J. Phys. Chem. 1988, 92, 4667-4669
4667
Tunable Diode Laser Measurements of H02N02Absorption Coefficients near 12.5 pm R. D. May,* L. T. Molina, and C. R. Webster Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91 109 (Received: January 21, 1988)
A tunable diode laser spectrometer has been used to measure absorption coefficients of peroxynitric acid (HO,NO,) near the 803-cm-l Q branch. H02N02concentrations in a low-pressure flowing gas mixture were determined from chemical titration procedures and UV absorption spectroscopy. The diode laser measured absorption coefficients, at a spectral resolution of better than 0.001 cm-', are about 10% larger than previous Fourier transform infrared measurements made at a spectral resolution of 0.06 cm-'.
Introduction Oxides of nitrogen play key roles in the photochemical processes occurring in the Earth's upper atmosphere and are responsible for a significant fraction of ozone depletion in the middle stratosphere.' The interactions of NO, NO2,and H N 0 3 with each other and with the ozone layer have been studied extensively in the laboratory, from rocket, aircraft, and balloon platforms, and from space.2 H 0 2 N 0 2is a temporary reservoir for both HOz and NO2,tying up increasingly large fractions of NO2 at altitudes below about 35 km. In the stratosphere H O z N 0 2 is formed primarily from the three-body recombination reaction
H02
+ NO2 + M
+
H02N02
+M
(1)
where M is a suitable third body, and is destroyed by photolysis, thermal decomposition, and chemical reaction, mainly with OH. N o experimental studies of the photolysis products as functions of wavelength and temperature have been carried out. Since first identified spectroscopically by Niki et al.,3 several workers have recorded the infrared spectrum of H 0 2 N 0 2during the course of kinetic studies using Fourier transform infrared (FTIR) spectrometers.e8 The strong absorption band centered at 803 cm-', identified as v6 by Baldwin and Golden? is relatively free from spectral interferences by contaminates and is suitable for use in atmospheric monitoring. Absorption features of H 0 2 N 0 2 were recently observed by the ATMOS Fourier transform instrument from a near-Earth orbit aboard Spacelab 3, and preliminary vertical profiles and total column densities have been reported.I0 Absorption coefficients required for quantitative determinations of atmospheric abundances have been measured at room temperatures by Molina and Molina' using a Fourier transform spectrometer at total system pressures near 1 atm, and at a spectral resolution of 1 cm-'. Graham6 carried out similar measurements at lower system pressures (1-20 Torr) and studied the absorption cross sections as a function of spectral resolution from 0.06 to 4 cm-'. Absorption cross sections for the 803-cm-' Q branch were observed to vary considerably with spectral resolution. In this (1.) National Research Council. Causes and Effects of Stratospheric
Ozone:An Update Report; National Academy Press: Washington, DC, 1982. (2) World Meteorological Organization. Global Ozone Research and Monitoring Project, Report No. 16, Vol. 11, 1985. (3) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Chem. Phys. Lett. 1977. 45. 564. -. . (4) Lcv'lnc,'S.-Z.;Uselman, W. M.; Chan, W. H.; Calvert, J. G . ;Shaw, J. H.Chem. Phys. Lett. 1977, 48, 528. (5) Graham, R. A.; Winer, A. M.; Pitts, J. N., Jr. Chem. Phys. Lett. 1977, 51, 215. (6) Graham, R. A.; Winer, A. M.; Pitts, J. N . , Jr. Geophys. Res. Lett. 1978, 5, 909. (7) Molina, L. T.; Molina, M. J. J . Photochem. 1981, 15, 97. 18) Barnes, I.; Bastian, V.; Becker, K. H.; Fink, E. H.; Zabel, F. Atmos. Environ. 1982, 16, 545. (9) Baldwin, A. C.; Golden, D. M. J . Phys. Chem. 1978, 82, 644. (10) Rinsland, C. P.; Zander, R.; Farmer, C. B.; Norton, R. H.; Brown, L. R.; Russell, J. M., 111; Park, J. H. Geophys. Res. Lett. 1986, 13, 761.
