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The role of ion-molecule reactions in the conversion of N2O5 to HNO3 in the stratosphere
00324633/83/02018547$03.00/0 0 1983 Pergamon Press Ltd.
Planer. SpoceSci., Vol. 31, No. 2, pp. 185-191, 1983 Printed in Great Britain.
THE ROLE OF ION-MOLECULE REACTIONS IN THE CONVERSION OF N,O, TO HNO, IN THE STRATOSPHERE H. BbHRINGER, * D. W. FAHEY,? F. C. FEHSENFELD and E. E. FERGUSON
Aeronomy Laboratory, NOAA Environmental Research Laboratories, Boulder, CO 80303, U.S.A. (Received 26 July 1982)
Abstract-We have investigated the role of several ion-molecule reactions in the conversion of N,O, to HNO,. In the proposed conversion, an N,O, molecule would react with an H,O molecule clustered to an inert ion to produce two HNO, molecules. Subsequent clustering of an Hz0 molecule to the inert ion would make the reaction catalytic. If such an ion-catalysed conversion of N,O, to HNO, occurs, it would probably play a role in the stratospheric chemistry at high latitudes in winter. In this paper we present reaction rate constant measurements made in a flowing afterglow apparatus for hydrated H30+, H+(CH,CN), (m = 1,2, 3), and several negative ions reacting with N,O,. Slow rate constants were found for these ions for hydration levels that are predominant in the stratosphere. With the known stratospheric ion density, these slow rate constants preclude significant N,O, conversion by ion-molecule reactions.
1. INTRODUCTION
Recently, two atmospheric neutral reactions were found to produce a large enhancement in the reaction rate constant when one of the neutrals was clustered to a chemically inert ion (Rowe et al., 1982). The reactions are X+(O,)+NO and
X+(N,OS)+NO
-+X+ +NO*+O,
(1)
-+ X+(N,OJ+NO,
(2)
where X+ is the alkali ion Li+, Na+, or K+. The rate constant for the neutral reaction of O3 with NO varies from 1.3(- 14) cm3 s-l to 2( - 17) cm3 s-l over the temperature range of 125-280 K (Birks et al., 1976). When O3 is clustered to an alkali ion, the rate constant increases by a factor in the range of 35&3(6). For the neutral reaction of N,O, with NO, the upper limit on the rate constant is l( - 20) cm3 s- ’ for temperatures <280 K (Viggiano, 1980). When N,O, is clustered to Li+ and Na+ ions, the rate constant increases by nine and seven orders of magnitude, respectively. The present study was undertaken to determine if the reaction N,O,
+H,O
--* 2HN0,
f9.1
kcal mol-’
(3)
extremely slow in the gas phase with k, < 1.3( - 20) cm3 s- 1(Morris and Niki, 1973). If reaction (3) were to be enhanced in the stratosphere, then the ratio of the nitrogen oxides, NO,, to HNO, could be altered. Under normal conditions this ratio is determined by the steady state defined by the three-body association of NO, with OH to give HNO, and the loss of HNO, by photolysis and reaction with OH (Nicolet, 1975). A need for an enhancement of reaction (3) is suggested by the situation in the stratosphere at high latitudes during winter. Noxon (1979) has measured an abrupt drop in the total stratospheric column abundance of NO, near 50”N in winter. This “cliff’ in NO;, abundance is not predicted in current global models (Wofsy, 1978). The missing NO, is presumed to be stored as HNO, with the conversion involving N,O, as an intermediate species. Reaction (3), however, is too slow to participate in this conversion and no other reaction pathways are known. In the present study, the enhancement of reaction (3) by stratospheric ions in an ion-catalysed reaction was investigated as an alternate reaction pathway. The proposed catalytic reaction sequence is X+(HZO),+N,O,
+X+(H,O),_,(HNO,)+HNO, (4)
might be significantly enhanced by an ion-assisted or ion-catalysed reaction. Reaction (3) is known to be
X+(H20),_1(HN03)+H,0
+X+(H,O),+HNO, (5)
* Cooperative Institute for Research in Environmental Science (CIRES) Research Associate, University of Colorado, Boulder, CO, 80303. Alexander von Humbolt Fellowship, Max-Planck Institut fiir Kernphysik, Heidelberg, Germany. t Cooperative Institute for Research in Environmental Science (CIRES) Research Associate, University of Colorado, Boulder, CO, 80303, U.S.A.
where the reaction sum is equal to reaction (3). The possible importance of reactions (4) and (5) has been discussed previously by Ferguson et al. (1979) and Fehsenfeld and Albritton (1980). Candidates for X+(H,O), are the stratospherically abundant ions, 185
186
H.
B~HRINGER et al.
