4135
J. Phys. Chem. 1986, 90, 4135-4142
gradual decrease in the yield of HCOOH at [SOzl0> 0.5 mTorr should suggest the existence of another channel competing with the HCOOH-forming one. Therefore, we propose the mechanism of the reaction of the Criegee intermediate with SO, as follows C H 2 0 0 + SO2* adduct (12) HCOOH SO, (13) adduct (14) adduct SO2 H C H O SO3 SO2 although a clear picture of reaction 14 cannot be proposed at this stage. On the basis of this mechanism, the increase of HCOOH by the addition of a small amount of SO2 can be explained by the occurrence of reaction 13, and its gradual decrease is well understood by considering that reaction 14 becomes dominant over reaction 13 in the presence of excess SOz. Schulten and Schurathg reported the detection of the protonated adduct of CHzOO SO2 and C H 3 C H O 0 SO2 from 1-butene-O3-SO2 reactions by means of high-resolution mass spectroscopy. Martinez and Herronss20-z'proposed a similar addition mechanism for this reaction to form a cyclic product.
+
--
+ +
+
+
+
In the absence of SO2,the isomerization of H2Cl8O1*0gives only HC180180H. In the presence of excess SO2,the contribution of reaction 6 can be neglected in the C2H4-03-S02 reaction. Thus, we can derive the equations ~ H C O O H=
AHCOOH/AC#4 ~/*HCOOH
= Kk13/(k13
+ k14[S021) (18)
= (1/K)(1 + (k14/k13)[S021)
(19)
Here, K is the factor of the production of the ground-state C H 2 0 0 , which is defined by K = (1 - a - 0)+ B [ M l / ( b / h
+ [MI)
(20)
According to eq 19 the inverse of the yield of HCOOH should be linear to the concentration of SO2when excess SOz is present. This condition seems to be achieved in the region of [SO,] > 1 mTorr as is shown in Figure 3. From the intercept and the slope of the plot of eq 19 for [SO,] > 1 mTorr, the ratio kI4/kl3was calculated to be (4.9 f 2.0) X cm3/molecule. Additional evidence for the addition mechanism for the CH@O SOz reaction was obtained from the reaction system of C2H4 (3 or 4 mTorr)-03 (4 mTorr)-CH3CH0 (13 mTorr)-SO, (0-1.6 mTorr). If the reaction of CH200 + SO2proceeds via a simple bimolecular reaction, the Stern-Volmer-type plot for the inverse of the yield of POZ vs. SO, concentration should give a straight line. However, as depicted in Figure 4, no linearity was observed. From this result, also, we can conclude that the reaction of CH200 with SOz is not a simple bimolecular 0 atom transfer reaction. The mechanism proposed in eq 1-9 can explain all the experimental results at least qualitatively.
+
The experiment employing l s 0 3 provided clear evidence for cyclic adduct formation. Thus, the formation of HC1800H and HC0I80H in addition to HC180180Hin the CZH4-'*O3-SOz reaction strongly suggests that the exchange of 0 atoms between C H 2 0 0 and SOz takes place. The detailed mechanism to form HCOOH is proposed as follows.
-
CH2180180 t SO2 18
18
18 18
H\C/o--qS-O H'
HO'
H C O % H or HC1800H
-
+
(16)
OS180 (17)
(20) Martinez, R. I.; Herron, J. T. J. Emiron. Sci. Health, Parr A 1983, A18, 739. (21) Martinez, R. I.; Herron, J. T. Chem. Phys. Lett. 1980, 72, 77.
Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research (No. 58030076 and 59030074) from the Japanese Minestry of Education, Science and Culture. Registry No. CH,OO, 56077-92-0;03,37222-66-5; H2C=CH2, 7485-1; H2SO4, 7664-93-9; HCHO, 50-00-0; SO,,7446-11-9; HCOZH, 64-18-6; SO2, 7446-09-5.
Temperature Dependence of the NO:, Absorption Spectrum Stanley P. Sander Jet Propulsion Laboratory,t California Institute of Technology, Pasadena, California 91 109 (Received: December 1 1 , 1985; In Final Form: February 13, 1986)
The absorption spectrum of the gas-phase NO3 radical has been studied between 220 and 700 nm by using both flash photolysis and discharge flow reactors for the production of NO3. In the flash photolysis method, cross sections at the peak of the (0,O)band at 661.9 nm were measured relative to the cross section of ClON0, at several different wavelengths. From the best current measurements of the CIONOzspectrum, the NO3 cross section at 661.9 nm was determined to be (2.28 0.34) X 1O-I' cmz molecule-' at 298 K. Measurements at 230 K indicated that the cross section increases by a factor of 1.18 at the peak of the (0,O) band. The discharge flow method was used both to obtain absolute cross sections at 661.9 nm and to obtain relative absorption spectra between 300 and 700 nm at 298 and 230 K. A value of (1.83 0.27) X 1O-I' cmz molecule-' was obtained for uN0, at 661.9 nm at 298 K. Upper limits to the NO3 cross sections were also measured between 220 and 260 nm with the discharge flow method.
*
Introduction The visible absorption spectrum of the ~0~radical consists of a number of diffuse bands extending from 400 to 700 nm. Early work by Jones and Wulf' established the visible band structure of NO3 using the N205catalyzed destruction of 0,as a steady'Mailing address: Mail Stop 183-601, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109.
state source. Ramsay subsequently examined the spectrum in high resolution, assigning the major vibrational band features., Schott and Davidson studied the No3 S P t m m in the temperature range 650-1050 K using the thermal decomposition of NzOs in a shock tube as an N o 3 Source and made the first r n e m m ~ ~ ~ofNO3 nts (1) Jones, E. J.; Wulf, 0. R. J . Chem. Phys. 1937, 5, 873. (2) Ramsay, D. A. Proc. Colloq. Spectrosc. Int. 1962, 10, 583.
