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J. Phys. Chem. 1992,96,9441-9446 (16) (a) Rosinek, M.P. Cutul. Rev.-Sci. Eng. 1977,16, 111. (b) Yao, J. C.; Yao, J. F. J . Cutul. 1984,86, 254. (c) Summers, J. C.; Ausen, S. J . Cutul. 1979, 58, 131. (d) Rieck, T. S.; Bell, A. T. J. Cutul. 1986, 99, 278. (17) (a) Fwher, H. E.; Schwartz, J. J. Am. Chem. Soc. 1989,111,7644. (b) Cannon, K. C.; Jo, S. K.; White, J. H. J. Am. Chem.Soc. 1989,111,5064. (18) Kung, M.C.; Kung, H. H. Curd. Reo.-Sci. Eng. 1985, 27, 425. (19) (a) Solymosi, F.; Kiss, J. Sut$ Sci. 1985,17, 149. (b) Solymosi, F.; Kiw,J. J . Chem. Phys. Lett. 1984, 110,639. (20) (a) Henderson, M.A.; Worley, S. J. Phys. Chem. 1985,89,1417. (b) Solymosi, F.; Erdohely, A.; Ban& T. J. Chem. Soc.,Faruduy Trans. 1 1981, 77,2645. (c) Solymosi, F.; Erdohely, A.; Bansad, T. J. Coral. 1981.68, 371. (d) Iizuka, T.; Tanaka, Y.; Tanabe, K. J. Mol. Cutul. 1982, 17, 381. (e) Solymosi, F.; Pasztor, M.J. Curd. 1987,104, 312. (f) Ichikawa, S.Coral. Lett. 1989, 3, 197. (9) Ichikawa, S.J. Mof. Curd.1989,53, 53. (21) (a) Solymosi, F.; Bugyi, L. J. Chem. Soc., Furuduy Truns. 1 1987, 83,2015. (b) Kiss, J.; Rencan, K.; Solymosi, F. Surf.Sci. 1988,207, 36. (c) Liu, 2.M.;Zhan, Y.; Solymoai, F.; White, J. M.Surf.Sci. 1981, 245, 289. (22) (a) Mahan, G. D.; Lucas, A. A. J. Chem. Phys. 1979.68, 1344. (b) Persson, B. N. J.; Ryberg, R. Phys. Rev. 1981.824, 6954. (c) Persson, B. N. J.; Liebisch, A. Surf. Sci. 1981, 110, 356. (d) Woodruff, D. P.; Heiden, B. E.; Prince, K.; Bradshaw, A. M.Surf. Sci. 1982,123,397. (e) Ueba, H. Surf.Sci. 1987, 188, 42 1. (23) (a) Solymosi, F.; Erdohely, A. J. Mol. Curd. 1980,8,47. (b) Vannice, M. A. J . Curd.1975, 37, 449. (c) Bardet, R.; Trambouze, Y. C.R. Acud. Sci. Paris, Ser. C 1979, 288, 101. (24) (a) Biloen, P.; Sacbtler, W. H. H. Adu. Cutul. 1981, 30, 165. (b)
Vannice, M. A. Cutul. Rev.-Sci. Eng. 1976,14, 153. (c) Bell, A. T. Cutul. Rev.-Sci. Eng. 1981,23,203. (d) Mills, G. A,; Steffgen, F. W. Curd. Mu. 1978, 10, 139. (25) (a) Weatherbee, G. D.; Bartholomew, C. H. J. Coral. 1981,67. (b) Solymosi, E;Erdohely, A.; Basagi, T. J. Cutuf. 1981,68,371. (c) Solymosi, F.; Erdohelyi, A,; Basagi, T. J . Cutul. 1981, 62, 165. (26) (a) Bcrkb, A,; Solymosi, F. Swf. Sci. 1981,187,359. (b) Liu, 2.M.; Zhou, Y.; Solymosi, F.; White, J. M. Surf.Sci. 1991,245,289. (c) Kiss, J.; R b b z , K.; Solymosi, F. Surf.Sci. 1988,207,36. (d) Berkb, A.; Solymwi, F. Surf.Sci. Lett. 1986, 171, L498. (e) Solymosi, F. J. Mol. Cutul. 1991, 65, 337. (27) (a) Behm, R. J.; Christmann, K.; Ertl, G. Surf. Sci. 1990,99,320. (b') Behn, R. J.; Fenka,V.; Cattania, M.G.; Christmann, K.; Ertl, G. J. Chem. Phys. 1983,78,7486. (c) Solymosi, F.; Kovacs, I. J. Phys. Chem. 1989,93, 7937. (d) Ehsasi, M.; Christmann, K. Surf. Sci. 1988, 194, 172. (e) Christmann, K.; Ehsasi, M.;Hirschwald, W.; Block, J. H. Chem. Phys. Lcrr. 1!W, 136,192. (f) Lauth, G.; Schvartz, E.; Cbristrnann, K. J. Chem. Phys. 1989, 91, 3729. (28) (a) Eisenberg, R.; Hendrihn. Adu. Card. 1987,28, 119. (b) Darensbourg, J.; Ovalles, C. J . Am. Chem. Soc. 1984, 106, 3750. (c) h g h , J. R.; Bruce, M.R. M.;Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1991,30,86. (tl) Braunstein, P.; Hatt, D.; Nobel, D. Chem. Rev. 1988, 88, 747. (e) Darmsbourg, D. J.; Wiengrcffe, H.P.; Wiengrcffe, P. W. J. Am. Chem. Soc. 1990, 112, 9252. (f) Darenabourg, D. J.; Kudaroski, R. Adu. Orgunonact. Chem. 1983,22,129. (g) Palmer, D. A.; van Eldik, R. Chem. Rev. 1983,83, 6.51.
