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No. 25, 1979
(15) J. A. Coxon and D. A. Ramsay, Can. J Phys., 54, 1034 (1976), cited in ref 13. (16) R. Alqasmi, H.-D. Knauth, and D. Rohlack, Ber. Bunsenges. Phys. Chem., 82, 217 (1978).
O’Brien, Green, and Doty (17) R. H. Miller, D. L. Bernitt, and I. C. Hisatsune, Spectrochim. Acta, Part A , 23, 223 (1967). (18) W. S. Smith, C. C. Chou, and F. S . Rowland, Geophys. Res. Lett., 4 , 517 (1977).
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Rate Constant for the Reaction NO2 4- OH 3- M HNO, Measured under Simulated Atmospheric Conditions Using a Novel Analysis Procedure Robert J. O’Brlen,“ Patrick J. Green, and Richard A. Doty Department of Chemistry, Portland State University, Portland, Oregon 97207 (Received April 19, 1979)
-
-
For the reactions OH + NO2 + M HNOB+ M (1)and OH + toluene products (51, we have measured the ratio of rate constants k l / k 5 = 2.2 f 0.2 under simulated atmospheric conditions (2‘ = 301 f 2 K, P = 780 torr, air, HzO = 11torr). This measurement was made possible by the development of a novel analysis procedure. Three literature values for the absolute value of k5 agree to within f 5 % . Thus we obtain a 1-atm value for k l of (1.3 f 0.3) X 10-l’ cmW3 s-l. This value, determined under realistic atmospheric conditions in a seemingly complex kinetic system, is in excellent agreement with more direct kinetic measurements. The agreement illustrates the validity of applying basic kinetic data to atmospheric processes and reinforces our understanding of the fate of NO2 in the troposphere.
Introduction The radical termination reaction OH + NO2 + M HN03 + M +
(1)
is a key process in the removal of nitrogen oxides from polluted atmospheres, the clean troposphere, and the stratosphere. Furthermore, it represents a major chaintermination reaction for hydroxyl radical in the atmosphere, and hence atmospheric models should be very sensitive to its rate constant. This reaction produces gas-phase nitric acid which may be removed from the atmosphere by rain to contribute to the phenomenon known as “acid rain”.l Gas-phase nitric acid may alternately be converted to the solid phase by reaction with gaseous NH3 to produce “,NO3 aerosol. This latter process is thought to be of great importance in areas such as Los Angeles where persistent hazes are caused by aerosols containing a large fraction of NH4N03.2 Another major route to nitric acid formation in the atmosphere is the sequence NOz + O3 NO3 + Oz (2)
-
NO3 + NO2
Nz06 + H 2 0
-
NzOj
(3)
2HN03
(4)
+
The importance of reaction 4 in clean and polluted atmospheres is not clearly defined. The homogeneous rate of this reaction has been measured to be extremely However, the reaction is known to proceed rapidly on surfaces. The relative efficiency of different ambient surfaces (the ground, plants, buildings, and atmospheric aerosols) is not known. We have been studying the reactions of aromatic hydrocarbons, in particular, toluene, under simulated atmospheric conditions where toluene is removed almost entirely via its reaction with hydroxyl radical. toluene + OH products (5)
-
Hydroxyl is generated in these experiments, as well as in the atmosphere, by several processes, some of which may 0022-3654/79/2083-3302$0 1.OO/O
be heterogeneous in nature. Its concentration is preserved by a variety of chain-propagation and chain-branching reactions involving hydrocarbons and some inorganic compounds. A discussion of this subject may be found in Doyle et al.17 In the course of this study we have been concerned with the fate of nitrogen oxides (NO, = NO NO2) in this system. This has led us to develop a novel method of data analysis which yields directly the rate constant ratio kl/kj for simulated atmospheric photochemistry when ozone is absent. This method also allows us to differentiate between the contribution to the nitrogen balance of reaction 1 compared to that of reactions 2-4. This method has wider utility, and a more complete development will be presented elsewhere. Knowledge of the actual rate of reaction 1 under ambient conditions of pressure and temperature is essential for understanding the chemistry of clean and polluted atmospheres. Recently, the recommended value of this rate constant has been increased from 4.9 X to (1.1 f 0.3) X 10-115-6cm-3 s-l for P = 1 atm and T = 300 K. This change was prompted by recent high-pressure meas u r e m e n t ~ . ~More - ~ recently, a pressure-dependence equation has appeared30which yields a value of 1.5 X at 780 torr of Nz. This value has a stated uncertainty of 40% and does not include a contribution from atmospheric O2 as a third body.
