286
Rowland et
(23) T. A. Walter, C. Lifshitz, W. A. Chupka, and J. Berkowitz, J. Cbem. Phys., 51, 3531 (1969). (24) J. L. Franklin, J. G. Dillard, H. M. Rosenstock, J. T. Herron, K. Draxl,
and F. H. Field, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 26 (June 1969). (25) J. 6.Pedby and 6.S. Iseard, “CATCH Tables for Silicon Compounds,” University of Sussex, 1972. (26) J. W. C. Johns and R. F. Barrow, froc. fbys. SOC.(London),71, 476
(1958). (27) T. M. H. Cheng and F. W. Lampe, J. fbys. Cbem., 77, 2841 (1973). (28) R. J. Cotter and W. S. Koski, J. Cbem. Phys., 59, 784 (1973). (29) C. E. Moore, Natl. Bur. Stand. (U.S.), Circ., No. 467 (1949). (30) W. N. Allen, T. M. H. Cheng, and F. W. Lampe, J. Chem. Phys., submitted for publication.
al.
(31) B. R. Turner, J. A. Rutherford, and D. M. J. Compton, J. Cbem, fbys., 48, 1602 (1968). (32) T. M. Mayer and F. W. Lampe, J. fbys. Cbem., 78, 2195 (1974). (33) T. M. Mayer and F. W. Larnpe, J. Phys. Cbem., 78, 2429 (1974). (34) R. Wolfgang, Acc. Cbem. Res., 3 , 48 (1970). (35) G. W. Stewart, J. M. S. Henis, and P. P.Gaspar, J. Chem. Phys., 57, 1990 (1972). (36) T. M. Mayer and F. W. Lampe, J. fbys. Cbem., 78, 2433 (1974). (37) J. R. Krause and P. Potzinger, Int. J. Mass Spectrom. Ion fbys., 18, 303 (1975). (38) B. L. Shaw, “Inorganic Hydrides,” Pergamon Press, New York, N.Y., 1967. (39) R. J. P. Corriu and M. Henner, J. Organometal. Cbem., 74, 1 (1974). (40) G. Giournousisand D. P. Stevenson, J. Cbem fbys., 29, 294 (1958).
The Temperature Dependences of the Ultraviolet Absorption Cross Sections of CC12F2 and CC13F, and Their Stratospheric Significance
t
C. C. Chou, W. S. Smith, H. Vera Ruiz, K. Moe, G. Crescentini, M. J. Molina, and F. S. Rowland” Department of Cbemistry, University of California, Irvine, California 927 17 (Received Ju/y 26, 1976) Publication GOStS assisted by the U S . Energy Research and Development Administration
The photochemical absorption cross sections of CC12F2 and CC13Fhave been measured between 1900 and 2200 A for stratospheric temperatures in the range from 212 to 257 K. The cross sections are monotonically lower with lower temperatures, with larger decreases for the longer wavelengths. The calculated photodissociation coefficients at each stratospheric altitude are correspondinglylower than previously estimated. The atmospheric residence times for CClzFz are 8-17 % longer than estimated from room temperature absorption cross sections. The changes are much smaller for CCl,F, corresponding to about 3-6% increases in residence times.