Heterogeneous Chemistry of HBt and HF David R. Hamon* and A. R. Ravisbankara NOAA Aeronomy Lab, 325 Broadway, Boulder, Colorado 80303, and Cooperative Ijutitute for Research in Environmental Sciences University of Colorado, Boulder, Colorado 80309 (Received: May 21, 1992; In Final Form: August 19, 1992)
The heterogeneous chemistry of HBr and HF was studied on glass and ice surfaces at 200 K. Physical and reactive uptake were investigated using a cylindrical fast-flow reactor in conjunction with a chemical-ionization mass spectrometer. HF exhibited no measurable uptake on ice or NAT nor did it significantlyreact with ClONO2 or HOCl. It is inferred that HF would be unreactive on polar stratospheric cloud particle surfaces. Physical adsorption or absorption of HBr was observed onto glass, adsorbed H20 layers on glass, and ice. For the relatively high [HBr] used, a very large uptake by ice was obsemd, and we detected no saturation of the surface. HBr reacts efficiently with CIONOz, Clz, and NzOs, especially on ice and nitric acid ice surfaces. BrCl was observed as the primary product for the reaction HBr + CION02. The products HCl and Brz were observed for the reaction of HBr with C12. Qn the basis of these results, it is likely that HBr would be p r d efficiently on ice particles.
Inaoductioa The heterogeneous processing of reservoir chlorine species, mainly HC1 and CIONOz, into easily photolyzed forms is essential for the occumnceof the Antarctic ozone hole.' The heterogmeous chemistry of HCl has recently come under extensive investigation2-'0 and reactions of HOCl and C1ONO2with HCl adsorbed on the surface of ice were found to be very effi~ient.~*'~ The bromine analogues of the chlorine reservoirs, HBr and BrONOZ, are short lived due to rapid gas-phase processes." Yet, it is of interest to study the possible heterogeneous reactions of these species. Heterogeneous processes may be especially important when gas-phase processing is slow such as during the polar night. In addition, heterogeneous procesSing of bromine compounds may play a role in the rapid loss of ozone observed in the winter arctic troposphere.I2 Heterogeneous processing of H F would have a large impact on its chemistry and its use as a tracer, as it is currently thought to be inert.I3 It is believed that the majority of fluorine released in the degradation of fluorocarbon molecules ends up as HF. At present, stratospheric levels of H F are comparable to HCI, especially in the Antarctic stratosphere. The abundance of H F is expected to increase or remain high as the current CFCs (chlorofluorocarbons) are replaced with compounds that contain less chlorine and in many cases, more fl~0rine.l~ The gas-phase re0022-3654/92/2096-9441$03.00/0
action of HF with C10N02is endothermic, unlike the case of HCl .+ CION02. Yet H F may react on the surface of a particle via dissociation of HF or C1ONOz on the surface. Therefore, it is important to know if reactions such as HF CIONOz can OCCUT heterogeneously and if so, how efficiently. This is the first investigation of the heterogeneous reactions of HBr and HF. The primary aim of this work was to qualitatively assess if such processes can occur and, if they do take place, their importance in the atmosphere. Therefore, mechanistic information was the major goal. Quantitative loss rate parameters were determined only when the process was determined to be important and also experimentally feasible. Our first step in aswsing the heteroGenaous reactivities of HBr and H F was to study their physical uptake onto water ice near 200 K. We are aware of no studies of this kind for HBr or HF. The physical uptake of HCI by ice has been investigated by a number of researchers (refs 9 and 10 and references therein) and the uptake phenomena reported for HCl help us interpret our measurements for HBr and HF by ice surfaces. To investigate the reactivity of HBr and HF, we studied the reactions of CIONOl with HBr and H F on ice. In addition, the reactions of HBr with Clz and Nz05 were explored. We also performed some experiments over a cold Blasp surface for reactions involving the HBr molecule. In some of these experiments, water
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9442 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
vapor was added to the camer gas to allow for adsorption of water onto the glass surface and its ansequent effect on the uptake rates. These latter investigations are interesting because they provide lower HBr surface concentrations than those in the experiments over ice.
