J. Phys. Chem. 1995, 99, 16264-16275
16264
Reaction between HOz and ClO: Product Formation between 210 and 300 K M. Finkbeiner, J. N. Crowley," 0. Horie, R. Muller, G. K. Moortgat, and P. J. Crutzen Max-Planck-Institut fur Chemie, Division of Atmospheric Chemistry, Postjach 3060, 55020 Mainz, Germany Received: April 20, 1995; In Final Form: July 26, 1995@ The products of the gas-phase reaction between H02 and C10 have been measured by matrix-isolation/FTIR spectroscopy at a total pressure of 700 Torr and at 210, 240, 270, and 300 K. HOCl was detected as the dominant product, accounting for 295% of reaction at all temperatures, showing that H 0 1 f C10 HOCl 0 2 is the major reaction pathway at high pressures and low temperatures. Evidence was found for ozone formation in this reaction at low temperatures, implying the presence of a second, minor ( 1 5 % ) reaction HC1 0 3 . N o evidence for other product channels was obtained, precluding the channel: HOz C10 formation of long-lived isomers of HC103. The atmospheric implications of a second channel in which O i and HC1 are products are discussed.
-
+
+
-
+
Introduction Anthropogenic emission of chlorofluorocarbons has resulted in a stratospheric chlorine content that has increased from a natural level of ca. 0.6 ppb (parts per billion) to a contemporary level of almost 4 ppb.' The enhanced chlorine burden has contributed to a significant negative trend in ozone concentrations at midlatitude (see e.g. ref 2) and, more spectacularly, in the polar regions., In both cases C10 radicals are implicated in the 0 3 depletion mechanisms. Both HO, and C10 are recognized as key species in stratospheric ozone chemistry. They take part in well established catalytic reaction cycle^^.^ that lead to midlatitude ozone destruction:
+ HO, - O H + O2 OH + 0, -HOz + O2 0
(1)
(2)
+ c 1 0 - c1+ o2 Cl + 0, c 1 0 + 0, 0
(3)
'-'' HO, + C10 - HCI + 0,
(8b)
(4)
OH
(9b)
Both cycles are coupled with a further ozone depletion sequence involving NO,, (where NO,, = NO NO?) through the formation of temporary reservoir species such as HN03, HNO4, or CION02 for example (reactions 5-7) and with each other via reactions 8a and 9
+
+ NO, + M -. HNO, + M
OH
+ NO2 + M --NO, + M C10 + NO, + M - ClONO, + M HO, + C10 - HOCl + 0, OH + C10 - HO, f C1
(5)
HO,
(6) (7) (84
(9)
The HOCl formed in reaction 8a is a temporary reservoir, as it can be photolyzed to retum C10, and HO, in the form of C1 and OH, HOCl
+ hv -.O H + C1
and is present only at low mixing ratios (-100 ppt at 35 km) in the stratosphere.h The net of reactions 2,4, 8a, and 10 also represents a catalytic ozone-destroying c y ~ l e ,in~ ,which ~ the rate-limiting step is reaction 8a. The relative importance of the various cycles for ozone depletion varies with altitude, with HO, related processes dominant at low altitudes ('20 km). Above 25 km, NO., and C10, related processes are most important.' The degree to which the cycles are coupled controls the partitioning between the reactive odd chlorine species (C10, C1) and the reservoirs, and thus also the rate of the halogen-initiated ozone depletion (see e.g. ref 10). This in tum depends on the relative rates of several reactions and on the nature of the products. For instance, it has been shown that a small fraction of reactions 8 and 9 forming HCl (Le. reactions 8b and 9b) can alter the Cl,/HCl partitioning sufficiently to have a significant effect on the calculated ozone seasonal cycle and on the longterm midlatitude ozone trends.j.'
(10)
Abstract published in Adr'unce ACS Ahtracrc. October I . 1995.
