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Catalytic destruction of ozone by chlorofluorocarbons and partial restoration by methane in large laboratory experiments
PhysicsLettersA 168 (1992) 423—428 North-Holland
PHYSICS LETTERS A
Catalytic destruction of ozone by chiorofluorocarbons and partial restoration by methane in large laboratory experiments A.Y. Wong, R.G. Suchannek and R. Kanner Department ofPhysics, University of Cal(fornia, Los Angeles, LosAngeles, CA 90024-1547, USA Received 15 June 1992; accepted for publication 25 June 1992 Communicated by M. Porkolab
The catalytic destruction ofozone by CFC was quantitatively measured in alarge laboratory device which has controlled ultraviolet radiation and gas composition.Experimental results obtained in a pureoxygen atmosphereagreed with theoretical estimates within a factor of two. A destruction ratio of one chlorine to 4 x 1O~ozone molecules was found. The introduction of methane reduced the density ofatomic chlorine resulting in a higher equilibrium ozone density. Even with a methane density six times the CFC density, only partial restoration of ozone is possible.
1. Introduction Stratospheric ozone is ofcritical importance to the earth’s ecology. Ozone has strong absorption bands in the ultraviolet region of the solar spectrum at wavelengths ranging from 240 to 290 nm. This portion of the solar radiation, if transmitted through the atmosphere, would have damaging effects on living organisms on the surface of the earth, The first photoelectric measurement of the amount ofozone in the atmosphere was made by Dobson and Harrison [1]. The nonuniform distribution of the ozone in the atmosphere is maximum in the stratosphere (at an altitude of approximately 25 km, depending on the latitude). The first theoretical explanation for the ozone layer was given by Chapman [2] who proposed a pure-oxygen photochemical model for the generation and destruction of ozone. His reaction scheme is described by the following set of equations:
Equation (2) describes the formation of an ozone molecule in a three-body collision of an oxygen atom with an oxygen molecule and a third collision partner M. The third reaction is the photodissociation of ozone. Equation (4) is the loss of one oxygen atom and one ozone molecule in a collision leading to the formation of two oxygen molecules. The last reaction is the loss of two oxygen atoms in a three-body collision resulting in the formation of one oxygen molecule. Although Chapman’s model can explain the formation of the ozone layer, the pure-oxygen model leads to calculated values for the ozone abundance in the atmosphere which are higher than experimental results [3]. It was concluded that an additional loss mechanism is affecting the ozone concentration which is not included in the pure oxygen-model. Researchers have been considering catalytic reactions of trace constituents of the atmosphere with ozone as an additional loss mechanism. The simultaneous
02+ h v—O + 0
(1)
observation [4] of ozone depletion and increased concentration of chlonne oxide in the stratosphere
O + 02+ M-+03 + M,
(2)
suggested that catalytic reactions of chlorine atoms
O 3+ h
(3)
with following The ozone molecules reaction contribute chain hastobeen the loss proposed ofozone. as
0+03-4202,
(4)
a dominant catalytic cycle:
0+O+M~O2+M.
(5)
Cl+O3~ClO+O2,
V*
~+
2,
0375-9601/92/S 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
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Volume 168, number 5,6
PHYSICS LETIERS A
(7)
ClO + O—’Cl + 02.
The net effect of reactions (6), (7) is the conversion of ozone molecules into oxygen molecules. Molina and Rowland [5] showed that atomic chlorine in the contemporary atmosphere primarily originates from the photolysis of two chlorofluorocarbons C13CF (CFC-11) and C12CF2 (CFC- 12). Both substances are produced on an industrial basis and have no natural origin. They are very inert and volatile. Their estimated lifetime in the atmosphere ranges from 40 to 150 years. They are released at the surface of the earth and transported by convection into the stratosphere, where the exposure to ultraviolet radiation leads to their photodissociation into atomic chlorine and the radicals C12CF and C1CF2. The atomic chlorine can then participate in the catalytic reactions (6), (7) which destroy ozone. Because of the large reservoir of CFC already existing in the atmosphere and the continued emission of CFC or related products, it might be necessary to consider mitigation measures. One method [6] suggested the conversion of chlorine atoms into negative ions to reduce their reactivity. Another method [7] is to use hydrocarbons to scavenge the chlorine radical. In order to conduct controlled studies ofcatalytic destructive processes and mitigation measures
ozone generator
02
~
7 September 1992
in the upper-atmosphere, we have built a laboratory chamber of sufficient dimensions in which ozone and reactive species have sufficient lifetimes to react repetitively. Simplified theories such as the Chapman model can be examined first and the concentration of minority species can be controlled. It is expected that in subsequent experiments more species will be included to make our laboratory modeling more realistic and to compare our results with computer modeling [81. This paper reports our first attempt of using a laboratory chamber to model critical concepts; however, the laboratory experiments were not designed to fully simulate the stratosphere.