0022-3654/86/2090-4135$01.50/0 0 1986 American Chemical Society
4136 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
cross sections., Subsequent determinations of uNO, have used the technique of molecular modulation$*5steady-state (slow flow) ab~orption,6-~ and discharge-flow absorption.I0 While these studies are in near agreement concerning the peak cross sections and integrated band strengths of several NO3 absorption features, most of these determinations have been indirect; i.e., it has been necessary to model a complex reaction scheme to calculate the absolute NO, concentration in the system. The only study to substantially avoid these complications is that of Ravishankara and Winelowho used a dischargeflow system to measure the NO3 spectrum under conditions of relatively well-controlled chemistry a t 298 K. Due to its importance in several key atmospheric cycles involving the NO, family of species, the kinetics and photochemistry of NO, have been studied intensively in the last several years. An accurate determination of the absolute NO, absorption spectrum is essential because the NO, cross section is a key parameter in many field and laboratory studies of NO, chemistry. For example, long-path differential absorption measurements of NO3 in the troposphere and stratosphere typically require the NO, cross section at the peak of the (0,O)band a t 662 nm to retrieve the column NO, mixing rati0.l' A number of NO3 laboratory studies have recently been published which have relied upon existing cross section data. These include several direct st~dies'"'~of the equilibrium constant for the reaction NOz + NO, s NzOs the determination of primary quantum yields in the photodissociation of and N2O5,I6and the branching ratio for NO, formation in the reaction OH
+ HNO,
-
products
(ref 17)
In this study, the visible absorption spectrum of the NO3 radical was measured by using two different methods. In the first method, NO3 radicals were produced by flash photolysis. The formation and disappearance of reactants and products were observed and used to determine NO, cross sections at 298,250, and 230 K in the range 610-670 nm. In the second method, NO, radicals were produced in a low-pressure discharge-flow reactor and monitored by long-path absorption. An optical multichannel analyzer (OMA) using an intensified silicon photodiode array detector was used to record the NO, spectrum between 300 and 700 nm. Cross-section measurements were also made in the range 220-240 nm by this method. It was observed that the cross sections obtained by the discharge flow and flash photolysis methods at the peak of the (0,O)band differed by about 30%. While the discharge flow results agreed extremely well with a previous determination (3) Schott, G.; Davidson, N . J. Chem. Phys. 1958, 80, 1841. (4) Graham, R. A.; Johnston, H. S. J . Phys. Chem. 1978,82, 254. (5) Burrows, J. P.; Tyndall, G. S.; Moortgat, G. K. J . Phys. Chem. 1985, 89, 4848. (6) Johnston, H. S.;Graham, R. A. Con. J . Chem. 1974,52, 1415. ( 7 ) Mitchell, D. N.; Wayne, R. P.; Allen, P. J.; Harrison, R. P.; Twin, R. J. J . Chem. Soc., Foraday Trans. 2 1982, 78, 1239. (8) Marinelli, W. J.; Swanson, D. M.; Johnston, H. S.J . Chem. Phys. 1982, 76, 2864. (9) Cox, R. A,; Barton, R. A.; Ljungstrom, E.; Stocker, D. W. Chem. Phys. . . Lerr. 1984, 108, 228. (10) Ravishankara, A. R.; Wine, P. H. Chem. Phys. Lerr. 1983,101,73. (11) For example: Noxon, J. F.; Norton, R. B.; Marovitch, E. Geophys. Res. Letf. 1980, 7, 125. Platt, U.; Perner, D.; Schroder, J.; Kessler, C.; Tonnison, A. J . Geophys. Res. 1981,86, 11965. Platt, U.; Perner, D.; Winer, A. M.; Harris, G. W.; Pitts, Jr., J. N . Geophys. Res. Len. 1980, 7, 89. (12) Tuazon, E. C.; Sanhueza, E.; Atkinson, R.; Carter, W. P. L.; Winer, A. M.; Pitts, Jr., J. N. J . Phys. Chem. 1984, 88, 3095. (13) Burrows, J. P.; Tyndall, G. S.;Moortgat, G. K. Chem. Phys. Letr. 1985, 119, 193. (14) Perner, D.; Schrneltekopf, A.; Winkler, R. H.; Johnston, H. S.; Calvert, J. G.; Cantrell, C. A.; Stockwell, W. R. J . Geophys. Res., 1985, 90, 3807. (15) Magnotta, F.; Johnston, H. S. Geophys. Res. Leu. 1980, 7, 769. (16) Swanson, D.; Kan, B.; Johnston, H. S. J. Phys. Chem. 1984,88,3115. (17) Ravishankara, A. R.; Eisele, F. L.; Wine, P. H. J . Phys. Chem. 1982, 86, 1854.
Sander using the same technique,I0the flash photolysis method is preferred for reasons to be discussed.
Experimental Method Flash Photolysis. The flash photolysis system has been described in detail previously.ls The flash assembly consisted of four concentric Pyrex tubes comprising the reaction cell (1 in. id.), photolyzing light filter, xenon flash lamp, and cooling/heating jacket. The cell was operated in the continuously flowing mode with all reagent and carrier gas flows being measured with calibrated mass flowmeters. The analytical lamp was a 150-W xenon arc lamp. After passage through the reaction cell, the analytical beam could be directed to a 1/4-mmonochromator (150-pm slit width, 0.6 nm fwhm resolution) for the observation of NO3 or to a l/z-m monochromator for the analysis of C1ONOz. The absorption path length from eight traversals of the reaction cell was determined from Beer's law plots obtained by measuring the absorbance of known concentrations of NOCl at 600 nm and CH3Br at 227.5 nm. The cross sections of NOCl and CH,Br at these wavelengths were determined in separate measurements using a Cary 14 spectrophotometer with a 10-cm cell. In this manner, the path lengths of the reaction cell at both wavelengths were found to be the same (675 f 20 cm). For the flash photolysis experiment, only the relative, rather than the absolute, path lengths are important. The temperature of the photolysis cell was adjusted by circulating ethylene glycol or methanol through the cell's temperature control jacket. The mechanism for the formation of NO, radicals was identical with that used in a previous study of the reaction NOz + NO, + M N 2 0 5 + M.19 NO3 radicals were produced from the photolysis of C1z-CIONOz mixtures at wavelengths longer than 300 nm:
-
Clz + hv(D300 nm) C1 + ClONOz
k, = 6.3
-+
Clz
-+
2C1
NO3
(1)
exp(lSO/T) cm3 molecule-' s-' (ref 20)
X
Sufficient CIONOz ((5-12) X 1014molecule cm-,) was used to ensure that NO, was formed on a time scale 100-1OOO times faster than its loss by reaction with itself or the walls. Because photolysis was restricted to wavelengths longer than about 300 nm, photodissociation of C1ONOZwas negligible. Initial NO, concentrations varied from about 4 X 1Ol2 to 4 X loi3 molecule cmT3. The synthesis, purification, handling, and measurement of CIONOz were identical with the methods described previo~sly.'~ Discharge Flow. The discharge flow system was functionally very similar to the design of Ravishankara and Wine.Io The flow tube consisted of a 3.8-cm-i.d., 70-cm-long side arm for the formation of NO3 and a jacketed 3.0-cm-i.d., 100-cm-long main section for absorption measurements pumped by a 100 cfm rotary pump through a liquid nitrogen trap. NO3 radicals were produced mainly by the reaction F
k2
-
+ HNO,
+
HF
+ NO,
1 X lo-" cm3 molecule-l
(2) s-l
+
although a few experiments used the C1 CIONOz source. Fluorine and chlorine atoms were produced by flowing dilute F, and Clz mixtures in helium through alumina or Pyrex microwave discharge tubes, respectively, in a discharge bypass arrangement. H N 0 3 or C10N02 was added 3-5 cm downstream from the discharge while the titrant gas, 2,3-dimeihyl-2-butene (tetramethylethylene, TME) or NO, was added 16 cm further downstream. Total pressure was measured by using a capacitance (18) Watson, R. T.; Sander, S. P.; Yung, Y. L. J . Phys. Chem. 1979,83, 2936. (19) Kircher, C. C.; Margitan, J. J.; Sander, S. P. J . Phys. Chem. 1984, 88, 4370. (20) NASA Panel for Data Evaluation, "Chemical Kinetics and Photochemical Data for Use in Stratospheric Modelling, Evaluation No. 7". JPL Publication 85-37, Jet Propulsion Laboratory, 1985.