0022-3654/95/2099- 16264$09.00/0
+ C10 - HC1 f 0,
The springtime polar stratosphere is denoxified following heterogeneous conversion of NO, to HN0?*.l5and temporary removal of HNO3 from the gas phase through the formation of nitric acid trihydrate (NAT). Low NO, levels, in combination with activation of chlorine reservoirs via heterogeneous reactions, result in highly elevated C10 concentrations.' The HO, chemistry is thus much simplified, and only C10 can be considered an important reaction partner for HO:. If the product of the reaction is assumed to be HOCl (as written in reaction 8a), a partly gas-phase, partly heterogeneous reaction sequence is possible that can activate the major stable chlorine reservoir, HCl.Ih.'' The rate of activation will be modified if the products are HCl and 0 3 , as written in reaction 8b. In addition to reactions 8a and 8b, several possible bimolecular product channels for reaction 8 are listed in Table 1. However. the reaction enthalpies indicate that only reactions 8a and 8b are expected to be important. Previous investigations of the reaction between H02 and C10 have mainly been concerned with measuring kinetic parameters,I8-?? although it has also been established that reaction 8a represents by far the most important reaction Estimates of the upper limit of the branching ratio of channel (8b) vary between 0.3% and 3%.14
0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 44, 1995 16265
Reaction between HO2 and C10 D Lamp
Cold Finger
\
Thermostatted fluid
IfqFFk, Pumps
Radiation shield
Gas Lines
oclo (110)
Monochromator and Photomultiplier
Figure 1. Schematic representation of the experimental setup.
TABLE 1: Possible Bimolecular Reaction Channels for H02 CIO" channel AH(298 K)/kcal mol-' 8a HOz+CIO -HOCl+02 -46 8b HCI + 03 - I5 8c -.ClOO + OH +2.9 8d OClO + OH +6.0 8e HClO + 0 2 +17 a Thermochemical data from Buttar and Hirst."3
+
--.
In this article we present new measurements of the temperature dependent formation of stable products in the title reaction. The pertinent results of the earlier investigations listed above and a comparison to the present work shall be discussed in detail later. In a separate paper25we describe a low-pressure flow tube kinetic/product investigation of the reactions of HO2 and DO2 with C10 between 225 and 295 K.
Experimental Section The reaction between HO2 and C10 was initiated by the continuous photolysis of a slow-flowing mixture of suitable precursors at 700 Torr, with products detected by matrixisolation/FTIR spectroscopy (MIFTIR). This method has been successfully employed in our laboratory to study temperature dependent product formation and product branching ratios in other radical-radical reaction^.^^.^^ As several modifications have been made to this apparatus, it is described here in some detail. A schematic representation of the apparatus is given in Figure 1. Reactor. A cylindrical, double-jacketed quartz tube ca. 120 cm long with a 3.5 cm internal diameter served as reaction vessel. The outermost chamber was evacuated to prevent condensation of ambient water vapor and to reduce the load on the cryostat when working at low temperatures. The middle chamber was used to circulate thermostated silicon oil around the wall of the reaction chamber. The temperature of the slowflowing gas mixture in the reaction chamber was assumed to be that of the silicon oil, which was measured both in the cryostat unit and after the oil had circulated through the system. The temperature could be varied between 210 and 300 K with an estimated precision of f 2 K. Gas-flows into the cell were maintained by calibrated Tylan flow-controllers. Photolytic radiation was provided by a set of eight radially mounted fluorescent lamps (Philips TLOS), which emit between 310 and 400 nm (see Figure 2). The time for which the gas mixture was exposed to the photolysis light was determined by the linear flow velocity of the gas mixture (typically between 2.7 and 3.8 cm/s) and the length of the illuminated volume. The former could be varied by changing the total gas flow rate, and the latter by altering the length of a radiation shield positioned between the lamps and the cell. In practice this provides only a first approximation of the reaction time, as
I
.
200
.
I
I
250
300
'
I
350
400
450
Wavelength [nm] Figure 2. UV absorption spectra of the CISand C120 photolytic radical precursors and the OClO product and the emission spectrum of the TL08 lamps. The OClO spectrum was taken from the work of Wahner et aL30and the Cl2 spectrum from Maric et aL3' The Cl20 absorption was measured as part of the present study. The UV spectra are offset from the baseline for clarity. The TU)8 curve shows the relative lamp intensity as a function of wavelength (Le. no y-axis units).