Introduction The primary known terrestrial process for the destruction of the widely used chlorofluoromethanes CClpFz and CC13F is through stratospheric photolysis by solar ultraviolet radiation, as in (1) and (2).’12 The release of CCI,F, t hv CC1,F t hv
-+
-+
C1 t CClF,
C1 t CC1,F
(1) (2)
C1 atoms at stratospheric altitudes initiates the ClO,-chain removal of 0 and O3 by reactions 3 and 4, providing
c1 t 0, c10 t 0, c10 t o + c1 t 0, -+
(3)
(41
substantial environmental importance to the details of reactions 1and 2.1-3 In previous estimates of the altitudes for photodissociation and of the average atmospheric residence times for these molecules, the calculations have been based upon the absorption cross sections measured at about 296 K, even though actual stratospheric ‘temperatures range as low as 210 K.’ Measurements by Rebbert and Ausloos of these absorption cross sections at 2139 A over a range of temperatures have shown that absorption can be appreciably less at lower temperature^,^ introducing some error into the earlier estimates. However, the temperature effect is observed to be largest near the threshold for absorption (about 2200 A for CC12Fzand 2300 A for CC13F),with lesser effects at shorter wavelengths. Since the stratospheric absorption of ultraviolet by CCl3F and CClzF2is spread rather evenly over the wavelength range from 1900 to 2100 A,5 the variations measured a t
t Alfred P. Sloan Fellow. The Journal of fhysical Chemistry, Vol SI, No. 4, 1077
2139 A clearly represent more change with temperature than can be expected in the actual stratosphere. Furthermore, since ultraviolet intensity changes very rapidly with altitude in the 20-40-km region important for chlorofluoromethane photolysis, even a decrease of a factor of 3 in average cross section would lead only to an increase of about 2 km in average altitude of photolysis, and to an increase of about a factor of 1.5 in the average atmospheric residence time.6 Preliminary measurements of the temperature dependence of these ultraviolet cross sections have indeed shown that the average decrease in cross section, weighted by stratospheric ultraviolet fluxes, is much less than a factor of 3, and that the overall effect on lifetimes and altitudes of photolysis is relatively m i n ~ r . ~Although ,~ some statements to the contrary have a ~ p e a r e dthe , ~ primary effect of reduced stratospheric cross sections is an increase in the estimated potential for destruction of stratospheric ozone per molecule of CC12F2or CC13F, since the C10, decomposition products are thereby estimated to be released at higher altitudes. In general, of course, the higher the altitude of injection of a potential stratospheric pollutant, the greater the destruction per molecule during the longer time required for diffusion to the troposphere and removal there by tropospheric processes such as rainoutS2 Although our preliminary estimates gave only about 10% as the overall increase in potential destructive capability of C10, released from CC12F2,7,8 we have refined both our measurements and our calculations to provide a reasonable prototype of the importance of stratospheric temperatures on such absorption processes. The more detailed study described here confirms the semiquantitative estimates
287
UV Absorption Cross Sections of CClpFpand CCl3F
lo- 18 -
@&-::*==:
0-
CC13F pw.””
sp .6 ,de AI‘-
IO+10-20
-
.+*
&,/
,“go PI-
p‘
P
o THIS WORK
x ROBBINSET&.
0’
p‘
Midpoint of interval,
p.ox
P
voa
TABLE I : Absorption Cross Sections for CCl,F, and CCl,F in the Wavelength Range 1850-2272 A at 296 K (Units of cm2)
PiCClzFz
A, A
10-21 10-22
i
2300
2200
2100
2000
a
1900
1800
Figure 1. Ultraviolet absorption cross sections for CCI,Fp and CCl3F this work; (X) measurements by Robbins and Stolarski, at 296 K: (0) ref 10.
made earlier that the errors introduced by using measurements of ultraviolet absorption cross sections made at 296 K rather than 210-275 K are rather small, especially relative to the other errors inherent in such calculations.