+
Experimental Conditions Photochemical reactions as they occur in the polluted troposphere were simulated in a 239-L, spherical, glass reaction vessel. The vessel is evacuable and is surrounded by a bank of 24 4-ft fluorescent black lights (GE-F40-BL) and 24 4-ft fluorescent sunlamps (GE-FS-40). The light intensity produced within the reaction vessel is comparable to that of tropospheric sunlight in its ability to photodissociate N0210but falls off more rapidly in intensity at short wavelengths. Gases were introduced into the reaction vessel by measuring known pressures in calibrated volumes on a vacuum line and expanding into the 239-L vessel. Air zero gas (Airco) was used as a diluent, at a pressure of 780 torr. 0 1979 American
Chemical Society
Rate Constant for NOp
-
+ OH (+M)
The Journal of Physical Chemistry, Vol. 83, No. 25, 1979 3303
HNO,
0 + NO2 + N O
1488
CONC.
6ee DDb
489
2ee
e
e
se
tse 2ea 258 3ee 358 TIME nin Figure 1. NO, and toluene data for a typical experiment. In most of these experiments the photooxidation of NOp was inhibited by the relatively small [toluene] to [NO,] ratio. lea
For these reactions water was added as a liquid in a volume . corresponds to sufficient to give 3.5 X 1017 ~ m - ~This about 38% relative humidity for the reaction temperature which was 301 f 2 K. These experiments consisted of monitoring the disappearance of total oxides of nitrogen (NO,), disappearance of toluene, and interconversion of NO and NOz. Toluene was measured by gas chromatography using a 36-cm3 sample loop. Nitric oxide and total NO, were measured via the chemiluminescence method1' using a Thermo Electron Model 12-A NO, analyzer. This instrument measures NO directly by its chemiluminescent reaction with O3 and total NO, by reducing NOz to NO in a 450 "C stainless steel catalytic converter. We checked the response of the overall system to nitric acid by depressurizing the reaction vessel to 600 torr and injecting 1FL of concentrated nitric acid which was flushed into the reactor while repressurizing to 780 torr. No response of the NO, analyzer to the added nitric acid ("1 ppm) was observed. This lack of response may be due to loss of HN03 on the walls of the reaction vessel, the sample line, or in the NO, analyzer and is not necessarily due to a failure of the catalyst to convert HN03 to NO. The catalytic converter is known to convert peroxyacetyl nitrate (PAN) to and thus PAN gives a 100% response as NO,. Since PAN is a known product of the breakdown of toluene in the presence of NO,,'3 we have designed these experiments to inhibit PAN formation by maintaining a sufficiently high NO to NOz ratio so that all PAN precursor peroxy radicals react preferentially with NO.14 Other types of nitrogen-containing products such as nitrotoluene and benzyl nitrate are formed in this system, but their yields are quite lowISfor the concentration range employed and hence have minimal impact on the NO, balance. Results, Theoretical Treatment, and Discussion A typical reaction profile for NO, and toluene is shown in Figure 1. Although the polluted troposphere is usually characterized by the photooxidation of NO and the ultimate buildup of ozone, this plot initially shows a reduction of NOz to NO. The overall process can be summarized by the sequence NO2 + h~ NO + 0 0 + Oz + M O3 M 0 3 + NO NO2 + 0 2 --+
+
+
(6) (7) (8)
(9) (10)
ROz + NO Reactions 6-8 are fast, quickly producing equilibrium, and this equilibrium is maintained during the slow photooxidation of N016or photoreduction of NOz. Reaction 10 involves hydrocarbon oxidation products and causes NO photooxidation and ozone buildup. Reaction 9 involves a photoreduction of NO2. Since the rate of reaction 9 in higher [NO,] coupled this system is proportional to with lower or unreactive hydrocarbons will inhibit O3 production for long periods of time. It is k n ~ w n ' that ~ , ~the ~ loss rates of many species in the polluted atmosphere are controlled by their reaction with hydroxyl radical. Other free radicals of potential importance are