The Journal of
Physical Chemistry
0 Copyright, 1986, by the American Chemical Society
VOLUME 90, NUMBER 10 MAY 8,1986
LETTERS The Hydrolysis of Chlorine Nitrate and Its Possible Atmospheric Significance F. Sherwood Rowland,* Haruo Sato, Haidet Khwaja, and Scott M. Elliott Department of Chemistry, University of California, Irvine, California 9271 7 (Received: November 6, 1985; In Final Form: February 6, 1986)
The hydrolysis of CIONOz takes place very readily on a variety of laboratory surfaces and may also occur catalytically on particulate surfaces in the stratosphere. The reaction can be considered as an oxide exchange between two X-0-Y molecules with X and Y = H, CI, or NOz. Two other reactions in this class which might occur in the stratosphere are HOC1 plus HOC1, and HOCl plus C1ONOz. Each of these three is approximately thermoneutral and should be accompanied by the reverse reaction with a comparable reaction rate constant. Current atmospheric models have not explained the very large ozone depletions which have taken place during Antarctic spring in the past decade. The chemical reactions included in these models may need to include heterogeneous catalysis of one or more of these oxide exchange reactions.
Introduction The decomposition in the stratosphere of organochlorine compounds emitted into the troposphere, e.g. CC12F2,CC13F, CCI4, CH3CI, and CH3CC13,releases chlorine which can then be found among at least five important inorganic chemical species: C1, C10, HC1, HOCl, and C 1 0 N 0 2 (chlorine nitrate). The first two are highly reactive odd-electron species capable of producing the CIO, free radical chain of (1) and (2), whose net result is the removal C1 + O3 C10 O2 (1)
c10
+ 0
+ CI + 0 2
(2)
of odd oxygen (= O3or 0) from the stratosphere in the altitude range from 30 to 50 km.'S2 The other three chlorinated compounds are even-electron species which serve as relatively unreactive reservoir molecules, temporarily withholding chlorine from the free radical chain processes. The two molecules C10N02 and (1) "Causes and Effects of Changes in Stratospheric Ozone: Update 1983"; National Academy of Sciences: Washington, DC,1984.
HOCl are formed in situ in the stratosphere by the cross termination of the C10, chain with the NO, chain in (3) or with
+ NO2 + M C10 + HO2
C10
-+
---c
ClON02
HOCl
+M
+0 2
(3) (4)
the HO, chain in (4), in each case removing the chain carriers for two chain reactions during the lifetime of the reservoir species. A full understanding of all of the atmospheric processes affecting these compounds has always been necessary for the modeling of stratospheric chemistry with the intent either of representation as it exists or prediction of possible future changes, e.g. depletion of stratospheric ozone. The need for understanding has been increased by the observation between 1974 and 1985 of substantial decreases in total ozone over with diminution of (2) "The Stratosphere 1981, Theory and Measurements", WMO Global Ozone Research and Monitoring Project, Report No. 11; World Meteorological Organization, 1982. (3) Farman, J. C.; Gardiner, B. G.; Shanklin, J. D. Nature (London) 1985, 315, 207.
