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Ultraviolet absorption spectra of FNO3 and HOF
Armosphrric
Enuiromenr
Vol
17, No. 4, PP. 759-761.
$03.00/o ooLw6981/83/0407s9-03 0 1983 Pcrgamon Press Ltd.
1983
Printed in Great Britain.
ULTRAVIOLET
ABSORPTION
SPECTRA OF FNO,
AND HOF
SCOTT ELLIOTT Department of Chemistry, University of California, Irvine, CA 92717, U.S.A. (First received 26 July 1982 and infinaljorm
9 September 1982)
Abstract-Ultraviolet absorption cross sections of gas phase FNO, and HOF have been measured over the wavelength range 180 to 400 nm at 295 K. The maximum phololysis lifetime for FNO, in the stratosphere is calculated to be in the order of days, and for HOF in the order of hours.
INTRODUCTION
In the few models of stratospheric fluorine chemistry which have appeared (Rowland and Molina, 1975; Stolarski and Rundell, 1975; Watson et al., 1978; Sze, 1978) HF has been the only non-radical reservoir or sink provided for fluorine atoms which are transported to the stratosphere and then enter the FO, cycle. By analogy with the much better understood chlorine chemistry of the stratosphere, the possibility of FNO, and HOF reservoirs may be considered as well; FNO, may be formed by termolecular recombination of FO and NO, radicals, and HOF through hydrogen abstraction by FO. This communication reports the first ultraviolet (u.v.) cross section measurements on fluorine nitrate, FNO, and hypofluorous acid, HOF and estimates their stratospheric photolysis lifetimes.
EXPERIMENTAL
Fluorine nitrate was prepared by combining and agitating fluorine gas and solid KNO,, following the method of Miller et al. (1967). Extreme care is recommended in handling; the compound may explode violently. It is safest never to allow fluorine nitrate to pass from the liquid to the solid phase. Fluorine nitrate was identified by its characteristic infrared (i.r.) spectrum (Miller et al., 1967). In addition to previously reported impurities HN03 and HF, NO, was present as indicated by its highly structured visible spectrum. Fluorine nitrate could be prepared free of any observable impurity (i.r. or u.v.) by careful distillation from an ethanol slush bath. Preliminary runs were made using a 1Ocm quartz cell with FN03 at pressures from 0.1 to 500 torr. Beer’s law was obeyed over this range. Low absorbance values in the long wavelength region were confirmed in a 180cm double pass long path cell. Appelman and Studier (1971) prepared HOF in a closed all-Teflon circulating system. In this work an open stainless steel apparatus was used, otherwise the methods are similar. Nitrogen carrying l-5 % fluorine was flowed at around 100,~ torr cm3 s-l through a
U-tube at -45°C filled with Rgshig rings which had been previously sprinkled with water. Product stream proceeded through traps at - 50°C and - 80°C which removed water and some HF. An HF-HOF mixture was collected at - 190°C. Hypofluorous acid was identified by its characteristic i.r. spectrum (Appelman and Kim, 1972). The vapor pressures of HOF and HF are very similar making separation impossible. The amount of HF in any HF-HOF mixture was quantified in the i.r. On some occasions CFI 0 was observed as an impurity. Runs with any trace were discarded and not considered in subsequent analysis and calculations. In the metal system used for purification and transfer of the HOF-HF mixture, HOF was found to be of unpredictable stability (explosive) at greater than -40°C. Consequently the maximum amount available for any one measurement was its vapor pressure at this temperature, around 30 torr. Adding methane to the mixture to 1 atmosphere improved stability. Hypofluorous acid spectroscopy was conducted in 1Ocm monel cells coated inside with halocarbon wax and fitted with sapphire windows. All cross-section measurements for both molecules were made at 295 K, and were taken on a Cary 2 19 u.v.visible spectrophotometer interfaced to a Nova 3 computer for data manipulation.
759
RESULTS AND DlSCIJSSlON
Figures 1 and 2 give the absorption spectra of FNO, and HOF determined in this work. Uncertainties discussed below have been treated according to Hudson (1971). In the FNO, case estimated uncertainty in the 20&350 nm range is + 15 %, bracketing general differences between runs. In the longer wavelength regions cross-sections disappear into instrumental uncertainties and are indicated as vertical bars. In the HOF experiment, relative uncertainty in cross-section between 200 and 350 nm arises mainly from uncertainty in HF pressure measured in the cell. Estimates represent lu deviations and again appear as vertical bars.