2. The reaction chamber A schematic drawing of the reaction chamber is shown in fig. 1. The chamber has an inner diameter of 1.40 m, and its overall length including the two extensions is 4.03 m. The chamber can be evacuated by a turbomolecular and a rotary vane pump to a pressure of 10—s Torn For operation under static conditions, the pumps can be valved off with a gate valve. The main vessel is equipped with several types of vacuum gauges to permit pressure measurements over the range from hundreds of Torr to 10—i Torr.
v.u.v. quartz
window
window
a Hg lamp
/
Hg
fl
ll
control photocell
r.IIOCtOqS
V~U•V~
00
tank
CFC
light source
R.F. discharge tub.
Fig. 1. Schematic drawing of the reaction chamber. The chamber has an inner diameter of 1.40 m, and length 4.03 m. The chamber can be evacuated to pressures ofthe order of 10~Tort. Two 140 W mercury lamps provide the ultraviolet radiation for studies of photochemical reactions. Absorption of the 253.7 nm line emitted by a low power mercury lamp is used to measure the ozone concentration in the chamber.
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Volume 168, number 5,6
PHYSICS LETTERS A
7 September 1992
An ozone generator is connected to the chamber, which can produce a mixture of 2.5% ozone and 97.5% oxygen at chamber pressures ranging from a few mTorr to hundred Torr. This external injection of ozone is mainly for calibration purpose. Internal solar simulators are used to generate ozone by their radiation in the UV range. The ozone concentration n in the chamber is determined from absorption measurements of the 253.7 nm line of mercury, which is almost at the center of the Hartley band of ozone. The ozone density n can be obtained from the equation / = ~exp ( an), (8)
the chamber for spectroscopic investigations in the visible, ultraviolet, and vacuum-ultraviolet region of the spectrum. A mass spectrometer connected to the chamber allows independent determinations of the reaction products.
where I is the optical path length of the analyzing beam in the chamber, a is the absorption cross section, and I and 1o are the transmitted and the unattenuated intensity, respectively. Absorption cross sections for this line have been measured by several authors [9]. For absorption measurements, a light beam emitted from a low pressure mercury lamp is passed through the reaction chamber, and the transmitted intensity of the 253.7 nm component is detected with a monochromator and a photomultiplier tube. The light beam is chopped at a frequency of 215 Hz, and before the beam enters the chamber, a portion ofits intensity is deflected by a beam splitter into a control monochromator, which monitors the lamp intensity at the 253.7 nm line. A lock-in amplifier locked to the chopping frequency of the light beam is used to determine the ratio of the photomultiplier signal to the output voltage of the control monochromator. Measuring this ratio eliminates fluctuations in the absorption measurements caused by the slight variations in the intensity of the light source. For studies of photochemical processes, the reaction chamber is provided with two 140 W mercury lamps (Light Source Inc. Model GSL 1524 T5 VH/HO) which are the solar simulators. The lamps are mounted inside the chamber and have significant intensity at the 185 nm line. The lamps are operated with 60 Hz line voltage, and their intensity is therefore modulated with 120 Hz. The high selectivity ofthe lock-in amplifier prevents any interference of the mercury lamps with the intensity measurements of the analyzing beam. Several portholes in the vacuum envelope of the chamber permit access of optical spectrometers to
sure regime, the ultraviolet light sources were started when the reaction chamber was still evacuated in order to avoid the high start-up voltage ofthe mercury lamps at almost critical pressures. This procedure was followed during the first measurements. During later experiments, we were able to start the light sources when the chamber was already filled with 10 Torr of oxygen. When oxygen was leaked into the chamber in the presence of ultraviolet light, the transmitted intensity ofthe 253.7 nm line in the analyzingbeam decreased exponentially with the ozone density n according to eq. (10). A plot of the transmitted intensity of this line as a function of time yields the time evolution of the ozone density as shown in fig. 2. At point A, the flow of oxygen into the chamber was started, and the sharp drop of the transmitted intensity indicated a sharp rise in the formation of ozone
—
~•
Expenmental procedures and results
The photochemical generation of ozone was measured at oxygen pressures from 0.5 to 10.1 Torr when the chamber was valved off from the pumps. Since electric breakdown voltages are very low in this pres-
___________________________________
A 6
~
2
8 B ~ 00
300 t me
m
Fig. 2. Time evolution ofthe number density ofozone during the photochemical generation and subsequent destruction of ozone by trichlorofluoromethane. The number densities were derived from absorption measurements ofthe 253.7 nm line.