0022-3654/86/2090-1985$01.50/0 0 1986 American Chemical Society
1986
The Journal of Physical Chemistry, Vol. 90, No. 10, 1986
more than 40% found in October 1985 at Halley Bay, Antarctica (76°S).5%6The rapidity of the decrease in Antarctic ozone in successive Octobers over the past decade implies causation by some other rapidly changing characteristic of the atmosphere and circumstantially points toward the organochlorine concentration of the atmosphere which has tripled in the past 15 years.' Ozone depletions as large as 40% in 1985 have not been found in any of the published calculations by the existing atmospheric models, suggesting that some important ozone-removingchemical reactions are not yet included. These missing reactions seem likely to involve significant participation by one or more halogenated species, with chlorine nitrate a prime candidate. Although chlorine nitrate was initially proposed as a potentially important stratospheric molecule a decade conclusive qualitative identification of its presence in the atmosphere has proven elusive'*I0and quantitative measurement of its concentration under a variety of conditions is still sought. The stratospheric burden of HOCl has proven even more difficult to assay, with only upper limits placed observationally on its concentrations to date.'," In the published calculations with stratospheric models, the only processes affecting any of these five chlorine species are homogeneous gas-phase reactions, either chemical or photochemical in nature. The formation of C10N02 is assumed to occur solely through (3), and removal is almost entirely by photolysis in (5), together with a much lesser photolysis route releasing 0 C 1 0 N 0 2 + hv
+ HzO CION02 + HCl
CION02
-+
-
+ NO, HOCl + HONO2 C1, + H O N 0 2
-+
CI
(5) (6) (7)
atoms, and quantitatively minor chemical reactions with unpaired electron species such as 0, HO, and CL6.12 The laboratory handling of chlorine nitrate is well-known to require great care to avoid heterogeneous hydrolysis to nitric a ~ i d , ~and , ' ~ both homogeneous and heterogeneous processes need to be. considered for C10N02 in the stratosphere. Consequently, we have performed a series of measurements on the reactions of C 1 0 N 0 2 with H20 to form nitric acid by ( 6 ) ,as well as with other nonradical species The inclusion of reaction 7 at such as HC1 by (7)14915and o3.I4 cm3 molecule-I s-] or reaction 6 at 3 X cm3 1 X molecule-' s-I in an otherwise standard one-dimensional stratospheric model raised the calculated steady-state ozone depletion from continued injection of chlorofluoromethanes from about 4% to more than 20% in each case.I6
Experimental Section Thorough drying of all of the apparatus is a prerequisite for (4) Heath, D. F.; Bhartia, P.; Schlesinger, B. Trans. A m . Geophys. Union 1985, 66, 1009.
(5) Krueger, A. J.; Stolarski, R. S.; Alpert, J. C.; Heath, D. F.; Chandra, S. Trans. Am. Geophys. Union 1985, 66, 838. (6) Farman, J. C., private communication. (7) Rowland, F. S. Origins Life 1985, 15, 279. (8) Rowland, F. S. Working Meeting of Panel on Atmospheric Chemistry, National Academy of Sciences, Snowmass, CO, July 1975. (9) Rowland, F. S.; Spencer, J. E.; Molina, M. J. J . Phys. Chem. 1976, 80, 2711. (IO) Rinsland, C. P.; Goldman, A.; Murcray, D. G.; Murcray, F. J.; Bonomo, F. S.; Blatherwick, R. D.; Malathy Devi, V.; Smith, M. A. H.; Rinsland, P. L. J . Geophys. Res. 1985, 90, 7931. (1 1 ) Larsen, J. C.; Rinsland, C. P.; Goldman, A,; Murcray, D. G.; Murcray, F. J. Geophys. Res. Lett. 1985, 12, 663. (12) "Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling, Evaluation Number 7", Demore, W. B., Ed.; Jet Propulsion Laboratory: Pasadena, CA, July 1, 1985; JPL 85-37. (13) Knauth, H. D.; Martin, H.; Stockman, W. 2.Nufurforsch. 1974, 29, 200. (14) Rowland, F. S.; Sato, H. Presented at the International Meeting on Current Issues in Our Understanding of the Stratosphere and the Future of the Ozone Layer, Feldafing, West Germany, June 11-16, 1984. (15) Molina, L. T.; Molina, M. J.; Stachnik, R. A,; Tom, R. D. J . Phys. Chem. 1985, 89, 3779. (16) Wuebbles, D.; Connell, P.; Rowland, F. S. Presented at the International Meeting on Current Issues in Our Understanding of the Stratosphere and the Future of the Ozone Layer, Feldafing, West Germany, June 11-16, 1984.
Letters CION02
00:02:54
H2O
+ CION02
HON02
+ HOC1
Figure 1. Successive FTIR spectra in the 500-1600-cm-' range at 10-s intervals following the addition of H 2 0 to CIONO,, showing the formation of H O N 0 2 .