H,O+(H,O),(n = 3,4), H+(CH,CN)(H,O),(n = 2,3), H+(CH&N),(H,O),,(n = 1,2),andH+(CH,CN),(H,O) (Arnold et al. 1981 amd Arijs et al. 1982). Since the exothermicity of reaction (3) is less than the cluster bond enthalpies of H,O to the above ion candidates (Kebarle, 1977), one of the HNO, product molecules must cluster to the product ion in order for reaction (4) to beexothermic. Reaction(4)isfollowed byaswitching reaction (reaction (5)) in which the HNO, molecule is displaced by an H,O molecule. Reaction (5) is expected to be exothermic since Hz0 generally bonds more strongly to positive ions than does HNO, (Fehsenfeld et al., 1975). The lifetime of the intermediate ion, H30+ (H20),_,HN0,, against reaction will be short due to the abundance of H,O in the stratosphere. For k, = l(-9) cm3 s-l and for [Hz01 = l(12) cmm3 at 35 km (Ackerman, 1979), this lifetime is l( - 3) s. The negative ion chemistry is not favorable for catalysis of reaction (3) in the stratosphere since HNO, bonds more strongly to negative ions than does H,O (Fehsenfeld et al., 1975). Thus, reaction (5) would not occur when X+ is replaced by X- and, therefore, would preclude a catalytic process. However, the present study includes the reactions of Cl-, O;, NO;, and NO; and their hydrates with N,O,. The requirement that an ion-molecule conversion process be catalytic is simple to show. Ifreaction (5) did not occur, the fate of the product ion of reaction (4), H,O+(H,O),_ iHN03, would be recombination with the ambient negative ions. The source of both positive and negative ions at 30 km is ionization by cosmic rays at a rate in the range of 10-50 cmM3 s ~’ (Heaps, 1978). Assuming a rate for reaction (4) in the range of l( - 12) cm3 s-i to l( -9) cm3 s-l, the removal rate of N,O, and hence NO, will be limited by the ionization rate. For [NO,] + [N205] = 5(9) cm3 (Solomon, 1982) and for an ionization rate of 50 cmm3 s- ‘, a removal lifetime of l(8) s, - 3 years, is calculated. Thus, a noncatalytic conversion process cannot account for the missing NO, that must disappear on the time scale of approx. one month (Noxon, 1979). If the process were not catalytic, then the ions H,O+(H,O),_ i(HNOJ should be observed in the stratosphere. In situ measurements (Arnold et al. 1981 and Arijs et al. 1982) clearly show that such ions are not present in the stratosphere at 44”N. We can further show that the rate of a catalytic conversion process must be within a specific range. The upper limit on this rate is set by the requirement that ambient N,O, not be catalytically removed during a single night. If N,O, were removed at night, the diurnal variation of NO, would not show a photolytic production during the day (Noxon, 1978). An ambient ion concentration of 2(3) cm-j at 30 km at
mid-latitudes and a time constant of 1 day will yield and upper limit of 6( - 9) cm3 s- ’ for reaction (4). This limit is larger than ion-molecule collision rates and therefore is not likely to be exceeded. The lower limit for the rate of the conversion process is given by the stratospheric chemistry in the latitude region (a4.5”) above the NO, cliff. In order for ions to be effective in converting N,O, to HNO,, the production rate of HNO, by reactions (4) and (5) must be larger than the production rate of HNO, in the reaction OH +NOz The production
+ M + HNO,
+ M.
(6)
rates are equal for
kdx’l CN,O,IR = MOW iTJO,
(7)
where [X’] is the ambient positive ion concentration and k, is the effective binary rate constant for reaction (6). The factor R in equation (7) is a correction factor to account for the diurnal variation of [OH] and for the regions of total darkness that occur above the cliff. Since no OH is produced in the absence of sunlight and since N,OS is photolyzed in the presence of sunlight, R is equal to the average ratio of hours of darkness to hours of sunlight. For the region of the cliff (45-65”), this ratio is taken to be 3. A further correction results from the regions above the cliff (26.5”) that are in constant darkness during winter. The lifetime of a stratospheric air parcel against transport to lower latitudes below the cliff is many days due to the existence of the polar vortex and, further, the air above the cliff is well mixed due to this vortex (Noxon, 1979). This strong mixing results in an air parcel residing in regions of total darkness approx. one-half of the time during winter and thus increases R from 3 to 6. Model calculations indicate that the daytime NO, and nighttime N205 initial concentrations near 30 km that must be converted to HNO, are roughly comparable (Solomon, 1982). Values for k, were taken from the NASA Evaluation (1981). At 30 km at 54” latitude, [X’] is 4(3) cmm3, [OH] is 2.7(6) cmm3 (Solomon, 1982) and k, is 2( - 12) cm3 s I, yielding k, =2.3(-10)cm3s-1.Similarly,at20kmk,is8.5(-11) cm3 s-r while at 35 km k, is 3.3( - 10) cm3 s- i. Since the majority of the column abundance of NOz will be included in the 2&35 km region, the value of k, must be in the range 8.5( - 11) < k, ,< 3.3( - 10) cm3 s-i for equal production rates. Since k, must be several times larger than the values in this range in order to be effective in forming the cliff, a firm lower limit fork, can be taken as 3( - 10) cm3 s-l. Thus, the existence of an ion-catalyzed conversion process is a reasonable speculation. For k, > 3( - 10)
Ion-molecule
reactions
cm3sm1 and for k, - l(-9) cm3 s-i, the conversion would be sufficiently fast to account for the cliff formation but slow enough not to alter the diurnal variation of NO,. The results of the present study, however, show that k, is small and outside of the above range for the ion clusters that predominate in the stratosphere. The following sections provide details of the experimental technique and measurement results and discussions. 2. EXPERIMENTAL
APPARATUS AND TECHNIQUE