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Volume 168, number 5,6
PHYSICS LETTERS A
according to the reactions in eqs. (1) and (2). At B, a pressure of 10.1 Torr was reached, and the flow of oxygen was stopped. The ozone density continued to increase until an equilibrium was reached at point C. From measurements of the time rate of change of the ozone density and its equilibrium values we can deduce in a self-consistent manner the rate of generation and the rate of destruction in response to various additions of trace constituents or change in the environment. In addition to the ozone losses describes by eqs. (3)—(5), ozone was lost in the reaction chamber during collisions of ozone molecules with the chamber wall. This loss rate was measured by turning the solar simulator off. A typical loss of ozone density 9 (cm—3 s~),which duetothe wall effectwas 6.5x10 is much less than the typical reaction rates of 3 ~ loll (cm—3 s~)among species inside the chamber, and therefore permits repetitive reactions to occur. The equilibrium ozone concentration at C in fig. 2 was 4.6 x l0’~cm—3, i.e. approximately 0.1% of the 02 molecules were converted into ozone molecules. At point C of fig. 2 approximately 3 mTorr of chlorofluorocarbon (also known as trichlorofluoromethane C1 vapora were into ber, and we 3CF) observed rapid leaked decrease of the the chamozone density. This loss of ozone was observed only when C1 3CF vapor was irradiated by ultraviolet light from the solar simulator. We deduced that the ozone destruction was caused by the reaction of ozone with products ofphotodissociated Cl~CFmolecules. As it had been proposed for the generation of atomic chlorine in the stratosphere, the photolysis of C13CF by ultraviolet radiation resulted in atomic chlorine and the radical C12CF. The ozone loss was caused by reactions of atomic chlorine with ozone according to eq(6) Since atomic chlorine is restored in the subsequent reactions (7) of the product molecule ClO, the relatively small addition of C13CF to the gases in the reaction chamber significantly reduced the ozone concentration. The catalytic destruction of ozone by atomic chlorine led to the establishment of a new equilibrium of ozone generation and destruction at point D. The ozone concentration at D was reduced to 5.7 x 1 013 cm which is only 12% of the concentration prior to the addition of C13CF. A plot of ozone concentration n as function of the density of ~,
426
7 September 1992
C13CF is shown in fig. 3. The ozone concentrations were measured at the equilibria between ozone generation and destruction, which we observed for increasing amounts of C13CF vapor in the reaction chamber. The partial pressure of C13CF was measured with a differential capacitance manometer (MKS Instruments Inc., Model 1 2OAD), which permitted measurements of pressure changes of the order of 3 x I 0~Torr when the pressure in the chamber was 10.1 Torr. At the lowest measurable change in partial pressure, 0.3 mTorr, the curve in fig. 3 exhibits a very sharp drop of the ozone concentration n, followed by a slower decrease at higher CFC densities n~.The data points could be fitted by a function of the form. n = 1/ (An~+B) (9) where the coefficients A and B were obtained from a least squares fit and the constraints that the function passes through the data point for n~—0.This constraint was chosen because the data point for = 0 was obtained with much higher accuracy than all other points. The following values were obtained for A and B: A=l.833 (10_28 cm6), B=O.16l 3). The solid line in fig. 3 was obtained (10~14 from eq.cm(11) with the above-given values for the coefficients.
We can explain the functional dependence of the equilibrium ozone concentration n on the number
c~’ ~ C t H -4
0 C
N 0
2 0
0 _____________________________ 0.0 0.5 1.0 1.5 2.0
density of CCI3F [1014 cm —3 I Fig. 3. Equilibrium ozone concentration versus density of tnchlorofluoromethane. The ozone concentrations were measured at the equilibria between photochemical ozone generation and destruction for increasing amounts of CI3CF in the reaction chamber.
Volume 168, number 5,6
PHYSICS LETTERS A
density n, of CFC molecules as follows: dn/dt=k2Jn~2—v~n—k~nnc1,
A plot of the ozone density as function of time is shown in fig. 4. After ozone was photochemically (10)
where k2 n~2is the quantum yield for the generation of ozone, v~describes the wall losses, k~is the rate constant for ozone destruction by chlorine atoms, and n02 and n~,are the number densities of molecular oxygen and atomic chlorine, respectively; J is the photon flux. Under equilibrium conditions dn/dt = 0,
(11)
we find the relation
=
(
k~nc,/n~ ~ + ~ k2Jn~2 k2Jn~2)
7 September 1992
—1
(12)
which has the same form as eq. (9). Values for k2Jn~2were obtained from measurements of the slope dn/dt at the beginning of the ozone generation when ozone losses are still negligible, and values for ~a/flc were obtained from measurements of the slope of n after injection of trichlorofluorocarbon vapor into the reaction chamber. For k~we used a published rate constant, the value3ofs’ which ternandatweroom obtained perature is 1.189 x 10—” cm— v,~,from the ozone decay rate after shutting off the ultraviolet light sources. With these parameters, we obtained for the coefficients A and B the following values: A=l.08 (1028 cm6), B=0.072 (10_14 cm3). The calculated values of A and B differ from the measured values by a factor of approximately 2. The theoretical estimate according to eq. (12) is shown in fig. 3 as a dotted curve. From our measurements of ~ we can deduce the multiplicative destructive factor of ozone molecules by Cl radical. Using the average ratio of n!n~~ 1 we find that one chlorine atom destroys up to 4x l0~ozone molecules. If the photodissociation of C1 3CF in the reaction chamber leads to the catalytic destruction of ozone by atomic chlorine, it should be possible to reduce the ozone destruction by adding hydrocarbons to the chamber, which react with atomic chlorine to form the less reactive HC1 molecule. HC1 acts as a sink for atomic chlorine. Binding chlorine atoms in this molecule reduces the number of free chlorine atoms which can destroy ozone. This effect was indeed observed.