Experiment The uptake of a species by a surface was studied by monitoring its gas-phase concentration as its contact time with that surface was varied. These measurements yield the first-order rate coefficient for the law of the species. The growth in the reaction products signal was also monitored. In one instance, the measured growth rate coefficient of the product was used to derive a reaction probability. The surface was located on the inner wall of a cylindrical flow tube and the gas-surface exposure was varied by changing the position of an injector through which the species was introduced into the flow tube. The uptake coefficient was calculated from the measured fust-order loss rate coefficient using the standard procedure for flow tubeds and when required, the more involved procedure of Brown.I6 The gas-phase species in the flow tube were detected with a chemical-ionization mass spectrometer. This detection technique has proved to be a very selective, sensitive, and versatile detector for many of the species of interest. The main parts of the experimental apparatus have been described b e f ~ r ehere ; ~ ~we ~ provide only a brief summary. Two different cylindrical flow tubes were used in the course of this study; one had a 1.89-cm i.d. and the other a 2.2-cm i.d. The temperature of the flow tubes were controlled by circulating a cooling fluid through a jacket surrounding the tube. There were two ports at opposite ends of the flow tube for movable injectors: a double-walled injector at the upstream end for separately introducing reactants and an insulated injector at the other end to supply water vapor for preparing ice surfaces. For some experiments, HBr was introduced through an insulated injector at the upstream end while C1ONO2entered the flow tube through a port at its upstream end. The ice surfaces, which were between 2 and 10 pm thick and covered the entire circumference of a 5-10-cm length of the flow tube wall, were prepared as described earlier.9 The experiments were camed out at 201 (fl) K,with the average carrier gas velocity between 1500 and 2400 cm s-'. Reactants HBr and H F were drawn from -5% mixtures in He and diluted to approximately 0.01% inside the injector, and their initial concentrations were estimated from known ionmolecule reaction rate coefficients." Because of the relatively low detection sensitivity for HF, its concentration was on the order of 101L1013molecules HF was detected via the threebody reaction with F (producing FsHF) or its reaction with 0-producing F and OH.'" HBr was detected as HF*Br-using the SFC reactant ion. The initial concentration of HBr, [HBr],, was between - 5 X lo9 and 10" molecules CION02and N2Os were introduced from cold traps at 163 (f10) and 198 (f5) K, respectively. Their initial concentrations, between 1Oloand 10" molecules ~ m - were ~ , estimated from their vapor pressures,*flow rates, and total pressures. C12 was introduced from a -1% mixture in He. The concentrations of HC1, C4, and Br2 were estimated from known ion-molecule reaction rate coefficients;17b we estimate the concentrations to have an absolute accuracy of f50%. Some species, such as BrCl and HN03, were observed but not quantified. CI2, Br2, and BrCl were detected on their parent ion peaks via charge exchangewith SF6- while HC1, €€NO3, and C1ONO2were detected via F exchange with the SF6-reactant ion. The relative detection sensitivities, given by the relative reaction rate coefficients, for the species C135C13S:HBr79:C1350N02:HC135 are 0.5:1:2:3. Because of mass discrimination within the spectrometer, the actual relative sensitivitiescan vary by as much as a factor of 2 from these estimates. For experiments conducted over ice, a flow of 1 STP cm3 s-I of He (approximately 20%of the total camer flow) was flowed through the injector. This was necessary to prevent HBr from adhering to the bottom few millimeters of the injector. Condensation did not take place when water vapor was absent (or when the insulated injector was used), implying that an HBr/H20
-
H B r on ice 1000
c
1
0
1
J
100
10 2
3
4
5
6
Inj. Pos. cm Figure 1. h s of HBr as it was increasingly exposed to an ice surface. Average carrier flow velocity was 2000 cm s-I. The uptake did not decrease with exposure time.
mixture was formed on the injector when low He flows were used. Some experiments on the glass surface were performed with water vapor added to the carrier gas. This was accomplished by flowing the He carrier gas over an ice deposit on the flow tube wall far upstream of the measurement region. This flow of He saturated with H20 at the ice vapor pressure was diluted by an equal flow of dry He through the injector so that the gas in the measurement region was humidified to approximately SO(flO)% of the ice vapor pressure.