strong concentration gradients make the back-diffusion of products important and because the photolysis light is not perfectly normal to the surface of the reactor. Corrections to the reaction time were carried out by computer simulation of product formation in a relatively simple reaction system which is described later. Radical Generation. The radicals HO2 and C10 were formed in multistep reaction sequences initiated by the photolysis of C12/H2/C120/02/Ar mixtures. C12
+hv-Cl+
c1
+ H2 HC1+ H H + 0, + M- HO, + M c 1 + c1,o - c10 + c1, Cl
(1 1)
(12)
(13) (14)
The role of Ar is that of buffer in the gas phase, and it was chosen rather than N2 because it forms optically superior lowtemperature matrices2*(see below). In addition the detection of HCI in a nitrogen matrix is complicated by weak interactions with the matrix material.29 0 2 flows were regulated to keep its partial pressure at ca. 20% of the total pressure. In this scheme, H2 and C120 compete for the C1 atoms produced in reaction 11. The relative rate constant k14/k12 is ca. 6000 at 298 K, increasing to 150 000 at 210 K.24 For this reason, partial pressures of H2 of greater than 100 Torr were used at low temperatures, whilst C120 concentrations were kept at ca. 1-5 x 1014 molecules ~ m - ~Several . other sources of HO2 were tested, including the oxidation of H202, CH30H, or HCHO, that avoided the use of highly concentrated H2/02 mixtures. However, all proved to be unfeasible at low temperatures due to aerosol formation following condensation of the precursor (e.g. H202) or secondary products such as HCOOH (formed in the oxidation of HCHO) or due to interfering IR absorptions from products formed in secondary reactions. In addition to C120, both OClO and 0 3 react rapidly with C1 atoms to generate However, in these experiments the choice of precursor was restricted by the need to detect products from reactions 8a and 8b, from which the IR-active species are HOC1 (reaction 8a) and 0 3 and HCI (reaction 8b), immediately
Finkbeiner et al.
16266 J. Phvs. Chem., Vol. 99, No. 44, 1995 ruling out 03 as a precursor, OClO possesses very strong absorption bands in a spectral region similar to that of the main CI? absorption30.3'and the TL08 emission (see Figure 2 ) and would be photolyzed rapidly to generate 0 3 , precluding its use in these experiments.
+ hv - c10 + 0 0 + 0, + M -03 + M OClO
(15) (16)
The gas-phase concentrations of the Cl? and CllO precursors were measured in situ by absorption of collimated UV light (deuterium lamp) which transversed the major axis of the reaction cell (optical path length = 120 cm) before dispersion in a 0.25 m monochromator and detection by a photomultiplier at either 330 nm (C12) or 250 nm (ClzO). The concentrations of precursor species which do not absorb UV light above 200 nm (Le. H,, 0 2 , Ar) were determined from their partial flows and the total pressure. Detection. Once leaving the illuminated reaction volume, the gas mixture, now comprising unreacted precursors, products, and carrier gas, passed through a Pyrex connection that was thermostated to the same temperature as that of the main cell, and into a molecular beam sampling-unit. Initially at 700 Torr, the gas is expanded through a nozzle into a chamber held at 1-3 Torr by a rotary pump. A molecular beam, directed orthogonally onto the surface of a gold-plated copper finger held at 5 K, was generated by further expansion through a skimmer into a chamber maintained at 5 x 10-4 Torr by a turbomolecular pump. The gold-plated finger at 5 K serves as an unreactive surface on which the gases may condense to form a crystalline solid and as a highly reflective surface for midinfrared radiation. Gases which are condensable at 5 K include the main carrier gases argon and all reactants and products with the exception of H?. As products and reactants represent only a small fraction of the total gas mixture, they may be considered isolated in a "matrix" of Ado:. The composition of the matrix was determined by FTIR spectroscopy (Bomem DA008) using the optical arrangement shown in Figure 1. The sensitivity of this method is limited by the thickness of the matrix, which determines the optical path length of the IR radiation through the absorbing medium. Thin matrices imply short path lengths, whereas thick matrices result in extensive scattering of the IR radiation and thus loss of intensity. Typically, matrix growth times were restricted to ca. 15-25 min; the thickness of the matrix was not determined. The low temperature and rigid nature of the matrix generally result in transitions from the ground rotationhibration levels of the trapped species only, resulting in a loss of rotational structure and thus a loss of qualitative information when compared to the gas-phase spectrum.2x On the other hand, as only one rotationalhibrational state is populated, the transition from this state is correspondingly more intense. HOCI, 0 3 , OCIO, H202, HCI, and C l ~ were 0 measured at 1235, 1038, 1105, 1271, 2869, and 961 cm-I, respectively. Detection limits (S/N = 1) for a matrix growth period of 2.5 min were approximately 6 x I O 1 ' , 6 x IO", and 1 x I O ' ? molecule cm-3 for the products HOCI, 03, and OC10, respectively. Photolysis Rates. Along with the initial concentration of C12 and the length of the photolysis period, the photolysis rate constant of CI, (J-C1.1 controls the rate of removal of the HZ and ClrO reactants and also the extent of product formation. A knowledge of this parameter is thus essential to avoid excessive depletion of the radical precursors such as C1'0 and the ensuing reaction of C1 atoms with the products. J-CI2 was determined as described previously?' by measuring at 280 nm the firstorder decay of CI, in static mixtures of Cl#2HJ02 under