-
Temperature Dependence of the NO3 Absorption Spectrum CI + CION02
,
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4137 0.5
C12 + NOg
I
I
I
I
225 nm
CION02 DISAPPEARANCE AT 225 nm
).
.
.
....,...,... . ..,.. ..... .".,..,,..'.:..:.'':..:.. ... ..(,
."'
"
......,.,...,I.
'f FLASH I
LO
0.5
0
I
o 1.5
I 2.0
TIME, ms
0.01
I
I
0.02 O.M ao4 ClNO3 ABSORBANCE
I
0.05
J 0.06
Figure 2. Plots of the NO, absorbance change at 661.9 nm vs. the CIONOzabsorbance change monitored at 220,225,235, and 240 nm at 298 K.
NO3 FORMATION AT 661.9 nm RASH
~
_..... ~
-.l._.....C.
4
TABLE I:
Temperature at 661.9 nm by the Flash Photolysis
1 T
temp, no. of XCIONO~, K expts nm
5%
t -
ON% VS.
Metbod'
i
manometer at the entrance of the main flow tube. The reactor walls were coated with halocarbon wax to minimize the heterogeneous loss of NO3. The optical configuration, detection electronics, and gas handling system were the same as that used for the flash photolysis experiment. The absorption path length was 800 f 20 cm. A few experiments were carried out using an optical multichannel analyzer (OMA) for broad-band analysis of the NO3 spectrum over the 300-700-nm wavelength range. In this configuration, a spectrograph (Jarrell-Ash Mark X, 1200 l/mm grating, 500-nm blaze, 25-pm entrance slit) was coupled to an intensified 1024-channel diode array detector (Tracor-Northern TN-1710). Spectra were taken in nine overlapping wavelength ranges with wavelength calibration being accomplished by the use of emission lines from mercury, neon, argon, and xenon atomic resonance lamps. In every spectral region, 10 Z, and Zo spectra were separately acquired, ratioed, and co-added. Spectra were interpolated to give points every 0.1 nm with the overall spectral resolution being about 0.4 nm. Since the rapid reaction
+ NO3
-
FO
QNo39
slope'
molecule-'
molecule-I 2.30 f 0.12 2.31 f 0.12 2.26 f 0.12 2.24 f 0.12 av 2.28 f 0.12 2.62 f 0.13 2.70 f 0.14
9 28 8 10
220 225 235 240
6.69 f 0.33 8.09 f 0.40 15.2 f 0.76 21.2 f 1.1
3.44 2.86 1.49 1.06
250
12
225
9.25 f 0.46 9.57 f 0.48
2.83 2.82
bFrom ref 21. 'Slope of plot of MNo,vs.
A A C ~ Oas ~O in ~Figure 2.
Figure 1. Analytical beam intensity vs. time showing C10N02 disappearance at 225 nm and NO, formation at 661.9 nm. The rise time for the NO, signal is limited by the photomultiplier recovery time rather than the NO, formation rate. [NO,],, is determined by extrapolation of the linear portion of the decay signal back to t = 0.
F
b
10-l' cm2
298
230 13 225 5 0 5 10 15 20 "Uncertainties are f20. TIME, ms
'JCIONOy
IO-'* cm2
+ NO2
unavoidably produced a significant amount of NO2, an NO2 reference spectrum was acquired and subtracted from the NO3 spectrum by using NO2 lines in the 300-400-nm region as an index for the subtraction. Anhydrous nitric acid was prepared by reacting N a N 0 3 with H2S04 under vacuum and collecting the product at 77 K. A portion of the main camer gas flow was bubbled through the liquid HN03 at a few degrees below ambient temperature. Direct measurements of [HN03] by UV absorption in the flow tube were consistent with complete saturation of the carrier gas in the bubbler. Tetramethylethylene (Aldrich) was degassed several times before use. All other reagents were used as supplied.
Results Flush Photolysis. In the photolysis of C12-C!10NO2 mixtures, both the formation of NO3 and the disappearance of C10N02 were readily observed (Figure 1). The NO3 cross section at 661.9 nm was determined by relating the increase in absorbance due to NO3 formation to the decrease in UV absorbance due to C10N02 disappearance, extrapolated to t = 0. If it is assumed that one NO3 molecule is formed for every ClONO2 molecule lost, than the NO3 cross section is given by
where U N O 3 a n d M a o N q are the changes in NO3and C10N02 absorbances, respectively, and U~IONO, is the chlorine nitrate cross section. The experimental procedure consisted of measuring both UNO, and M c Iat different ~ ~ ~ C12 (and ~ hence [NOJo) concentrations at a given CIONOz analyzing wavelength. Individual absorbance measurements were made by averaging about 50 flashes and extrapolating the time-dependent intensity profiles to t = 0. Because both C10N02 and NO3 decayed very slowly, these extrapolations introduced virtually no error into the measurement. Initially, measurements of U C I O N 0 2 at a given C12 concentration were sandwiched between measurements of UNO,, which were averaged. This was later found to be unnecessary as the consecutive measurements of UNO, were found to be reproducible within a few percent. A total of 80 experiments was carried out at temperatures of 230, 250, and 298 K using analyzing wavelengths for C10N02 between 220 and 240 nm. The total pressure in the reaction cell were linear over was 350 Torr of N2. Plots of UNO^ vs. UCIONO~ a wide range of [N0310as indicated in Figure 2. NO3 cross sections were calculated from eq I by using the slopes of these plots and CION02 cross sections measured by Molina and Molina21 (Table I). At 298 K, despite the variation of more than (21) Molina, L. T.; Molina, M. J. J . Phofochem. 1979, ZZ, 139.
