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J. Phys. Chem. 1996, 100, 190-194
Reactions of CF3O- with Atmospheric Trace Gases L. Gregory Huey,*,† Peter W. Villalta,† Edward J. Dunlea,† David R. Hanson,† and Carleton J. Howard NOAA, Aeronomy Laboratory, Boulder, Colorado 80303 ReceiVed: July 12, 1995; In Final Form: September 21, 1995X
The rate coefficients and product yields for the reactions of CF3O- with ClONO2, HNO3, HCl, N2O5, SO2, HI, and H2O were measured at 295 K and ∼0.4 Torr using the flowing afterglow technique. The reactions of CF3O- with HO2NO2 and H2SO4 were also studied qualitatively. CF3O- reacts rapidly with ClONO2, HO2NO2, SO2, and HCl by only fluoride transfer and with HNO3, HI, and H2SO4 by both fluoride and proton transfer. CF3O- also reacts with HI to form IF2- and with N2O5 to produce NO3-. CF3O- undergoes a slow clustering reaction with H2O and is transformed within water clusters into F-‚HF and F-‚(HF)2. CF3O- is unreactive with CH3NO3, CH3C(O)O2NO2, O3, NO2, CO2, and O2. These results demonstrate that CF3O- is an excellent candidate as a reagent ion for the selective detection of ClONO2, HCl, and HNO3 in the upper troposphere and stratosphere with a chemical ionization mass spectrometer. The observed reaction of CF3Owith H2O within water clusters indicates that CF3O- will hydrolyze in aqueous solution to form F-, HF, and CO2. This provides insight into the mechanism for the heterogeneous loss of CF3OH in the atmosphere.
Introduction ClONO2, HCl, and HNO3 are important trace species involved in the heterogeneous chemistry that leads to the Antarctic polar ozone hole.1 The relatively inert chlorine reservoir species, ClONO2 and HCl, are converted into the photolabile species HOCl and Cl2 by heterogeneous reactions 1 and 2.2
ClONO2 + HCl f Cl2 + HNO3
(1)
ClONO2 + H2O f HOCl + HNO3
(2)
These reactions are essential for the occurrence of the dramatic ozone loss during the Antarctic spring. In addition, the partitioning of the reservoir chlorine compounds (i.e., [ClONO2]/ [HCl]) at midlatitudes is not well understood.3 Despite the importance of these compounds, there are few specific in situ measurements of ClONO2 and HNO3 in the stratosphere. These species can be detected using chemical ionization mass spectrometry (CIMS) as has been demonstrated by Hanson and Ravishankara,4 Arnold and co-workers,5 and Viggiano and coworkers.6 We have undertaken the development of a chemical ionization scheme that can be used for the simultaneous in situ measurement of ClONO2, HNO3, and HCl in the stratosphere. HNO3, ClONO2, HCl, and many other species have been selectively detected in laboratory studies using CIMS and fluoride transfer reactions of SF6-.4,7 For example, HNO3 and ClONO2 are detected using the following reactions.
SF6- + HNO3 f NO3-‚HF + SF5
(3)
SF6- + ClONO2 f NO3-‚FCl + SF5
(4)
These reactions allow specific detection of these compounds with a mass spectrometer at the parent mass plus 19 amu. However, the fast charge transfer reaction of SF6- with O3 makes this ion unsuitable for the in situ detection of trace species in the atmosphere.7 Thus, it is desirable to identify an ion with †Also affiliated with Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO. X Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0190$12.00/0
fluoride transfer properties similar to SF6- that is unreactive toward relatively abundant atmospheric species such as CO2 or O3. An ion that potentially meets these requirements is CF3O-. The fluoride affinities of CF2O and SF5 are 42.6 ( 28 and 38.2 ( 49 kcal mol-1, respectively. This suggests that CF3O- and SF6- may fluoride transfer in a similar manner. CF3O has a very large electron affinity (4.2 ( 0.2 eV 9), and consequently it is unlikely that CF3O- will charge transfer to any atmospheric species. However, CF3O- may undergo proton transfer with a strong acid such as HNO3 to form NO3-. This could destroy the selectivity of CF3O- as a reagent ion for the detection of HNO3, because there are many species in the atmosphere such as N2O5, ClONO2, and organic nitrates which might also react with CF3O- to give NO3-. Therefore, the chemistry of CF3Owas studied to determine whether it reacts selectively with atmospheric species. The rate coefficients and product yields were measured for the reactions of CF3O- with a series of compounds. This series includes the primary candidates for CIMS detection, HNO3 and ClONO2, and other atmospheric constituents such as CH3NO3, HO2NO2, and H2O which could possibly interfere with CF3O- detection schemes. The reactivity of CF3O- is also of fundamental interest because of the formation of CF3O and CF3OH in the atmosphere. CF3O radicals are a key intermediate in the degradation of many hydrofluorocarbon and hydrochlorofluorocarbon compounds which are currently being used as replacements for chlorofluorocarbons.10 The primary loss for CF3O radicals in the lower atmosphere is reaction with hydrocarbons11 and possibly water12 to form CF3OH. The only efficient loss process for CF3OH in the atmosphere that has been identified is reactive uptake onto water droplets.13-16 It is likely that CF3OH dissociates in aqueous solution because it is a relatively strong acid.9,17
CF3OH(aq) + H2O(1) T H3O+(aq) + CF3O-(aq) (5) Lovejoy et al.13 have postulated that CF3OH may be lost in aqueous solution due to the reaction of CF3O- with water. For this reason, another goal of this work is to examine the reactivity of CF3O- with water in the gas-phase to provide insight into the reactivity of CF3O- in aqueous solution. © 1996 American Chemical Society
Reactions of CF3O- with Atmospheric Trace Gases
J. Phys. Chem., Vol. 100, No. 1, 1996 191
Experimental Section
TABLE 1: Reactions of CF3O-
The reaction rate coefficients for CF3O- were measured using a flowing-afterglow apparatus. Both the experimental technique and apparatus have been described thoroughly7,18 and are only briefly discussed here. The ion-molecule reactions were carried out in a 130 cm long × 7.3 cm i.d. flow tube which was sampled into a quadrupole mass filter. Helium was used as the carrier gas at a flow rate of 100 STP cm3 s-1 (STP ) 0 °C, 1 atm) and a pressure of ∼0.4 Torr. This yielded a flow velocity of ∼5000 cm s-1 and reaction times on the order of 10 ms. The ion source was a heated thoriated-iridium filament biased at -50 V and regulated to give a constant emission current of 5-10 µA. CF3O- was generated by electron attachment to CF3OOCF3, which was added to the flow tube just downstream of the filament.19,20 CF3O- was the only ion identified as a product of electron attachment to CF3OOCF3. CF3O-‚HF was the only secondary ion generated by this method and was typically