generated in the reaction chamber, approximately 6.0 mTorr of C13CF vapor were leaked into the chamber in the presence ofultraviolet radiation. The addition of C13CF, which is indicated in fig. 4 by point C, caused the ozone concentration to decrease. After a new equilibrium concentration of ozonewas reached at D, we added approximately 5.1 mTorr of methane to the gases in the chamber, and, as expected, the ozone density began to rise and leveled out at a higher value. We leaked again 5.3 mTorr of methane into the chamber at point E and repeated the procedure successively until the partial pressure of methane reached 30 mTorr. The rise of the equilibrium ozone density with increasing methane density is shown in fig. 5. We note that it takes much higher methane densities to scavenge a given density of chlorine atorns. Assuming the linear relationship to hold it would take 136 mTorr of methane to scavenge completely the chlorine atoms and restore the ozone to its initial density. The density ratio of methane! chlorine would be 5.4 X 1 07, and the density ratio of methane/CFC would be approximately 23.found A possible explanation for this high ratio can be in I
‘~
I I
I
I
6
H _
Q) C
2 0 N 0
0
i....i 100
200
300
400
time [m n] Fig. 4. Time variation ofthe number density ofozone during the photochemical generation of ozone and after addition of CI3CF vapor and methane. After the equilibrium concentration of ozone had been reached at point C, 6.0 mTorr of C13CF vapor were leaked into the chamber, and the ozone density dropped to the new equilibrium at D. At this point, 5.1 mTorr ofmethane were introduced into the chamber, and binding ofatomic chlorine in HCI molecules manifested itself in an increased ozone concentration. The addition of methane was repeated at the points E, F, G, H, and I.
427
Volume 168, number 5,6
PHYSICS LETTERS A
1.5 I
~c cm
0 C
I
lutants such as rocket exhausts on the ozone layer. Such studies are currently underway.
I
4 C12
1 94x10~
Acknowledgement
-4
~
7 September 1992
1.0
C 0
rence Weadvice Livermore wish toDr. acknowledge National the encouragement of Harry pert Darlington of Ralph IV of the R. Laboratory. Wuerker Ozone Society, andThe Dr.help the Glenn exRosenthal of UCLA, and Dr. Darwin Ho of Law-
~ 0.5 ~
C C 0 0
ci,
C 0 N 0
0.0
I,,,,
0
2
I,,,,
I,,,,
I
.
6
I
8 densIty of methane [1014 cm3] 4
10
Fig. 5. Equilibrium ozone concentration versus number density of methane.
the relatively large rate constant (1.4 x 10_b cm ~ s—’) for the reaction ofmethane with atomic oxygen [10]. Methane and hydrocarbons, in general, are potential greenhouse gases and the injection of large quantitietrof such gases into the atmosphere requires further careful studies.
4. Summary We have presented an experimental procedure by which the catalytic destruction of ozone by CFC can be studied in a controlled laboratory environment. It helps to focus on the essential physics and is useful as a check of computer modeling. Future expenments will include more species and heterogeneous reactions in order to systematically approach the more complex atmospheric situation. The demonstration of the catalytic reactions will allow us to study methods of slowing such reactions. The first one is to introduce methane to scavenge the chlorine radical. Our experiments support that possibility and more work is needed to assess its practicality. Another is to introduce electrons to form negative chlorime ions whose interactions with ozone are slower by several orders of magnitude. An experiment of injecting electrons through a current channel is being conducted. A further use of this chamber is to test various CFC substitutes or the effects of specific poi-
428
Marvin Drandell is very much appreciated. This work is supported by the National Science Foundation, the Ozone Society, and the UCLA Department of Physics.
References [1] G.M.B. Dobson and D.N. Harrison, Proc. R. Soc. A 110 (1926) 660. [2] S. Chapman, Mem. R. Meteorol. Soc. 3(Oxford (1930)103. [3] R.P. Wayne, Chemistryofatmospheres Univ. Press, Oxford, 1985). [4] J.G. Anderson, D.W. Toohey and W.H. Brune, Science 251 (1991) 39. [5] M.J. Molina and F.S. Rowland, Nature 249 (1974) 810. [6] A.Y. Wong, J. Steinhauer, R. Close, T. Fukuchi and G.M. Milikh, Comm. Plasma Phys. Controll. Fusion 12 (1989) 223; A.Y. Wong, R. Wuerker, J. Sabutis, R. Suchannek, C.D. Hendricks and P. Gottlieb, Ion dynamics and ozone, in: Proc. mt. Workshop on Controlled active global experiments (CAGE), Varenna, September 1990, eds. E. Sindoni and A.Y. Wong (Editrice Compositori, Bologna, 1991). [7] R.J. Cicerone, 5. Elliot and R.P. Turco, Scinece 254 (1991) 1191. [81K.T. Tsang, D.D.-M. Ho, A.Y. Wong and R.J. Siverson, in: Proc. Workshop on Controlled active global experiments (CAGE), Varenna, September 1990, eds. E. Sindoni and A.Y. Wong, (Editrice Compositori, Bologna, 1991) pp. 143— 156; D.D.-M. K.T. Tsang, A.Y. Wong and R. Siverson, in: Proc. mt. Ho, Workshop on Controlled active global experiments (CAGE), Varenna, September 1990, eds. E. Sindoni and A.Y. Wong (Editnice Compositori, Bologna, 1991) pp. 157— 174. [9] L.T. Molina and M.J. Molina, J. Geophys. Res. 91(1986) 14501; G. Brasseur and S. Solomon, Aeronomy of the middle atmosphere (Reidel, Dordrecht, 1986). [10] D. Ho, Private communications. [11] W.B. DeMore et al., JPL-Pub. 85-37 (1985).