Experimental Section Absolute Cross Sections at 296 K . Our basic absorption measurements have been made with a nitrogen-purged Cary 14 ultraviolet spectrometer and a temperature controlled absorption cell equipped with a cooling jacket and Suprasil quartz windows. The temperature of the cell was controlled by circulating refrigerated liquid (methanol or ethanol) through the cooling jacket. The temperature of the gas within the cell was measured to be within *1 “C of the temperature of the refrigerant at its emergence from the cooling jacket. The absorption cross sections of these molecules vary by as much as a factor of lo3 over the wavelengths investigated here, thereby requiring a range of filling pressures for optimum cross section measurements. While no difficulties are encountered at room temperature, for which both CClzFzand CC1,F have high vapor pressures, serious limitations are found (especially for CC1,F) in the amount of gas available at 210-240 K. (The vapor pressure of CC13F at 210 K is 7 Torr.) The overall procedure which was adopted involves (a) evaluation of the absolute cross section for ultraviolet absorption at 296 K for all wavelengths; and (b) evaluation of the averages of the relative absorption cross sections for temperature T K vs. 296 K for each particular cell-filling. Experimentally, of course, it is possible to obtain an absolute cross section at temperature T K directly from the measurement of transmitted light for a particular filling. However, the reproducibility of the relative cross sections a t T K vs. 296 K was better, since possible errors in composition were automatically cancelled. Measurements of absolute cross sections were considerably easier at 296 K and were consequently much more numerous, leading to quite reliable estimates of absolute cross sections at that temperature. The absolute cross sections for absorption of ultraviolet radiation between 1850 and 2272 A are summarized in Table I, expressed as cross sections a t the midpoint of 500-cm-l intervals. We have also included for comparison purposes the earlier values given by Rowland and Molina. Our new data are graphed in Figure 1 together with the data of Robbins and Stolarski.lo Direct absorption measurements at 296 K have also been recently reported by Bass and Ledford,l’ with essential agreement to the data of Table 1. In our earlier review article, we also
2260 2235 2210 2186 2162 2139 2116 2094 2073 2051 2030 2010 1990 1970 1951 1932 1914 1896 1878 1860
Interval boundCCI,F, aries, u , 1 0 3 This cm-’ work ref 2 44.0
s-l x
0.04 0.23 0.72 2.7 6.9 15.3 24.1 23.1 19.2 18.8 20.6 21.5 16.1 9.8 4.2 3.6 2.6 0.58 0.07 190
0.03 0.19 0.56 2.1 5.6 12.7 20.3 19.8 16.7 16.7 18.6 19.7 15.0 9.3 4.0 3.5 2.5 0.58 0.07 168
CCl,F,
+ O(ID) + O('D)
-+
+
C10 + CClF, C10 t CC1,F
0
252'K 8 232'K A 213'K o Ref. 1 1 a t 223'K 0
0.8
A
m
. B
A
O
0
0
4
8
A
I
m
A
0.71 I
0
0.61
I
I
I
I
I
1900 2000 2100 2200 Flgure 5. Temperature effect on ultraviolet absorption cross sections of CC13F (expressed relative to cross sections at 296 K), this work: (0)252 K; (H) 232 K; (A)212 K; (0) ref 11 at 223 K.
weighted effect of using the cross sections at actual stratospheric temperatures is illustrated in Table 111, for 30 km, 235 K, and overhead sun. For these conditions the value of J found for CClzFzwith 6235 is reduced by about 28% from that found using 6296, while that for CC13F is reduced by only about 12%. This calculation can then be repeated for all zenith angles and altitudes with the appropriate solar fluxes, and new average values of J obtained. These averaged photodissociation coefficients, together with the overhead sun values, are compared in Table IV. The averaged photodissociation coefficients have then been used in a steady-state calculation of the type described in ref 2, using a variety of suggested eddy diffusion coefficients, as before. The molecules have also been assumed to be decomposed by reactions with O(lD) atoms by eq 6 and 7, using 2.6 X cm3/molecule s and 3.3 CC1,F
A D
0.9
s-l x
.....