HOz, NO3, O(3P),O(lD), RCH20, RCH2OZ,RC(0)02, and others. Another major reactive species is ozone. In the case of toluene, as well as with many other atmospheric hydrocarbon species, the OH concentration coupled with the speed of its reaction makes OH responsible for virtually all of the hydrocarbon removal. Rate constants have been measured for the reaction of toluene with OH,17-20031 21 O(3P),23N03,24and HOzSz2The rates of the reaction of the latter four species are quite slow compared with that of OH for ambient conditions. The toluene + OH rate constant, k5, has been measured by flash photolysis-resonance fluorescence in a direct fashion18-20 and by its disappearance relative to another hydrocarbon under simulated atmospheric ~0nditions.l~ Three of these measurements lie within f6% of their average. The fourth measurement,17which is based only upon a few data points and a reference value for n-butane, lies 30% below the average. We may use the fact that toluene (T) loss is controlled by reaction with OH to obtain a time-dependent equation for OH concentration -+
1288
+ 02 NO2 + RO
When this equation is combined with the rate expression for NOz loss via reaction 1, we eliminate both time and hydroxyl concentration as variables. If we then take into account that NO and NOz are being interconverted and write the rate expression in terms of NO, loss ([NO,] = [NO] + [NO,]), we obtain
The integral on the right-hand side of eq 12 may be evaluated numerically by expressing [NO,] as a polynomial in [toluene], dividing by [toluene], and integrating analytically. In Figure 2 we have plotted [NO,] vs. [toluene] for the reaction presented in Figure 1 and the solid line represents a fifth-order polynomial fit of the data. Thus a plot of A[NO,] vs. the integrated polynomial should generate a straight line of slope equal to the rate constant ratio. To place the plot in the positive quadrant, the negative of the terms in eq 12 is plotted and shown in Figure 3a. The linearity of this plot indicates that all of the NO, loss may be attributed to reaction 1. A summary of the results of ten experiments is given in Table I. These experiments generated very small amounts of nitrotoluenes, benzyl nitrate, and possibly other nitrogen-containing products as well. Loss of NO, to these compounds is not accounted for by eq 12, and hence the slopes of the plots similar to Figure 3a really represent an upper limit for the rate constant ratio. We account for this effect on a crude basis by considering the likely mecha-
3304
The Journal of Physical Chemistry, Vol. 83, No. 25, 1979
ieee 9ee
O'Brien, Green, and Doty
ieee ,
tI
r
/I
a
*
*
. 7ee . we
(N02) DPb
see
.
see
.
/111/
2ae
. 388 .
4ee
4ee
E
see
6ee
7ee
aee
(TOLUENE)
9ee
ieee iiee
opb
Figure 2. Plot of [NO2] vs. [toluene] from data in Figure 1. The solid line is a fifth-order polynomial fit of the data.
TABLE I: Summary of Toluene/NOx Reactions run 35 36 44 45 46 41 48 49 50 54
[Noxlinittu [toluenelinit PPmb PPmb 1.64 1.10 1.23 1.20 1.22 3.55 5.07 4.66 5.37 0.586
0.82 1.05 1.08 1.02 1.03 3.43 4.35 3.66 3.31 0.528
9
k,lk,
a
iao
zae
388
- f(NOe)/(T)
4ee d(T)
e
lee
3ee
200 DPb
Flgure 3. (a) Plot of eq 12 fpr the data of Figure 1. The linearity indicates that all of the NO, loss may be attributed to reaction 1. The slope of the line gives the value for k,lk,. T = toluene. Note: I n order to place the plot in the positive quadrant, the negatives of both A[NO,] and the right-hand slde of eq 12 are plotted. (b) Similar plot for a typical experiment conducted by Pitts et aLZ8which generated ozone. The initial slope agrees with that of part a and the upward curvature coincldes with the onset of O3formation. This curvature indicates NO, loss in addition to reaction 1, presumably due to reactions 2-4.
2.17 2.41 2.11 1.97 2.02 2.58 2.35 2.20 2.16 1.92 av= 2.2 s = 0.2
a All experiments except 36 started with [NO,]/[NO] y 8; experiment 36 used [NO,]/[NO] = 0.2. b Parts per million by volume are traditional units. We here define 1 ppm = 2.45 X 10'' cm-3 for T = 300 K .