4138 The Journal of Physical Chemistry, Vol. 90, No. 17, I986 I
I
I
OMA
298K
230K
~
I
Sander I
I
I
I
FP 0
----
WAVELENGTH, rim
Figure 3. Comparison of the temperature dependence of the NO3 absorption spectra from 600 to 690 nm at 230 and 298 K using the flash photolysis and discharge flow methods.
a factor of 3 in acIONOz, there is no systematic variation in the NO3cross section, the average value being (2.28 f 0.12) X lo-'' cm2 molecule-' (2a random uncertainty). With the inclusion of other systematic uncertainties (absorbance measurement f5%, uncertainty in u ~f 10%) ~ the overall ~ uncertainty ~ ~ is estimated ~ , to be f15% (2a) giving a value for oN0,of (2.28 f 0.34) X cm2 molecule-'. The cross section at 661.9 nm increases with decreasing temperature with values of 2.62 and 2.70 X cm2 molecule-' being measured a t 250 and 230 K, respectively. The NO3 absorption spectrum between 610 and 670 nm was recorded a t 0.4-nm intervals by measuring the NO3 absorbance relative to thevalue at its peak. The absorbance at 661.9 nm was noted periodically during the scan to check for any change in the output of the flash lamp. Figure 3 shows the NO, absorption spectrum recorded in this manner at 230 and 298 K. Discharge Flow. The cross section of NO3 at 661.9 nm was measured in a discharge flow system by measuring the absorbance arising from a known concentration of NO3 in the flow. The NO3 concentration was determined by titration with known concentrations of 2,3-dimethyl-2-butene (tetramethylethylene, TME) which reacts rapidly with NO,: NO3 (CH3)2C=C(CH3)z products (3)
-
+
k3 = 3.4
X lo-"
-
-+
k4 > 5
+ NO,
+ NO,
(4)
lo-" cm3 molecule-] s-I (ref 10)
X
FO
k5 = 2.6
FO
X
+ NO
---L
F
+ NOz
(5)
lo-" cm3 molecule-' s-I (ref 20)
With CIONOz used as the NO3 precursor and NO as the titrant, anomalously low values for the NO, cross section were measured (22) Atkinson, R.; Aschmann, S. M.; Winer, A. M.; Pitts, Jr., J. N. Enuiron. Sci. Technol. 1984, 18, 370.
I
[TME], 1013 molecule cm 3
Figure 4. Titration of NO3 with 2,4-dimethyl-2-butene(TME) using the discharge flow method.
because of NO, regeneration by the analogous mechanism involving chlorine: C1 + ClONO, C12 + NO, (1) C1 + NO3 C10 + NO2 (6)
-
--+
k6 = 7.6
cm3 molecule-' s-' (ref 22)
While the nature of the products arising from this reaction has not been studied there were no indications of any secondary removal or production of NO,. For this reason, TME was preferred as a titrant to NO because of the possibility of F, and consequently NO3, regeneration by the following mechanism: F + "03 H F + NO, F
I
X
lo-" cm3 molecule-'
C10
k7 = 1.7
X
+ NO
---L
s-l
(ref 20)
C1+ NO2
(7)
lo-" cm3 molecule-' s-' (ref 20)
Thir was due in large part to the limitation on the maximum CIONOz concentration that could be used, about 1 X lOI4 molecule ~ m - ~While . considerably higher HNO, concentrations ~ ) be used, minimizing the (up to 1 X lOI5 molecule ~ m - could competitive loss of F by reaction with NO3 relative to C1, it was felt that TME was a better titrant than NO because the potential regeneration step was absent. In fact, it was observed that measurements made with the ClONO, source and TME as a titrant gave results consistent with those obtained by using the HNO, source, suggesting that NO, regeneration did not take place. Titrations were carried out by setting an NO, flow and increasing the TME flow in discrete steps until the NO3 absorption at 661.9 nm approached zero. Total absorbance was determined from measurements of photomultiplier anode current with the fluorine discharge switched on and off. Complications due to the
1. 5
I
I
I
I
2.5
I
10
-QJ
3
u
05
E
-s5 N
h
-m
0
0
b'
500
400
700
600 WAVELENGTH nm
Figure 5. NO, absorption spectra at 298 K in the range 400-700 nm from the discharge flow system using diode array detection (solid line) and the
flash photolysis system (+). The diode array spectrum was scaled to the flash photolysis measurement at the peak of the (0,O) band (see text) The contribution of NO2 to the absorption spectrum between 400 and 550 nm is indicated. TABLE II: Measurements of uNoj by the Discharge Flow-Titration Method at 298 K 10-4.
TABLE 111: Upper Limits to the NO3 Cross Section in the Wavelength Range 220-260 nm
10-13.
uN03r
flow ["O3I, fTME1, uN03 velocity, press., molecule initial molecule W 7cm2 Torr cm-3 absorbance molecule-' cm s-l F + HN03 Source 744 744 750 750 750 1330 1340 1370 1420 1440 1440 1450 1450 1450 1450 1450 1550 1550 1550 2024
1.70 1.70 1.69 1.69 1.69 1.26 1.25 1.21 1.17 1.15 1.15 1.14 1.14 1.14 1.15 1.15 1.09 1.10 1.10 1.62
8.4 8.4 3.3 3.3 3.3 2.6 2.6 2.6 2.6 5.5 5.5 5.5 5.5 3.6 3.6 3.6 1.6 1.6 1.6 2.1
1500 1500
1.05 1.07
1.04" 1.04
0:076 0.307 0.306 0.199 0.088 0.31 1 0.069 0.096 0.214 0.246 0.136 0.289 0.246 0.3 18 0.216 0.090 0.088 0.310 0.202 0.099
0.578 2.00 1.95 1.39 0.661 2.06 0.499 0.685 1.52 1.65 0.958 1.83 1.65 2.06 1.35 0.700 6.50 1.86 1.51 0.595
1.64 1.92 1.96 1.79 1.66 1.89 1.73 1.75 1.76 1.86 1.77 1.97 1.86 1.93 2.00 1.61 1.70 2.08 1.67 2.05
0.731 0.620
1.53 1.75
C1 + C10N02 Source
a
0.0898 0.0872
Concentrations are [C10N02].