The reaction rate constant measurements were made with a variable-temperature flowing afterglow apparatus that has been described previously (Ferguson et ul., 1969). The flow tube was modified by the addition of a high-pressure ion source as described in a recent study (Fahey et al., 1982). A helium buffer gas and a flow tube pressure of 0.4 T were used for all measurements. The high-pressure ion source was necessary for the present measurements for two reasons. First, the use of the source permits a reliable measurement of the reactivity of highly clustered ions. At sufficiently low temperatures, the source produces a distribution of cluster ions that is reactively uncoupled in the flow tube reaction volume. Therefore, the loss of the reactant cluster ion with N,O, is measured in the absence of its further production by association reactions. Second, the high pressure source can produce highly clustered ions with a small amount of the clustering species in the buffer gas flow. In the present study, the H,O flow tube concentration must be low enough that reaction (5) is negligible in the flow tube reaction volume. The product ion of reaction (4) must not react with H,O in reaction (5), but instead must be sampled and detected as evidence that the conversion to HNO, occurred. The experiments do not include measurements of the neutral gas composition and hence cannot directly measure the production of gas phase HNO,. The addition of H,O to the buffer gas was carefully controlled by adding a small He flow that was saturated with H,O. The He was saturated under constant pressure and temperature conditions in a flask containing liquid H,O. The absolute H,O flow rates were derived from H,O vapor pressure data. In this way, the concentration of H,O in the flow tube reaction volume was kept below 5 ppmv by H,O flow rates less than 2(-4) STP cm3 s-i. The effect of H,O in the flow tube is to reduce the apparent rate of reaction (4) by participating in reaction (5). Reaction (5) provides a distributed source of X+(H,O), ions in the reaction volume and thereby
187
in the stratosphere
alters the apparent loss of X+(H,O), ions with N,05. A computer model of the flow tube reaction kinetics was utilized to determine the effect of this distributed source. The magnitude of the effect is determined by the lifetimes against reaction, (k4[N205])-i and (k,[H,O]))‘, for reactions (4) and (5), respectively. We assume in the model that k, = l(-9) cm3 s-‘. The magnitude of the effect is small in the present measurements because of the low value of [Hz01 in the reaction volume. However, adjustments were calculated for the measured rate constants and were found to be less than a factor of two in all cases. The reactant ions in the present study were produced in the high-pressure ion source by electronimpact of the helium buffer mixed with small amounts of source gases. The source gases used were H,O for H30+, CH,CN for CH4CN+, Ccl, for Cl-, 0, for O;, NO, for NO;, and a mixture of 0,, O,, and NO, for NO;. These ions were hydrated by adding a wellcontrolled flow of a He/H,0 mixture to the source as discussed above and by lowering the flow tube temperature. An N,O, sample was prepared and stored by methods used in previous studies (Davidson et al., 1978 and Viggiano et al., 1981). A known flow rate of N,O, was obtained by saturating a He carrier gas flow in a constant temperature bath. N,O, flow rates used in the measurements were in the range of 5( -4) STP cm3 s- ’ to 3( - 2) STP cm3 s ‘. The HNO, content in the N,O, sample was measured to be less than 4% by chemical ionization techniques (Davidson et al., 1978). The measurement of a reaction rate constant was obtained in the usual way of observing the decline of the primary ion signal as a function of the absolute N20, flow rate. Product ions were observed but reaction channels were not always easily identified due to the presence of several primary ions and due to small reaction rate constants. 3. RESULTS
The measured rate constants for reaction with N,O, are shown in Table 1 for positive ions and in Table 2 for negative ions. The tables include the temperature range of the how tube for each reaction measurement. The reaction rate constants include any correction calculated from the flow tube computer model. These rate constant values are estimated to be accurate within a factor of two (+ lOO’/& - 50%). The largest contribution to this uncertainty is the uncertainty in the N,O, flow rate. The results of earlier measurements by Davidson et al. (1978) are shown in Tables 1 and 2 and are found to be in good agreement with the present results.
188
H. B~HRINGERet al. TABLE1. REACTION RATE CONSTANTS FOR POSITIVE IONSWITWN,O 5
k&m3 s-l) 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
T(K)
8.9( - 10) l.l(-9) 8.q - 10) 4.5( - 11)
0:
H30+ H,O+(H,O) H,O+(H,O), *H,O+(H,O), *H,O +(HzO)z, %O+(HzO), CH,CN + CH,CN+(H,O) *CH4CN+(H20)2 *CH,CN+(H,O), CH,CN+(H20), CH,CN+(CH,CN) *CH,CN+(CH$N)(H,O) *CH,CN+(CH&N)(H,O), *H+(CH,CN),(H,O)
Previous measurementst at3OOK
300 300 300 222-300 222-300 215-243 215 300 300 220-300 220-243 220 300 300 222 222-300
8.8( - 10) 1.3(-9)
*Abundant ions in the stratosphere at 35 km. t From Davidson et al. (1978).
TABLE2. REACTION RATECONSTANTS FORNEGATIVE IONSWITHN,O,
24. 25. 26. 27. 28. 29. 30. 31. 32.
ClCl-(H,O) 0; O;(H,O) O;(H,O), NO; NO;(H,O) NO;(H,O) NO;(H,O)*
k(cm3 s-‘)
T(K)
9.3( - 10) 8.2(-10) l.l(-9) 1.q-9) 9( - 10) 6( - 10) 5(-10)
300 300 300 300 300 300 273 215 215
Previous measurements* at3OOK 9.q - 10)
7(-10)
* From Davidson et al. (1978).
a. Positive ions In reaction (8), 0;
ions react with a rate of 8.9( - 10)
cm3s-l with N,O, to yield the product ion NO:. Good agreement is found with previous results. In reaction (9), H30’ ions react with a rate of l.l(-9) cm3sm1 at 300 K with N,O,. There are three exothermic reaction channels in which the product ions NO:, NOi(HNO,), and H,NO: are formed.
H30+ +N,O,
-+NO:
+HNO,+H,O +1.5kcalmol-’
(9a)
--f NO;(HN03)+H20 +(>lSkcalmol-‘) + H,NO:
+HNO,
+21.7 kcalmol-’
(9b) (9c)
The exothermicity of the reactions follows from the thermodynamic data cited by Davidson et al. (1978). Previous studies have shown the equivalence of protonated nitric acid and hydrated NO;, H,NO: z NO;(H,O) (Fehsenfeld et al., 1975). The ion NOf(HN0,) can be thought of as the stable form of protonated N,O,. All three product ions of reaction (9) were observed in the experiment. In the previous study, only reaction channels (9a) and (SC) were observed. The observation of the NO: .HNO, indicated that the H,O concentration in the flow tube is low since H,O displaces HNO, in positive ion clusters. Since NO: . HNO, may also be easily collisionally dissociated at 300 K in He, branching ratios for reaction (9) could not be determined. In reaction (lo), H,O+(H,O) ions react with a rate
189