nismls for formation of nitrogen-containing organic reaction products. T + OH I (several possible)
-
+ NOz slow N-containing products fast I + Oz non-N-containing products I
-
Using methods similar to those used in deriving eq 12, we find the total NO, loss to organic products is then proportional to the integral J[N02] d(T). A plot of the ratio k l / k 6 vs. this integral is given in Figure 4. As expected, there is an apparent small decrease in the rate constant ratio at smaller NO2concentration. The average of the ten experiments gives k l / & = 2.2 f 0.2 (standard deviation), while the intercept of Figure 4 is 2.1. The difference of the two values does not justify a more detailed correction. The ten experiments listed in Table I caused no measured ozone buildup; thus reactions 2-4 do not occur. We have examined the case where ozone is formed as well, since this situation is more characteristic of photochemical air pollution. To do this we analyzed data taken by Pitts et al. in the large evacuable smog chamber at the University of California, Riverside.26 The data for these experiments are available in a US. Environmental Protection Agency Report by Whitten and H0g0.~' This particular set of experiments consisted of ten toluene reactions, nine of which generated appreciable ozone. A plot similar to Figure 3a is shown for run EC-82, a typical reaction, in
1
1
The Journal of Physical Chemistry, Vol. 83, No. 25, 1979 3305
Communications to the Editor
by the average of three literature values18-20for reaction 5. The average is 6.08 X cm-l s-l which yields kl = 1.3 f 0.3 X cm-I s?. The uncertainty quoted is the sum of the 6% range in k6 and the 9% standard deviation in our measurements of k l / k 5 .
Conclusions Previous ~ o r k e r s ~have ~ p used ~ ~ ,relative ~~ pseudo-firstorder disappearance rates for hydrocarbons in a system similar to this to obtain rate constant ratios for reaction with hydroxyl radical. Although similar to this work in principle, our analysis (eq 12) has the advantage of not depending upon the existence of pseudo-first-order kinetics or constant OH concentration. The analysis is completely dimensionless so it is also independent of instrument calibration for hydrocarbon, NO, or NO, (since both NO, and NOz [NO,] = [NO,.. - [NO] have the same calibration factor). The excellent agreement between our value for the rate constant for reaction 1 [(1.3 f 0.3) X cm-3 s-l] and the value recommended by Hudson6 from a summary of more direct measurements [(1.1f 0.3) X lo-.” cm-3 s-l] is quite significant. It provides support for the conviction that basic rate data can be applied with confidence to the analysis and prediction of atmospheric chemical processes and it gives confidence in the extent of our understanding of the importance of reaction 1 in determining the atmospheric fate of NO,. and also as a major chain-termination process for hydroxyl radical. Other nitrogen-containing acids are known to be formed in the atmosphere, in particular HONO and H02N02. However, these compounds are not stable with respect to photolysis or thermal decomposition and hence do not accumulate to an appreciable extent. Since they do not represent a sink for NO, as does HN03, their presence does not affect this analysis. Acknowledgment. This work was supported by U S . Environmental Protection Agency Grant No. R804764, Marcia Dodge project officer. References and Notes (1) S. Oden, Water, A t , SOilPoWut., 6, 137 (1976), and references therein. (2) W. H. White and P. T. Roberts, Atmos. Environ., 11, 803 (1977).