possible formation of NO3 from a thermal reaction between Fz and H N 0 3 were not observed. The NO3concentration in the flow was determined by extrapolating the linear portion of the absorbance vs.,[TME] curve to zero absorbance, as in Figure 4. Twenty titrations were performed at 298 K over the following ranges of experimental parameters: flow velocity, 744-2024 cm s-l; pressure, 1.1&1.70Torr of helium; [HNO,], (1.6-8.4) X 1014 molecule ~ m - [NO,], ~; (5.8-21) X 10l2 molecule ~ m - ~As. indicated in Table 11, the average value obtained for UNO^ at 298 K was (1.83 f 0.14) X 1O-I' cmz molecule-' (1 6,random uncertainty only). Several measurements of aN03were made in the wavelength range 220-260 nm using the discharge flow system. With [NO,]
[Prn
1
in13 IV
nm
cni3
AA,d
220 225 240 260
1.o 1.o 2.0 1 .o
-1.5 f 0.5 -1.5 f 0.5 -1.6 f 0.5 Of1
..
1 0 - l ~cm2
AAUNn.b obsdC readd
1.2 0.7 0.2 0.15
3.4 2.6 0.97 1.2
7.8 6.6 2.2
NA
"Observed absorbance change with discharge ON/OFF; base e. bAbsorbancechange expected from A[HN03]due to F + "0,; base - AAHNO,Iand e. 'Upper limit for uNo3based on the larger of laAobsd d N 0 3cross section that would be required the uncertainty in Mobs,+ to raise the apparent cross section from the value obtained from the discharge flow method (1.83 X lo-'' cm2) to the value obtained by the flash photolysis method at the indicated wavelength. in the range (1-2) X loi3molecule cm-, as determined by photometry at 662 nm and [HNO,] = 4 X lOI4 molecule cm-,, measurements of the analytical beam intensity in the ultraviolet were made under discharge on/discharge off conditions. The measured absorbance changes are indicated in Table 111. The NO, cross section at each wavelength was calculated based on the observed absorbance change with a correction to account for the expected change in HNO, absorbance arising from the F + HNO, reaction. Since there was also a small contribution to the absorbance from the formation of NOz, the calculated cross section is an upper limit to the true value. The calculated upper limits range from 1 X lo-'' cmz molecule-I at 260 nm to 3.4 X 1O-I' cm2 molecule-' at 220 nm. The NO, spectra acquired by the diode array spectrometer at 230 and 298 K are shown with the spectra acquired by the flash photolysis system in Figures 3 and 5 . Under the experimental 1 X l O I 5 molecule ~ m - [NO,] ~, conditions employed ([HNO,] 5 X lo1, molecule cm-,), the NO2concentration approximately equalled that of the NO,, requiring a subtraction of the NO2 contribution below 400 nm. The cross section at the peak of the (0,O)band at each temperature has been scaled to the result cm2 obtained from the flash photolysis system, Le., 2.28 X molecule-' at 298 K and 2.70 X cm2 molecule-I at 230 K. Where they overlap, the spectra obtained by the two methods are in good agreement. NO3 cross sections are observed to increase with decreasing temperature over the entire visible spectrum except
-
-
4140 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
Sander
TABLE IV: NO2 Cross Sections (em2 molecule-') Every Nanometer at 298 and 230 K Obtained by Using the Diode Array SpectrometeP A, nm
298 K
230 K
A, nm
298 K
400 40 1 402 403 404 405 406 107 408 409 410 41 1 412 413 414 415 416 417 418 419 420 42 1 422 423 424 425 426 427 428 429 430 43 1 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 45 1 452 453 454 455 456 457
0.0 0.0 0.0 0.2 0.0 0.3 0.2 0.1 0.3 0.0 0.1 0.2 0.5 0.5 0.2 0.6 0.7 0.8 0.5 0.9 0.9 0.9 1.o 1.2 1.o 0.8 1.5 1.5 1.3 1.2 1.8 1.4 1.6 1.9 2.0 1.7 1.6 2.0 2.3 2.2 2.1 2.0 2.3 1.9 2.1 2.2 2.6 3.1 2.6 3.0 3.1 3.3 3.6 3.4 3.9 3.9 3.9 4.3
0.4 0.5 0.5 0.5 0.3 0.: 0.6 0.5 0.5 0.8 0.5 0.8 0.4 0.7 1.2 0.8 0.8 1.1 1.1 1.1 1.4 1.3 1.3 1.3 1.4 1.7 1.6 1.3 1.6 1.4 1.7 1.8 1.8 2.0 2.2 2.4 2.3 2.0 2.2 2.8 2.4 2.5 2.3 2.3 2.4 2.9 2.9 3.3 3.6 3.3 3.3 3.7 4.0 3.7 4.0 4.1 3.6 4.2
458 459 460 46 1 462 463 464 46 5 466 467 468 469 470 47 1 472 473 474 475 476 477 478 479 480 48 1 482 483 484 48 5 486 487 488 489 490 49 1 492 493 494 49 5 496 497 498 499 500 50 1 502 503 504 505 506 507 508 509 510 51 1 512 513 514 515 516