Ion-molecule reactions in the stratosphere with N,Os. With a cluster bond of 8(-10) cm3s-’ energy of 32 kcal mall’ for H,O+(H,O) (Kebarle, 1977), reaction (10) has only one exothermic channel leading to the production of NOi(H,0)2. This product ion is observed but is expected to dissociate to NOl(H,O) in the flow tube at room temperature. The production of NOi(H,O) directly by H,O+ product ions in reaction (9) interferes with the detection of this ion as the product of NO;(H,O), dissociation. In reaction (11) H30’(Hz0)2 ions react with a rate of 4.5(-11) cm3se1 with N,O, in the range of 222300 K. No temperature dependence of the rate constant was observed in this temperature range. The low reactivity may be due to a small endothermicity of reaction (11). If one assumes, however, that the reaction is endothermic and that the reaction rate constant approaches the collision rate at infinite temperature, one should find a stronger temperature dependence than observed with a rate constant decreasing to near 1.5( - 11) cm3 s- ’ at 222 K. The product ions of this reaction could not be determined. The likely product ion, H30+(H,0)(HN03), was not observed, presumably due to its low stability or a further reaction with N,O,. No reaction was found for the larger proton hydrates with N,O, in reactions (12-14). The rate constant values in Table 1 for these reactions are upper limits. In reactions (15519) involving protonated acetonitits hydrates, only CH,CN+ and rile and CH,CN+(H*O) were found to react with N,O,. The rate constant for CH,CN+(H,O) in reaction (16) is 4.1(- 10) cm3 s-r which is one-half of the value for CH,CN+ in reaction (15). For reaction (15) the reaction channels are CH,CN+
+N,O,
--f NO;
+CH3CN+HN03 - 16 kcal mall ’
(154
-+ NO,+(CH,CN)+HNO,-16kcalmoll’ + D:(NO;
-CH,CN)
UW
where the production of NO; is endothermic. The endoergicity follows thermodynamic data cited by Davidson et nl. (1978) and from a value of the proton affinity of CH,CN of 187 kcal mol- ’ from Wolf et al. (1977). Since the data taken from Davidson et al. (1978) and Wolf et al. (1977) are based on different values of the proton affinity of water, 168 and 170 kcal mol-‘, respectively, we have used a value of 185 kcal mol-’ for the proton affinity of CH,CN to be consistent with the thermodynamical data of Davidson et al. (1978). The product ion NO:(CH,CN) is observed in reaction (15) indicating that channel (15b) is exothermic and
that the NO: - CH,CN cluster bond energy is larger than 16 kcal mol- ‘. The product ions in reaction (16) could not be identified. h. Negative ions In reactions (24-29) the reactions of Cl-, O;, and NO; and their hydrates with N,O, were found to be fast, k > l( - 10) cm3 s- i. Some reduction in the rate constant occurred as a result of hydration. For Cland NO;, good agreement was found with previous measurements by Davidson et al. (1978). No reaction was found for NO; hydrates in reaction (31) and (32). 4. DISCUSSION
The exothermicity is given by
of reaction
AH = 9.1 -D:[X+(H,O),_, +DE[X’(H,O),_
(4) for X+(H,O),
ions
-(H,O)]
1-HNO,]
kcal mol-’
(33)
where 9.1 kcal mol- ’ is the exothermicity of reaction (3) and D”, is the indicated bond dissociation energy. For Xt = H30+, the hydration energies are known for n < 6 (Kebarle, 1977). The values of Dz[X’(H,O),_ 1-HNO,] are not known but are expected to be much less than the first several H,O’ hydration energies. For n = O,l, reaction (4) is exothermic for the production of H,NOi or NO;(H,O) since the proton affinity of HNO, exceeds that of H,O (Fehsenfeld et al., 1975) and thus equation (33) is not appropriate. For n > 2, the formation of H,NO: is not favorable because H,O+(H,O), will be more stable than NO:(H,O),. For n = 4-7, reaction (4) is likely to change from near-thermoneutral to exothermic as determined by equation 33. For n 2 7, reaction (4) will certainly be exothermic. Eventually, one expects that hydrated cluster ions exceeding a certain size will react rapidly with N,O, to produce 2HN0, since N,O, reacts rapidly with H,O adsorbed on surfaces (Morris and Niki, 1973). For X+ = H+(CH,CN), (m = t,2,3), much less is known about the thermochemistry of the H,O cluster bonds. Bohringer and Arnold (1981) have measured equilibrium constants, K,,, for these heteromolecular cluster ions. Their results show that K,, is much smaller for H+(CH,CN),,,(H,O), ions than H,O+(H,O), ions with the same number of ligands. In addition, the HNO, bond energies are also not known for these cluster ions. Thus, for the same number of ligands, the reactions of H+(CH,CN),(H,O), ions with N,O, in reaction (4) are more likely to be exothermic than the H,O+(H,O), ion reactions with N,O,.
H. BOHRLINGER
190
On the basis of the known thermochemistry, the exothermicity of reaction (4) for stratospherically abundant positive ions cannot be precluded. The results of the reaction rate constant measurements, however, revealed no significant reaction of these ions. Therefore, considering the uncertainty in the thermochemistry, some if not all of the reactions not observed may be endothermic. The majority of negative ion reactions in Table 2 were found to be fast. Although product ions were not determined, the exothermicity of these reactions is favored by the strong bond of HNO, to negative ions. The lack of reaction of NO; hydrates is thus surprising. The exothermicity of the reactions NO;(H,0)+N,05
-+ NO;(HNO,) +HNO,+18.2
NO;(H,O),
+N,O,
-+ NO;(HNO,),
kcal mol-’
(31)
+H,O 1
+ NO;(HNO,)(H,O)
+ HNO,
+(> 18.2 kcal mol-‘)
J
(32)
is large as calculated from the known bond dissociation energies (Yamdagni and Kebarle, 1971 and Fehsenfeld et al., 1975). The lack of reaction in these cases suggests that a barrier exists for the N,O, + H,O reaction that is not overcome by the presence of the NO; ion. McCrumb and Arnold (1982) have applied a steady state treatment to the reaction system of stratospheric negative ions to derive the HNO, concentration. In the derivation, HNO, is assumed to be the only reactant that leads to the formation of NO;(HNO,), ions. The results in the present study for reaction (31)(32), ensure that N,05 does not interfere in the reaction scheme. 5.
CONCLUSIONS
A proposed reaction scheme in which ions would catalytically convert NzO, to HN03 in the stratosphere is not supported by laboratory measurements. Combined with the “window” of reactivity presented in the Introduction, the measured lack of reactivity of the stratospherically abundant ions preclude a significant N,O, conversion by ion reactions in the stratosphere. Acknowledgements~The authors are grateful for helpful discussions with S. Solomon, J. F. Noxon, and C. J. Howard of the NOAA Aeronomy Laboratory. This research is supported by the Defense Nuclear Agency. REFERENCES
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l?td.
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191
between water and ammonia from pulsed ion cyclotron resonance thermal equilibria measurements. J. Am. Chem. Sot. 99,5417-5429. Yamdagni, R. and Kebarle, P. (1971) Hydrogen-bonding energies to negative ions from gas-phase measurements of ionic equilibria. J. Am. Chem. Sot. 93,7139-7143.
Planer. SpoceSci., Vol. 31, No. 2, pp. 185-191, 1983 Printed in Great Britain.
