., 2016). However, the OH concentration remarkably differ from daytime to nighttime (Bey et al., 1997; Stone et al., 2012). Therefore, we consider the concentration ratio between HO₂ and OH during the nighttime and daytime. During the nighttime, as shown in Fig. 9, the concentration ratio of $\left[{\mathrm{HO}}_{\mathrm{2}}\right]/\left[\mathrm{OH}\right]$ exceeds two orders of magnitude in industrial regions such as Russia, Malaysia, and parts of Africa, with values reaching as high as 410–1200 in the Amazon. This markedly enhances the HO₂-to-OH degradation rate ratios for C₂F₅CHO and C₃F₇CHO, reaching values of 88.5–259 and 56.0–164, respectively. This suggest that HO₂-initiated degradation exceeds OH-initiated pathways by more than a factor of 50 during nighttime in these regions. In contrast, over oceanic regions such as the Atlantic and Pacific, the $\left[{\mathrm{HO}}_{\mathrm{2}}\right]/\left[\mathrm{OH}\right]$ ratio falls below unity, substantially reducing the contribution of HO₂ to degradation processes in these areas.

Figure 9 The annual average ratio of ${\mathrm{HO}}_{\mathrm{2}}/\mathrm{OH}$ at night globally.

Table 3 Atmospheric lifetimes (τ, hours) of C₂F₅CHO and C₃F₇CHO against major degradation pathways.

^a Gas-phase hydrolysis of C₂F₅CHO with H₂O. ^b HCOOH-catalyzed gas-phase hydrolysis of ${\mathrm{C}}_{\mathrm{2}}{\mathrm{F}}_{\mathrm{5}}\mathrm{CHO}/{\mathrm{C}}_{\mathrm{3}}{\mathrm{F}}_{\mathrm{7}}\mathrm{CHO}$ with H₂O. ^c Hydrolysis of C₂F₅CHO with water dimer at the air-water interface.

During the nighttime, as shown in Fig. S4, the concentration ratios between HO₂ and OH generally favor HO₂, exhibiting maxima up to three orders of magnitude along the western coast of South America and ranging from one to two orders of magnitude in industrialized and African regions. These elevated ratios are closely associated with localized emission sources. However, During the daytime, photolysis is also an important route for removal of C₂F₅CHO and C₃F₇CHO. For C₂F₅CHO and C₃F₇CHO, photolysis represents the dominant atmospheric degradation route. C₂F₅CHO exhibits a high photolysis quantum yield of 0.81±0.09 at 254 nm, corresponding to an estimated atmospheric lifetime of less than 2 d in Table 3. Similarly, C₃F₇CHO displays a measured photolysis lifetime of 21±10 h under sunlight, confirming the efficiency of this removal mechanism (Chiappero et al., 2006; Solignac et al., 2007).

Chlorine atom reactions represent an additional potential atmospheric degradation pathway for linear perfluoroaldehydes. Kinetic measurements report rate constants of k(Cl + C₂F₅CHO) = (1.96±0.28) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ (Sulbaek Andersen et al., 2003) and k(Cl + C₃F₇CHO) = (2.03±0.23) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ (Andersen et al., 2004), which are approximately one order of magnitude higher than those for the corresponding HO₂-initiated reactions. Although Cl atoms exhibit intrinsically faster reaction kinetics, the atmospheric relevance of this degradation pathway is limited by the relatively low concentrations of Cl in the troposphere. Typical Cl atom concentrations range from 1.0×10⁴ to 3.0×10⁵ molecules cm⁻³ (Chang et al., 2004; Hossaini et al., 2016; Wang et al., 2019), leading to estimated atmospheric lifetimes of approximately 400 h (∼17 d) for both C₂F₅CHO and C₃F₇CHO. This stands in sharp contrast to the significantly shorter HO₂-driven lifetimes, which are typically less than 79.2 h in the lower troposphere in Table 3. The predominance of HO₂-mediated degradation arises from the relatively high ambient concentrations of HO₂, which compensate for its slower reaction kinetics – particularly in regions such as the Amazon, where $\left[{\mathrm{HO}}_{\mathrm{2}}\right]/\left[\mathrm{Cl}\right]$ ratios may exceed ∼10³ (Li et al., 2018; Wang et al., 2019). Under such conditions, the HO₂ initiated reaction rate can exceed that of Cl by 2–3 orders of magnitude. Moreover, the atmospheric relevance of Cl-initiated degradation is further constrained by its spatial heterogeneity, being primarily restricted to marine boundary layers and polluted coastal environments (Hossaini et al., 2016; Yang et al., 2022). In contrast, HO₂-driven degradation is effective across continental interiors and industrialized regions.

