5040
J. Phys. Chem. A 1999, 103, 5040-5048
Rate Coefficient Measurements for the Reaction OH + ClO f Products Carla S. Kegley-Owen,† Mary K. Gilles,* James B. Burkholder, and A. R. Ravishankara*,† Aeronomy Laboratory, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, Colorado 80303, and CooperatiVe Institute for Research in EnVironmental Sciences, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed: February 4, 1999; In Final Form: May 4, 1999
The rate coefficient for the reaction OH + ClO f products (1) was measured under pseudo-first-order conditions in OH. A discharge flow system was used to produce ClO, and its concentration was measured by UV/visible absorption. OH was produced by pulsed laser photolysis of O3 (or ClO) at 248 nm in the presence of H2O and was monitored by laser-induced fluorescence. The value of k1 between 234 and 356 K is given by k1(T) ) (8.9 ( 2.7) × 10-12 exp[(295 ( 95)/T] cm3 molecule-1 s-1, where uncertainties are 95% confidence limits and include estimated systematic uncertainties. Our value is compared with those from previous investigations.
Introduction The reaction of OH with ClO
OH + ClO f HO2 + Cl
(1a)
f HCl + O2
(1b)
may play a significant role in the partitioning of chlorine in the upper stratosphere. The conversion of ClO to Cl in reaction 1a propagates ozone destruction via
Cl + O3 f ClO +O2
Poulet et al.5 (at 298 K). Previous studies, all of which were carried out under pseudo-first-order conditions in OH, used discharge flow systems for the radical source (or sources) followed by detection of OH via resonance fluorescence, laserinduced fluorescence, or laser magnetic resonance. Several experiments have used reaction 2 with an excess of Cl atoms as the ClO source and assumed that the initial ozone concentration was equal to the initial ClO produced, [O3]0 ) [ClO]0.4,6,7 We will examine this assumption in this work. Other experiments done in excess Cl atoms applied corrections4,5,7 to k1 to account for the regeneration of OH through the reaction
(2)
while reactive chlorine (ClO and Cl) is converted to the reservoir species HCl in reaction 1b. The branching ratios in reaction 1 are of particular importance, since they directly affect partitioning between active and reservoir, HCl and ClONO2, chlorine species in the upper stratosphere. Recently, Dubey et al.1 performed a box model sensitivity analysis using the currently recommended value2 of k1 and found that a 7% branching ratio for channel 1b reduces the modeled [ClO]/[HCl] ratio to that observed in some field studies. Revisions in the value of k1 and its temperature dependence would change the value of the branching ratio required to bring field observations and model calculations into agreement. Revisions in the overall value of k1 would also have other consequences to chlorine chemistry. Previous measurements of k1 contain discrepancies in both its magnitude and temperature dependence. Determinations of k1 at room temperature range from 0.91 × 10-11 to 1.99 × 10-11 cm3 molecule-1 s-1.2 The value of k1 currently recommended for atmospheric models is k1(T) ) 1.1 × 10-11 exp[(120 ( 150)/ T] cm3 molecule-1 s-1, with k1(298 K) ) 1.7 × 10-11 cm3 molecule-1 s-1.2 This recommendation is based on the data of Hills and Howard3 (who observed a negative temperature dependence (E/R ) 235 ( 46 K)), of Burrows et al.4 (who reported no dependence of k1 on temperature), and the value of * To whom correspondence should be addressed. Address: NOAA/ERL, R/E/AL2, 325 Broadway, Boulder, CO 80303. E-mail:
[email protected] or
[email protected]. † Also affiliated with the Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309.
Cl + HO2 f ClO + OH f HCl + O2
(3a) (3b)
A few studies have used excess O3, [O3] > [Cl]0, when employing reaction 2 as the ClO radical source.3,5,8 In this paper, we present results from the study of the temperature dependence of the rate coefficient for reaction 1. Reaction 1 was studied under pseudo-first-order conditions in OH. ClO was generated via reaction 2 under conditions where (a) excess O3 was present or (b) where O3 was completely depleted during ClO production. In these experiments ClO and O3 were monitored simultaneously via UV/visible absorption in situ. Experiments Owing to the relatively slow self-reaction of ClO and the high sensitivity with which OH can be detected by laser-induced fluorescence, we studied this reaction under conditions of [ClO] > 10[OH]0 (pseudo-first-order in OH). The apparatus used was a combination of a discharge flow system for producing ClO, a UV/visible absorption spectrometer to quantify ClO concentration, and a pulsed laser photolysis and pulsed laser-induced fluorescence system for producing and detecting OH. The apparatus and methods of operation are described in detail in a recent publication.9 ClO Production. Cl atoms were generated in a side arm of a flow tube (2.54 cm i.d.) by passing a dilute mixture of Cl2 in
10.1021/jp9904320 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/15/1999
OH + ClO f Products
J. Phys. Chem. A, Vol. 103, No. 26, 1999 5041 kinetic experiments. The concentration of ClO was calculated using the Beer-Lambert law:
A ) σl[ClO]
Figure 1. (a) Reference spectra of O3, ClO, and Cl2. (b) Example of the spectral subtraction used to obtain [ClO]. (c) Residual (×1000) obtained after subtracting the scaled ClO and O3 reference spectra from the measured spectrum that contained both ClO and O3.
