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TFA Deposition From Potential Use Of 2-Bromo-3,3,3-Trifluoropropene (2-BTP) In Aircrafts September 18, 2023 Swarnali Sanyal, Rachana Kulkarni and Donald J. Wuebbles Department of Atmospheric Sciences University of Illinois Urbana, IL 61801 The chemical 2-Bromo-3,3,3-Trifluoropropene (2-BTP) has emerged as a commercialized replacement for halons as a firefighting agent in aircraft. Previous research indicates that, at midlatitudes, the average lifespan of 2-BTP is estimated to be around 7 days. Additionally, its Ozone Depletion Potential (ODP) is 0.0028 and it has a Global Warming Potential (GWP) of 0.0050 over a 100-year period [1,6]. These figures suggest that 2-BTP is unlikely to exert a significant influence on ozone levels and climate. However, the potential production of Trifluoroacetic Acid (TFA) from 2-BTP has not been extensively studied. Given the potential increase in the use of 2-BTP, it is important to understand the effects of TFA formation and deposition. TFA is a potent organic acid with phototoxic properties and lacks a known degradation mechanism in water. In high concentrations, it can adversely impact aquatic ecosystems, particularly algae. The primary source of atmospheric TFA in the current atmosphere is through chemical production, specifically via the oxidation of hydrofluorocarbons (HFCs) utilized as refrigerants and coolants. In this study, we embark on a first-principles analysis of potential degradation products of 2-BTP and the plausible production of TFA. Equations labeled from R1 to R26 represent our most comprehensive understanding of the atmospheric breakdown of 2-BTP, and Table 1 have the rate coefficients for the reactions. Below are the key reactions: Chemical pathway for 2-BTP decomposition OH + CF3C(Br)=CH2CF3C(Br)(OO.)CH2OH CF3C(Br)(OO.)CH2OH + NO CF3C(Br)(O.)CH2OH + NO2 CF3C(Br)(OO.)CH2OH + HO2CF3C(Br)(OOH)CH2OH + O2 OH + CF3C(Br)(OOH)CH2OHCF3C(Br)(OO.)CH2OH + H2O CF3C(Br)(O.)CH2OH CF3C(=O)Br + CH2OH CH2OH + O2CH2O + HO2 CF3C(=O)Br hydrolysis CF3C(=O)OH + HBr CF3C(=O)Br + h CF3C(=O)OO. + Br CF3C(Br)(O.)CH2OH CF3C(=O)CH2OH + Br OH + CF3C(=O)CH2OHCF3C(=O)CH=O + H2O + HO2 OH + CF3C(=O)CH=OCF3C(=O)OO. + CO + H2O (R1) (*R2) (R3) (R4) (*R5) (R6) (*R7) (*R8) (*R9) (*R10) (*R11) 1 CF3C(=O)OO. + NO2 CF3C(=O)OONO2 CF3C(=O)OONO2 CF3C(=O)OO. + NO2 CF3C(=O)OO. + NO CF3OO. + CO2 + NO2 CF3OO. + NO F + CF2O + NO2 CF3C(=O)OO. + HO2 CF3C(=O)OOH + O2 CF3C(=O)OH + O3 CF3OO. + CO2 + OH + O2 CF3C(=O)CH=O + h CF3C(=O)OO. + HCO HCO + O2 HO2 + CO OH + CF3C(Br)=CH2 CF3C(Br)(OH)CH2(OO.) CF3C(Br)(OH)CH2(OO.) + HO2CF3C(Br)(OH)CH2(OOH) + O2 OH + CF3C(Br)(OH)CH2(OOH) CF3C(Br)(OH)CH2(OO.) + H2O CF3C(Br)(OH)CH2(OO.) + NOCF3C(Br)(OH)CH2(O.) + NO2 CF3C(Br)(OH)CH2(O.) CF3C.(Br)(OH) + CH2O CF3C.