V. (Since several minor procedural changes have been made in the computing process since the calculations of ref 2, all have been repeated again so that the only differences between the individual pairs of Table V are the result of using 4296 in one case and UT in the other.) As expected, the calculated altitudes for decomposition are slightly higher. Consequently,the lifetimes for CClzFzare 8-17% longer, and for CC13F 3-6% longer. Since the values of J are lower than before, the importance of decomposition by O(lD) atoms becomes fractionally more important. No temperature variations have been measured for O(lD) reactions with CClzF2or CC13F, but such effects must be very small since the collision efficiencies approach unity at room temperatures. At steady state, the concentration of ClX, the chlorine-containing decomposition products from CClzFzand CC13F,will be proportional to the btmospheric lifetimes given fixed imput fluxes. Since the fraction of C1X to be found as C10, etc., is only a slowly varying function of the amount of C1X itself, the steady-state estimate of C10 concentrations from each molecule will be increased by approximately the ratios of atmospheric residence times. A reasonable estimate of the overall effect at steady state can be obtained by summing the C1X concentrations at 40 km from CClzFz and CC13F. The calculated percentage
(6) (7)
X 10-l' cm3/molecules as the respective rate constants for reactions 6 and 7.13 The overall effect on these calculations can be described by the changes in average atmospheric residence times or by the percentage reaction by photolysis, as shown in Table
TABLE IV: Calculated Values for Photodissociation Coefficients, J , for CCI,F, and CCI,F Vs. Altitude, Using Temperature Dependent Absorption Cross Sections Photodissociation coeff, J? s-' Altitude, Temp, __ km
K
15 20 25 30 35 40 45
210.8 218.9 227.1 235.2 251.7 268.2 274.5
-
CC1,F , Overhead sun '296
1.5(-10) 5.6(- 9) 4.9(-8) 2.1(- 7) 5.7(- 7) l.l(-6) 1.6(-6) 1.5(- 1 0 ) signifies 1.5 X
CC1,F Worldwide av u296
9.0(- 11) 3.5(- 9) 3.3(- 8) 1.5(- 7) 4.4(- 7) 9.3(- 7) 1.4(-6)
7.7(-12) 4.3(- 10) 5.3(-9) 3.2(-8) 1.2(-7) 2.9(-7) 5.1(-7)
'T
4.5(-12) 2.6(- 10) 3.6(- 9) 2.2(-8) 8.8(- 8) 2.4(-7) 4.5(- 7)
Overhead sun
'
'T
196
2.0(-9) 6.2(-8) 4.8(- 7) 1.9(-6) 4.8(- 6) 8.7(- 6) 1.2(- 5)
1.7(-9) 5.4(-8) 4.3(- 7) 1.7(-6) 4.6(-6) 8.3(-6) 1.2(- 5)
Worldwide av '296
1.0(- 10) 4.7(- 9) 5.3(-8) 2.9(- 7) 1.0(-6) 2.5(-6) 4.1(-6)
U T
8.4(- 11) 4 4 - 9) 4.7(- 8) 2.6(- 7) 9.7(- 7) 2.3(- 6) 4.0(-6)
The Journal of Physical Chemistv, Vol. 81, No. 4, 1977
Rowland et al.
290
TABLE V : C o m p a r i s o n of A t m o s p h e r i c Residence T i m e s Calculated U s i n g A b s o r p t i o n Cross Sections of CCl,F, a n d CCI 3Fa t 296 K a n d a t A c t u a l Stratospheric Temperatures Stratospherica mixing r a t e CCl,F, Lifetime,years
% photolysisC
Temp A (296 K)b 71.9 78.0 (TK) (296 K) 91.4 ( T K ) 89.3
B 98.5 106.7 90.9 88.6
D 143 157 89.6 86.7
C
155 182 88.7 85.2 ~~~
CC1SF L i f e t i m e , years %photolysis
(296 K ) (TK) (296 K) (TK)
42.4 43.7 98.2 98.1
58.0 60.0 97.9 97.7
59.4 62.9 97.2 97.0
77.9 80.8 97.1 96.9
a T h e e d d y d i f f u s i o n c o e f f i c i e n t s u s e d by stratospheric modelers are f r e q u e n t l y adjusted to give b e t t e r f i t s to n e w d a t a as acquired. These m o d e l s are those of W o f s y (A), C r u t z e n (B), Chang (C), and H u n t e n (D) as described in r e f 2. E a c h has b e e n m o d i f e d since, but t h e e f f e c t s are generally applicable. All cross sections a t 296 K, or a t a c t u a l stratospheric temperatures. R e m a i n d e r is r e m o v e d by r e a c t i o n with o(lD).