TABLE I: Observed Chemical Reaction Rate Constants for CIONOl plus H20at 27 OC
other
rate constant,' cm3 molecule-' s-I x 1019
(Halocarb wax cell coating) (boric acid cell coating) 58 N 2 (Halocarb wax coating)
3.4 1.7 13 2.6 1.9 1.7
press., torr CION02 HI0
1.0 1.0 0.54 0.62 0.61 0.56
0.5 0.4 0.50 0.51 0.52 0.51
Kel-F cell
OObserved bimolecular rate constant for loss of CIONO,.
making any quantitative kinetic or photochemical rate measurements with C10N02. Our kinetic measurements of reaction 6 have been made with a IO-cm Pyrex infrared absorption cell with KBr windows and a small sidearm, each attached to a standard high vacuum line through a grease-free stopcock. Chlorine nitrate was synthesized from C l 2 0 and N 2 0 5by L. T. Molina of the Jet Propulsion Laboratory1' and was extensively degassed in our laboratory before each set of experiments. Between experiments, the IR cell was cleaned and dried in an oven at 270 "C, evacuated at 100 O C for at least 12 h, dried by exposure to BF, for 2 h, evacuated and then exposed to C 1 0 N 0 2 for 30 min, and evacuated once more. Finally, the required pressure of C1ONO2 was introduced into the cell, and FTIR spectra of C 1 0 N 0 2 were recorded for several scans, showing strong absorption features near 1300 and 1700 cm-'. After such preparation, C1ONO2 can be held in this infrared cell for periods of many hours with negligible loss, and without the appearance of H O N 0 2 . When H 2 0 stored in the sidearm is released into the IR cell, both the growth of H O N 0 2 and diminution of C 1 0 N 0 2 can be seen immediately, as illustrated in the successive scans of Figure 1. The initial data of Figure 1 are consistent with a loss of C10N02 by a reaction such as ( 6 ) with an equivalent bimolecular rate cm3 molecule-' SKI. The kinetic constant of about 3 X reaction rate constants for the loss of C 1 0 N 0 2 during the early portions of some of our other hydrolysis experiments are given in Table I for a variety of initial conditions, and none of our whole set of experiments has given an initial rate of disappearance for C10N02which corresponds to less than 1 X cm3 molecule-' s-l. In contrast, experiments carried out at the University of California Riverside with large volume reactors (2500 to 5800 L) have shown that the homogeneous rate constant k6 is at least (17) Molina, L. T.; Spencer, J. E.; Molina, M. J. Chem. Phys. Left. 1977, 45, 158.
The Journal of Physical Chemistry, Vol. 90, No. 10, 1986 1987
Letters 50-fold slower than our lower limit, with an upper limit of 2 X cm3 molecule-' s-I.I8 None of our four alternative surface treatments (Pyrex cell, Kel-F cell, Halocarb wax, or boric acid coatings on the Pyrex cell) has appreciably altered the observed initial bimolecular rate constants in Table I, indicating that a rapid heterogeneous reaction occurs in our system on a wide variety of surfaces, including some which are often unreactive in other systems. The ease with which reaction 6 can be catalyzed on generally inert surfaces in the laboratory invites extensive consideration of whether this reaction can be similarly catalyzed on any or all of the various types of particulate surface present in the stratosphere. Discussion Some major problems complicate any interpretation of the heterogeneous reactions reported in Figure 1 and Table I: (a) Reaction 6 is approximately thermoneutral and cannot go to completion, but only to an equilibrium position with major residual concentrations of CIONOz and/or HzO.I9 Nevertheless, the loss of CIONOz (e.g., in Figure 1) exceeded that expected from the equilibrium between (6) and (-6), indicating that additional reactions were occurring. (b) Reaction 6 and its reverse are only two of a general class of reactions, at least four more of which are likely to be present whenever (6) occurs. The combination of water vapor with chlorine nitrate belongs to an extensive class of reactions involving chemical substituent exchanges between molecules X-0-Y and X'-0-Y'. Even with H, C1, and NOz as the only substituents, 27 different bimolecular exchanges are possible, of which (6) is one and its reverse (-6) is another. Five more such forward/reverse pairs of bimolecular exchanges are possible, including the synthesis of C 1 0 N 0 2 from ClzO and N,05, the hydrolysis of N z 0 5in (8), the hydrolysis of H20
+ N2OS
-
HONO,
+ C120 HOCl + CION02 H,O
(8)
+ HOCl ( 2 1 2 0 + HONO2
HOCl
-
+
+ HONOZ
(9) (10)