continuous photolysis using eight TL08 lamps. This method was viable at temperatures above 260 K.. Below this temperature, aerosol formation perturbed the UV measurements and the photolysis rate was normalized to the amount of light from the TL08 lamps that passed through the cooling fluid and was scattered into the detection region. In addition to Clz, some precursors and products have absorption spectra that overlap with the lamp emission spectrum, notably C 1 ~ 0HOCl,Oi, , and OC10. The loss of HOCl and 0 3 via photolysis would reduce the true product yield. The photolysis rates of these species were determined relative to that of Cll by numerical integration of the overlap of the UV absorption spectrum of each molecule and the emission spectrum of the TL08 lamps. The photolysis of OClO produces 0 atoms with a quantum efficiency of unity14 and is thus a source of 0 3 in the presence of 0 2 . The rate constant for this process is important, as it may falsify the amount of 03 formed in reaction 8b. We therefore carried out the photolysis of OCIO/Or/Ar and C12ICHJOzIAr mixtures in a separate a p p a r a t ~ s ~which ~ , ~ ' was equipped with a UVlvisible diode array detector capable of simultaneously measuring all wavelengths in a spectral region between 220 and 380 nm. With this method the decay of OClO and the formation of 03 could be simultaneously monitored. Calibration of Product Signals. In the absence of an easily measurable optical path length or infrared absorption cross sections for the matrix-isolated species, use of an intemal standard was necessary to convert infrared signals to concentrations. This was accomplished by utilizing the 1549 and 15.51 cm-' absorption lines of an asymmetric crystalline phase of molecular oxygen,35as described previously.36 A strictly linear relationship between the duration of matrix growth and the intensity of the 154911551 cm-' absorption confirmed the viability of this method for 20% 02 in Ar mixtures. Product signals (in the matrix) were converted to gas-phase concentrations by comparing integrated peak areas (after normalizing to the intemal standard) with calibration curves obtained under identical conditions of total pressure and oxygen partial pressure. For stable, easily handled substances such as CH30H, HCHO. and HC1, the gas-phase concentration was calculated using volumetric methods. For HlO?, 03, and OClO the gas-phase concentration was derived from measurements of extinction in the UV and a knowledge of the optical path length and absorption cross section at the wavelength of the measurement. Possible errors from such methods are related to the accuracy of the flow controllers and pressure gauges or to the absorption cross sections and are estimated as 5%. Attempts to calibrate the HOC1 infrared signal by similar methods failed due to the difficulties associated with generating a pure flow of this substance by wet-chemical methods. Pure HOCl is converted to H20 and C1.0 in a surface catalyzed reaction
+
2HOC1- H,O Cl20 (17, -17) and HOCl is always accompanied by water vapor impurity. On the other hand, equilibrium 17, -17 can be employed as a gasphase HOCl source rea~tion,~' as can the reaction of Clz with aqueous CaCO3 solution or the reaction between NaOCl and MgS04*7H20.3*,39Although UV measurements of HOCl are unperturbed by the presence of water vapor, MI-FTIR spectra revealed extensive in-matrix complex formation between HOCl and HrO. Weak interactions such as hydrogen bonding are not reflected in changes in the UV absorption spectrum when compared to that of the nonhydrated monomer but are easily observed in matrix vibrational spectra and resulted in this instance in several distinct IR absorption bands due to different complexes. As the extent of hydration of HOCl either in the gas-phase or upon matrix growth is unknown, the UV spectra
Reaction between H02 and C10
J. Phys. Chem., Vol. 99, No. 44, 1995 16267
cannot be used to calibrate the IR spectra. The chosen method for HOCl calibration was thus in-situ photochemical generation in low concentration ranges which make reaction 17 inefficient. This procedure is described in the Results section. Chemicals. Commercially obtained gases and chemicals were Ar (99.999%), He (99.999%), H2 (99.999%), 2% C12/He (99.8%/99.999%),0 2 (99.99%), C2H6 (99.95%), HC1(99.96%), HgO (yellow, 99%), NaClO2 (80%), NaOCl (aqueous, 5% active chlorine), CH3OH (99.9%), and MgS04-7H20 (98%) and were used without further purification. H202 was concentrated from an 85% aqueous solution to -95% by vacuum distillation at room temperature. OClO and C120 were made as described p r e v i o u ~ l y . Stored ~~ as a 10% mixture in He in a blackened bulb, C120 showed no significant decomposition over a period of several days. 0 3 was generated in situ in a low-frequency discharge through 0 2 . HCHO was prepared by the gentle heating of paraformaldehyde (-100 "C) and, following dilution in helium, was stored in a 6 L blackened glass bulb.