TABLE VI: Calculated Percentage Increase in Steady-State CIX C o n c e n t r a t i o n s a t 40 km ( u n i t s of 10’c m M 3 )
A
B
C
D
3.08 3.26 5.8
4.39 4.66 6.2
7.30 8.54 17.0
6.72 7.34 9.2
%increase
1.35 1.39 3.0
1.85 1.92 3.8
1.90 2.01 5.8
2.54 2.63 3.5
Total [ClX],,6p [CIX]m
4.43 4.65
6.24 6.58
9.20 10.55
9.26 9.97
5.0
5.5
14.7
7.7
CCl,F, [‘lx1296K
[CIXITK % increase C C l 3F [cIX],g6K
[C!X],
% increase
increases in C1X at 40 km for each of the eddy diffusion models is given in Table VI. The increases in C1X range from 5 to 15% and therefore represent increases in estimated depletion of ozone at steady state by similar amounts. However, such changes are well within the estimated accuracy of such calculations, and do not materially alter the understanding of the situation. The calculated variations are of the magnitude
The Journal of Physical Chemistv, Vol. 81, No. 4, 1977
that is neither of major importance nor completely negligible, indicating that some assessment of temperature variations of cross section should be made for any other fluorocarbon molecule with an absorption threshold less than 2300 A. These calculations have also been performed without any feedback, and are actually applicable only in situations in which the stratospheric composition is essentially unperturbed from the present situation. If the current rate of release of CC12F2and CC13F were permitted to continue to concentrations approaching steady state, then the depletion of ozone at high altitudes would be so severe that much larger amounts of 1900-2200-A radiation would penetrate to lower altitudes and the values of the photodissociation coefficients would be appreciably increased at many altitudes. This variation in J with time is automatically programmed into most long-term estimates of ozone depletion. Typically, however, the change in stratospheric temperature with loss of ozone in the 4050-km range is not included in such calculations. The stratospheric mixing processes (e.g., eddy diffusion coefficients in one-dimensional calculations) are also assumed to be unchanged despite rather drastic modifications in the physicochemical structure of the upper stratosphere.
Acknowledgment. This research was supported by ERDA Contract AT-(04-3)-34,P.A. 126.
References and Notes (1) M. J. Molina and F. S. Rowland, Nature(London),249, 810 (1974). (2) F. S. Rowland and M. J. Molina, Rev. Geophys. Space Phys., 13, l(1975). (3) “Fluorocarbons and the Environment”, Report of Federal Task Force on Inadvertent Modification of the Stratosphere (IMOS), Council on Environmental Quality, Washington, D.C., June, 1975. (4) R. E. Rebbert and P. J. Ausloos, J. Photochem., 4, 419 (1975). (5) Figure 5 of ref 2, p 9. (6) Table 8 of ref 2, p 13. (7) F. S. Rowland, New Scientist, 08, 8 (1975). (8) C. C. Chou, M. J. Molina, G. Crescentini, H. Vera Ruiz, and F. S. Rowland, Abstracts of Papers, First Chemical Congress of the North American Continent, Nov 30-Dec 5, 1975, Mexico City. (9) R. L. Schuyler, E. I. DuPont de Nemours and Co., Hearings before the Subcommittee on the Upper Atmosphere of the Committee on Aeronautical and Space Sciences, United States Senate, Sept 18, 1975, p 547-548. (10) D. Robbins and R. Stolarskl, Geophys. Res. Lett, 3 , 603 (1976). (11) A. M. Bass and A. E. Ledford, Jr., 12th Informal Conference on Photochemistry, June 28-July 1 , 1976, Gaithersburg, Md. (12) R. H. Huebner, D. L. Bushnell, R. J. Celotte, S. R. Mielczarek, and C. E. Kuyatt, Nature (London), 257, 376 (1976). (13) R. K. M. Jayanty, R. Simonaitis, and J. Heicklen, J. Photochem., 4, 203 (1975).