0022-365418812092-4667$01.50/0
paper we present high-resolution FTIR and tunable diode laser (TDL) spectra of the Q-branch region, and report TDL measurements of absorption coefficients at room temperatures for the HO2NO2803-cm-' Q branch at total system pressures below 5 Torr.
Experimental Details The TDL spectrometer has been described in detail in ref 1 1 , so only an abbreviated description will be given here. Figure 1 illustrates the experimental setup. A single longitudinal mode of a stripe-geometry TDL was isolated by using a 0.5-m Jarrell-Ash monochromator. The collimated TDL beam made a single pass along the axis of a 97.2-cm Pyrex absorption cell, which was enclosed in an aluminum housing to avoid photodecomposition of the sample. The diameter of the absorption cell was 3.8 cm, and the interior cell walls were silanized to minimize wall adsorption and decomposition. AgCl windows were vacuum sealed to the absorption cell with Teflon-coated O-rings. All measurements were carried out at room temperatures. Total cell pressures were measured with a calibrated MKS Baratron gauge (0-10 Torr) attached to a central side port on the cell. After passing through the absorption cell the TDL beam was focused onto a HgCdTe detector whose output was amplified and applied to one of two signal-processing paths. For fast scans of relatively small regions (C0.2 cm-') the sweep integration technique', was employed in which successive scans were accumulated in a dual-channel signal averager (PAR 4203). In this technique, high signal to noise ratio spectra could be recorded in very short periods of time (seconds) and deterioration of the absorption signal due to wall reactions or other possible mechanisms could be readily identified. It was found that a mixture containing a stable H 0 2 N 0 2 concentration could be maintained within the absorption cell for periods of 20-30 min after initiation of the reaction in a well-conditioned cell. Slower scans (2-5 min) over longer spectral regions were recorded by increasing the speed of the chopper to several hundred hertz and using conventional phase-sensitive detection. The 100% transmission levels were determined by flushing the absorption cell with pure helium after recording the H 0 2 N O z spectrum, evacuating the cell, and then recording a spectrum of the empty cell. A 7.62-cm germanium etalon (free spectral range nominally 0.016 cm-') provided a fringe pattern for calibration of the frequency scale. In both of the data collection schemes, data were transferred to a laboratory computer (Hewlett-Packard Model 1000F)for processing and plotting. HOzNOz was prepared from the reaction of nitronium tetrafluoroborate (NO,BF,) with 90% H202inside a nitrogen-filled glove box as described by Molina and Molina.' This generation technique has advantages over alternative methods, for spectroscopic studies, as described in ref 7. Specifically, a greater proportion of HOzNOzis produced from (1.1) May, R. D.; Webster, C. R.; Molina, L. T. J . Quant. Specrrosc. Radrat. Transfer 1987, 5, 381. (12) Jennings, D. E. Appl. Opt. 1980, 19, 2695.
0 1988 American Chemical Society
4668 The Journal of Physical Chemistry, Vol. 92, No. 16, 19'88
May et al.
CHART
HO2NO2 RES 0006 cm-'
FTIR l Y Y l
REFERENCE
SIGNAL AVERAGER
GENERATOF
CHOPPER
JARATRON
~
'I 04'1
L
801 0
I 802 0
803 0
804 0
805 0
WAVENUMBER icm ' I
BUBBLER
Figure 1. Experimental arrangement for tunable diode laser measurements of H 0 2 N 0 2absorption coefficients.