In addition to photolysis and radical-initiated oxidation, hydrolysis constitutes another potential atmospheric sink for C₂F₅CHO and C₃F₇CHO. Taking C₂F₅CHO hydrolysis as an example, its gas phase hydrolysis proceeds extremely slowly, with an estimated atmospheric lifetime exceeding 5.39×10⁶ h (Tables 3 and S11). This removal pathway is negligible, aligning with findings reported for CF₃CHO (Sulbaek Andersen et al., 2006). Hydrolysis catalyzed by atmospheric acids could potentially enhance hydrolysis rates through its ability to reduce the reaction barriers (Hazra et al., 2013; Liu et al., 2021). Formic acid (HCOOH), a ubiquitous atmospheric component, forms stable complexes with water (HCOOH⋯H₂O). Even at elevated concentrations of HCOOH⋯H₂O complexes (e.g., 10¹¹ molecule cm⁻³), the estimated hydrolysis lifetimes of C₂F₅CHO and C₃F₇CHO exceed 10⁷ h in Table 3. These timescales remain orders of magnitude longer than those associated with HO₂-initiated degradation, suggesting that the acid-catalyzed hydrolysis is insufficient to promote significant atmospheric removal of these compounds. Therefore, although acid catalysis effectively reduces the reaction barrier, its impact on the gas-phase degradation of C₂F₅CHO and C₃F₇CHO is negligible under typical tropospheric conditions.

Hydrolysis at air–water interfaces, such as those present on aerosol particles and cloud droplets, proceeds with markedly enhanced efficiency. Laboratory experiments have shown that passing gaseous CF₃CHO through liquid water results in over 80 % conversion to CF₃CH(OH)₂ within seconds, highlighting the potential importance of heterogeneous processes in atmospheric removal pathways (Sulbaek Andersen et al., 2006). Similarly, ¹H NMR measurements reveal that C₂F₅CHO rapidly converts to its gem-diol form, CF₃CF₂CH(OH)₂, within 3 min upon contact with liquid water, further confirming the efficient aqueous-phase hydration of perfluoroaldehydes. (Sulbaek Andersen et al., 2006). Here, we estimate a low Gibbs free-energy barrier (ΔG=9.8 kcal mol⁻¹) for C₂F₅CHO + 3H₂O at air-water interfaces, proceeding via a cyclic proton-transfer mechanism by using ab initio molecular dynamics, compared to a much higher barrier of 25.5 kcal mol⁻¹ for the corresponding gas-phase reaction (see Fig. S5a, b). More details are provided in Supplement. This results in significantly shorter atmospheric lifetimes, on the order of $\mathrm{3.47}\times {\mathrm{10}}^{-\mathrm{4}}$ h. Under humid conditions, such air–water interfacial hydrolysis is likely to dominate and may effectively compete with HO₂-mediated degradation pathways. Once formed, C_nF_2n+1CH(OH)₂ reacts with OH, ultimately leading to the formation of perfluorocarboxylic acids (PFCAs). This hydrolysis-oxidation pathway represents a significant indirect source of persistent PFCAs, particularly given the ubiquity of aqueous phases in the atmosphere.

We can conclude that HO₂ initiated degradation pathways dominate the gas-phase degradation of C₂F₅CHO and C₃F₇CHO. We further consider the final product in the HO₂+ C_nF_2n+1CHO reactions. As mentioned, the HO₂ reaction generates intermediate perfluoroalkyl radicals C_nF_2n+1, which can subsequently undergo a carbon-shortening process to form the more stable COF₂, as depicted in Fig. 10. Taking the example of C₂F₅, the process starts with C₂F₅ reacting with O₂ to form C₂F₅O₂. Subsequently, C₂F₅O₂ reacts with NO to produce C₂F₅O, which then undergoes C-C bond cleavage to generate CF₃ and COF₂. The CF₃ further reacts to eventually yield COF₂ through a similar reaction pathway. However, the absence of quantified rate constants for these reactions prevents a robust assessment of their global or regional impacts. A comprehensive evaluation of the role of NO would require integrating the kinetics of RO₂ + NO reactions (e.g., $\mathrm{M}\mathrm{1}/\mathrm{M}\mathrm{2}+\mathrm{NO}$) into atmospheric models, which is beyond the scope of this study.

Figure 10 Atmospheric degradation mechanism for C_nF_2n+1 with C₂F₅ used as a representative example.

