David A. Good

Lifetimes and global warming potentials for dimethyl ether and for fluorinated ethers: CH3OCF3 (E143a), CHF2OCHF2 (E134), CHF2OCF3 (E125)

Using recent kinetic data, two-dimensional (2-D) chemical-transport modeling of the atmospheric lifetimes of dimethyl ether and fluorinated ethers CH 3 OCF 3 (E143a), CHF 2 OCHF 2 (E134), and CHF 2 OCF 3 (E125) shows that E134 and E125 have substantially larger lifetimes than previously estimated. Dimethyl ether has a short atmospheric lifetime of 5.1 days and a relatively insignificant radiative forcing leading to a relatively low global warming potential. Increasing fluorination is accompanied by slower rates of reaction with hydroxyl radical and ultimately longer lifetimes. E143a, E134, and E125 were found to have lifetimes of 5.7, 29.7, and 165 years, respectively. In addition, our work uses ah initio methodology to determine IR absorption cross sections for each ether. Our study finds that E134 and E125 have significant infrared absorption and thus relatively high radiative forcing values. Thcse two propertics together yield global warming potentials for E134 and E125 of 5720 and 14,000, respectively, integrated over a 100 year period.

HEAT OF FORMATION OF CH3OCH2 RADICAL

Abstract The heats of formation of dimethyl ether and dimethyl ether radical have been investigated via ab initio methodology. The heat of formation of dimethyl ether was found to agree with experimental values of both 0 and 298 K (−39.8 and −44.0 kcal mol−1, respectively). The heat of formation of dimethyl ether radical was determined to be 0.9 kcal mol−1 at 298 K. Corresponding literature values range rom −1.3 to −6.9 kcal mol−1, thus demonstrating considerable disagreement. The heat of formation of dimethyl ether radical was determined to be 4.2 kcal mol−1 at 0 K.

Global warming potential assessment for CF3OCF=CF2

We have examined CF3OCF = CF2 regarding its reactivity toward OH radical, its infrared spectroscopic properties, its atmospheric lifetime, and its radiative forcing. From these we then determined the Global Warming Potentials (GWPs) for CF3OCF = CF2. The examination is completed using a combination of discharge flow coupled with mass spectrometer and resonance fluorescence (DF/MS/RF), Fourier transform infrared (FTIR) spectroscopy, ab initio molecular orbital calculation, and atmospheric and radiative transfer modeling. Mass spectral evidence suggests that both HF and CF3OCFC(O)F are products from the reaction of CF3OCF = CF2 with OH. The Arrhenius expression for CF3OCF = CF2 + OH is determined to be k1 = (6.41±0.82)×10−11 exp[(−868±40)/T] cm3 molecule−1 s−1 in the temperature range of 253–348 K. The atmospheric lifetime of CF3OCF = CF2 is estimated to be less than 5 days due to the OH attack. The calculated vibrational frequencies using ab initio molecular orbital calculations are in good agreement with FTIR experimental observation for the CF3OCF = CF2 molecule. Both C-O and C-F stretching modes in the CF3OCF = CF2 contribute to prominent absorption in the atmospheric window region. The absolute adjusted radiative forcing at the tropopause due to an increase in the concentration of CF3OCF = CF2 by one part per billion by volume (ppbv) is calculated to be 0.041 W m−2 ppbv−1. The Global Warming Potential for CF3OCF = CF2 is evaluated to be 0.004 for 100-year time horizon.

Rate constant for the reactions of CF3OCHFCF3 with OH and Cl

Abstract The kinetics of reactions of CF 3 OCHFCF 3 with hydroxyl radicals and chlorine atoms has been investigated using a discharged flow combined with both mass spectrometer and resonance fluorescence technique and using a relative rate technique, respectively, at 298 K. The rate constant for the reactions of CF 3 OCHFCF 3 with OH and Cl was determined to be k 1 =(4.98±1.64)×10 −15 cm 3 molecule −1 s −1 and k 2 =(3.1±2.5)×10 −14 cm 3 molecule −1 s −1 , respectively. On the basis of our kinetics measurements, the tropospheric lifetime of CF 3 OCHFCF 3 is calculated to be about 8 years, primarily due to reaction with the hydroxyl radicals in the troposphere.

Location of the first excited states of fluorinated ethers, E143a (CH3OCF3), E134 (CHF2OCHF2), and E125 (CHF2OCF3)

Abstract The first excited states for dimethyl ether, E143a (CH 3 OCF 3 ), E134 (CHF 2 OCHF 2 ), and E125 (CHF 2 OCF 3 ) have been located using the state-averaged CASSCF method. The first excited states have been found to lie at 185, 138, 122, and 114 nm for dimethyl ether, E143a, E134 and E125, respectively. The value for dimethyl ether is in good agreement with literature values. The effect of fluorinating dimethyl ether to form E143a is to blue shift the position of the first excited state by over 40 nm. These results are consistent with the methanol system, which sees an analogous shift of approximately 40 nm as methanol is fluorinated to form CF 3 OH. Implications for the photolysis of these fluorinated ethers in the atmosphere is discussed.

Evaluation of the atmospheric lifetime and radiative forcing on climate for 1,2,2,2‐tetrafluoroethyl trifluoromethyl ether (CF3OCHFCF3)

The compound 1,2,2,2-Tetrafluoroethyl Trifluoromethyl Ether, CF3OCHFCF3 (HFE-227), is currently being considered as a potential replacement for certain halocarbons, particularly for perfluorocarbons (PFCs), as a dry etching gas in the semiconductor industry. For this reason, it is important to determine the potential environmental effects resulting from the use and emissions of this compound. In this paper, the atmospheric lifetime, radiative forcing, and Global Warming Potentials (GWPs), an important measure of the potential effects of a gas on climate, are evaluated for this compound using our zonally averaged chemical transport and radiative transfer models of the atmosphere. To our knowledge, this is the first time this compound has been evaluated with such atmospheric models. In order to calculate the lifetime and radiative forcing, the rate constants and infrared cross sections of this compound were measured in laboratories at Illinois and Purdue, and results are reported here. The model-evaluated atmospheric lifetime is 11.3 years, mainly due to reaction with OH radicals. The model-evaluated instantaneous-clear-sky radiative forcing is 0.38 W m−2 ppbv−1, about 45% lower than previously estimated [Imasu et al., 1995]. However, the model-estimated cloudy-sky adjusted forcing, needed to calculated GWPs, is about 25% lower than the model-estimated instantaneous-clear-sky forcing. The GWPs are calculated to be 3400, 1200, and 370 for 20, 100, and 500 year time horizons, respectively.