Figure 2. High-resolution(0.006 cm-') Fourier transform spectrum of the H 0 2 N 0 2Q branch at 803 cm-I. The optical path length was 42 m.
the reaction, resulting in stronger absorptions at low pressures, and significantly less H N 0 3 is generated, which is the most troublesome contaminate in the 12.5-pm region. The final solution was transferred to a glass bubbler maintained at 8 O C . A stream of helium gas forced through the bubbler carried the H 0 2 N 0 2 vapor through the absorption cell. The relative concentration of H 0 2 N 0 2in the absorption cell was controlled by varying the helium flow rate, monitored with a Hasting mass-flow meter, and by adjustment of the total cell pressure. Total system pressures were in the 2-5-Torr range, and H 0 2 N 0 2partial pressures were in the range 10-70 mTorr, for the measurements reported here. The residence time of H 0 2 N 0 2in the absorption cells was less than 0.5 min.
that all Br2 liberated was due to oxidization by H 0 2 N 0 2 . Second, pure anhydrous HNO, was placed in the sample bubbler, and a IO-cm quartz cell with Suprasil windows was placed in series with the TDL absorption cell. The quartz cell was also contained inside the sample chamber of the UV-visible spectrometer. The HN03/helium mixture was flowed at a few Torr total pressure through the two cells. The flow of HNO, was then diverted into the trap in the same manner as was done for H 0 2 N 0 2 , and titrated with a standard NaOH solution upon thawing. Using the UV absorption cross sections of Molina and Molina7 for HNO,, the cell concentration was determined both from the UV absorption and from the result of the titration. The agreement was consistently within 10% and gave confidence at this level in the procedures used to determine H 0 2 N 0 2concentrations.
Concentration Measurements Because it was not possible to obtain a pure gaseous sample, the partial pressure of H 0 2 N 0 2in the absorption cell had to be determined by an indirect method. Spectral subtraction methods could not be appied due to the limited tuning range of the laser. Therefore a titration method originally developed by Molina and Molina7 was employed. In this method the cell effluent is passed through a measured volume of standardized KBr solution. The Br2 liberated from the oxidation of KBr is quantitatively determined from the UV absorption at 417 nm, and H 0 2 N 0 2 concentrations are derived from the appropriate stoichiometric relationships. At system pressures of 1 atm the effluent can be bubbled directly into the KBr solution. At the low total pressures used in the present work (e5 Torr) the effluent could not be directly bubbled into a KBr solution, and it was necessary to modify the titration procedure, although H 0 2 N 0 2concentrations were still determined by using the oxidization of KBr. After investigating several alternatives the following procedure was found to give consistent results. An evacuated glass trap at 77 K was placed immediately after the exit port on the absorption cell so that the cell effluent could be diverted into it by using a pair of Teflon stopcocks. The effluent was collected for a fixed amount of time dependent on the flow rates and total pressure, typically 2 min, and the trap was sealed off and removed from the system. A measured volume of KBr solution was then added to the frozen mixture and placed within the sample chamber of a UV-visible spectrometer (IBM Model 9430). As the mixture thawed the oxidation of KBr proceeded rapidly and H 0 2 N 0 2concentrations were determined from the measured absorption at 417 nm. The validity of this trapping method was tested by investigating the critical aspects in separate control experiments. First, pure HN03, and then 90% H202,were placed separately in the sample bubbler, instead of the H 0 2 N 0 2generation mixture, to assess the extent of KBr oxidation by these species. The rate of KBr oxidization by either H N 0 3 or H 2 0 2was less than for H02N02by a factor of 10, as found previously.' It was therefore assumed
Results Figure 2 shows an FTIR spectrum of the Q branch of the 12.5-pm band of H02N02 recorded with a Bomem Model DA3.002 spectrometer at a spectral resolution of 0.006 cm-I (unapodized). The H 0 2 N 0 2generation procedure was the same as for the spectra recorded with the TDL spectrometer. N o attempt was made to determine quantitative absorption coefficients from the FTIR spectrum due to the large cell volume (27 L) and the long time required to record the spectrum (66 min for 20 scans). Several survey spectra of this type were recorded in order to investigate the general appearance of the spectrum at 0.006 cm-' resolution, which is a higher spectral resolution than has been reported previously. In Figure 3, a and b, FTIR and TDL spectra of the Q-branch region are compared. Because an adequate theoretical description of the H 0 2 N 0 2spectrum in this spectral region is lacking, no attempt was made to assign any of the individual spectral features. The higher resolution TDL spectrum shows some additional structure, but there is still a large continuum absorption present. The continuum was proportional to the H 0 2 N 0 2 pressure. However, the relative amounts of comtaminate species also vary in proportion to the H 0 2 N 0 2concentration for the production method used. The measured absorption coefficient, k ( v ) , at 0.001 cm-I resolution is plotted in Figure 4. This plot represents an average of eight individual measurements whose standard deviation was 6.9%. The maximum value of k(v) was 2.24 X lo-'* cm2/molecule at 802.76 cm-]. This value can be compared to the FTIR results of Graham et a1.6 (2.1 X cm2/molecule at 0.06 cm-I resolution) and Molina and Molina' (1.0 X cm2/molecule at 1 cm-l resolution). We observe only a slight increase in k ( v ) relative to the result of Graham et a1.,6 but the measurements overlap within the experimental uncertainties. Thus we do not observe a significant increase in the absorption coefficient at the much higher resolution obtained here. This is primarily due to the large continuum, which accounted for 40-50% of the total absorption.