ClzO in (9),the reaction of HOCl with ClONO, in (lo), and that of HOCl with NzOs, plus the reverse of each. In addition to these six pairs, 15 bimolecular exchange reactions can be written which HOD involve only isotopic exchange, e.g. D 2 0 HOCl DOCI. The list of such oxide exchange reactions can be greatly extended by the consideration of additional substituents such as Br, NO, etc. Although the homogeneous gas-phase rate constants are not definitely known for any of these 27 reactions, it seems likely that none are rapid enough to be important in the stratosphere. The rate of homogeneous hydrolysis for N,OS in (8) has proven very difficult to measure because of its sensitivity toward heterogeneous cm3 molecule-' s-l has catalysis, but an upper limit of 2 X been placed on the homogeneous rate constant, and even this may represent a residual heterogeneous contribution.18,20 Reaction 9 and its reverse (-9) are well-known to occur, as evidenced by the formation of equilibrium concentrations of H 2 0 , HOCl, and C I 2 0 from experimental syntheses of HOCl.21-24 The rapid reaction of HCI with ClzO in (1 1) has been used as a water-free
+
HCl
+ Cl20
+
Clz
+ HOCl
-
+
( 1 1)
(18) Atkinson, R.; Tuazon, E. C.; MacLeod, H.; Aschmann, S. M.; Winer, A. M. Geophys. Res. Lett. 1986, 13, 1 1 7. (19) Baulch, D. L.; Cox, R. A.; Hampson, R. F.; Kerr, J. A.; Troe, J.; Watson, R. T. J . Phys. Chem. Ref. Data 1984, 13, 1259. (20) Tuazon, E. C.; Atkinson, R.; Plum, C. W.; Winer, A. M.; Pitts, J. N. Geophys. Res. Lett. 1983, I O , 953, (21) Molina, L. T.; Molina, M. J. J . Phys. Chem. 1978, 82, 2410. (22) Knauth, H. D.; Alberti, H.; Clausen, H. J . Phys. Chem. 1979, 83, 1604. ~ .
.
(23) Maker, P. D.; Niki, H.; Savage, C. M.; Breitenbach, L. P. Chem. Phys. Lett. 1979, 66, 325. (24) Molina, M. J.; Ishiwata, T.; Molina, L. T. J . Phys. Chem. 1980,84, 821.
synthetic approach to the formation of HOCl.z2324The self-reaction of HOCl by (-9) has at most a relatively slow homogeneous rate of reacti0n.2~The H/D isotopic exchanges of DzO with HzO, HOCl, and HON02 are all very rapid and are usually considered to occur by heterogeneous pathways. Three of the six forward/reverse pairs of oxide exchange reactions (Le., N 2 0 5with H 2 0 , HOCI, or Cl2O) are each exothermic by about 40 kJ/mol in the direction away from N2O5I9and therefore will proceed to equilibrium positions containing negligible residual quantities of N,OS. In the absence of any N 2 0 Sinitially, the reverse reactions are not expected to form more than trace quantities of it. However, the other three oxide exchange pairs (i.e., reactions 6,9, and 10 and their reverses) are approximately thermoneutral so that the reverse reactions are always important in laboratory experiments. For example, the forward reaction 6 cannot approach completion either homogeneously or heterogeneously because the reverse reaction -6 of HOCl and H O N 0 2 must have for thermodynamic reasons a rate constant two or three times faster than (6) at 298 K. Experiments with the H 2 0 / C 1 0 N 0 , system, such as that illustrated in Figure l , are further complicated by the possibilities that HOCl formed in (6) can either react with another molecule of C 1 0 N 0 2 in (lo), or with itself in (-9) producing additional H 2 0 for reaction 6 . Kinetic treatments of a reaction mixture of H 2 0 with CIONOz can begin with the assumption that only (6) is occurring but must soon include (-6) and probably also (9), (-9), (lo), and (-10). Moreover, each of these forward/reverse reaction pairs can in principle be heterogeneously catalyzed on different surfaces to different degrees, providing a very complex maze of reaction rate alternatives potentially applicable to the monotonic changes in C10N02 and HONO, found in the evolution of the experimental IR spectra. We are unable as yet to discriminate among such possible alternatives as (lo), or (-9) followed by (6), in explanation of the continuing loss of CIONO, found in the experiments of Table I. The apparent rate constants of Table I have been calculated from the initial portions of the recorded data in each experiment. Several other further potential complications may exist in this system. While an alternate pathway for H 2 0 with ClONO, to form HCI and H 0 2 N 0 2is quite endothermic, the comparable alternate route for HOCl plus C 1 0 N 0 2 through reaction 12 to HOCl + ClONO2 --+ Cl2 + HO2NOZ (12) form HO2NO2is thermoneutral ( A H = -2 A 20 kJ/mol) within the large present uncertainty (f20 kJ/mol) in the heat of formation of HO2NO2.I9 A reaction can also be written for ClzO