PHYSICS LETTERS A
Catalytic destruction of ozone by chiorofluorocarbons and partial restoration by methane in large laboratory experiments A.Y. Wong, R.G. Suchannek and R. Kanner Department ofPhysics, University of Cal(fornia, Los Angeles, LosAngeles, CA 90024-1547, USA Received 15 June 1992; accepted for publication 25 June 1992 Communicated by M. Porkolab
The catalytic destruction ofozone by CFC was quantitatively measured in alarge laboratory device which has controlled ultraviolet radiation and gas composition.Experimental results obtained in a pureoxygen atmosphereagreed with theoretical estimates within a factor of two. A destruction ratio of one chlorine to 4 x 1O~ozone molecules was found. The introduction of methane reduced the density ofatomic chlorine resulting in a higher equilibrium ozone density. Even with a methane density six times the CFC density, only partial restoration of ozone is possible.
1. Introduction Stratospheric ozone is ofcritical importance to the earth’s ecology. Ozone has strong absorption bands in the ultraviolet region of the solar spectrum at wavelengths ranging from 240 to 290 nm. This portion of the solar radiation, if transmitted through the atmosphere, would have damaging effects on living organisms on the surface of the earth, The first photoelectric measurement of the amount ofozone in the atmosphere was made by Dobson and Harrison [1]. The nonuniform distribution of the ozone in the atmosphere is maximum in the stratosphere (at an altitude of approximately 25 km, depending on the latitude). The first theoretical explanation for the ozone layer was given by Chapman [2] who proposed a pure-oxygen photochemical model for the generation and destruction of ozone. His reaction scheme is described by the following set of equations:
Equation (2) describes the formation of an ozone molecule in a three-body collision of an oxygen atom with an oxygen molecule and a third collision partner M. The third reaction is the photodissociation of ozone. Equation (4) is the loss of one oxygen atom and one ozone molecule in a collision leading to the formation of two oxygen molecules. The last reaction is the loss of two oxygen atoms in a three-body collision resulting in the formation of one oxygen molecule. Although Chapman’s model can explain the formation of the ozone layer, the pure-oxygen model leads to calculated values for the ozone abundance in the atmosphere which are higher than experimental results [3]. It was concluded that an additional loss mechanism is affecting the ozone concentration which is not included in the pure oxygen-model. Researchers have been considering catalytic reactions of trace constituents of the atmosphere with ozone as an additional loss mechanism. The simultaneous
02+ h v—O + 0
(1)
observation [4] of ozone depletion and increased concentration of chlonne oxide in the stratosphere
O + 02+ M-+03 + M,
(2)
suggested that catalytic reactions of chlorine atoms
O 3+ h
(3)
with following The ozone molecules reaction contribute chain hastobeen the loss proposed ofozone. as
0+03-4202,
(4)
a dominant catalytic cycle:
0+O+M~O2+M.
(5)
Cl+O3~ClO+O2,
V*
~+
2,
0375-9601/92/S 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
(6) 423
Volume 168, number 5,6
PHYSICS LETIERS A
(7)
ClO + O—’Cl + 02.
The net effect of reactions (6), (7) is the conversion of ozone molecules into oxygen molecules. Molina and Rowland [5] showed that atomic chlorine in the contemporary atmosphere primarily originates from the photolysis of two chlorofluorocarbons C13CF (CFC-11) and C12CF2 (CFC- 12). Both substances are produced on an industrial basis and have no natural origin. They are very inert and volatile. Their estimated lifetime in the atmosphere ranges from 40 to 150 years. They are released at the surface of the earth and transported by convection into the stratosphere, where the exposure to ultraviolet radiation leads to their photodissociation into atomic chlorine and the radicals C12CF and C1CF2. The atomic chlorine can then participate in the catalytic reactions (6), (7) which destroy ozone. Because of the large reservoir of CFC already existing in the atmosphere and the continued emission of CFC or related products, it might be necessary to consider mitigation measures. One method [6] suggested the conversion of chlorine atoms into negative ions to reduce their reactivity. Another method [7] is to use hydrocarbons to scavenge the chlorine radical. In order to conduct controlled studies ofcatalytic destructive processes and mitigation measures
ozone generator
02
~
7 September 1992
in the upper-atmosphere, we have built a laboratory chamber of sufficient dimensions in which ozone and reactive species have sufficient lifetimes to react repetitively. Simplified theories such as the Chapman model can be examined first and the concentration of minority species can be controlled. It is expected that in subsequent experiments more species will be included to make our laboratory modeling more realistic and to compare our results with computer modeling [81. This paper reports our first attempt of using a laboratory chamber to model critical concepts; however, the laboratory experiments were not designed to fully simulate the stratosphere.