THE ROLE OF ION-MOLECULE REACTIONS IN THE CONVERSION OF N,O, TO HNO, IN THE STRATOSPHERE H. BbHRINGER, * D. W. FAHEY,? F. C. FEHSENFELD and E. E. FERGUSON
Aeronomy Laboratory, NOAA Environmental Research Laboratories, Boulder, CO 80303, U.S.A. (Received 26 July 1982)
Abstract-We have investigated the role of several ion-molecule reactions in the conversion of N,O, to HNO,. In the proposed conversion, an N,O, molecule would react with an H,O molecule clustered to an inert ion to produce two HNO, molecules. Subsequent clustering of an Hz0 molecule to the inert ion would make the reaction catalytic. If such an ion-catalysed conversion of N,O, to HNO, occurs, it would probably play a role in the stratospheric chemistry at high latitudes in winter. In this paper we present reaction rate constant measurements made in a flowing afterglow apparatus for hydrated H30+, H+(CH,CN), (m = 1,2, 3), and several negative ions reacting with N,O,. Slow rate constants were found for these ions for hydration levels that are predominant in the stratosphere. With the known stratospheric ion density, these slow rate constants preclude significant N,O, conversion by ion-molecule reactions.
1. INTRODUCTION
Recently, two atmospheric neutral reactions were found to produce a large enhancement in the reaction rate constant when one of the neutrals was clustered to a chemically inert ion (Rowe et al., 1982). The reactions are X+(O,)+NO and
X+(N,OS)+NO
-+X+ +NO*+O,
(1)
-+ X+(N,OJ+NO,
(2)
where X+ is the alkali ion Li+, Na+, or K+. The rate constant for the neutral reaction of O3 with NO varies from 1.3(- 14) cm3 s-l to 2( - 17) cm3 s-l over the temperature range of 125-280 K (Birks et al., 1976). When O3 is clustered to an alkali ion, the rate constant increases by a factor in the range of 35&3(6). For the neutral reaction of N,O, with NO, the upper limit on the rate constant is l( - 20) cm3 s- ’ for temperatures <280 K (Viggiano, 1980). When N,O, is clustered to Li+ and Na+ ions, the rate constant increases by nine and seven orders of magnitude, respectively. The present study was undertaken to determine if the reaction N,O,
+H,O
--* 2HN0,
f9.1
kcal mol-’
(3)
extremely slow in the gas phase with k, < 1.3( - 20) cm3 s- 1(Morris and Niki, 1973). If reaction (3) were to be enhanced in the stratosphere, then the ratio of the nitrogen oxides, NO,, to HNO, could be altered. Under normal conditions this ratio is determined by the steady state defined by the three-body association of NO, with OH to give HNO, and the loss of HNO, by photolysis and reaction with OH (Nicolet, 1975). A need for an enhancement of reaction (3) is suggested by the situation in the stratosphere at high latitudes during winter. Noxon (1979) has measured an abrupt drop in the total stratospheric column abundance of NO, near 50”N in winter. This “cliff’ in NO;, abundance is not predicted in current global models (Wofsy, 1978). The missing NO, is presumed to be stored as HNO, with the conversion involving N,O, as an intermediate species. Reaction (3), however, is too slow to participate in this conversion and no other reaction pathways are known. In the present study, the enhancement of reaction (3) by stratospheric ions in an ion-catalysed reaction was investigated as an alternate reaction pathway. The proposed catalytic reaction sequence is X+(HZO),+N,O,
+X+(H,O),_,(HNO,)+HNO, (4)
might be significantly enhanced by an ion-assisted or ion-catalysed reaction. Reaction (3) is known to be
X+(H20),_1(HN03)+H,0
+X+(H,O),+HNO, (5)
* Cooperative Institute for Research in Environmental Science (CIRES) Research Associate, University of Colorado, Boulder, CO, 80303. Alexander von Humbolt Fellowship, Max-Planck Institut fiir Kernphysik, Heidelberg, Germany. t Cooperative Institute for Research in Environmental Science (CIRES) Research Associate, University of Colorado, Boulder, CO, 80303, U.S.A.
where the reaction sum is equal to reaction (3). The possible importance of reactions (4) and (5) has been discussed previously by Ferguson et al. (1979) and Fehsenfeld and Albritton (1980). Candidates for X+(H,O), are the stratospherically abundant ions, 185
186
H.
B~HRINGER et al.
H,O+(H,O),(n = 3,4), H+(CH,CN)(H,O),(n = 2,3), H+(CH&N),(H,O),,(n = 1,2),andH+(CH,CN),(H,O) (Arnold et al. 1981 amd Arijs et al. 1982). Since the exothermicity of reaction (3) is less than the cluster bond enthalpies of H,O to the above ion candidates (Kebarle, 1977), one of the HNO, product molecules must cluster to the product ion in order for reaction (4) to beexothermic. Reaction(4)isfollowed byaswitching reaction (reaction (5)) in which the HNO, molecule is displaced by an H,O molecule. Reaction (5) is expected to be exothermic since Hz0 generally bonds more strongly to positive ions than does HNO, (Fehsenfeld et al., 1975). The lifetime of the intermediate ion, H30+ (H20),_,HN0,, against reaction will be short due to the abundance of H,O in the stratosphere. For k, = l(-9) cm3 s-l and for [Hz01 = l(12) cmm3 at 35 km (Ackerman, 1979), this lifetime is l( - 3) s. The negative ion chemistry is not favorable for catalysis of reaction (3) in the stratosphere since HNO, bonds more strongly to negative ions than does H,O (Fehsenfeld et al., 1975). Thus, reaction (5) would not occur when X+ is replaced by X- and, therefore, would preclude a catalytic process. However, the present study includes the reactions of Cl-, O;, NO;, and NO; and their hydrates with N,O,. The requirement that an ion-molecule conversion process be catalytic is simple to show. Ifreaction (5) did not occur, the fate of the product ion of reaction (4), H,O+(H,O),_ iHN03, would be recombination with the ambient negative ions. The source of both positive and negative ions at 30 km is ionization by cosmic rays at a rate in the range of 10-50 cmM3 s ~’ (Heaps, 1978). Assuming a rate for reaction (4) in the range of l( - 12) cm3 s-i to l( -9) cm3 s-l, the removal rate of N,O, and hence NO, will be limited by the ionization rate. For [NO,] + [N205] = 5(9) cm3 (Solomon, 1982) and for an ionization rate of 50 cmm3 s- ‘, a removal lifetime of l(8) s, - 3 years, is calculated. Thus, a noncatalytic conversion process cannot account for the missing NO, that must disappear on the time scale of approx. one month (Noxon, 1979). If the process were not catalytic, then the ions H,O+(H,O),_ i(HNOJ should be observed in the stratosphere. In situ measurements (Arnold et al. 1981 and Arijs et al. 1982) clearly show that such ions are not present in the stratosphere at 44”N. We can further show that the rate of a catalytic conversion process must be within a specific range. The upper limit on this rate is set by the requirement that ambient N,O, not be catalytically removed during a single night. If N,O, were removed at night, the diurnal variation of NO, would not show a photolytic production during the day (Noxon, 1978). An ambient ion concentration of 2(3) cm-j at 30 km at
mid-latitudes and a time constant of 1 day will yield and upper limit of 6( - 9) cm3 s- ’ for reaction (4). This limit is larger than ion-molecule collision rates and therefore is not likely to be exceeded. The lower limit for the rate of the conversion process is given by the stratospheric chemistry in the latitude region (a4.5”) above the NO, cliff. In order for ions to be effective in converting N,O, to HNO,, the production rate of HNO, by reactions (4) and (5) must be larger than the production rate of HNO, in the reaction OH +NOz The production
+ M + HNO,
+ M.