with C 1 0 N 0 2 to form the unknown molecule chloryl nitrate, C102N02,in analogy with the known bromyl nitrate (Br02N02) formed by the reaction of BrONO, with O3at -78 0C.z5 Detailed laboratory experiments will be needed to determine whether such alternative reaction routes exist for some of the substituent exchange reactions. The frequent observation among these oxide-exchange processes of very readily catalyzed heterogeneous reactions raises the question of whether they can also occur on particulate surfaces in the stratosphere. Knauth et al., for instance, have commented on their laboratory observations of increased rates of equilibration between (9) and (-9) at lower temperatures.22 The procedures for handling homogeneous gas-phase chemical reactions in stratospheric models are well established, and the minimum basic set of equations has grown steadily over the past decade as new reactions were introduced.'J Catalytic heterogeneous reaction paths, however, pose a difficult problem for atmospheric modeling. Because of the differences in the nature of the surfaces in each milieu, reactions which readily occur heterogeneously in the laboratory may not occur in the atmosphere, or vice versa, or the reactions may occur under both sets of conditions but at different rates. Precision laboratory evaluation of a heterogeneous rate constant applicable to stratospheric conditions will be very difficult because of the problems in duplication of such environments even (25) Schmeisser, M.; Brandle, K. Angew. Chem. 1961, 73, 388
J. Phys. Chem. 1986, 90, 1988-1990
1988
if the detailed nature of such stratospheric surfaces were firmly established. Both reactions -9 and 10 form Cl20, a molecule not normally included in present atmospheric models. While reaction 9 should furnish a rapid removal route for Cl2O in the atmosphere, other exothermic reactions competitive with it may also exist for C120. The reaction of CI20with HCI by (1 1) is a known rapid reaction and would tend to convert the C1 trapped in relatively inert HCI into an easily photolyzable form. Exothermic reactions can also whose stratospheric concentration is be found for ClzO with 03, comparable to that of H20. One very important attribute of the heterogeneous oxide exchange reactions is that they do not require sunlight and can occur equally well at night. This could result in substantial shifts in chlorine distributions from those expected during the daylight periods dominated by free radical reactions driven by solar radiation. The total organochlorine concentration of the troposphere has increased by a factor of three in the past 15 years to a 1986 level of about 3500 pptv (l0-l2) in the northern hemisphere and is continuing to increase at a rate of at least 1000 pptv per decade.'
The recent rapid change in the yearly decrement in total ozone in the Antarctic ~ p r i n g ~appears -~ to correspond more to a quadratic than to a first-power dependence on the total tropospheric chlorine content. Several of the reactions, e.g. (7), (lo), and (1 l ) , involve the interaction of two chlorinated species and could, if fast enough, contribute to a higher than first-order dependence on total organochlorine concentration. Reasonable simulations of the Antarctic ozone observations have recently been attained by inclusion in an atmospheric model of heterogeneous reactions of C 1 0 N 0 2 with either HCl or H20.26
Acknowledgment. This research was supported initially by the Office of Basic Energy Sciences of the Department of Energy through Contract No. DE-AT-03-76ER-70126, and then by NASA Contract NAGW-668. The authors thank Drs. Atkinson and Winer for discussions and a preprint of their work on chlorine nitrate. (26) Solomon, S.;Garcia, R.; Rowland, F. S.; Wuebbles, D. Nature (London),in press.
Reaction Rates for Thermal Chlorine Atoms wlth H,S from 232 to 359 K by a Radiochemical Technlque Eric C. C. Lu, R. Subramonia Iyer, and F. S. Rowland* Department of Chemistry, University of California, Irvine, California 9271 7 (Received: December 3, 1985; In Final Form: January 28, 1986)
The relative rate constants for the gas-phase reactions of thermal chlorine atoms have been measured with H2S, C2Hs, and CH2=CHBr over the temperature range from 232 to 359 K by using radioactive 38Clatoms. These data at a pressure of 4000 Torr of CClF, are consistent with a temperature-dependent rate constant of (10.5 0.4)X lo-" cm3 molecule-' s-l, based on the consensus literature values for the reaction of C1 with C&.