Results Photolysis Rates. The photolysis rate constant for C12 was found to decrease at lower temperatures, presumably due to the changing refractive index of the thermostating fluid (silicon oil). Values of 2.85 x and 2.65 x s-l were determined for J-Cl2 at 298 and 210 K, respectively. The relative photolysis rate constants for OC10, C120, HOC1, 03,and ClOOCl compared to that of Clz were calculated to be 27.5, 0.20, 0.09, 0.005, and 0.44, respectively. The photolysis of C120 and the symmetric C10 dimer (ClOOCl) contribute to the C1 and C10 production rates. C l 2 0 hv c1+ c 1 0 (18)
+ ClOOCl + hv C1+ C1+ 0,
-
(19)
The photolysis of Os is slow and in any case unimportant, as the 0 atom produced is rapidly converted back to 0 3 under our conditions of 0 2 partial pressure (I40 Torr) and total pressure (700 Torr). This was confirmed by observing stable 0 3 concentrations when 03/02/Ar mixtures were subjected to irradiation in the absence of chlorine chemistry. The photolysis rate constant of HOCl is ca. 9% of that of C12. It will be shown later that the rate of HOCl removal due to photolysis is however negligible compared to its production rate. The strong UV absorption of OClO overlaps well with the emission spectrum of the TL08 lamps, and this is reflected in the high value of J-OCIO relative to J-Cl2. To confirm the calculated J-value ratio at 300 K, OClO samples in 0 2 were irradiated using the same lamps but in a different apparatus (see Experimental Section). The following reactions are anticipated:
+ hv - 0 + c10 0 + 0, + M -.0, + M c10 + c 1 0 - c1, + 0, OClO
-
+
(15) (16) (204
OClO c 1 (2OC) OClO and 0 3 concentration profiles were obtained simultaneously at 5 s intervals by diode array UV absorption spectroscopy. The OClO removal is both photolytic (15) and through secondary reactions involving C1 atoms produced in reactions 20b and 20c and is also offset by its generation in reaction 20c. As the photolysis proceeds, the 0 3 reaches a concentration which enables it to compete with OClO for C1 atoms and the measured loss rate of OClO is a complex function of starting conditions, photolysis times, and branching ratios in
the C10 self-reaction. For this reason the OClO and O3profiles were analyzed by numerical modeling of a full reaction scheme of the chemistry, using literature values for the rate constants and branching ratio^^^,^' of the above reactions. Both the O3 production and the OClO decay could be accurately fitted using a OClO photolysis rate constant of (3.29 f 0.1 1) x IO-, s-l, where the errors are 95% confidence limits, as retumed by a fit to the data. The Cl2 photolysis rate constant was also measured s-I, yielding a in this apparatus and found to be 1.20 x relative value for OClO of 27.4 f 0.9, in excellent agreement with the calculated value of 27.5. The use of recently determined values for branching ratios in the C10 self-reaction at room temperture in oxygen34resulted in a photolysis rate constant of (3.28 f 0.13) x lo-* s-I, within error limits indistinguishable from that using the C10 data of Nickolaisen et aL4I Effective Reaction Time. As mentioned above, both diffusion and a poorly defined reaction length result in effective reaction times that are longer than those estimated using the linear velocity (velocity of the gas mixture along the major axis of the cell) of the gas mixture. The effective reaction time was determined at all temperatures by product analysis in a very simple reaction system in which C12/H2/02/Ar mixtures were photolyzed. C12
+ hv - c 1 + c1
(1 1)
H
(12)
C1+ H, -HC1+
H
+ 0, + M--0, + M H + C12 - HC1+ C1
(13)
(21)
The high 0 2 concentration ensured that all H atoms generated in reaction 12 are removed via reaction 13 and not reaction 21. Under these conditions, assuming no C1 wall losses, the HC1 production rate is simply J-C12[Cl,]i. As both J-Cl2 and [C12]i are accurately known, and HC1 can be accurately calibrated, the effective reaction time can be calculated from analysis of HCl time-profiles. Effective reaction times were always larger than those calculated from the linear velocity, and a correction factor of 1.2 was necessary at 210 K, increasing to 1.9 at 300 K. The decrease in the correction at 210 compared to 298 K presumably reflects a lower diffusion constant at this temperature. Reaction times in the numerical simulations were corrected using the appropriate factor obtained in the Clz/H2/ 0 2 system. Calibration of HOCl. The HOCl signal was calibrated using the in-situ gas-phase generation of HOCl in the photolysis of flowing Cl2/O3/H2/Ar/O2 mixtures at 300 K and numerical simulation of HOCI, H202, and HCl concentration profiles. HOCl is expected to be formed by reactions 11- 13 along with