Results and Discussion Physical Uptake of HBr and HF by Ice. Figure 1 is a plot of HBr signal vs injector position for the exposure of HBr to an ice surface. The average carrier gas velocity was 2000 cm s-' and [HBr], was -2 X 1OO ' molecules The loss rate coefficient for HBr shown in Figure 1 did not change with time: we observed no change in the uptake of HBr by the ice surface for HBr surface coverages up to 5 X molecules cm-2 (- 10 monolayers). In similar experiments with large concentrations of HCI, [HCl], >10l2 molecules we also observed a large, timeindependent uptakee9 However, at lower [HCl], the uptake ceased after - 5 X 1014molecules of HCI, approximately a monolayer, was deposited on the surface? The unlimited uptake at high [HCI] was interpreted to be due to the formation of an HCI/H20 phase on the ice surface. This phase could be a stable crystalline hydrate or metastable supercooled liquid.l8 The appearance of a phase other than HBr-in-ice solid solution is also likely to have oocurred in the present experiments. There are no reported measurements of the HBr or H20partial pressures of HBr-H20 solids or solutions at low temperatures; thus we cannot firmy prove that they did form in our experiments. Although we cannot rule out the possibility of multilayer HBr adsorption onto ice, it is very unlikely as the vapor pressure of pure HBr is several hundred Torr at 200 KF3 Another possible explanation for the unlimited uptake is rapid diffusion of HBr through the bulk of the ice sample. However, this is unlikely to be fast enough to lead to the large uptake. In the case of HC1, such rapid diffusion was not observed?J9 If an HBr-H20 mixture was formed on the ice surface, the HBr-H2O binary phase must exist at very low [HBr]. In Figure 1, the HBr signal of 3 Hz for the injector at 7 cm corresponded to an [HBr] of approximately 3 X 10s molecules ( P H ~-6r )( Torr) and is a rough measure of the upper limit for the HBr concentration in equilibrium with such a phase. This low equilibrium vapor pressure is consistent with a value of 7 X l@ Torr obtained from a crude extrapolation of the partial pressure of HBr, PHBr, over a 38 wt % HBr solution at 29g20 to 202 K,which is its freezing point?' This value delineates the vapor pressure boundary between the HBr-in-ice solid solution and the IIBr-H20 liquid at 202 K. This is close to our operating temperature, and thus this value is also the pressure at which an I I B r H 2 0 liquid could be formed in our experiments. No stable
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Heterogeneous Chemistry of HBr and H F crystallineHBr-hydrates have been reported in this commition and temperature range. Therefore, PHB,was extrapolated according to the Clausius-Clapeyron equation using a value of 17 kcal mol-’ for the partial molar latent heat of evaporation of HBr. This value for the latent heat was estimated from the variation in PHB,over a 48% solution from 293 to 328 K20 using the Clausius-Clapeyron equation. The latent heat for HBr from a 38% solution is probably greater than that of the 48% solution, and thus the extrapolated PHB,of 7 X lo4 Torr can be viewed as an upper limit. For example, a value of 20 kcal mol-’ for the latent heat results in a value of 7 X Torr for P H ~This ~ . low value for PHB,is consistent with our results shown in Figure 1. Under atmospheric conditions, the gas-phase concentrations of HBr could be lower than the limit for PHB,derived above. Even if this is the case,it is likely that particles in the atmospherewould efficiently scavenge HBr. HBr uptake was also studied onto the glass flow tube wall at 200 K with and without H 2 0 vapor added to the carrier gas. Without water vapor present, a very small uptake was detected: a small decrease in the HBr signal (- 10%) was observed for a few seconds upon initial exposure of a 70 cmz area of the flow tube wall. The fractional surface coverage, 8, for HBr was estimated to be for an HBr pressure of 7 X lo-’ Torr. In later experiments conducted over the glass flow tube that had been treated with an H F solution, HBr uptake was much greater; 8 was -0.1 independent of pHBr ranging between 2 X to 3 X Torr. The lack of dependence on pressure may have been due to a filling of surface sites. HBr adsorption onto this freshly cleaned glass surface was observed to decrease with time. Furthermore, the contrast between the behavior on ice and glass indicates that the uptake on ice is not simply a physical uptake onto active sites. HBr uptake was much greater in the experiments with water vapor present in the carrier gas at 50% of the ice pressure. For [HBrIogreater than - 5 X Torr, there was a large uptake, three monolayer HBr coverage was observed with no indication of a decrease in the uptake. This behavior can be explained by the formation of a new phase: an HBr-H20 mixture on the glass surface that has a vapor pressure of HBr of (3-5) X Torr at 200 K with pH20 at 50% of the ice pressure. This threshold pressure is greater than the estimate of the minimum HBr pressure over a 38% liquid discussed above. A higher PHB,is required in this case as less water vapor is present. In addition, a higher HBr pressure would be required if the glass surface is a poorer surface than ice on which to grow the liquid (or crystalline) mixture. Also, formation