2. The reaction chamber A schematic drawing of the reaction chamber is shown in fig. 1. The chamber has an inner diameter of 1.40 m, and its overall length including the two extensions is 4.03 m. The chamber can be evacuated by a turbomolecular and a rotary vane pump to a pressure of 10—s Torn For operation under static conditions, the pumps can be valved off with a gate valve. The main vessel is equipped with several types of vacuum gauges to permit pressure measurements over the range from hundreds of Torr to 10—i Torr.
v.u.v. quartz
window
window
a Hg lamp
/
Hg
fl
ll
control photocell
r.IIOCtOqS
V~U•V~
00
tank
CFC
light source
R.F. discharge tub.
Fig. 1. Schematic drawing of the reaction chamber. The chamber has an inner diameter of 1.40 m, and length 4.03 m. The chamber can be evacuated to pressures ofthe order of 10~Tort. Two 140 W mercury lamps provide the ultraviolet radiation for studies of photochemical reactions. Absorption of the 253.7 nm line emitted by a low power mercury lamp is used to measure the ozone concentration in the chamber.
424
Volume 168, number 5,6
PHYSICS LETTERS A
7 September 1992
An ozone generator is connected to the chamber, which can produce a mixture of 2.5% ozone and 97.5% oxygen at chamber pressures ranging from a few mTorr to hundred Torr. This external injection of ozone is mainly for calibration purpose. Internal solar simulators are used to generate ozone by their radiation in the UV range. The ozone concentration n in the chamber is determined from absorption measurements of the 253.7 nm line of mercury, which is almost at the center of the Hartley band of ozone. The ozone density n can be obtained from the equation / = ~exp ( an), (8)
the chamber for spectroscopic investigations in the visible, ultraviolet, and vacuum-ultraviolet region of the spectrum. A mass spectrometer connected to the chamber allows independent determinations of the reaction products.
where I is the optical path length of the analyzing beam in the chamber, a is the absorption cross section, and I and 1o are the transmitted and the unattenuated intensity, respectively. Absorption cross sections for this line have been measured by several authors [9]. For absorption measurements, a light beam emitted from a low pressure mercury lamp is passed through the reaction chamber, and the transmitted intensity of the 253.7 nm component is detected with a monochromator and a photomultiplier tube. The light beam is chopped at a frequency of 215 Hz, and before the beam enters the chamber, a portion ofits intensity is deflected by a beam splitter into a control monochromator, which monitors the lamp intensity at the 253.7 nm line. A lock-in amplifier locked to the chopping frequency of the light beam is used to determine the ratio of the photomultiplier signal to the output voltage of the control monochromator. Measuring this ratio eliminates fluctuations in the absorption measurements caused by the slight variations in the intensity of the light source. For studies of photochemical processes, the reaction chamber is provided with two 140 W mercury lamps (Light Source Inc. Model GSL 1524 T5 VH/HO) which are the solar simulators. The lamps are mounted inside the chamber and have significant intensity at the 185 nm line. The lamps are operated with 60 Hz line voltage, and their intensity is therefore modulated with 120 Hz. The high selectivity ofthe lock-in amplifier prevents any interference of the mercury lamps with the intensity measurements of the analyzing beam. Several portholes in the vacuum envelope of the chamber permit access of optical spectrometers to
sure regime, the ultraviolet light sources were started when the reaction chamber was still evacuated in order to avoid the high start-up voltage ofthe mercury lamps at almost critical pressures. This procedure was followed during the first measurements. During later experiments, we were able to start the light sources when the chamber was already filled with 10 Torr of oxygen. When oxygen was leaked into the chamber in the presence of ultraviolet light, the transmitted intensity ofthe 253.7 nm line in the analyzingbeam decreased exponentially with the ozone density n according to eq. (10). A plot of the transmitted intensity of this line as a function of time yields the time evolution of the ozone density as shown in fig. 2. At point A, the flow of oxygen into the chamber was started, and the sharp drop of the transmitted intensity indicated a sharp rise in the formation of ozone
—
~•
Expenmental procedures and results
The photochemical generation of ozone was measured at oxygen pressures from 0.5 to 10.1 Torr when the chamber was valved off from the pumps. Since electric breakdown voltages are very low in this pres-
___________________________________
A 6
~
2
8 B ~ 00
300 t me
m
Fig. 2. Time evolution ofthe number density ofozone during the photochemical generation and subsequent destruction of ozone by trichlorofluoromethane. The number densities were derived from absorption measurements ofthe 253.7 nm line.