SCOTT ELLIOTT
760
2
I
200
I 250
I
I
I
I
300
350
WAVELENGTH
(nml
factor of 5-10. Their residence times in the stratosphere should be correspondingly longer. Average daily and noon values of the unimolecular photodissociation coefficients for FN03 and HOF have been calculated as a function of altitude using a computer program written by M. J. Molina (private communication, 1981). Altitude and wavelengthdependent solar flux values used were the latest available at the time of computation (D. Wuebbles, private communi~tion, 1981). Lower limits for crosssections given in the figures were taken in order to put upper bounds on stratospheric photolysis lifetimes. Results are given in Tables 1 and 2.
I
400
Fig. 1. Absorption spectrum of FNOJ at 295 K.
Table 1. Solar photodissociation coefficients for fluorine nitrate, FNO,, as a function of altitude, in units of 10-5s-’ Altitude (km) 10 15 20 25 30 35 40 45 50
200
Fig.
2.
300
350
WAVELENGTH
250
km4
400
Absorption spectrum of HOF at 295 K.
Decomposition of HOF was monitored to completion in both the i.r. and U.V.In most cases, correlation of i.r. and U.V.bands permitted the unambiguous assignment of all initial U.V.absorption to HOF at all wavelengths. After one such decomposition an unidentified absorption remained beyond 350nm, but 75 “/, of initial absorption was still attributable to HOF in the 300-350nm spectra1 region critical to calculation of stratospheric photolysis lifetimes. Lengthening vertical bars at long wavelengths reflect the results of these band correlation experiments. Continuous spectra for both FNO, and HOF indicate absorption to unbound states. The processes should be dissociative with quantum yields of unity. From HOF there are only two possible sets of nonr~rrangement products, H + OF and OH f F. The H + OF pathway is energeti~lly inaccessible at the measured HOF threshold, and in the 300-350nm region. None of the possible non-rearrangement products from FN03 can be excluded on the basis of thermodynamics. Both the FNO, and HOF spectra are blue shifted relative to those of the analogous chlorine compounds CINOS and HOC1 (Rowland et al., 1976; Molina and Molina, 1978). Over most of the 2&34OOnm range cross-sections of FNO, and HOF are smaller by a
Average 0.36 0.36 0.37 0.43 0.71 1.7 3.5 5.4 6.6
Noon 1.1 1.1 1.1 1.3 2.5 5.5 10 15 18
Table 2. Solar phot~~ss~iation coefficients for hypofluorous acid, HOF, as a function of altitude, in units of 10-5s-’ Altitude (km) 10 15 20 25 30 35 40 45 50
Average
Noon
2.9 2.9 2.9 3.0 3.2 3.5 4.0 4.4 4.7
8.7 8.7 8.7 9.0 9.5 10 11 13 14
It is apparent that throughout the stratosphere, maximum lifetimes are on the order of days for FN03 and hours for HOF. These may be compared with typical vertical mixing times in the stratosphere, which are on the order of months to years. The gross conclusion can be drawn that neither FNO, nor HOF represents a fluorine sink of the magnitude of HF. Further interpretation awaits resumed modeling of stratospheric fluorine. Acknowledgement-This number NSG-7208.
work was supported by NASA grant
REFERENCES Appelman E. H. and Studier M. H. (1971) HypoAuorous acid. J. Am. them. Sot. 93, 2349-2352. Appelman E. H. and Kim H. (1972) Gas phase infrared spectra of HOF and DOF. J. them. Phys. 57, 3772-3’775.
Ultraviolet
absorption
Hudson R. D. (1971) Critical review of ultraviolet photoabsorption cross sections for molecules of astrophysical and aeronomic interest. Reo. Geophys. Space Res. 9, 305-381. Miller R. H., Bernitt D. L. and Hisatsune S. (1967) Infrared spectra of isotopic halogen nitrates. Spectrochim. Acta 238, 2233236. Molina M. J. and Molina L. T. (1978) Ultraviolet Spectrum of HOCI. J. phys. Chem. 82, 241&2414. Rowland F. S. and Molina M. J. (1975) Chlorofluoromethanes in the environment. Reo. Geophys. Space Phys. 13, l-35. Rowland F. S., Spencer J. E. and Mohna M. J. (1976)
spectra
of FNO,
and HOF
761
Stratospheric formation and photolysis of chlorine nitrate. J. phys. Chem. 80, 2711-2713. Stolarski R. S. and Rundell R. D. (1975) Fluorine photochemistry in the stratosphere. Geophys. Res. Left. 2, 443445. Sze N. D. (1978) Stratospheric fluorine, a comparison between theory and measurement. Geophys. Rex Left. 5, 781-784. Watson R. T., Smokler P. E. and Demore W. B. (1978) An assessment of a fluorine or N, 0, injection from an aborted space shuttle mission. NASA contract report NASA-CR 156164.