4.0 4.6 4.3 4.2 4.3 4.4 5.2 5.5 5.9 6.2 6.1 6.3 6.4 6.7 7.0 6.7 6.7 7.4 8.5 8.4 7.9 7.9 7.6 7.7 7.7 7.8 8.4 8.9 9.9 10.0 10.3 10.4 11.2 10.7 10.8 11.0 11.0 11.5 13.1 13.2 13.0 12.7 12.3 12.0 12.0 12.1 13.7 13.9 14.5 13.9 13.8 14.7 16.4 18.8 19.2 17.4 17.1 17.2 16.9
230 K 4.7 4.5 4.6 4.3 4.0 4.8 5.1 5.4 5.7 6.0 5.9 6.0 5.7 6.2 6.5 6.5 6.4 7.4 8.3 8.2 7.4 7.4 7.5 7.4 7.3 7.1 7.4 8.2 9.5 9.4 9.2 10.6 11.2 10.3 10.6 10.9 10.1 11.1 12.9 14.0 13.2 12.6 12.3 11.4 11.1 11.9 13.3 14.0 15.0 14.0 13.0 14.1 16.5 20.0 21.1 19.2 17.3 17.0 17.5
A, nm
517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 55 1 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574
298 K 16.2 15.6 16.7 18.2 19.9 20.9 19.2 17.8 17.1 17.7 19.6 22.8 26.0 24.2 22.7 21.9 21.2 22.1 25.0 27.9 28.0 25.4 22.2 22.8 22.2 20.4 18.2 18.5 21.3 26.3 31.6 32.4 29.4 26.9 26.4 26.8 27.5 30.2 33.8 35.4 35.7 38.1 40.4 36.0 32.4 31.5 30.4 29.5 29.6 30.9 30.5 30.9 31.4 30.3 30.0 29.8 30.2 31.0
230 K 15.4 14.9 15.9 17.3 18.9 20.6 19.1 16.8 16.0 16.8 19.3 23.8 27.3 24.7 22.7 22.0 21.1 22.7 26.6 30.6 30.5 26.0 22.4 22.6 21.8 19.7 17.5 17.3 21.3 26.5 33.0 33.4 29.7 27.8 27.6 28.5 29.4 33.1 38.0 39.2 39.3 42.2 45.3 38.5 33.8 32.7 32.1 30.8 31.0 33.0 31.4 32.0 32.6 31.1 30.9 30.5 30.9 31.9
X, nm
575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 500 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632
298 K 33.4 35.5 36.7 35.9 35.2 36.3 38.5 35.6 31.8 30.6 31.4 36.0 45.2 54.7 66.5 64.7 59.1 55.5 49.7 45.5 46.6 50.2 47.3 39.8 33.7 30.0 31.1 36.0 41.3 47.4 47.3 36.0 26.1 20.1 18.6 19.2 20.7 24.2 28.6 27.7 24.5 22.7 22.9 25.9 27.8 35.5 56.9 110.5 159.9 130.8 91.0 79.3 81.7 80.0 75.8 73.4 52.5 35.5
230 K 36.0 38.7 39.5 38.5 38.3 39.9 43.9 39.5 34.6 32.8 34.0 39.7 51.8 63.8 77.3 71.8 64.6 60.2 53.2 50.2 52.8 58.1 54.0 43.7 36.5 29.7 30.4 35.7 43.0 51.4 53.2 39.6 26.5 19.1 17.7 18.5 20.7 25.2 32.0 30.5 25.8 22.5 22.0 24.4 27.1 35.8 62.9 121.3 174.5 138.7 100.7 88.2 96.1 94.3 90.3 89.7 61.0 39.8
A, nm 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 686 686 687 688 689 690 69 1
298 K 23.6 17.8 15.6 18.3 22.5 22.0 17.1 13.4 10.9 10.0 10.5 10.3 9.3 8.1 7.6 6.7 5.9 5.4 6.0 6.6 7.7 8.9 10.6 14.4 18.6 26.3 44.2 80.9 157.2 228.0 189.4 122.6 80.5 53.9 33.0 20.6 13.6 10.3 8.6 8.2 7.0 5.6 5.2 5.3 6.4 8.1 8.5 7.5 5.8 4.3 3.3 2.8 1.9 1.7 1.3 1.3 1.3 1.1 0.8
230 K 25.1 17.3 14.0 16.2 20.1 18.9 14.2 11.3 9.5 8.4 8.1 8.4 8 .O 6.9 6.8 6.3 5.3 5.0 5.6 6.6 7.9 9.2 11.0 14.4 18.5 25.9 42.7 79.0 167.5 266.9 229.7 145.5 92.9 62.9 37.4 23.3 14.5 11.2 9.4 9.7 8.1 6.3 5.5 5.2 6.2 7.2 7.3 6.4 5.3 4.4 3.2 2.8 2.4 1.5 2.3 2.0 1.9 2.1 1.7
"The listed points are extracted directly from the spectrum with no wavelength averaging. The estimated 2u uncertainty from random and systematic errors is the larger of i 1 5 % and i 0 . 2 X cmz molecule-'.
at two bands at 678.8 and 637.4 nm which are substantially weaker a t 230 K than at 298 K. Values of the cross section from 400 to 7 0 0 nm at 1-nm increments are listed in Table IV. Discussion Both the flash photolysis and discharge flow experiments have several sources of possible systematic error that must be considered. The flash photolysis experiment will be discussed first. Flash Photolysis. (a) Incorrect ClONO2 Cross Sections. The derived value for UNO, is directly proportional to the value used for ffCiONo2 at the analyzing wavelength for C10N02. C10N02 cross sections have been reviewed recently by DeMore et aLzowith
the recommended values being taken from the study of Molina and Molina.21 The only other published spectrum for which tabulated cross sections are available is that of Rowland et al.23 whose results are 7-10% higher in the 220-250-nm range. The Molina and Molina results are preferred, however, because their measurements cover a wider temperature range with greater attention being paid to possible impurities and heterogeneous effects. These measurements have an estimated experimental uncertainty of 10% (random plus systematic error, 2a) in the ~
~~
~~~
(23) Rowland, F. S.; Spencer, J. E.; Molina, M. J. J . Phys. Chem. 1976, 80, 2711.