of a different H20-HBr phase than was formed on ice is possible. We performed identical experiments to investigate the loss of H F onto ice and observed very little, if any, loss. These experiments were performed with high [HFIo. The measurements were carried out within a few seconds and hence saturation upon reaching a monolayer (5 X 1014molecules should have been detected. However, saturation upon formation of 10.05 monolayer of HF would not have been detectable for the observational time scales used. The obrserved lack of uptake implies that the [HFIo we used was below that needed for HF-H20 mixtures to grow. These results yield an upper limit of 3 X lo3 atm-’ for the adsorption constant for H F onto ice. The PHF over HF/H20 solutions pertinent to our experiment have been reported over a temperature range 273-313 K.22 Extrapolating these to our experimental temperature, 200 K,assuming a constant latent heat of evaporation for HF, yields a PHF 1 3 X lo4 Torr ( 10” molecules ~ m - for ~ )an HF/H20 solid mixture to form. We used [HFIo at or below this value so the appearance of such a phase is not likely to have affected our experiment. In addition, if an HF-hydrate had formed, it is likely that the uptake of HF would have been observable as was the case for HC19 and HBr (discussed above). The lack of H F uptake also suggests that the H F coverage of ice crystals in the stratosphere is very low. Assuming that the fractional surface coverage is proportional to partial pressure in the gas phase, our upper limit for the H F adsorption constant gives
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N
The .lournu1 of Physical Chemistry, Vol. 96, No.23, 1992 9443 H B r + ClONO,
V
V
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BrCl
20
Injector Pos. cm F’lgme 2. Loss of HBr (solid circles) and C1ONO2(open circles) and the production of BrCl (triangles) over a cold glass surface.
an HF coverage of less than 10” monolayer on stratospheric ice crystals. Therefore, physical removal of H F from the stratosphere due to adsorption on ice particles followed by sedimentation is not efficient. The low amount of physical adsorption of HF onto ice surfaces is in stark contrast to the situation for HC19and HBr (discussed above). H F is a highly polar molecule as are HCl and HBr; however, it does not easily dissociate as can HCl and HBr. It appears that the efficient adsorption of HCl and HBr by ice is due to their dissociation at the surface which would not be @ble for HF which is a weak acid. Reaction of CION@ with HBr. Because we used relatively high [HBrIo, in comparison to atmospheric concentrations, the experiments carried out over ice surfaces most likely had higher HBr surface coverages than would be found in the atmosphere. Reaction probabilities measured with lower HBr surface coverages (Le., cold glass with adsorbed water) may better represent the heterogeneous reactivity of HBr in the atmosphere. Therefore, the reaction of ClONO2 with HBr was studied on cold glass, cold glass with adsorbed HzO, and on ice. (i) Cold Glass. We studied the reaction HBr CIONOz on cold glass to assess the role of H 2 0 in the reaction and also to study the effect of lower HBr surface coverage. In addition, we can determine whether these reactions are fast enough (if they go at all) in the gas phase to affect our ice uptake measurements. We introduced both HBr and C10N02 into the flow tube at 200 K,through separate parts of the injector, at concentrations of about 5 X lo9 molecule In this experiment, no water vapor was added to the He carrier gas and it contained only trace levels of H20; thus adsorbed water vapor on the flow tube wall was minimal. Figure 2 shows the signals for HBr, C10N02,and BrCl as a function of injector position. As [HBr] and [CION02] decrease, the signal due to product BrCl incrat9es. An increase in the HN03 signal was also noted, it is not shown in Figure 2 because of a high variability in [HN03] exiting the flow tube presumably due to adsorption onto and desorption from the surface. These ohervations suggest the direct reaction
+
HBr
-
+ C1ONOZ
BrCl
+ HN03
(1)
takes place on the surface. The reactive uptake coefficient, y, for C10N02 in the presence of HBr on glass from the data in Figure 2 is Note that C1ONOz was not taken up onto the cold glass at 200 K in the absence of HBr. At 220 K and even with 10 times higher concentrations than those used at 200 K,no detectable decreases in HBr or CIONOz or increases in BrCl were observed. This result indicates (1) that if a gas-phase reaction occurs between HBr and ClONOz, it is too slow to affect our experiments and (2) that HBr adsorption on this glass surface was much lower at 220 K than at 200 K. These experiments were repeated on the HF-treated flow tube wall, for the case where HBr adsorption was observed to lead to
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9444 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
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Hanson and Ravishankara
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2
HBr
4
6
8
-
Inj. Pos. cm
Figwe 3. Reaction HBr + C10N02 products is shown as it was catalyzed by a glass surface with adsorbed water layers. Symbols are as in Figure 2 with the additional product Br2 (solid trianglca).