(3) E. D. Morris and H. Niki, J . Phys. Chem., 77, 1929 (1973). (4) R. F. Hampson, Jr., and D. Garvin, Eds., Natl. Bur. Stand. ( U . S . ) , Tech. Note, No. 886 (1975). (5) R. D. Hudson. Ed., NASA Ref. Pub/. 7070 (1977). (6) R. F. Hampson, Jr:, and D. Garvin, Eds., Nail. Bur. Stand. ( U . S . ) , Spec. Pub!., No. 513 (1977). (7) C. Anastasi and I. W. M. Smith, J . Chem. SOC.,Faraday Trans. 2, 72, 1459 (1976). (8) R. Atkinson, R. A. Perry, and J. N. Pitts, Jr., J. Chem. Phys., 65, 306 (1976). (9) D. D. Davis, “Absolute Rate Constants for Elementary Reactions of Atmospheric Importance: Results from the University of Maryland Gas Kinetics Laboratory”, Report No. 3, University of Maryland, Cdlege Park, Md., 1976. (10) J. R. Holmes, R. J. O’Brien, J. H. Crabtree, T. A. Hecht, and J. H. Seinfeld, Environ. Sci. Techno/., 7, 519 (1973). (11) D. H. Stedman, E. E. Daby, F. Stuhl, and H. Niki, J. Air Po/lut. Control Assoc., 22, 260 (1972). (12) A. M. Winer, J. W. Peters, J. P. Smith, and J. N. Pitts, Jr., Envlron. Sci. Techno/., 8, 1118 (1974). (13) A. P. Aitshuller, S. L. Kopczynski, W. A. Lonneman, F. D. Sutterfield, and D. L. Wilson, Environ. Sci. Techno/., 4, 44 (1970). (14) D. G. Hendry and R. A. Kenley, J. Am. C b m . Soc., 99, 3198 (1977). (15) R. A. Kenley, J. E. Davenport, and D. G. Hendry, J. Phys. Chem., 82, 1095 (1978). (16) R. J. O’Brien, Environ. Sci. Techno/., 8, 579 (1974). (17) G. J. Doyle, A. C. Lloyd, K. R. Darnail, A. M.Winer, and J. N. Pitts, Jr., Envifon. Scl. Technol., 9, 237 (1975). (18) D. A. Hansen, R. Atkinson, and J. N. Pitts, Jr., J. Phys. Chem., 79, 1763 (1975). (19) R. A. Perry, R. Atkinson, and J. N. Pitts, Jr., J . Phys. Chem., 81, 296 (1977). (20) D. D. Davis, W. Bollinger, and S. Fischer, J. Phys. Chem., 79, 293 ( 1975). (21) C. T. Pate, R. Atkinson, and J. N. Pitts, Jr., J . Envlron. Sci. Hea/th-€nvifon. Sci. Eng., A l l , 1 (1976). (22) D. G. Hendry, T. Mill, L. Piszkiewicz, J. A. Howard, and H. K. Eigenmann, J . Phys. Chem. Ref. Data, 3, 937 (1978). (23) A. J. Colussi, D. L. Singleton, R. S. Irwin, and R. J. Cvetanovic, J . Phys. Chem., 79, 1900 (1975); R. Atkinson and J. N. Pks, Jr., iM., 79, 295 (1975). (24) S. M. Japar and H. Niki, J. Phys. Chem., 79, 1629 (1975). (25) A. C. Lloyd, K. R. Darnall, A. M. Winer, and J. N. Pltts, Jr., J. Phys. Chem., 80, 789 (1976). (26) J. N. Pitts, Jr., et al., “Mechanisms of Photochemical Reactlons In Urban Air”, US.-E.P.A. Grant No. R-800649. (27) G. Z.Whitten and H. Hogo, “Mathematical Modeling of Simulated Photochemical Smog”, US.-EPA-600/3-77-011, 1977. (28) R. J. O’Brien, P. J. Green, and R. A. Doty, ”Chemical and Biological Implications of Nitrogeneous Air Pollutants”, D. Grosjean, Ed., Ann Arbor Science, Publishers, Ann Arbor, Mich., 1979, Chapter 11. (29) R. Atkinson, K. R. Darnall, and J. N. Pitts, Jr., J. Phys. Chem., 82, 2759 (1978). (30) NASA Panel for Data Evaluation, “Chemical Kinetic and Photochemical Data for Use in Stratospherlc Modeling, Evaluation No. 2”, Jet Propulsion Laboratoty, California Institute of Technology, Pasadena, Calif., 1979.
COMMUNICATIONS TO THE EDITOR Determination of the Hydrodynamic Volume of Inverted Micelles Containing Water by the Fluorescence Polarization Technique Publication costs assisted by the Centre National de /a Recherche Scientlfique (France)
Sir: Sodium bis(2-ethylhexyl)sulfosuccinate (aerosol OT, AOT) can form inverted micelles in apolar organic solvents and large amounts of water can be solubilized in such solutions. Water is confined in spherical pools encased by surfactant molecules. Our objective is to use the fluorescence polarization technique as a straightforward method of determination of the hydrodynamic micellar volume as 0022-3654/79/2083-3305$0 1.00/0
a function of the [H20]/[AOT]ratio. This can be achieved by using a hydrophylic fluorescent probe which is localized in the aqueous part of the micelle. Such a method was originally proposed by Singleterry and Weinbergerl but only for very low water contents so that the degree of polarization of the emitted fluorescence reflects only the rotational behavior of the micelle as a whole. At higher water contents, there is an additional fluorescence depolarization due to the motions of the dye within the water p00l.~9~ We report here a method which permits simultaneous determination of the two rotational rates, thus allowing a straightforward estimate of the micellar hydrodynamic volume and the fluidity of the water pool. For a given [H20]/ [AOT] ratio, this is indeed 0 1979 American
Chemical Society