Temperature Dependence of the NO3 Absorption Spectrum
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4141
wavelength region of interest here.24 A number of other unpublished measurements of ClONOz cross sections have been made in this laboratory in connection with other studies. Using both the original Molina CIONOz samples as well as subsequent preparations, these measurements have always agreed with the It should also be noted Molina and Molina results within that because of the proportionality between UNO> and UCIONO~, the use of the Rowland et al. spectrum would result in proportionately higher NO, cross sections than those based on the Molina and Molina study. ( b ) Effect of Secondary Chemistry. The flash photolysis method for the determination of UNO, relies on the existence of a one to one relationship between CIONOz removed and NO3 formed. The two most likely kinetic complications affecting this stoichiometry are (1) the formation of less than one NO, molecule for every ClON0, molecule removed, and (2) the removal of NO3 by reactions which are fast on the scale of the NO3 measurement. Both effects cause uN0, to be underestimated. Only processes which cause chain regeneration of NO3 can result in an overestimation of ~ ~ 0 , . With respect to the formation of NO3, the channel forming C10 + ClONO from reaction 1 is endothermic by 8.9 kcal mol-'. MargitanZ5has shown that C10 cannot be a significant product a t 298 K. There is no reason to believe, then, that the products of reaction 1 are anything other than ClZ NO,. The destruction of NO3 by reaction 6 can, in principle, reduce C1 + NO3
-
ducing ClONO which has an absorption in the 220-250-nm region. However, the required fractional impurity would have to be of the same order of magnitude as the relative error in the cross section it caused since the rate constant for ClONO formation is roughly the same as kl and the intensity of the ClONO spectrum is similar to that of C1ONOz. Such large NO, impurities are extremely unlikely, and would have been revealed in periodic checks of ClONO, by UV spectroscopy. Also, impurity concentrations of NO, were sufficiently low to rule out the secondary formation of NO3 by the reactions NO,
O(,P)
(6)
the stoichiometry to values less than one. The system governed by reactions 1 and 6 was numerically integrated to estimate the magnitude of this effect. In simulations using [CIONO,] = 7 X 1014 molecule cm-3 in which [Cl], was varied from 1 X loiz to 3 X loi3 molecule cm-,, it was found that the C1+ NO3 reaction could cause deviations in the stoichiometry greater than 10% only for [NO,] > - 2 X loi3molecule ~ m - ~This . effect, if important, would result in downward curvature in plots of UNO, vs. AAWNQ. Such curvature was not observed (Figure 2). Moreover, because most of the data were taken under condtions where [NO,] was less than 2 X IOl3 molecule and [CIONO,] was greater than 7 X lOI4 molecule an-,,the effect of reaction 6 on the overall stoichiometry is expected to be negligible. In any case, this effect would cause the measured cross sections to be too low. (c) Interfering Absorption in the Ultraviolet. A species formed by a secondary process, or NO3 itself, could subtract from the true CIONOz absorbance change resulting in a measured cross section that was too large. In the case of NO3, the cross sections required to raise the apparent NO3 cross section at 661.9 nm from 1.8 X to 2.3 X cm2 molecule-' are listed in Table 111 along with the upper limits measured using the discharge flow system. The required cross sections are about a factor of 2.4 higher than the observed upper limits at each wavelength implying that the ultraviolet absorption, if attributable entirely to NO,, can account for only about 40% of the difference between the results obtained for the flash photolysis and discharge flow methods. Because the origin of the absorption is in doubt and there is a relatively large correction due to the change in [ H N 0 3 ] , the derived upper limit cross sections were not used to correct values of uNo3from the flash photolysis experiment. It should also be pointed out that because the measurements at four widely separated wavelengths for the analysis of ClON0, give the same value of uN03despite more than a factor of 3 variation in u ~ ~the ~ interfering absorber would have to have the same spectral shape as ClON0,. If NO, were present as a substantial impurity in the ClONO,, the reaction C1 + NO2 + M ClONO + M
-
NO
+ NOz + M
+ O(,P) NO3 + M
-
As indicated in the discussion of secondary chemistry, a small amount of NO, is formed from reaction 6. Under worst-case conditions (A = 220 nm, [Cll0 = 4 X lo', molecule cm-,) the absorbance change due to NOz formation is less than 3% of that due to ClONO, removal. The NO, absorption would normally be much less than this making this effect negligible. (d)Other Systematic Effects. A species formed with a visible absorption band would have the effect of raising the apparent cross section. The formation of such a species is unlikely due to the large cross section that a trace product would be required to have (u > cm2 molecule-') and the fact that a wavelength scan reproduced the known NO3 spectrum. The relatively long time scale for the observation of NO3 ( t > 1 ms) and the high bath gas pressure (350 Torr of N,) also rule out the possibility of NO3 vibrational excitation affecting the visible spectrum. Discharge Flow. Ravishankara and Wine', have discussed potential sources of error in the discharge flow/titration experiment including wall loss of NO3, absorption from NO, and inaccuracies in determining the titrant concentration. While the wall loss rate of NO3 could not be determined directly in this experiment, the apparent insensitivity of the measured cross section to a variation in the flow velocity of a factor of 3 suggests that the axial NO3 concentration gradient was small. This is consistent with other observations of negligible NO, wall loss rates in both uncoatedz7 and halocarbon wax-coatedZ8flow tubes. As discussed by Ravishankara and Wine, the presence of NO, from secondary reactions at concentrations approaching that of [NO,] can have no effect on the measured cross section. While not measured directly, viscous pressure drop along the flow tube in this study was estimated to be no greater than 6%." The estimated uncertainty in the cross section from both random errors and systematic errors associated with the measurement of the titrant concentration and the NO3 absorbance is f15%. As indicated above, the use of NO as a titrant with the F + HNO, source can, in theory, lead to errors due to the regeneration of F, and consequently NO,, through the reaction FO + NO F NO, (5)
+
C10 + NO,
+ hv
-
~
+
Estimates of the magnitude of this error were made by numerically simulating the experiment of Ravishankara and Winelo through two reaction zones: the NO3 generation region (first 30 ms) to determine the initial condition for [FO], and the N O titration region (30 ms to the point where the titration reaction effectively stops). The most uncertain factor in the simulation is the rate constant and product branching ratio for the reaction FO FO 2F + 0, ~
,
,
+
-
-
Fz + 02 (8b) This reaction has the effect of reducing the error caused by NO3 regeneration by suppressing the FO concentration before the titration region. The two reported rate constants for reaction 830*31
-+
would compete with reaction 1 for atomic chlorine thereby pro(24) Molina, L. T., private communication. (25) Margitan, J. J. J . Phys. Chem. 1983, 87, 674. (26) Kircher, C. C.; Sander, S.P., unpublished results.
(27) Hammer, P. D.; Dlugokencky, E. J.; Howard, C. J., submitted for publication in J . Phys. Chem. (28) Ravishankara, A. R.; Mauldin, R. L. J . Phys. Chem. 1985,89,3144. (29) Kaufman, F. Prog. React. Kinet. 1961, 1 , 1 . (30) Wagner, H. G.; Zetsch, C.; Warnatz, J. Ber. Bunsenges. Phys. Chem. 1972, 76, 526.