+ 0, HO, + C10 - HOCl + 0, C l + 0, -ClO
(4) @a)
Additional reactions include H02
+ H 0 2 - H 2 0 2+ 0,
(22)
and reactions 19 and 20. In this scheme, the rate of generation of C1 atoms is controlled by the Cl2 concentration and the Cl2 photolysis rate. In order to calculate the rates of H02 and CIO formation, we need to know the rate constants for the reactions of C1 with H2 and 0 3 and the absolute concentrationsof both reactants. As the roomtemperature rate constants for reaction of C1 with H2 and 0 3
Finkbeiner et al.
16268 J. Phys. Chem., Vol. 99, No. 44, 1995 HOC1
- 210 K
1
+ I
270K
\
300 K
A
II
'0
0.0
0.2
0.4
0.6
1230
1240
1250
0.8
Normalised Peak Area Figure 3. Calibration curve for HOC1 showing the linear dependence of the signal on calculated Concentration.
are among the best characterized in the kinetic literature24and both 0 3 (by UV absorption) and H2 (by partial flow) can be accurately measured, we can calculate the partitioning of C1 atoms in this simple system. Complications arise due to the CI atom production in reaction 20, once again necessitating correction of the calculated HOCl concentration profile by numerical modeling. A series of experiments were carried out in which 0 3 , HOCl, Hz02, and HCl were measured following 40 s photolysis. This experiment was then repeated with identical initial 0 3 and H2 concentrations and a different initial Cl2 concentration and thus different extents of reaction. The MIFTIR HOCl signal could then be converted to a concentration by comparison with the predicted concentration from the numerical simulations. The following arguments give us confidence in this rather indirect calibration method. The 03, HCI, and H202 profiles were found to be well reproduced by the numerical simulations. From reactions 4 and 12 we see that G(Cl0)ldt = -6(03)/dt and G(H)ldt = d(HC1)l dt. As the 03 and HCl concentrations were well described, we infer that the C10 and H atom production rates are correct in the model. The correct simulation of the H202 profile implies that the HO2 flux into its self-reaction (reaction 22) is accurately predicted. This implies that the flux into competing reaction 8 is also correct and therefore that the model HOCl concentration is correct. The calibration factor for HOCl was obtained by plotting the MIFTIR HOCl signal (at 1239 cm-I) against the calculated HOCl concentration. The good linearity of the plot (Figure 3) infers no deviation from the expected chemistry as the reaction progresses. It should be noted that we assume that HOCl is the only product of the H02 reaction with C10, although the aims of this study are to measure the product branching in the same reaction. We will show later that the errors inherent in this assumption are very small as HOCl does appear, within error limits, to be the sole product at 300 K. In addition, a small contribution (Le. 5 % ) of another reaction pathway would only introduce an error of 5% into the HOCl calibration. The total estimated error in the HOCl calibration by this method is 115%, certainly better than that obtainable by wet-chemical methods of generation. Product Measurements. As expected, and consistent with previous product measurements, HOCl was observed as the major product at all temperatures, confirming that reaction 8a is the dominant pathway for the reaction between H02 and C10. The infrared absorption at 1235 cm-l (see Figure 4) was used
1
, , , , , , , , , , , , , , , , . , , ,
1080
1100 1110 Wavenumber [cm-'1
1090
1120
Figure 4. Infrared absorptions of HOC1 (1235 cm-I), 03 (1038 cm-I), and OClO (1 105 cm-') products at each temperature. The absorbance for each species depends on both its gas-phase concentration and the duration of the matrix growth. Gas-phase concentrations, calculated folowing a procedure which accounts for the thickness of the matrix (see text) are listed in Table 3.