*
We have measured the reaction rate constants for chlorine atoms with H2S by (1) over the temperature range from 232 to 3sCl
+ H2S
-
+
H38C1 SH
(1)
359 K utilizing a competitive radiochemical technique at pressures of approximately 4000 Torr.I4 The reaction of chlorine atoms with H2S is potentially important in the atmosphere of the earth and of other planets.56 Several previous experiments have provided measurements of the reaction rate constant at 298 K ranging from 4 X lo-" to >7.3 X lo-" cm3 molecule-' s-IS5-l1 The one study over a temperature range has indicated a rate constant of (6.29 f 0.46) X lo-" cm3 molecule-' s-l from 21 1 to 353 K at pressures (1) Lee, F. S. C.; Rowland, F. S.J . Phys. Chem. 1977, 81, 1235. (2) Lee, F. S. C.; Rowland, F. S. J. Phys. Chem. 1977,81, 1229. (3) Lee, F. S. C.; Rowland, F. S. J . Phys. Chem. 1977, 81, 86. (4) Lee, F. S. C.; Rowland, F. S. J . Phys. Chem. 1980, 84, 1876. (5) DeMore, W. B.,Ed. "Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling. Evaluation Number 7"; Jet Propulsion Laboratory: Pasadena, CA, 1985; JPL-85-37. (6) Baulch, D. L.; Cox, R. A.; Crutzen, P. J.; Hampson, R. F.; Kerr, J.; Troe, J.; Watson, R. T. J. Phys. Chem. Ref.Data 1982, 11, 327. (7) Braithwaite, M.; Leone, S. R. J . Chem. Phys. 1978, 69, 839. (8) Nesbitt, D. J.; Leone, S. R. J. Chem. Phys. 1980, 72, 1722. (9) Clyne, M. A. A.; Ono, Y.Chem. Phys. Lett. 1983, 94, 597. (10) Clyne, M. A. A.; MacRobert, A. J.; Murrells, T. P.; Stief, L. J. J. Chem. SOC.,Faraday Trans. 2 1984, 80, 877. (11) Nava, D. F.; Brobst, W. D.; Stief, L. J. J . Phys. Chem. 1985, 89, 4703.
0022-3654/86/2090-1988$01.50/0
of 100 Torr of Ar or less.'' Our data at 4000 Torr of CCIF, are consistent with a temperature-independent rate constant of (10.5 f 0.4) X lo-" cm3 molecule-' s-l from 232 to 359 K. Radioactive 38Clatoms have been formed by thermal neutron irradiation of CClF3 and moderated to thermal energies by multiple collisions with CCIF3 prior to competitive reactions with one of two minor substrate molecules.'-" The basic competition has been between reaction 1 with H2S and reaction 2 with 38CI + CH2=CHBr
-
CH2=CH38Cl
+ Br
(2)
CH2=CHBr, the latter leading to the formation of the easily measured molecule CH2=CH3sCl.'2 Both reactions have been placed on an absolute reaction rate scale through separate study of the competition between (2) and the well-known abstraction reaction of H from C2H6 in (3).5*6 The monitor reaction with
+
38Cl CzH6
-
H38Cl
+ C2H5
(3)
CH2==CHBr in (2) has been chosen because it is approximately as fast as its two competitor reactions (1) and (3) and because it produces a substantial yield of CH2=CH38C1 which can be readily separated and assayed by radio gas chromat~graphy.'~-'~ (12) Iyer, R. S.; Rowland, F. S. Chem. Phys. Lett. 1983, 103, 213. (13) Iyer, R. S.; Rogers, P. J.; Rowland, F. S.J . Phys. Chem. 1983, 87, 3799. (14) Lee, J. K.; Lee, E. K. C.; Musgrave, B.; Tang, Y.-N.; Root, J. W.; Rowland, F. S. Anal. Chem. 1962, 34, 741.
0 1986 American Chemical Society