The Journal of Physical Chemistry, Vol. 92, No. 16. 1988 4669
,
1.oo
0.8909
I
HO2NO2
a! FTIR
1
[
HO2NO2 a) OBSERVED 802.6968
802.7256
802.7545
802.7833
802.8121
I
802.8409
WAVENUMBER (cm-’l
u ” 8 ’ ~ ,~ ~
I
0.475 802.4954
b! TDL
’
H02N02
I
802.8060
802.8836
I
1
802.5730
802.6506 802.7283 WAVENUMBER Icm-l!
bl DECONVOLVED
Figure 3. Comparison of (a) FTIR and (b) TDL spectra of the H02N02
Q branch at 803 cm-’ recorded with similar total system pressures and flow rates. Some additional structure is revealed in the higher resolution TDL spectrum, but a continuum absorption level equal to 4 0 4 0 % of the total absorption persists due to the high density of lines. 2.24 I
li
I
I
HO2NO2 absorption coefficient
0, ’ ~
73
I’h
0.6982 802.6968
I
I
1
I
802.7256
802.7545
802.7833
802.8121
802.8409
WAVENUMBER (cm-l)
Figure 5. Observed HOzNO2spectrum (upper trace), and appearance of the spectrum after deconvolution to remove the instrumental broadening (lower trace).
The spectrometer response function was measuredlS to be 28 MHz hwhm and very nearly Gaussian in shape. The excess laser width was due to the combination of acoustical shocks associated with the closed cycle refrigeration unit, and noise on the TDL current supply. Deconvolution of the instrumental broadening produces the spectrum in Figure 5. For this procedure the spectrum was recorded at a high point density (3 MHz/point) and with a high signal-to-noise ratio (SNR) (“500), as required for successful deconvolution. There are not large changes in the absolute transmission levels in deconvoluted spectrum due to the severe line overlap, but additional structure is revealed which should give a reasonable representation of the spectrum at near-Doppler-limited resolution. The helium buffer gas at 3 Torr pressure adds a small amount of residual pressure broadening. More experimental work at high resolution is needed before line parameters can be obtained for use in atmospheric modelling efforts. The recent microwave studies of Suenram et a1.I6 should help to provide a starting point for a better theoretical description of the infrared spectrum of this. important atmospheric molecule. Acknowledgment. The research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Registry No. H 0 2 N 0 2 , 26404-66-0; NO2BF4, 13826-86-3; H202,
7722-84-1.
(1 3) Blass, W. E.; Halsey, G. W. Deconuolution of Adsorption Spectra; Academic: New York, 1981.
(14) Jansson, P. A., Ed.; Deconuolution: with Applications in Spectroscopy; Academic: New York, 1984. (15) May, R. D. J. Quant. Spectrosc. Radiat. Transfer 1988, 39, 247. (16) Suenram, R. D.; Lovas, F. J.; Pickett, H. M. J. Mol. Spectrosc. 1986,
116, 1986.