(6)
rates are equal for
kdx’l CN,O,IR = MOW iTJO,
(7)
where [X’] is the ambient positive ion concentration and k, is the effective binary rate constant for reaction (6). The factor R in equation (7) is a correction factor to account for the diurnal variation of [OH] and for the regions of total darkness that occur above the cliff. Since no OH is produced in the absence of sunlight and since N,OS is photolyzed in the presence of sunlight, R is equal to the average ratio of hours of darkness to hours of sunlight. For the region of the cliff (45-65”), this ratio is taken to be 3. A further correction results from the regions above the cliff (26.5”) that are in constant darkness during winter. The lifetime of a stratospheric air parcel against transport to lower latitudes below the cliff is many days due to the existence of the polar vortex and, further, the air above the cliff is well mixed due to this vortex (Noxon, 1979). This strong mixing results in an air parcel residing in regions of total darkness approx. one-half of the time during winter and thus increases R from 3 to 6. Model calculations indicate that the daytime NO, and nighttime N205 initial concentrations near 30 km that must be converted to HNO, are roughly comparable (Solomon, 1982). Values for k, were taken from the NASA Evaluation (1981). At 30 km at 54” latitude, [X’] is 4(3) cmm3, [OH] is 2.7(6) cmm3 (Solomon, 1982) and k, is 2( - 12) cm3 s I, yielding k, =2.3(-10)cm3s-1.Similarly,at20kmk,is8.5(-11) cm3 s-r while at 35 km k, is 3.3( - 10) cm3 s- i. Since the majority of the column abundance of NOz will be included in the 2&35 km region, the value of k, must be in the range 8.5( - 11) < k, ,< 3.3( - 10) cm3 s-i for equal production rates. Since k, must be several times larger than the values in this range in order to be effective in forming the cliff, a firm lower limit fork, can be taken as 3( - 10) cm3 s-l. Thus, the existence of an ion-catalyzed conversion process is a reasonable speculation. For k, > 3( - 10)
Ion-molecule
reactions
cm3sm1 and for k, - l(-9) cm3 s-i, the conversion would be sufficiently fast to account for the cliff formation but slow enough not to alter the diurnal variation of NO,. The results of the present study, however, show that k, is small and outside of the above range for the ion clusters that predominate in the stratosphere. The following sections provide details of the experimental technique and measurement results and discussions. 2. EXPERIMENTAL
APPARATUS AND TECHNIQUE
The reaction rate constant measurements were made with a variable-temperature flowing afterglow apparatus that has been described previously (Ferguson et ul., 1969). The flow tube was modified by the addition of a high-pressure ion source as described in a recent study (Fahey et al., 1982). A helium buffer gas and a flow tube pressure of 0.4 T were used for all measurements. The high-pressure ion source was necessary for the present measurements for two reasons. First, the use of the source permits a reliable measurement of the reactivity of highly clustered ions. At sufficiently low temperatures, the source produces a distribution of cluster ions that is reactively uncoupled in the flow tube reaction volume. Therefore, the loss of the reactant cluster ion with N,O, is measured in the absence of its further production by association reactions. Second, the high pressure source can produce highly clustered ions with a small amount of the clustering species in the buffer gas flow. In the present study, the H,O flow tube concentration must be low enough that reaction (5) is negligible in the flow tube reaction volume. The product ion of reaction (4) must not react with H,O in reaction (5), but instead must be sampled and detected as evidence that the conversion to HNO, occurred. The experiments do not include measurements of the neutral gas composition and hence cannot directly measure the production of gas phase HNO,. The addition of H,O to the buffer gas was carefully controlled by adding a small He flow that was saturated with H,O. The He was saturated under constant pressure and temperature conditions in a flask containing liquid H,O. The absolute H,O flow rates were derived from H,O vapor pressure data. In this way, the concentration of H,O in the flow tube reaction volume was kept below 5 ppmv by H,O flow rates less than 2(-4) STP cm3 s-i. The effect of H,O in the flow tube is to reduce the apparent rate of reaction (4) by participating in reaction (5). Reaction (5) provides a distributed source of X+(H,O), ions in the reaction volume and thereby
187
in the stratosphere
alters the apparent loss of X+(H,O), ions with N,05. A computer model of the flow tube reaction kinetics was utilized to determine the effect of this distributed source. The magnitude of the effect is determined by the lifetimes against reaction, (k4[N205])-i and (k,[H,O]))‘, for reactions (4) and (5), respectively. We assume in the model that k, = l(-9) cm3 s-‘. The magnitude of the effect is small in the present measurements because of the low value of [Hz01 in the reaction volume. However, adjustments were calculated for the measured rate constants and were found to be less than a factor of two in all cases. The reactant ions in the present study were produced in the high-pressure ion source by electronimpact of the helium buffer mixed with small amounts of source gases. The source gases used were H,O for H30+, CH,CN for CH4CN+, Ccl, for Cl-, 0, for O;, NO, for NO;, and a mixture of 0,, O,, and NO, for NO;. These ions were hydrated by adding a wellcontrolled flow of a He/H,0 mixture to the source as discussed above and by lowering the flow tube temperature. An N,O, sample was prepared and stored by methods used in previous studies (Davidson et al., 1978 and Viggiano et al., 1981). A known flow rate of N,O, was obtained by saturating a He carrier gas flow in a constant temperature bath. N,O, flow rates used in the measurements were in the range of 5( -4) STP cm3 s- ’ to 3( - 2) STP cm3 s ‘. The HNO, content in the N,O, sample was measured to be less than 4% by chemical ionization techniques (Davidson et al., 1978). The measurement of a reaction rate constant was obtained in the usual way of observing the decline of the primary ion signal as a function of the absolute N20, flow rate. Product ions were observed but reaction channels were not always easily identified due to the presence of several primary ions and due to small reaction rate constants. 3. RESULTS
The measured rate constants for reaction with N,O, are shown in Table 1 for positive ions and in Table 2 for negative ions. The tables include the temperature range of the how tube for each reaction measurement. The reaction rate constants include any correction calculated from the flow tube computer model. These rate constant values are estimated to be accurate within a factor of two (+ lOO’/& - 50%). The largest contribution to this uncertainty is the uncertainty in the N,O, flow rate. The results of earlier measurements by Davidson et al. (1978) are shown in Tables 1 and 2 and are found to be in good agreement with the present results.