to quantify the HOCl production; weaker absorptions at 721, 733, and 3586 cm-' 32 confirmed its identification. Both HCl and 0 3 are infrared active and could theoretically be used to study product formation from reaction 8b. In practice. the amount of HCI resulting from reaction 12 is much larger than that from reaction 8b and only the 0 3 product of this reaction can be used to estimate branching ratios. As for HOCl, 0 3 (1038 cm-I) was observed at all temperatures (Figure 4). Were the HOCl and O3 to come solely from reactions 8a and 8b, respectively, and the condition be fulfilled that neither undergo significant loss processes, then the branching ratio (asa) would simply be the ratio of the concentration of both species at a certain reaction time; i.e., ag,(in percent) = 100[HOCl]/ ([HOCl] [Oj]). However, as already indicated, some 0 3 formation is expected following OClO photolysis, where OClO is generated in the C10 self-reaction (reaction 20c). In order to estimate this contribution, we need detailed, temperature dependent, mechanistic information for the bimolecular C10 self-reaction (reaction 20), including the termolecular channel:
+
C10
+ C10 + M
Q
ClOOCl
+M
(20, -20)
Kinetic parameters for the forward reaction rates of each channel have recently been remeasured4' and show a marked temperature dependence, as does the equilibrium coefficient of at the termolecular channel. The ratio k20clk20d is -6 x at 210 K. At the room temperature, decreasing to -2 x same time the value for k-2od changes from 68 s-I at 300 K to 1.1 x 10-3 at 210 K, implying that OClO formation is strongly dependent upon the temperature. At 210 K, the lowest temperature reached in these experiments, OClO is not expected to be formed in measurable amounts. This was confirmed experimentally (see below). At all other temperatures an OClO
J. Phys. Chem., Vol. 99, No. 44, I995 16269
Reaction between HO2 and C10
TABLE 2: Reaction Scheme Used in Numerical Simulations! desired reaction
c12
Cl Clz C10 -HCl+H -H02+M HOCl 0 2 HC1+ 0 3
c1+ Cl20 C1+ H2
H+Oz+M HO2 HO2
+ C10 + C10
+
0
5
10
15
20
time [SI Figure 5. Concentration profiles of HOCl, HCl,OClO, and 0 3 at 270 K. The solid lines are the results of numerical simulations of the 03 concentrations (see Table 2). The dashed line is a simulation of the
OClO concentration. signal was observed, and thus a correction to the O3 formation via OClO photolysis was necessary. Product Formation at 300 and 270 K. Figure 5 shows experimentally determined concentrations of HC1, HOCl, OC10, and 03 at 270 K and at reaction times of 8.5, 11, 13.5, and 16.5 s. Precursor concentrations for this particular experiment were [Clz], 5 x lOI5; [C120], 8.0 x lOI4; [Hz], 1.90 x lo'* molecule ~ m - ~With . these conditions, the fate of the C1 atoms generated in the photolysis of Clz and secondary reactions is well controlled as long as the conversion of C120 is kept low. Simulations showed that, after 16 s, the C120 concentration was reduced to -50% of its starting concentration. The H2 remains essentially unchanged, and these two precursors account for >98% of C1 atom reactions, with the flux toward H2 (and thus HO2 formation) increasing as the reaction progresses. Even at the longest reaction times (16.5 s), calculations show that [CIONO&,,): dashed lines, asb= 0 ; solid lines, a 8 b =
5%.