He through a microwave discharge. Within the flow tube, Cl atoms reacted with O3 added through the movable injector via reaction 2, where k2(298 K) ) 1.2 × 10-11 cm3 molecule-1 s-1.2 Typical flow rates of He in the flow tube were 16-38 STP cm3 s-1 at pressures of 4.2-15.4 Torr. This resulted in linear gas flow velocities in the flow tube and reaction cell (also 2.54 cm i.d.) of 330-800 cm s-1. ClO concentrations ranged from 0.3 × 1013 to 12 × 1013 molecule cm-3. UV Absorption Spectroscopy. The absorbance due to ClO was measured in situ by UV/visible absorption spectroscopy. The output of a 30 W D2 lamp was collimated and passed through the cell along the axis of the flow (counter to the direction of the photolysis laser beam and perpendicular to the probe laser beam). The D2 beam was then focused onto the entrance slit of a 0.35 m spectrometer. The spectrometer employed a 600 grooves/mm grating blazed at 300 nm and a cooled 1024 element diode array detector. For monitoring ClO and O3, the spectrometer was set to observe from 250 to 365 nm at a spectral resolution of 0.8 nm (fwhm). The wavelength was calibrated using the emission lines from a low-pressure Hg lamp. The ClO concentration was quantified using the unstructured portion of its spectrum, since the cross section in this region is well-known and is independent of temperature and resolution.2 However, the spectral subtraction, discussed later, utilized the structured region of the spectrum. Reference spectra of O3, ClO, and Cl2 are shown in Figure 1 and are in good agreement with literature values.2 A ClO reference spectrum was measured prior to each experiment by titrating O3 (or in some cases Cl2O) with a large excess of Cl atoms. These spectra were recorded under flow, pressure, and temperature conditions that were identical to the subsequent
(I)
where A was the absorbance, σ was the absorption cross section (σ253.7nm ) 4.25 × 10-18 cm2 molecule-1),2 and l was the path length (28.0-38.8 cm). After each OH temporal profile was recorded, the photolysis laser was blocked and the beam from the deuterium lamp passed through the cell to measure the absorbance due to ClO and O3. The absorbance due to the initial O3 could be measured by turning off the microwave discharge. Finally, the flows of O3 and Cl2 were shut off and I0, the intensity in the absence of absorbers, was measured. These spectra were used to determine the initial ozone concentration ([O3]0) prior to ClO formation, the concentration of ClO, and the concentration of ozone remaining after ClO production, “[O3]excess”. The absorbance due to ClO was extracted from the measured spectrum by subtracting a scaled ClO reference spectrum until all of the ClO structured absorption was completely removed. Since the ClO spectra is structured, this is easily accomplished. The spectral subtraction was normally done by eye immediately after taking the spectra to check for any variation in the deuterium lamp intensity. An example of the spectral subtraction is shown in the lower part of Figure 1. The residual shown in the middle and lower part (×1000) of Figure 1 was obtained by subtracting the remaining O3. The abundance of ClO (as inferred from its known absorption cross section at 253.7 nm) subtracted from the reference spectra to obtain an unstructured residual was used to determine [ClO]. The concentrations of Cl2 used, (0.2-3) × 1014 molecule cm-3, were too small to be measured by UV absorption and were calculated from calibrated flows and the total pressure. Some experiments were carried out under conditions in which ozone was consumed during the ClO radical production. These measurements were performed to test the dependence of k1 on the presence of excess ozone and the assumption of stoichiometric conversion of O3 to ClO (reaction 2). Under these conditions, there was no O3 remaining in the reaction cell. The concentration of remaining Cl atoms was not measured. We could ensure that all O3 was depleted by comparing the ratio of the measured ClO absorbance under these conditions at 260.0 and 253.7 nm. The absorbance at these two wavelengths for ClO was determined from ClO spectra measured in a large excess of Cl by adding Cl2O or O3 to be 1.20 ( 0.01. Because the O3 cross section2 at 253.7 nm is ∼2.4 times larger than that of ClO, the ratio would deviate from 1.20 if O3 were present. Because the measured ratio of absorbance was essentially 1.20, we place an upper limit of 7 × 1011 molecule cm-3 of O3 in the cell. OH Production. In most experiments, OH was produced via photolysis of O3 in the presence of H2O that was added downstream of the flow tube:
O3 + hν f O(1D) + O2
(4)
O(1D) + H2O f 2OH
(5)
The rate coefficient2 for the reaction of O(1D) with H2O is k5 ) 2.2 × 10-10 cm3 molecule-1 s-1, and this reaction went to >95% completion in 99.9995%), and He (>99.997%) were all obtained commercially. Helium was passed through a liquid nitrogen trap, and all other gases were used as supplied. Ozone was produced in a commercial ozonizer and stored on a silica gel trap kept in a dry ice/ethanol bath. Cl2O was synthesized following techniques in the literature,14 and its purity (>92%) was determined via UV absorption. Mass flow controllers were used to measure the gas flows into the cell, and pressure was measured with capacitance manometers. HNO3 was introduced into the reaction cell by bubbling He through a mixture of concentrated sulfuric and nitric acids. Results OH temporal profiles in the presence of excess ClO and O3 were influenced by the reactions
OH + ClO f products
(1)
OH f loss
(10)
where reaction 10 represents the first-order rate coefficient for loss of OH due to diffusion and flow out of the detection region. Other reactions that could influence the OH temporal profile were
OH + Cl2 f HOCl + Cl
(11)
OH + O3 f HO2 + O2
(12)
Contributions to the OH loss rate coefficients from reactions 11 and 12 were estimated from the calculated [Cl2] and measured [O3]excess along with the respective rate coefficients.2,15 These losses were estimated to be