Enuiromenr
Vol
17, No. 4, PP. 759-761.
$03.00/o ooLw6981/83/0407s9-03 0 1983 Pcrgamon Press Ltd.
1983
Printed in Great Britain.
ULTRAVIOLET
ABSORPTION
SPECTRA OF FNO,
AND HOF
SCOTT ELLIOTT Department of Chemistry, University of California, Irvine, CA 92717, U.S.A. (First received 26 July 1982 and infinaljorm
9 September 1982)
Abstract-Ultraviolet absorption cross sections of gas phase FNO, and HOF have been measured over the wavelength range 180 to 400 nm at 295 K. The maximum phololysis lifetime for FNO, in the stratosphere is calculated to be in the order of days, and for HOF in the order of hours.
INTRODUCTION
In the few models of stratospheric fluorine chemistry which have appeared (Rowland and Molina, 1975; Stolarski and Rundell, 1975; Watson et al., 1978; Sze, 1978) HF has been the only non-radical reservoir or sink provided for fluorine atoms which are transported to the stratosphere and then enter the FO, cycle. By analogy with the much better understood chlorine chemistry of the stratosphere, the possibility of FNO, and HOF reservoirs may be considered as well; FNO, may be formed by termolecular recombination of FO and NO, radicals, and HOF through hydrogen abstraction by FO. This communication reports the first ultraviolet (u.v.) cross section measurements on fluorine nitrate, FNO, and hypofluorous acid, HOF and estimates their stratospheric photolysis lifetimes.
EXPERIMENTAL
Fluorine nitrate was prepared by combining and agitating fluorine gas and solid KNO,, following the method of Miller et al. (1967). Extreme care is recommended in handling; the compound may explode violently. It is safest never to allow fluorine nitrate to pass from the liquid to the solid phase. Fluorine nitrate was identified by its characteristic infrared (i.r.) spectrum (Miller et al., 1967). In addition to previously reported impurities HN03 and HF, NO, was present as indicated by its highly structured visible spectrum. Fluorine nitrate could be prepared free of any observable impurity (i.r. or u.v.) by careful distillation from an ethanol slush bath. Preliminary runs were made using a 1Ocm quartz cell with FN03 at pressures from 0.1 to 500 torr. Beer’s law was obeyed over this range. Low absorbance values in the long wavelength region were confirmed in a 180cm double pass long path cell. Appelman and Studier (1971) prepared HOF in a closed all-Teflon circulating system. In this work an open stainless steel apparatus was used, otherwise the methods are similar. Nitrogen carrying l-5 % fluorine was flowed at around 100,~ torr cm3 s-l through a
U-tube at -45°C filled with Rgshig rings which had been previously sprinkled with water. Product stream proceeded through traps at - 50°C and - 80°C which removed water and some HF. An HF-HOF mixture was collected at - 190°C. Hypofluorous acid was identified by its characteristic i.r. spectrum (Appelman and Kim, 1972). The vapor pressures of HOF and HF are very similar making separation impossible. The amount of HF in any HF-HOF mixture was quantified in the i.r. On some occasions CFI 0 was observed as an impurity. Runs with any trace were discarded and not considered in subsequent analysis and calculations. In the metal system used for purification and transfer of the HOF-HF mixture, HOF was found to be of unpredictable stability (explosive) at greater than -40°C. Consequently the maximum amount available for any one measurement was its vapor pressure at this temperature, around 30 torr. Adding methane to the mixture to 1 atmosphere improved stability. Hypofluorous acid spectroscopy was conducted in 1Ocm monel cells coated inside with halocarbon wax and fitted with sapphire windows. All cross-section measurements for both molecules were made at 295 K, and were taken on a Cary 2 19 u.v.visible spectrophotometer interfaced to a Nova 3 computer for data manipulation.
759
RESULTS AND DlSCIJSSlON
Figures 1 and 2 give the absorption spectra of FNO, and HOF determined in this work. Uncertainties discussed below have been treated according to Hudson (1971). In the FNO, case estimated uncertainty in the 20&350 nm range is + 15 %, bracketing general differences between runs. In the longer wavelength regions cross-sections disappear into instrumental uncertainties and are indicated as vertical bars. In the HOF experiment, relative uncertainty in cross-section between 200 and 350 nm arises mainly from uncertainty in HF pressure measured in the cell. Estimates represent lu deviations and again appear as vertical bars.