At day 5, temperatures are reached at which PSC formation is possible (Figure 8). Reactions 52 and 53 take place and immediately extinguish the CION02 reservoir (Figure 9b):
+ HCl - C1, + HNO, HOCl + HC1- C1, + H,O
ClONO,
(52)
(53)
The HCl reservoir (dashed line in Figure 9c) is extinguished within four PSC encounters (by day 25), similar to the case for Figure 2 of Muller et al.56where it was assumed that the reaction between HO2 and C10 proceeds via channel 8a only. The solid lines in Figure 9 show the situation when reaction 8b is included. The increased production rate of HC1 via reaction 8b has two effects: Firstly, the depletion of HCI due to reaction with OH before PSC formation occurs is offset; indeed the HCI reservoir increases slightly (Figure 9c). Secondly, even once PSCs have formed, the HC1 reservoir survives until day 45. As the CION02 reservoir is already fully depleted after one PSC encounter in this scenario, the increased partitioning of C10, and HO, into the HC1 reservoir due to reaction 8b results in a very slight reduction in 0 3 depletion until day 45, as seen in Figure 9a. Once the HC1 reservoir is empty at about day 46, the CION02 can build up. The HC1 generated in reaction 8b then serves to convert the ClONO2 back into active forms, and the 03 depletion is enhanced (solid curve in Figure 9a). Also noticeable is the more rapid recovery of the HC1 reservoir once PSCs evaporate at higher temperatures after day 58. Although the HCl profiles are very different with and without reaction 8b, the 0 3 depletion is not greatly affected. This reflects the fact that, in this scenario, the heterogeneous C10, production and thus the 0 3 depletion are controlled by the size of the ClONO2 reservoir and the HOCl production rate (see Crutzen et al.I6), neither of which are affected significantly by reaction 8b. Scenario 2 ([CION021 > [HCI]). In this case the model is initialized following the scenario proposed by Webster et al.59 for the Arctic with CION02 in excess of HCl. The dashed lines
50 Day of t h e y e a r
100
Figure 10. 0 3 CIONO2, and HCI mixing ratios generated in scenario 2 ([CION02llnlt> [HCI],,,,) dashed lines, a8h = 0. solid lines, a8b = 5%
in Figure 10 represent the case where reaction 8b is not included. On day 5, PSCs are formed and the HC1 reservoir is practically completely depleted after a single encounter with a PSC, due to efficient titration by CION02 via reaction.52 The addition of reaction 8b to the scheme (solid lines in Figure 10) increases the HCl concentration prior to PSC formation (Figure 10c) and further ensures that the CION02 is converted to active forms over the course of the entire winter (compare dashed and solid CION02 lines in Figure lob). After the initial titration of HCl by the excess ClON02, the rate-limiting step in ClO, production via reactions 53 is the rate of HCl production. Clearly this is increased due to reaction 8b, and the concomitant increase in the rate of ozone depletion is apparent. In summary, reaction 8b can have a substantial effect on HC1 reservoir concentrations and recovery rates. Furthermore, when, at the onset of the polar winter, CION02 is in excess of HC1. 03 depletion rates may be significantly enhanced. When this situation is reversed (i.e. [HCl]init > [ClON02],,,,), the HCl reservoir is increased but the C10, production rates and thus 0 3 depletion rates are not greatly influenced. Our results regarding the relevance of reaction 8b are qualitatively similar to those obtained by Lary et al.,I4who examined the effect of HCl formation in the reaction between OH and C10 (reaction 9b). The potential role of reaction 8b in midlatitude stratospheric ozone depleting cycles is more complicated to assess due to the coupled HO,/ClO,/NO, cycles, as discussed in the Introduction. However, a recent investigation60 of the various stratospheric 0 3 removal processes indicates that the reaction sequence reactions 2, 4, 8a, and 10 accounts for ca. 30% of halogen-controlled 0 3 loss. Leu23has estimated that, with a branching ratio of 3%, reaction 8b becomes the dominant source of HCl at altitudes between 25 and 35 km. This additional HC1 production changes the partitioning between active chlorine (CIO,) and the HCl reservoir, decreases the ClO,/HCl ratio, and thus reduces the efficiency of the ozone-depleting cycles. A proper assessment of the effects of HC1 formation in reaction 8b on midlatitude upper stratospheric ozone requires pressure and temperature dependent branching ratios for reaction 8.
Reaction between HOz and C10
Conclusions Product formation in the reaction between H02 and C10 has been investigated at temperatures of 210, 240, 270, and 300 K and at a total pressure of 700 Torr. The major product channel is that forming HOCl and 0 2 (reaction 8a) and contributes at least 95% at all temperatures. At 210 K a second reaction channel forming HC1 and O3 (reaction 8b) is proposed to account for 03 formation in our reaction system at this temperature. The branching ratio at 210 K is ca. 5 k 2% for this channel. At 240 K a branching ratio of 2 f 1% was found. The large error limits at this temperature reflect the need to correct the 0 3 profile for formation following photolysis of OC10. At 270 and 300 K OClO photolysis is shown to account for the O3 observed, and an upper limit of 1% is estimated for branching to reaction 8b. A small branching ratio to reaction 8b has consequences for both gas-phase midlatitude 0 3 destruction cycles and, under certain conditions, the rate of 0 3 destruction in the polar stratosphere in springtime.
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