fractional surface coverages of -0.01. The measured y were much higher; they ranged from a value of 0.04 to 10.3. In addition, the reactions did not slow appreciably on warming the glass surface to 220 K, which is markedly different than the observations discussed above. The reactions were observed to follow the stoichiometryof reaction 1 except when HBr was in excess. In this case, the production of BrCl was lower and production of HCl and Br2 were noted. This suggests the product BrCl reacted further with HBr: BrCl
+ HBr
-
HCl
+ Br2
1
- 2 0
4
6
8
1nj.Pos. cm Figure 4. Reaction of HBr with C10N02 over an HNO,-coated ice surface. (a) Signals due to HBr, ClON02, Br2 (solid triangles), and BrCl (triangles) with ClON02 in excess and (b) HBr, C10N02, Br2, BrCl, and HCI (squarca) with HBr in exms.
(2)
Reaction 2 has been demonstrated on a NAT substrate (below). (ii) Cold Glass with 50% Relative Humidity carrier Gas. We performed an experiment similar to that depicted in Figure 2, except that the carrier gas contained H20. The results from an experiment with pC10N02 >> pHBr are shown in Figure 3, as plots of the product and reactant signals versus injector position. HBr was lost very efficiently due to reaction with ClONO2and BrCl and a small amount of Br2 was produced. The probability for d v e uptake of HBr was >0.3.Not shown is the production of HNO,;as before, there were significant delays in the increase of the HN03 signal, indicating the glass/H20 surface retains it for some time. BrCl was most likely produced in reaction 1, while Br2 could be produced in reaction 2. Another possibility is that HBr also reacted with C10N02 to yield BrON02 and HCl and the BrON02 reacted further with HBr to yield Br2. (iii) Pure and HNOrDoped Ice at 200 K. A pure water ice layer was deposited on the flow tube wall, and the reactants were exposed to this surface. Becaw HN03adheres to the ice surface to form a NAT-type and because HN03was present in these experiments as an impurity in the C1ONO2 and/or as a product of the reactions, the pure ice surface became coated with nitric acid within minutes. The experiments were performed over pure ice and HNO,-coated ice with no noticeable difference in the results for the reactions of HBr with ClONO2 and C4. Therefore, the discussion below applies to both pure ice and HN03-dopedice surfaces. Typical results for these experiments are shown in Figure 4, which shows plots of reactant and product temporai profla similar to F i i 2 and 3. In this case the uptake rates were much higher than in the absence of ice and therefore required much shorter exposure lengths/times. Figure 4a shows the loss of HBr and C10N02 along with the production of Br2 and BrCl when pC10N02,approximately 1.5 X 10" Torr,was -4 times pHBr. Figure 4b shows the HBr and C10N02 loss along with the production of HCl, Br2. and BrCl when pHBr, approximately 4 X lo-' Torr, was -4 times pC10N02. The decrease in gas-phase HBr shown in Figure 4b was not only due to reaction with C10N 4 but also due to the large uptake on ice diecussdd earlier. The lower limit to the uptake coefficients for both HBr in Figure 4a and CION02 in Figure 4b was 0.3,and the uptake coefficients did not vary with exposure time.
2
[HF] =10
I\
13
molecule e m
ClONO,
:
m
10"
__---
___---- ---
-3
-.
5 10 Inj. Pos. c m IQwe 5. CIONOl loa and HOCl production on ice/NAT in the preaence of [HF] at 2 X 10') molecules an-). 0
An explanation for these observations is that C10N02 reacts with HBr very rapidly on the ice surface producing BrCl, and the measured uptake coefficient is the reactive uptake rate. A p parently, when HBr is in excess, it reacts rapidly with BrCl to produce Br2and HCl. The appearance of the HNO, signal, the product of reaction 1, was delayed much longer than in the a b " of ice. Furthermore, the peak HN03 signals here were much smaller than when no ice was present. This decrease is likely due to the efficient uptake of HNO, by ice and NAT surfaces, presumably forming NAT crystals. Upper Limit for tbe Reaction of CION& with HF. A pure ice surface was simultaneously exposed to ClONO2 and HF at concentrations of 3 X 10" and 2 X loL3molecules respectively. HOC1, the known product of the reaction of C1ONO2with HzO, was observed. No other new species was detected in this system. The gas-phase concentration of HOCl was monitored along with that for C10N02 and HF. The observed HOCl and ClONO2 ixmnccntrations versus injector position are plotted in F i i 5. The i i m a t of HOCl produced was equal to 5&75% of the amount of C1ONO2lost. The y for CION02 in this case was 0.015. This low value of y indicates that the surface was nearly completely converted to "AT" due to the high [C10N02]~ used. Even at
Heterogeneous Chemistry of HBr and HF HBr
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The Journal of Physical Chemistry, Vol. 96, No. 23, 1992 9445
C1, Q 50% r . h
HBr
+
C1,
on ice
t
10' 0
2
4 6 8 Inj. Pos. c m
1
0
-
Floplv 6. Reaction of HBr with CI2 as it occurred over a glass surface with adsorbed water layers. The products HCI and Br2 are also shown.