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according to the reactions in eqs. (1) and (2). At B, a pressure of 10.1 Torr was reached, and the flow of oxygen was stopped. The ozone density continued to increase until an equilibrium was reached at point C. From measurements of the time rate of change of the ozone density and its equilibrium values we can deduce in a self-consistent manner the rate of generation and the rate of destruction in response to various additions of trace constituents or change in the environment. In addition to the ozone losses describes by eqs. (3)—(5), ozone was lost in the reaction chamber during collisions of ozone molecules with the chamber wall. This loss rate was measured by turning the solar simulator off. A typical loss of ozone density 9 (cm—3 s~),which duetothe wall effectwas 6.5x10 is much less than the typical reaction rates of 3 ~ loll (cm—3 s~)among species inside the chamber, and therefore permits repetitive reactions to occur. The equilibrium ozone concentration at C in fig. 2 was 4.6 x l0’~cm—3, i.e. approximately 0.1% of the 02 molecules were converted into ozone molecules. At point C of fig. 2 approximately 3 mTorr of chlorofluorocarbon (also known as trichlorofluoromethane C1 vapora were into ber, and we 3CF) observed rapid leaked decrease of the the chamozone density. This loss of ozone was observed only when C1 3CF vapor was irradiated by ultraviolet light from the solar simulator. We deduced that the ozone destruction was caused by the reaction of ozone with products ofphotodissociated Cl~CFmolecules. As it had been proposed for the generation of atomic chlorine in the stratosphere, the photolysis of C13CF by ultraviolet radiation resulted in atomic chlorine and the radical C12CF. The ozone loss was caused by reactions of atomic chlorine with ozone according to eq(6) Since atomic chlorine is restored in the subsequent reactions (7) of the product molecule ClO, the relatively small addition of C13CF to the gases in the reaction chamber significantly reduced the ozone concentration. The catalytic destruction of ozone by atomic chlorine led to the establishment of a new equilibrium of ozone generation and destruction at point D. The ozone concentration at D was reduced to 5.7 x 1 013 cm which is only 12% of the concentration prior to the addition of C13CF. A plot of ozone concentration n as function of the density of ~,
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7 September 1992
C13CF is shown in fig. 3. The ozone concentrations were measured at the equilibria between ozone generation and destruction, which we observed for increasing amounts of C13CF vapor in the reaction chamber. The partial pressure of C13CF was measured with a differential capacitance manometer (MKS Instruments Inc., Model 1 2OAD), which permitted measurements of pressure changes of the order of 3 x I 0~Torr when the pressure in the chamber was 10.1 Torr. At the lowest measurable change in partial pressure, 0.3 mTorr, the curve in fig. 3 exhibits a very sharp drop of the ozone concentration n, followed by a slower decrease at higher CFC densities n~.The data points could be fitted by a function of the form. n = 1/ (An~+B) (9) where the coefficients A and B were obtained from a least squares fit and the constraints that the function passes through the data point for n~—0.This constraint was chosen because the data point for = 0 was obtained with much higher accuracy than all other points. The following values were obtained for A and B: A=l.833 (10_28 cm6), B=O.16l 3). The solid line in fig. 3 was obtained (10~14 from eq.cm(11) with the above-given values for the coefficients.
We can explain the functional dependence of the equilibrium ozone concentration n on the number
c~’ ~ C t H -4
0 C
N 0
2 0
0 _____________________________ 0.0 0.5 1.0 1.5 2.0
density of CCI3F [1014 cm —3 I Fig. 3. Equilibrium ozone concentration versus density of tnchlorofluoromethane. The ozone concentrations were measured at the equilibria between photochemical ozone generation and destruction for increasing amounts of CI3CF in the reaction chamber.
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density n, of CFC molecules as follows: dn/dt=k2Jn~2—v~n—k~nnc1,
A plot of the ozone density as function of time is shown in fig. 4. After ozone was photochemically (10)
where k2 n~2is the quantum yield for the generation of ozone, v~describes the wall losses, k~is the rate constant for ozone destruction by chlorine atoms, and n02 and n~,are the number densities of molecular oxygen and atomic chlorine, respectively; J is the photon flux. Under equilibrium conditions dn/dt = 0,
(11)
we find the relation
=
(
k~nc,/n~ ~ + ~ k2Jn~2 k2Jn~2)
7 September 1992
—1
(12)
which has the same form as eq. (9). Values for k2Jn~2were obtained from measurements of the slope dn/dt at the beginning of the ozone generation when ozone losses are still negligible, and values for ~a/flc were obtained from measurements of the slope of n after injection of trichlorofluorocarbon vapor into the reaction chamber. For k~we used a published rate constant, the value3ofs’ which ternandatweroom obtained perature is 1.189 x 10—” cm— v,~,from the ozone decay rate after shutting off the ultraviolet light sources. With these parameters, we obtained for the coefficients A and B the following values: A=l.08 (1028 cm6), B=0.072 (10_14 cm3). The calculated values of A and B differ from the measured values by a factor of approximately 2. The theoretical estimate according to eq. (12) is shown in fig. 3 as a dotted curve. From our measurements of ~ we can deduce the multiplicative destructive factor of ozone molecules by Cl radical. Using the average ratio of n!n~~ 1 we find that one chlorine atom destroys up to 4x l0~ozone molecules. If the photodissociation of C1 3CF in the reaction chamber leads to the catalytic destruction of ozone by atomic chlorine, it should be possible to reduce the ozone destruction by adding hydrocarbons to the chamber, which react with atomic chlorine to form the less reactive HC1 molecule. HC1 acts as a sink for atomic chlorine. Binding chlorine atoms in this molecule reduces the number of free chlorine atoms which can destroy ozone. This effect was indeed observed.