(Br)(OH) + O2CF3C(=O)Br + HO2 CF3C(=O)OH (gas phase) + OH loss CF3C(=O)OH (gas phase) + OH dry deposition (*R12) (*R13) (*R14) (R15) (R16) (*R17) (R18) (R19) (R20) (R21) (R22) (R23) (R24) (R25) (R26) (L1) (L2) Those marked with an asterisk (*) indicate the likely pathways for TFA formation from 2-BTP that we explore in this investigation. This will be followed by development of global emission database of 2-BTP and addition of the chemistry mechanism to atmospheric climate chemistry model CESM-CAM6. This will be used to simulate the degradation products, including the production and deposition of TFA. Since the atmospheric chemical degradation pathway is not very well known, we have to perform this extensive first principle analysis. Due to 2-BTP's rapid reactivity in the troposphere, it is anticipated that its mixing ratios will swiftly diminish beyond the emission region, both in terms of latitude and altitude. Conversion of 2-BTP to TFA includes an OH initiated reaction with a temperature dependent rate coefficient (0.9x1.05e12xexp(381/T) cm3/molec/s) producing CF3C(Br)(OO.)CH2OH. The initial reaction with OH is a rate limiting step. Table 1 summarizes the reaction rate coefficients for the probable pathway of 2BTP degradation in the atmosphere. CF3C(Br)(OO.)CH2OH has two possible pathways to TFA formation. It is either converted to CF3C(=O)Br (Equation R5) or formation of CF3C(=O)CH2OH (Equation R9). The second one is much quicker than the first pathway. The approximate near surface concentration of different relevant compounds used in this work are summarized in Table 2. In the first principle analyses we are assuming, an ambient temperature of 278 - 298K, tropospheric OH average values of 10.9 105 molecules/cm3 [1,3,5], HO2 concentration 2.61 x 108 molecules/cm3[1,3,5]. NOx concentration 37.5ppb, NO2 concentration 22.4ppb [4,5]. 2 Based on Equation R1, one mole of CF3C(Br)=CH2 produces one mole of CF3C(Br)(OO.)CH2OH. At assumed temperature of 278 - 298K, reaction rate with NO (Equation R2) is 8.53-7.94x1012 and with HO2 (Equation R3) is 9.3-7.86 x1012. At the higher range the reaction rate with NO is marginally higher, while at the lower range, reaction with HO2 is marginally faster. The relative atmospheric concentration of the two oxidants determines the more dominant reaction and considering the significantly higher atmospheric concentration of NO compared to HO2 ([NO]/[HO2] = 1.42x103), 99% of CF3C(Br)(OO.)CH2OH reacts with NO (Equation R2) and remaining with HO2 (Equation R3). Consequently 0.99 mole of CF3C(Br)(OO.)CH2OH produces 0.99 mole of CF3C(Br)(O.)CH2OH. The reaction rate of pathway shown in Equation R5 is considerably slower than that in Equation R9. To assume the probable pathway for TFA formation, we are assuming 99% of CF3C(Br)(O.)CH2OH follows the pathway of TFA formation from Equation R9. Therefore 0.98 mole CF3C(Br)(O.)CH2OH produces 0.98 moles of CF3C(=O)CH2OH. The hydrolysis rate of CF3C(=O)Br is not known (Equation R7). We are assuming a proxy of CF3C(O)F with a hydrolysis rate of 150s-1 and Henry's law solubility constant of 3Matm-1, but the fluorinated compound is assumed to be slower to react compared to the brominated compound. Hydrolysis of CF3C(=O)Br is the predominant reaction pathway, so we are ignoring the photolysis pathway. The pathway to formation of TFA follows Equation R11 (1.5x10-11-1.31x10-11) which is more than two times faster than pathway in Equation R10 (6.32x10-12-5.85x10-12). 