4142 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 TABLE V Simulation of NO3 Formation/Titration Kinetics Using the F + HNOJ Source Mechanism
reaction F + HNO, .?+ HF + NO, F+N03fFO+N02 FO FO
k(298) 1 x 10-1' 7 x 10-1' 2 x 10-1' 0,l.S x lo-''
+ NOLF + NO* + FO-% 2F + O2 2 F2 + O2
0
Simulation" kea = 0 [NO,],
loi3
ksa = 1.5 X
lo-''
[NO,], 10" molecule cm-)
% molecule cm-, % [Fl,, molecule cm-3 model measd error model measd error
1 3 6
0.935 2.49 4.30
1.00 2.92 5.65
6 17 31
0.940 2.61 4.74
1.00 2.77 5.05
6 6 7
Sander Schott and Davidson3 obtained cross sections between 650 and 1050 K in a shock tube. Ravishankara and M a ~ l d i nusing , ~ ~ the same technique employed by Ravishankara and Wine, have studied the (0,O) band at 220 and 240 K. Values of 1.54 and 1.29, respectively, were found for the ratio U~o,(T)/U~o,(298)at these temperatures. This is a considerably stronger dependence than that observed in this study in which values of 1.18 and 1.15 were observed for a~o,(T)/(T~o,(298) at 230 and 250 K, respectively. The decrease in intensity of the features at 678.5 and 637.4 nm as the temperature is decreased from 298 to 230 K indicates that these are hot bands. Two recent studies of NO3 by laserinduced f l u o r e ~ c e n c eare ~ ~in, ~excellent ~ agreement concerning the frequencies of the ground-state vibrational modes, although there is some disagreement concerning the ground-state geometry (C2"or D,,,). Both of these studies report a vibrational mode at 370 f 10 cm-' (vq/l). This, combined with the vl' progression identified by Ramsay,2 can be used to assign the band at 678.8 nm to the A(OOO0) X(OOO1) transition and the band at 637.4 nm to the A(1000) X(OOO1) transition (assuming D3* symmetry). The sensitivity of these bands, as well as the (0,O) band, to temperature means that the relative absorption of these bands could be used in field studies as a measure of the average temperature along the optical path. The use of a larger cross section would have a number of implications for laboratory and field studies involving NO,. (1) Several recent studies of the equilibrium constant for the reaction NO2 + NO, F? N2OS (9)
-
+-
taken after [N031actual taken after 30 ms reaction time. 60 ms reaction time. [HNO,] = 1 X loi5molecule cm-). differ by a factor of 4 with a recent evaluation20 recommending the weighted mean of the two measurements, ks = 1.5 X lo-" cm3 molecule-' s-', Table V presents the results of two sets of simulations with keataken to be 0 and 1.5 X lo-" cm3 molecule-' s-', and keb = 0. In the range of [F], between 1 and 6 X lo1, molecule cm-3 the minimum error expected in the [NO], measurement is 6% (k8a= l .5 X lo-'' cm3 molecule-' s-') with the error increasing to 31% in the extreme case where ks, = 0. In all cases this effect results in an underestimation of ~ ~ 0 , . While the use of TME as a titrant instead of NO should have eliminated the effects of NO3 regeneration through FO, the value of aNo, obtained from the discharge flow experiment in this work (1.83 f 0.27) X cm2 molecule-' at 661.9 nm is in extremely good agreement with the value obtained by Ravishankara and Wine,Io (1.78 f 0.23) X cm2 molecule-'. This agreement could mean that the regeneration error described above is at the lower limit of the estimate (as is likely if reaction 8 is rapid) or else the agreement is fortuitous. The results from both discharge and flow experiments differ from the value obtained by the flash photolysis method, (2.28 f 0.34) X cm2 molecule-', although the error limits overlap. There is no obvious reason for the discrepancy between the two methods. For several reasons, however, the preferred value is the one derived from flash photolysis. This method has the important advantages of simplicity and apparent freedom from unknown secondary chemistry and does not rely on any experimental measurements requiring calibration such as pressure or flow rate. The main disadvantage of this technique is its reliance on previously measured C10N02 cross sections although these are reasonably well-known and later improvements in the accuracy and precision of these measurements can be used to improve the estimates of from this study. Previous measurements of NO3 cross sections at 298 K have been extensively discussed and summarized by Burrows et aL5 The recommendation of the most current NASA data evaluation,20 which was published before the work of Burrows et aL5 and Cox et al.9, is based on the results of Ravishankara and Wine.lo These two studies, however, are in excellent agreement with those of Ravishankara and Wine, obtaining (1.85 f 0.56) X and (1.64 f 0.30) X cm2 molecule-', respectively, at the peak of the (0,O)band. The cross section obtained by the flash photolysis technique in this study is about 30% larger than the average of the three most recent studies. While the 2a error limits from this study overlap those of previous workers, the agreement is not as good as could be expected. There are no published measurements of the temperature dependence of the NO, spectrum near room temperature although (31) Clyne,
M. A. A.; Watson, R. T. J . Chem. SOC.,Faraday Trans. I
1974, 70, 1109.
have used absorption methods for the detection of N03.4*'2-'4 The effect of an increase in aNO,would be to increase the equilibrium constants derived from these studies. (2) A recent measurement by Swanson et a1.I6 of the NO, quantum yield from N205photolysis at 249 nm gave (0.89 f 0.15) for This value would be reduced to 0.74 if the NO, cross section from this work were used. (3) Magnotta and J o h n s t ~ nmeasured '~ quantum yields for the formation of 0 and N O from NO3 photolysis. The quantum yields derived from this experiment are sensitive to the ratio K ~ / L T N Owhere , K9 is the equilibrium constant for reaction 9. This experiment, which used values of K9 from Graham and Johnston, obtained combined quantum yields for the two channels significantly greater than unity. While an increase in uNo3of 50% (a somewhat larger change than that suggested by this study) would mitigate this problem, the effect of such a change on Graham's and Johnston's value of K9 would also have to be considered. (4) Ravishankara et al.I7 used NO, absorption to determine the branching ratio for NO3 product formation from the reaction OH + HNO, products
-
obtaining values of 0.98 f 0.35 and 1.17 f 0.34 at 298 and 251 K, respectively. The new value of uN0, would lower these values to 0.72 f 0.26 and 0.86 f 0.25. (5) As far as field measurements are concerned, the retrieved NO, mixing ratio integrated along the optical path is inversely proportional to the differential cross section. The use of a larger room temperature cross section, combined with the observed increase in the cross section with decreasing temperature, may have a significant impact on atmospheric measurements. Acknowledgment. We thank R. R. Friedl, J. J. Margitan, and M. J. Molina for several useful discussions, I. S. McDermid for the use of the optical multichannel analyzer, and J. P. Burrows and A. R. Ravishankara for communicating their results prior to publication. The research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Registry No. NO,, 12033-49-7. (32)Ravishankara, A. R.;Mauldin, R. L. J. Geophys. Res., in press. (33)Ishiwata, T.; Fujiwara, I.; Naruge, Y . ;Obi, K.; Tanaka, I. J . Phys. Chem. 1983,87, 1349. (34)Nelson, H.H.; Pasternak, L.; McDonald, J. R. J . Phys. Chem. 1983, 87, 1286.