188
H. B~HRINGERet al. TABLE1. REACTION RATE CONSTANTS FOR POSITIVE IONSWITWN,O 5
k&m3 s-l) 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
T(K)
8.9( - 10) l.l(-9) 8.q - 10) 4.5( - 11)
0:
H30+ H,O+(H,O) H,O+(H,O), *H,O+(H,O), *H,O +(HzO)z, %O+(HzO), CH,CN + CH,CN+(H,O) *CH4CN+(H20)2 *CH,CN+(H,O), CH,CN+(H20), CH,CN+(CH,CN) *CH,CN+(CH$N)(H,O) *CH,CN+(CH&N)(H,O), *H+(CH,CN),(H,O)
Previous measurementst at3OOK
300 300 300 222-300 222-300 215-243 215 300 300 220-300 220-243 220 300 300 222 222-300
8.8( - 10) 1.3(-9)
*Abundant ions in the stratosphere at 35 km. t From Davidson et al. (1978).
TABLE2. REACTION RATECONSTANTS FORNEGATIVE IONSWITHN,O,
24. 25. 26. 27. 28. 29. 30. 31. 32.
ClCl-(H,O) 0; O;(H,O) O;(H,O), NO; NO;(H,O) NO;(H,O) NO;(H,O)*
k(cm3 s-‘)
T(K)
9.3( - 10) 8.2(-10) l.l(-9) 1.q-9) 9( - 10) 6( - 10) 5(-10)
300 300 300 300 300 300 273 215 215
Previous measurements* at3OOK 9.q - 10)
7(-10)
* From Davidson et al. (1978).
a. Positive ions In reaction (8), 0;
ions react with a rate of 8.9( - 10)
cm3s-l with N,O, to yield the product ion NO:. Good agreement is found with previous results. In reaction (9), H30’ ions react with a rate of l.l(-9) cm3sm1 at 300 K with N,O,. There are three exothermic reaction channels in which the product ions NO:, NOi(HNO,), and H,NO: are formed.
H30+ +N,O,
-+NO:
+HNO,+H,O +1.5kcalmol-’
(9a)
--f NO;(HN03)+H20 +(>lSkcalmol-‘) + H,NO:
+HNO,
+21.7 kcalmol-’
(9b) (9c)
The exothermicity of the reactions follows from the thermodynamic data cited by Davidson et al. (1978). Previous studies have shown the equivalence of protonated nitric acid and hydrated NO;, H,NO: z NO;(H,O) (Fehsenfeld et al., 1975). The ion NOf(HN0,) can be thought of as the stable form of protonated N,O,. All three product ions of reaction (9) were observed in the experiment. In the previous study, only reaction channels (9a) and (SC) were observed. The observation of the NO: .HNO, indicated that the H,O concentration in the flow tube is low since H,O displaces HNO, in positive ion clusters. Since NO: . HNO, may also be easily collisionally dissociated at 300 K in He, branching ratios for reaction (9) could not be determined. In reaction (lo), H,O+(H,O) ions react with a rate
189
Ion-molecule reactions in the stratosphere with N,Os. With a cluster bond of 8(-10) cm3s-’ energy of 32 kcal mall’ for H,O+(H,O) (Kebarle, 1977), reaction (10) has only one exothermic channel leading to the production of NOi(H,0)2. This product ion is observed but is expected to dissociate to NOl(H,O) in the flow tube at room temperature. The production of NOi(H,O) directly by H,O+ product ions in reaction (9) interferes with the detection of this ion as the product of NO;(H,O), dissociation. In reaction (11) H30’(Hz0)2 ions react with a rate of 4.5(-11) cm3se1 with N,O, in the range of 222300 K. No temperature dependence of the rate constant was observed in this temperature range. The low reactivity may be due to a small endothermicity of reaction (11). If one assumes, however, that the reaction is endothermic and that the reaction rate constant approaches the collision rate at infinite temperature, one should find a stronger temperature dependence than observed with a rate constant decreasing to near 1.5( - 11) cm3 s- ’ at 222 K. The product ions of this reaction could not be determined. The likely product ion, H30+(H,0)(HN03), was not observed, presumably due to its low stability or a further reaction with N,O,. No reaction was found for the larger proton hydrates with N,O, in reactions (12-14). The rate constant values in Table 1 for these reactions are upper limits. In reactions (15519) involving protonated acetonitits hydrates, only CH,CN+ and rile and CH,CN+(H*O) were found to react with N,O,. The rate constant for CH,CN+(H,O) in reaction (16) is 4.1(- 10) cm3 s-r which is one-half of the value for CH,CN+ in reaction (15). For reaction (15) the reaction channels are CH,CN+
+N,O,
--f NO;
+CH3CN+HN03 - 16 kcal mall ’
(154
-+ NO,+(CH,CN)+HNO,-16kcalmoll’ + D:(NO;
-CH,CN)
UW
where the production of NO; is endothermic. The endoergicity follows thermodynamic data cited by Davidson et nl. (1978) and from a value of the proton affinity of CH,CN of 187 kcal mol- ’ from Wolf et al. (1977). Since the data taken from Davidson et al. (1978) and Wolf et al. (1977) are based on different values of the proton affinity of water, 168 and 170 kcal mol-‘, respectively, we have used a value of 185 kcal mol-’ for the proton affinity of CH,CN to be consistent with the thermodynamical data of Davidson et al. (1978). The product ion NO:(CH,CN) is observed in reaction (15) indicating that channel (15b) is exothermic and