SCOTT ELLIOTT
760
2
I
200
I 250
I
I
I
I
300
350
WAVELENGTH
(nml
factor of 5-10. Their residence times in the stratosphere should be correspondingly longer. Average daily and noon values of the unimolecular photodissociation coefficients for FN03 and HOF have been calculated as a function of altitude using a computer program written by M. J. Molina (private communication, 1981). Altitude and wavelengthdependent solar flux values used were the latest available at the time of computation (D. Wuebbles, private communi~tion, 1981). Lower limits for crosssections given in the figures were taken in order to put upper bounds on stratospheric photolysis lifetimes. Results are given in Tables 1 and 2.
I
400
Fig. 1. Absorption spectrum of FNOJ at 295 K.
Table 1. Solar photodissociation coefficients for fluorine nitrate, FNO,, as a function of altitude, in units of 10-5s-’ Altitude (km) 10 15 20 25 30 35 40 45 50
200
Fig.
2.
300
350
WAVELENGTH
250
km4
400
Absorption spectrum of HOF at 295 K.
Decomposition of HOF was monitored to completion in both the i.r. and U.V.In most cases, correlation of i.r. and U.V.bands permitted the unambiguous assignment of all initial U.V.absorption to HOF at all wavelengths. After one such decomposition an unidentified absorption remained beyond 350nm, but 75 “/, of initial absorption was still attributable to HOF in the 300-350nm spectra1 region critical to calculation of stratospheric photolysis lifetimes. Lengthening vertical bars at long wavelengths reflect the results of these band correlation experiments. Continuous spectra for both FNO, and HOF indicate absorption to unbound states. The processes should be dissociative with quantum yields of unity. From HOF there are only two possible sets of nonr~rrangement products, H + OF and OH f F. The H + OF pathway is energeti~lly inaccessible at the measured HOF threshold, and in the 300-350nm region. None of the possible non-rearrangement products from FN03 can be excluded on the basis of thermodynamics. Both the FNO, and HOF spectra are blue shifted relative to those of the analogous chlorine compounds CINOS and HOC1 (Rowland et al., 1976; Molina and Molina, 1978). Over most of the 2&34OOnm range cross-sections of FNO, and HOF are smaller by a
Average 0.36 0.36 0.37 0.43 0.71 1.7 3.5 5.4 6.6
Noon 1.1 1.1 1.1 1.3 2.5 5.5 10 15 18
Table 2. Solar phot~~ss~iation coefficients for hypofluorous acid, HOF, as a function of altitude, in units of 10-5s-’ Altitude (km) 10 15 20 25 30 35 40 45 50
Average
Noon
2.9 2.9 2.9 3.0 3.2 3.5 4.0 4.4 4.7
8.7 8.7 8.7 9.0 9.5 10 11 13 14
It is apparent that throughout the stratosphere, maximum lifetimes are on the order of days for FN03 and hours for HOF. These may be compared with typical vertical mixing times in the stratosphere, which are on the order of months to years. The gross conclusion can be drawn that neither FNO, nor HOF represents a fluorine sink of the magnitude of HF. Further interpretation awaits resumed modeling of stratospheric fluorine. Acknowledgement-This number NSG-7208.
work was supported by NASA grant
REFERENCES Appelman E. H. and Studier M. H. (1971) HypoAuorous acid. J. Am. them. Sot. 93, 2349-2352. Appelman E. H. and Kim H. (1972) Gas phase infrared spectra of HOF and DOF. J. them. Phys. 57, 3772-3’775.
Ultraviolet
absorption
Hudson R. D. (1971) Critical review of ultraviolet photoabsorption cross sections for molecules of astrophysical and aeronomic interest. Reo. Geophys. Space Res. 9, 305-381. Miller R. H., Bernitt D. L. and Hisatsune S. (1967) Infrared spectra of isotopic halogen nitrates. Spectrochim. Acta 238, 2233236. Molina M. J. and Molina L. T. (1978) Ultraviolet Spectrum of HOCI. J. phys. Chem. 82, 241&2414. Rowland F. S. and Molina M. J. (1975) Chlorofluoromethanes in the environment. Reo. Geophys. Space Phys. 13, l-35. Rowland F. S., Spencer J. E. and Mohna M. J. (1976)
spectra
of FNO,
and HOF
761
Stratospheric formation and photolysis of chlorine nitrate. J. phys. Chem. 80, 2711-2713. Stolarski R. S. and Rundell R. D. (1975) Fluorine photochemistry in the stratosphere. Geophys. Res. Left. 2, 443445. Sze N. D. (1978) Stratospheric fluorine, a comparison between theory and measurement. Geophys. Rex Left. 5, 781-784. Watson R. T., Smokler P. E. and Demore W. B. (1978) An assessment of a fluorine or N, 0, injection from an aborted space shuttle mission. NASA contract report NASA-CR 156164.