0.66-0.65 moles CF3C(=O)OO. forms in this pathway which leads to TFA. The major product of this pathway is production of CF3OO. (Equation R18), but also yields TFA (Equation R17). The rate of formation of CF3OO. (1.34x10-11-1.11x10-11) is higher than TFA (9.08x10-12-7.62x10-12), so only a fraction of 2-BTP is converted to TFA, proportional to the rate of the product formation, yielding 0.27-0.32% TFA from a mole of 2-BTP. In other words, for a mole emission of 2-BTP, there is 0.0027-0.0032 mole of TFA formed. TFA is highly soluble in cloud water. After evaporation of cloud water, TFA can be released into gas phase. Gas phase TFA can be lost through either dry or wet deposition. Dry deposition rate of TFA is assumed to be same as that of nitric acid. Equations L1 and L2 describes the loss of TFA from the atmosphere. Given the rapid removal of TFA from the atmosphere and the presence of alternative pathways for products stemming from the atmospheric breakdown of 2-BTP, our initial first-principle analysis indicates that the contribution of 2-BTP to atmospheric TFA production is quite low (only 0.270.32% of TFA molecule produced for emission of a 2-BTP molecule). However, a more in-depth simulation utilizing the identified chemistry pathway will offer a more comprehensive understanding of the decomposition process of 2-BTP. This subsequent investigation is expected to provide a clearer picture of the overall impact of 2-BTP on TFA levels in the atmosphere. 3 Table 1. Rate constants for the different steps atmospheric degradation of 2-BTP. Chemical Reaction R1[1] R2* R3* R4 R5 R6[2] R7 R8 R9 R10+ R11+ R12+ R13+ R14+ R15+ R16 R17 R18 R19 R20[2] R21[1] R22* R23* R24 R25 R26 L1[7] L2[7] [] Literature reference * Analogous to alkyl peroxy radical reactions + From IUPAC, https://iupac.aeris-data.fr/ Rate Coefficient (cm3/molec/sec) 0.9*1.05e-12*exp(381/T) 2.90e-12 exp(300/T) 7.50e-13 exp(700/T) 3.80e-12 exp(200/T) insignificant 9.1e-12 insignificant insignificant 1e6 s-1 (exact rate unimportant) 2.0e-12 exp(320/T) 1.9e-12 exp(575/T) 6.6e-12 1.75e14 exp(-12600/T) 4.0e-12 exp (560/T) 5.40e-12 exp(320/T) 0.09*1.73e-12 exp(730/T) 0.38*1.73e-12 exp(730/T) 0.56*1.73e-12 exp(730/T) insignificant 5.2e-12 0.1*1.05e-12*exp(381/T) 7.50e-13 exp(700/T) 3.80e-12 exp(200/T) 2.90e-12 exp(300/T) 1e6 s-1 (exact rate unimportant) insignificant 9.35e-14 Similar to nitric acid 4 Table 2. Ambient surface concentration of reactants. [] Literature reference OH [1,3,5] HO2[1,3,5] NOx (NO + NO2)[4,5] Concentration (molecule/cm3) 1.09 x 1006 2.61 x 1008 9.22 x 1011 References 1. Patten, K. O., V. G. Khamaganov, V. L. Orkin, S. L. Baughcum, and D. J. Wuebbles (2011), OH reaction rate constant, IR absorption spectrum, ozone depletion potentials and global warming potentials of 2-bromo-3,3,3-trifluoropropene, J. Geophys. Res., 116, D24307, doi:10.1029/2011JD016518 2. Burkholder, J. B., et al. Chemical kinetics and photochemical data for use in atmospheric studies; evaluation number 19. Pasadena, CA: Jet Propulsion Laboratory, National Aeronautics and Space Administration, 2020, 2020. 3. Pimlott, M. A., Pope, R. J., Kerridge, B. J., Latter, B. G., Knappett, D. S., Heard, D. E., Ventress, L. J., Siddans, R., Feng, W., and Chipperfield, M. P.: Investigating the global OH radical distribution using steady-state approximations and satellite data, Atmos. Chem. 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