In summary, we identify that the HO₂-initiated reaction represents an important atmospheric sink for linear perfluoroaldehydes in gas phase. Notably, recent studies suggest that HO₂ concentrations may be elevated at air–water interfaces compared to the bulk gas phase (Angelaki et al., 2024; Li et al., 2023). Given the enhanced reactivity at the air–water interface and the complex competition between OH and HO₂, interfacial HO₂-driven degradation may play a more significant role than previously recognized, potentially influencing atmospheric acidity. For example, Xia et al. (2024b) recently reported both single-carbon and double-carbon scission pathways during the degradation of C₇F₁₅ on water droplet surfaces. These heterogeneous processes may contribute not only to the atmospheric removal of perfluoroaldehydes but also to the broader degradation of polyfluoroalkyl substances (PFAS). Nevertheless, further experimental and theoretical studies on reaction kinetics and mechanisms are needed to better constrain this complex chemical processes. Incorporating such processes into atmospheric models is crucial for improving the prediction of PFAS environmental fate and secondary pollution, with important implications for emission control strategies and environmental risk assessment.

4 Summarizing Remarks

In this study, we have delved into the chemical reaction kinetics of linear perfluoroaldehydes (C₂F₅CHO and C₃F₇CHO) with hydroperoxyl radicals in the gas phase using ab initio calculation methods and reaction kinetics theory. We find that the activation enthalpies for the reactions of C₂F₅CHO and C₃F₇CHO with HO₂ at 0 K are both −2.7 kcal mol⁻¹, demonstrating that carbon chain elongation in linear perfluoroaldehydes has a negligible thermodynamic influence on their enthalpies of activation at 0 K. This is further shown in C₄F₉CHO and C₅F₁₁CHO with HO₂.

Further kinetic studies reveal that anharmonicity have a significant impact on the reaction rates, while the torsional anharmonicity, recross coefficient, and tunnelling effects contribute relatively little to the rate constants. It is particularly noteworthy that the reaction of C₃F₇CHO with HO₂ exhibits a distinct pressure dependence, whereas the reaction of C₂F₅CHO with HO₂ does not show such a pressure effect.

By integrating kinetics with the data based on GEOS-Chem modelling, we have identified some regions such as Russia, Malaysia, and parts of Africa, where HO₂ concentration exceeds OH concentration by 2–3 orders of magnitude. Therefore, the reactions of HO₂ with C₂F₅CHO and C₃F₇CHO can compete well with their corresponding reaction with OH. Specifically, the atmospheric lifetimes of C₂F₅CHO and C₃F₇CHO via HO₂ are shortened to be 14.4–31.3 and 21.6–51.8 h, respectively, with orders of magnitude shorter than that of the corresponding OH-mediated pathways. In addition, photolysis, typically occurring within 48 h, represents an efficient daytime removal pathway, while heterogeneous hydrolysis proceeds rapidly at air-water interfaces with characteristic timescales of less than 1 h. Accordingly, HO₂-initiated degradation should be considered a major gas-phase sink, particularly in continental source regions. Under high NO_x conditions, this pathway may contribute to tropospheric HCOOH and COF2 formation.

While the present investigation establishes the HO₂-mediated degradation pathway for linear perfluoroaldehydes (C₂F₅CHO/C₃F₇CHO), it simultaneously highlights critical gaps in our understanding of their atmospheric lifetimes. Notably, the current work focuses on gas-phase HO₂ reactions. However, the roles of heterogeneous interfacial processes (e.g., on aerosol surfaces or cloud droplets) remain unexplored (Zhang et al., 2024). The potential for HO₂-driven defluorination to generate reactive CF₃ radicals, which could initiate secondary reactions (e.g., with O₃ or NO₂), requires systematic investigation to assess implications for atmospheric oxidizing capacity and secondary aerosol formation. Additionally, the study focuses on radical-driven pathways but acknowledges that photolysis is a competing sink for linear perfluoroaldehydes. Future work should quantify photolysis rates under stratospheric UV conditions (e.g., 200–300 nm) to reconcile discrepancies between modeled and observed atmospheric lifetimes (Thomson et al., 2025). Addressing these limitations will require integrating advanced experimental techniques (e.g., synchrotron-based photoionization mass spectrometry) with multi-scale modeling frameworks, while prioritizing under sampled environments like the upper troposphere and polar regions where HO₂ reactivity anomalies could profoundly alter PFAS degradation trajectories (Alam et al., 2024; Zhou et al., 2024b). Such efforts are critical for refining environmental risk assessments of emerging HFOs and guiding the design of next-generation chemicals with minimized atmospheric persistence.