J. Phys. Chem. A 2001, 105, 9697-9703
9697
A Temperature-Dependent Kinetics Study of the Important Stratospheric Reaction O(3P) + NO2 f O2 + NO E. G. Estupin˜ a´ n,† J. M. Nicovich,‡ and P. H. Wine*,†,‡ School of Earth and Atmospheric Sciences and School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332 ReceiVed: May 21, 2001; In Final Form: August 14, 2001
A laser flash photolysis-resonance fluorescence technique has been employed to investigate the kinetics of the important stratospheric reaction O(3P) + NO2 f O2 + NO (k1) as a function of temperature (221-425 K) with particular attention paid to obtaining the most accurate values possible for k1(T). The following Arrhenius expression adequately describes the observed temperature dependence: k1(T) ) (4.21 ( 0.25) × 10-12 exp{(273 ( 18)/T} cm3 molecule-1 s-1, where the uncertainties represent precision at the 2σ level. The accuracy of the reported values for k1(T) is estimated to be (6% over the entire temperature range investigated. At room temperature the rate coefficient measured in this study is in excellent agreement with that reported in a recent paper by Gierczak et al. [J. Phys. Chem. A 1999, 103, 877] while the activation energy of the rate coefficient is somewhat more negative than that reported by Gierczak et al. Incorporation of the results of the present study into models of stratospheric chemistry would lead to somewhat lower mid-stratospheric ozone levels than would be obtained using results of previous studies.
Introduction Active nitrogen constituents, primarily NO and NO2 (together known as NOx) play important roles both in the troposphere and stratosphere. In the troposphere, NOx is necessary for the photochemical production of O3. In addition, NOx directly and indirectly affects the concentration of the hydroxyl radical, which is the primary tropospheric oxidant responsible for converting many primary emissions to secondary products. In the stratosphere, NOx-catalyzed destruction cycles in combination with those involving ClOx, BrOx, and HOx radicals strongly influence the concentration and vertical profile of O3. The catalytic ozone destruction cycle
O(3P) + NO2 f O2 + NO
(1)
O3 + NO f O2 + NO2
(2)
Net: O( P) + O3 f 2O2
(3)
3
is the most important NOx cycle in the stratosphere. Reaction 1 is the rate-determining step in the cycle and it accounts for the majority of odd-oxygen destruction in the 25 to 40 km altitude regime.1 Hence, it is desirable to know k1(T) with a very high degree of accuracy, especially at temperatures that are typical of the mid-stratosphere, i.e., 220-260 K. While several studies of the kinetics of reaction 1 have been reported in the literature,2-6 the 1997 NASA panel for chemical kinetics and photochemistry data evaluation7 suggested that the uncertainties in k1(T) values remained much higher than desired, particularly at temperatures below 240 K. Uncertainties * Author to whom correspondence should be addressed at School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400. E-mail:
[email protected]. † School of Earth and Atmospheric Sciences. ‡ School of Chemistry and Biochemistry.
in k1(T) have been identified as a major source of uncertainty in stratospheric models.8 The kinetics of reaction 1 were recently reinvestigated by Gierczak et al.9 These investigators obtained a value for k1(298 K) that is 10% faster than the previously recommended value, and report that k1 increases with decreasing temperature more rapidly than suggested by earlier studies. The findings of Gierczak et al.9 stimulated the present study of the kinetics of reaction 1 with the goal of measuring this critical rate coefficient to a high degree of accuracy over the temperature range 220-425 K. In this study, kinetic information was obtained by monitoring the temporal profile of oxygen atoms under pseudo-first-order conditions with NO2 in large excess. Special emphasis was placed on accurate determination of the NO2 concentration and on measurements of k1 at stratospheric temperatures. Experimental Technique The laser flash photolysis (LFP)-resonance fluorescence (RF) apparatus used in this study was similar to systems previously employed in this laboratory to study atom-molecule reactions involving O atoms.10 A schematic diagram of the current version of the apparatus is published elsewhere.11 Important features of the apparatus and experimental approach that are specific to this study are described below. A Pyrex, jacketed reaction cell with an internal volume of ∼150 cm3 was used in all experiments. The cell was maintained at a constant temperature by circulating ethylene glycol (T > 298 K) or methanol (T < 298 K) from a thermostated bath through the outer jacket. A copper-constantan thermocouple with a stainless steel jacket was periodically injected into the reaction zone through a vacuum seal to measure the gas temperature under the precise pressure and flow rate conditions of the experiment. The thermocouple was retracted during kinetic experiments and the temperature measurement in the reaction zone is estimated to be accurate to (1 K.
10.1021/jp011940o CCC: $20.00 © 2001 American Chemical Society Published on Web 09/27/2001
9698 J. Phys. Chem. A, Vol. 105, No. 42, 2001
Estupin˜a´n et al.