that the NO: - CH,CN cluster bond energy is larger than 16 kcal mol- ‘. The product ions in reaction (16) could not be identified. h. Negative ions In reactions (24-29) the reactions of Cl-, O;, and NO; and their hydrates with N,O, were found to be fast, k > l( - 10) cm3 s- i. Some reduction in the rate constant occurred as a result of hydration. For Cland NO;, good agreement was found with previous measurements by Davidson et al. (1978). No reaction was found for NO; hydrates in reaction (31) and (32). 4. DISCUSSION
The exothermicity is given by
of reaction
AH = 9.1 -D:[X+(H,O),_, +DE[X’(H,O),_
(4) for X+(H,O),
ions
-(H,O)]
1-HNO,]
kcal mol-’
(33)
where 9.1 kcal mol- ’ is the exothermicity of reaction (3) and D”, is the indicated bond dissociation energy. For Xt = H30+, the hydration energies are known for n < 6 (Kebarle, 1977). The values of Dz[X’(H,O),_ 1-HNO,] are not known but are expected to be much less than the first several H,O’ hydration energies. For n = O,l, reaction (4) is exothermic for the production of H,NOi or NO;(H,O) since the proton affinity of HNO, exceeds that of H,O (Fehsenfeld et al., 1975) and thus equation (33) is not appropriate. For n > 2, the formation of H,NO: is not favorable because H,O+(H,O), will be more stable than NO:(H,O),. For n = 4-7, reaction (4) is likely to change from near-thermoneutral to exothermic as determined by equation 33. For n 2 7, reaction (4) will certainly be exothermic. Eventually, one expects that hydrated cluster ions exceeding a certain size will react rapidly with N,O, to produce 2HN0, since N,O, reacts rapidly with H,O adsorbed on surfaces (Morris and Niki, 1973). For X+ = H+(CH,CN), (m = t,2,3), much less is known about the thermochemistry of the H,O cluster bonds. Bohringer and Arnold (1981) have measured equilibrium constants, K,,, for these heteromolecular cluster ions. Their results show that K,, is much smaller for H+(CH,CN),,,(H,O), ions than H,O+(H,O), ions with the same number of ligands. In addition, the HNO, bond energies are also not known for these cluster ions. Thus, for the same number of ligands, the reactions of H+(CH,CN),(H,O), ions with N,O, in reaction (4) are more likely to be exothermic than the H,O+(H,O), ion reactions with N,O,.
H. BOHRLINGER
190
On the basis of the known thermochemistry, the exothermicity of reaction (4) for stratospherically abundant positive ions cannot be precluded. The results of the reaction rate constant measurements, however, revealed no significant reaction of these ions. Therefore, considering the uncertainty in the thermochemistry, some if not all of the reactions not observed may be endothermic. The majority of negative ion reactions in Table 2 were found to be fast. Although product ions were not determined, the exothermicity of these reactions is favored by the strong bond of HNO, to negative ions. The lack of reaction of NO; hydrates is thus surprising. The exothermicity of the reactions NO;(H,0)+N,05
-+ NO;(HNO,) +HNO,+18.2
NO;(H,O),
+N,O,
-+ NO;(HNO,),
kcal mol-’
(31)
+H,O 1
+ NO;(HNO,)(H,O)
+ HNO,
+(> 18.2 kcal mol-‘)
J
(32)
is large as calculated from the known bond dissociation energies (Yamdagni and Kebarle, 1971 and Fehsenfeld et al., 1975). The lack of reaction in these cases suggests that a barrier exists for the N,O, + H,O reaction that is not overcome by the presence of the NO; ion. McCrumb and Arnold (1982) have applied a steady state treatment to the reaction system of stratospheric negative ions to derive the HNO, concentration. In the derivation, HNO, is assumed to be the only reactant that leads to the formation of NO;(HNO,), ions. The results in the present study for reaction (31)(32), ensure that N,05 does not interfere in the reaction scheme. 5.
CONCLUSIONS
A proposed reaction scheme in which ions would catalytically convert NzO, to HN03 in the stratosphere is not supported by laboratory measurements. Combined with the “window” of reactivity presented in the Introduction, the measured lack of reactivity of the stratospherically abundant ions preclude a significant N,O, conversion by ion reactions in the stratosphere. Acknowledgements~The authors are grateful for helpful discussions with S. Solomon, J. F. Noxon, and C. J. Howard of the NOAA Aeronomy Laboratory. This research is supported by the Defense Nuclear Agency. REFERENCES
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l?td.
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Wofsy, S. C. (1978) Temporal and latitudinal variations of stratospheric trace gases: a critical comparison between theory and experiments. J. geophys. Res. 83,364378. Wolf, J. F., Staley, R. H., Koppel, 1.; Taagepera, M., McIver, R. T., Jr., Beauchamp, J. L. and Taft, R. W. (1977) Gas phase basicities and relative proton affinities of compounds
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between water and ammonia from pulsed ion cyclotron resonance thermal equilibria measurements. J. Am. Chem. Sot. 99,5417-5429. Yamdagni, R. and Kebarle, P. (1971) Hydrogen-bonding energies to negative ions from gas-phase measurements of ionic equilibria. J. Am. Chem. Sot. 93,7139-7143.