+
these high [HFIo, the reactions C10N02 HF,b and HOCl + HFd were slow, and they may not have occurred at all. The above value of y for ClONO, uptake is about 3 times greater than the value of y for ClONO2loss on NAT that was previously measured in our laboratory? We attribute this larger value to HCI present as an impurity in the HF sample which we detected at a level of approximately 25% of the [CIONOz]o;thus some of the C10N02 and/or HOC1 product must have reacted with HCl. The measured value of 0.015 for y(ClON0J is consistent with the presence of HCl and/or the ice surface not being completely converted to NAT. The product yield for HOCl being less than unity (50-75%) is also consistent with the presence of HCI as HOCl reacts with HCl as does CION02? A conservative upper limit to y for the reaction C1ONO2 HF is 0.01 from this experiment. The value of y for the stratosphere, if the reaction occurs at all, is likely to be much lower than this upper limit as stratospheric HF concentrations are about 10' times lower than that used in this experiment. The lower HF concentrations should lead to -10' times smaller surface merages in the atmosphereand thus the potential reaction of C1ONO2with HF on the surface would be 10' slower. Reaction of HBr witb CI2. We observed the reaction of HBr with C12 occurring on a cold glass surface with the He carrier containing water vapor at 50% of the ice pressure. Figure 6 is a plot of the signals for HBr, C12, and the HCl and Br2products, as a function of injector position. We did not detect the formation of BrCl. The quantity of HCl produced in this experiment was equal to the amount of HBr and to twice the amount of CI2lost. Therefore, our results are consistent with the overall reaction of 2HBr C12 Br2 2HC1. The reaction most likely proceeds through the following sequence:
+
-
+
-
+
--
+ C12 HBr + BrCl HBr
BrCl
+ HC1
(3)
Br2
+ HCl
(2)
As no BrCl was detected, reaction 2 is likely to be much faster than reaction 3. The increase in the Br2signal was about a factor of 3 lower than that expected from the above sequence and is attributed to mass discrimination in these experiments. A lower voltage on the entrance orifice leading to the mass spectrometer was used in this experiment than for the experiment depicted in Figure 4b. This was found to be responsible for the mass discrimination due to inefficient collection of high mass ions. This was validated by other experiments which were carried out to check for mass discrimination. Figure 7 shows the results from an experiment performed over a pure ice surface. Losses of Clz and HBr and production of HCl and Br2 are evident. As in the case for the reaction of HBr with C10N02, the reaction occurs much more efficiently on solid ice. Note that the loss of HBr from the gas phase was not only due to reaction with C12but also due to its efficient uptake onto ice. The amounts of C12and HBr lost and HCl produced were again
1
0
1 2 3 4 5 6 7 Inj. Pos. cm FIgure 7. Same as in Figure 5 expect the surface was an ice deposit coating the glass surface. C12was in exccss (its signal is lower because af a lower detection sensitivity) and thus is not completely lost over this ice surface. The production of HCI in this figure matched the losa of HBr. HBr
+ N,O, cold glass
0
0
4
:100
4
M
.d
m
0
5
10 15 Inj. Pos. cm
20
Floplv 8. Three sets of data are shown for the loss of N2O5in the presence of HBr over: cold glaas (circle), adsorbed water layers and ice (solid circles), and anomalous uptake possibly due to the presence of a liquid layer (triangles).