Oxygen atoms were produced by 355 nm laser flash photolysis of NO2 using third harmonic radiation from a Nd:YAG laser (Quanta Ray model DCR-2A, pulse width ∼6 ns) as the photolytic light source:
NO2 + hV f O(3P) + NO
(4)
At λ ) 355 nm, the yield of O(3P) in reaction 4 is known to be unity.7 An atomic resonance lamp, situated perpendicular to the photolysis laser, excited resonance fluorescence in the photolytically produced atoms. The resonance lamp consisted of an electrodeless microwave discharge through ∼1 Torr of a flowing mixture containing a trace of O2 in helium. The flow of a 0.1% O2 in He mixture and pure He into the lamp were controlled by separate needle valves, thus allowing the total pressure and O2 concentration to be adjusted for optimum signalto-noise. Radiation was coupled out of the lamp through a MgF2 window and into the reaction cell through a MgF2 lens. Dry N2 was used as a purge gas in the volume between the lamp window and the reaction cell lens to exclude room air and thus allow transmission of vacuum-UV radiation. Fluorescence from excited O(3P) atoms within the reaction zone was collected by a MgF2 lens on an axis orthogonal to both the photolysis laser beam and the resonance lamp beam and imaged onto the photocathode of a solar blind photomultiplier. The region between the reaction cell and the photomultiplier was purged with dry N2 and contained a CaF2 window to prevent detection of Lyman-R emission from the resonance lamp. The fluorescence signals were processed using photoncounting techniques in conjunction with multichannel scaling. For each O(3P) decay rate measured, 500-5000 temporal profiles were co-added to obtain a well-defined temporal profile. The multichannel analyzer sweep was triggered approximately 3.2 ms prior to the photolysis laser in order to determine the background light level immediately before the laser flash. To avoid accumulation of photolysis or reaction products, all experiments were carried out under “slow-flow” conditions. The linear flow rate through the reactor was typically 3 cm s-1, and the laser repetition rate was typically 5 Hz. Since photolysis occurred on an axis perpendicular to the direction of flow, no volume element of the reaction mixture was subjected to more than a few laser shots. The reactant and photolytic precursor, NO2, was flowed into the reaction cell from a 12-L bulb containing dilute mixtures in nitrogen buffer gas, while N2 was flowed directly from a high-pressure tank. The NO2/N2 bulb was blackened to prevent photolysis by room lights. To minimize systematic error in the NO2 concentration measurement, two independent methods were employed. In one method, the flow of the NO2/N2 mixture and the flow of pure N2 were both measured with calibrated mass flow meters. The concentration of NO2 was calculated using measurements of the mass flow rates of the two components of the flow, the total pressure in the reaction cell (measured with a 1000 Torr full scale capacitance manometer), and the temperature in the reaction cell. The mole fraction of NO2 in the NO2/N2 mixture was checked frequently by UV photometry at 366 nm. The photometric measurements were carried out on a separate high vacuum gas handling system employing a mercury penray lamp and a photomultiplier tube equipped with an interference filter to isolate the three Hg transitions at λ ∼ 366 nm from other lamp emissions. The effective NO2 absorption cross section was measured using the same lamp and filter to be (6.05 ( 0.14) × 10-19 cm2 molecule-1. The concentration of NO2 was also determined by in situ long-path absorption measurements both upstream and down-
stream of the reaction cell. Using a partially reflecting optic, the output beam of the argon ion laser operating at 457.9 nm was split into two approximately equal components which were then multipassed using White cell optics12 through two separate Pyrex absorption cells. The windows on the absorption cells had antireflection coatings (400-500 nm) and the White cell mirrors were coated for high reflectivity (425-485 nm). For these experiments, the upstream cell was 37.3 cm in length with 52 passes of the laser beam and the downstream cell was 35.5 cm in length with 56 passes. The intensity of the laser light exiting the White cells was continuously monitored by separate silicon photodiodes. To reference the signals from these two photodiodes to the initial intensity of the laser, a thin, glass optic was used to pick off a portion of the laser beam before the beam splitter and direct it onto a third photodiode. Each photodiode was equipped with a stack of thin Teflon diffusers to prevent saturation. The detector signals were electronically amplified and processed through an analog-to-digital card in a personal computer. An electromechanical shutter allowed the laser beam to be periodically blocked in order to measure the small background for each detector. On a separate gas handling system, the NO2 absorption cross section at 457.9 nm and room temperature was measured using the same Ar+ laser, a 37.4 cm long absorption cell, and manometrically measured samples of NO2. Again, the light intensity of the laser beam exiting the absorption cell was referenced to the initial intensity using a pick-off mirror and a second photodiode. Absorption measurements were made over a wide range of [NO2] (0.11 to 0.87 Torr), path length (1 to 5 passes of the laser beam), and total pressure (N2 was added for some measurements). The pressure gauges used for these determinations were calibrated by the expansion of known masses of various gases into the known volumes of the gas manifold and absorption cell. Results of both methods of determining the NO2 concentrations in the kinetics experiments are discussed below. The N2 used in this study was UHP grade with a minimum purity of 99.999%; it was used as supplied. NO2 was prepared by reacting NO with excess O2 and leaving the mixture overnight. NO2 was then collected in a liquid nitrogen cooled trap while O2 and other impurities were pumped away. NO and O2 had minimum stated purities of 99.0% and 99.996%, respectively. Results and Discussion 1. NO2 Concentration Results. The NO2 absorption cross section at the argon ion laser wavelength (457.9 nm) was determined thirty-two times in order to maximize the precision of the measurement. These results are summarized in Figure 1, where the calculated NO2 cross section is plotted as a function of absorbance. Two parameters were varied to ensure that the measured NO2 cross section was independent of initial conditions; these were the initial NO2 pressure and the total laser absorption path length. No systematic change in cross section results was observed. The NO2 concentration was corrected for the formation of N2O4 using the equilibrium constant recommended by the NASA panel.7 The corrections were very small (