generated in the reaction chamber, approximately 6.0 mTorr of C13CF vapor were leaked into the chamber in the presence ofultraviolet radiation. The addition of C13CF, which is indicated in fig. 4 by point C, caused the ozone concentration to decrease. After a new equilibrium concentration of ozonewas reached at D, we added approximately 5.1 mTorr of methane to the gases in the chamber, and, as expected, the ozone density began to rise and leveled out at a higher value. We leaked again 5.3 mTorr of methane into the chamber at point E and repeated the procedure successively until the partial pressure of methane reached 30 mTorr. The rise of the equilibrium ozone density with increasing methane density is shown in fig. 5. We note that it takes much higher methane densities to scavenge a given density of chlorine atorns. Assuming the linear relationship to hold it would take 136 mTorr of methane to scavenge completely the chlorine atoms and restore the ozone to its initial density. The density ratio of methane! chlorine would be 5.4 X 1 07, and the density ratio of methane/CFC would be approximately 23.found A possible explanation for this high ratio can be in I
‘~
I I
I
I
6
H _
Q) C
2 0 N 0
0
i....i 100
200
300
400
time [m n] Fig. 4. Time variation ofthe number density ofozone during the photochemical generation of ozone and after addition of CI3CF vapor and methane. After the equilibrium concentration of ozone had been reached at point C, 6.0 mTorr of C13CF vapor were leaked into the chamber, and the ozone density dropped to the new equilibrium at D. At this point, 5.1 mTorr ofmethane were introduced into the chamber, and binding ofatomic chlorine in HCI molecules manifested itself in an increased ozone concentration. The addition of methane was repeated at the points E, F, G, H, and I.
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1.5 I
~c cm
0 C
I
lutants such as rocket exhausts on the ozone layer. Such studies are currently underway.
I
4 C12
1 94x10~
Acknowledgement
-4
~
7 September 1992
1.0
C 0
rence Weadvice Livermore wish toDr. acknowledge National the encouragement of Harry pert Darlington of Ralph IV of the R. Laboratory. Wuerker Ozone Society, andThe Dr.help the Glenn exRosenthal of UCLA, and Dr. Darwin Ho of Law-
~ 0.5 ~
C C 0 0
ci,
C 0 N 0
0.0
I,,,,
0
2
I,,,,
I,,,,
I
.
6
I
8 densIty of methane [1014 cm3] 4
10
Fig. 5. Equilibrium ozone concentration versus number density of methane.
the relatively large rate constant (1.4 x 10_b cm ~ s—’) for the reaction ofmethane with atomic oxygen [10]. Methane and hydrocarbons, in general, are potential greenhouse gases and the injection of large quantitietrof such gases into the atmosphere requires further careful studies.
4. Summary We have presented an experimental procedure by which the catalytic destruction of ozone by CFC can be studied in a controlled laboratory environment. It helps to focus on the essential physics and is useful as a check of computer modeling. Future expenments will include more species and heterogeneous reactions in order to systematically approach the more complex atmospheric situation. The demonstration of the catalytic reactions will allow us to study methods of slowing such reactions. The first one is to introduce methane to scavenge the chlorine radical. Our experiments support that possibility and more work is needed to assess its practicality. Another is to introduce electrons to form negative chlorime ions whose interactions with ozone are slower by several orders of magnitude. An experiment of injecting electrons through a current channel is being conducted. A further use of this chamber is to test various CFC substitutes or the effects of specific poi-
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Marvin Drandell is very much appreciated. This work is supported by the National Science Foundation, the Ozone Society, and the UCLA Department of Physics.
References [1] G.M.B. Dobson and D.N. Harrison, Proc. R. Soc. A 110 (1926) 660. [2] S. Chapman, Mem. R. Meteorol. Soc. 3(Oxford (1930)103. [3] R.P. Wayne, Chemistryofatmospheres Univ. Press, Oxford, 1985). [4] J.G. Anderson, D.W. Toohey and W.H. Brune, Science 251 (1991) 39. [5] M.J. Molina and F.S. Rowland, Nature 249 (1974) 810. [6] A.Y. Wong, J. Steinhauer, R. Close, T. Fukuchi and G.M. Milikh, Comm. Plasma Phys. Controll. Fusion 12 (1989) 223; A.Y. Wong, R. Wuerker, J. Sabutis, R. Suchannek, C.D. Hendricks and P. Gottlieb, Ion dynamics and ozone, in: Proc. mt. Workshop on Controlled active global experiments (CAGE), Varenna, September 1990, eds. E. Sindoni and A.Y. Wong (Editrice Compositori, Bologna, 1991). [7] R.J. Cicerone, 5. Elliot and R.P. Turco, Scinece 254 (1991) 1191. [81K.T. Tsang, D.D.-M. Ho, A.Y. Wong and R.J. Siverson, in: Proc. Workshop on Controlled active global experiments (CAGE), Varenna, September 1990, eds. E. Sindoni and A.Y. Wong, (Editrice Compositori, Bologna, 1991) pp. 143— 156; D.D.-M. K.T. Tsang, A.Y. Wong and R. Siverson, in: Proc. mt. Ho, Workshop on Controlled active global experiments (CAGE), Varenna, September 1990, eds. E. Sindoni and A.Y. Wong (Editnice Compositori, Bologna, 1991) pp. 157— 174. [9] L.T. Molina and M.J. Molina, J. Geophys. Res. 91(1986) 14501; G. Brasseur and S. Solomon, Aeronomy of the middle atmosphere (Reidel, Dordrecht, 1986). [10] D. Ho, Private communications. [11] W.B. DeMore et al., JPL-Pub. 85-37 (1985).