consistent with the overall stoichiometry of the sequence shown above. However, as before, the Br2 signal was affected by mass discrimination. No loss of Clz was observed over the ice surface when HBr was absent. An upper limit to y of 7 X lo-" (at the 95% confidence level) for the loss of C12on ice without HBr present was estimated. Reaction of HBr with N205. We measured the loss of N205 in the presence of 5 X 109-101*molecules HBr over the three surfaces: cold glass, water adsorbed on cold glass, and HN0,-coated ice (NAT). The results of these experiments are shown in F i i 8 which is a plot of N2O5 signal vs injector papition for the different cases. The N2O5 loss over the clean, cold glass surface was very slow yielding an upper limit to y of lo-". Therefore, we report a value for y of 2 x 10'O molecules cmP3. After an ice layer was deposited and completely coated with HN03the loss of NzO, was observed in the ptesence of HBr, and the results were identical to the data presented as circles in Figure 8. As in the case for the 5096 humidity experiments, we s o m e h a observed high values for y, ranging from 0.03 to 0.06. These measurements mostly occurred when high [HBr], were used. Without HBr present, y for the loss of N2O5 was 0.001 or less, consistent with the loss of NZO5 on NAT reported previously.* The reaction is most likely HBr
+ NzOs
-+
BrONO
+ HN03
(4)
although production of BrON02 HONO cannot be ruled out. We did not attempt to detect the Br-containing products of the above reaction. s-ry Table I summarizes the values of y determined for the listed process over the designated surfaces. HBr exhibits an interesting
propensity for surface chemistry, especially on ice surfaces. It should be emphasized that the y we measured for these reactions on ice/NAT are likely to be lower in the atmosphere because we used -100 times greater [HBrJothan found in nature. This allowed for the formation of liquid HBr/H20 mixtures in our experiments that may not be present in the atmosphere. The adsorbed water layers, however, provide a surface with relatively low HBr content (for low [HBrIo) and thus may be more representative of the atmospheric situation. Even if the HBr uptake is as low as on the glass surface, HBr would be effectively processed on PSCs by reaction with CION02 as well as taken up on tropospheric ice particles. HF is not efficiently taken up by ice surfaces, and hence it is not likely to be removed by sedimentation of ice particles from the stratosphere. HF is also not very reactive on surfaces. The possible heterogeneous reactions of H F with HOC1 or C1ONOz are probably not important for chlorine activation or for HF loss.
R e NO. NAT, 13444-83-2;HF, 7664-39-3;CION02, 14545H20,773272-3;HOCl, 7790-92-3;C12.7782-50-5; N205, 10102-03-1; 18-5; HBr, 10035-10-6.
References plld Notes (1) Solomon, S.Rea Geophys. 1988,26, 131. (2)Molina, M.J.; Tso, T.-L.; Molina,L. T.; Wang, F.C.-Y. Science 1987,
238, 1253. (3) Tolbert, M.A.; Rossi, M.J.; Malhotra, R.; Golden, D. M.Science 1987,238,1258. (4)Tolbert, M.A.;Rwi, M.J.; Golden, D. M.Science 1988,240,1018. (5) Ltu, M.-T. Geophys. Res. Lett. 1988,15,17. (6)Leu. M.-T. Geophys. Res. Lett. 1988, 15, 851. (7) Moore, S.B.; Keyeer, L. F.;Leu, M.-T.; Turco, R. P.; Smith, R. H. Nature 1990,345,333. (8) Hanson, D.R.;Ravishankara, A. R. J. Geophys. Res. 1991,96,5801. (9)Hanson, D.R.; Ravishankara, A. R. J. Phys. Chem. 1992,96,2682. (10)Abbatt, J. P. D.; Molina, M.J. Geophys. Res. Lett. 1992,19,461. (1 1) Photolysb of BrON02: Spencer, J. E.; Rowland, F. S . J. Phys. Chem. 1978,82,7-10. Reaction of OH with HBr: Ravishankara, A. R.; et al. J. Chem. Phys. 1985,83,447. (12)Bame, L. A.; Bottenhiem, J. W.; Schnell, R. C.; Crutzen, P. J.; Rasmuesen, R. A. Nature 1988, 334,138. 113) Mankin. W. G.: et al. J. Atmos. Chem. 1990.10. 219-236. (14jAtkinson, R.;Cox, R. A.; Leaclaux, R.; Nib, H.;bllneriR. WMO Report No. 20 1989,2,159-266. (15) Howard, C. J. J. Phys. Chem. 1979,83,3. (16)Brown, R. L.J. Res. Natl. Bur. Stand. 1978,83,1. 117) la) Beiubaum. V.:et al. Chem. Phvs. Lett. 1982. 84. 4. 1b) Streit. G. k J. C k m . Phys. i98g 77,826. (c) Webharr, J. C.; Zwi& T.S.; Leon< S.R. J. Chem. Phys. 1981,75,4873. (d) Babcock, L.M.;Streit, G. E. J. Chem. Phys. 1982,76,2407. (18)Hanson, D.R.; Mauersberger, K. J. Phys. Chem. 1990, 94,4700. Wofsy, S.C.; et al. J. Geophys. Res. 1988,34,2442. (19)WOE,E.W.; Mulvaney, R.; Oates, K. Geophys.Res. Lett. 1989,16, 487. (20)International Critical Tables; McGraw-Hill: New York, 1928;Vol. 3, p 306. (21)Pickering, S.U.Philos. Mag.1893,36, 111. (22)Encyclopedia of Chemical Technology, 3rd ai.;10,733,Wiley: New York. 1980 Vol. 10.D 733. (23)CRC Hand&& of Chemistry and Physics, 60th 4.CRC ; Rtss: Cleveland, 1979.
