ULTRAVIOLET ABSORPTION SPECTRUM OF METHYLCHLOROFORM IN THE VAPOR PHASE

VOL. 6, NO. 6 GEOPHYSICAL RESEARCH LETTERS JUNE 1979

ULTRAVIOLET ABSORPTION SPECTRUM OF METHYLCHLOROFORM IN THE VAPOR PHASE

N. Vanlaethem-Heur•e, J. Wisemberg and P.C. Simon

Institut d'A•ronomie Spatiale de Belgique, 1180 Bruxelles, Belgium

Abstract. Ultraviolet absorption cross sections cross sections may be observed in the range of

of methylchloroform CH3CC13 have been measured in stratospheric temperatures.

the wavelength interval 180-240 nm and tem-

perature range 210-295 K. Numerical values are Experimental given and compared with existing data at ambient

temperature. Photodissociation coefficients have The ultraviolet spectrum of methylchloroform been calculated for altitudes between 20 and in the 200 nm wavelength range has been studied 45 km and appear to ,be significantly lower than as a function of temperature with a classical generally considered. The substantial decrease in single beam equipment (deuterium light source, absorption cross sections observed at low tem- 1 m model 225 McPherson monochromator, absorption peratures and k > 210 nm has only a minor cell, photoelectric recording via a EMR type influence on overall photodissociation coeffi- 542 P-09-18 multiplier tube). A complete descrip- cients. Relative contributions of photo- tion of the experimental device has been given dissociation and reaction with OH radicals as previously by Wisember• and Vanlaethem-MEur•e

potential sinks for methylchloroform at strato- (1978).

spheric altitudes are briefly discussed. As condensation conditions can restrict the use of fairly high gas pressure at low tem- Introduction peratures and consequently the access to low

absorption cross sections, a refrigerated stain- Amongst the numerous chlorocompounds of less steel absorption cell with a 2 m optical

industrial origin released to the atmosphere and path •s been conceived. It can be evacuated down

considered as potentially harmful for strato- to 10 torrs by an ionic pump which prevents any spheric ozone, methylchloroform (1-1-1-tri- contamination by organic materials. Temperature

chloroethane, CH3CCi•) , whose world production regulation down to 220 K is achieved by circula-

will probably reach • million tons by 1980, has tion of cooled methylcyclohexane through a double been suggested to play an ever increasing role. jacket. Thermic equilibrium is usually obtained Substantial destruction of this molecule occurs after 3-4 hours, as revealed by a temperature in the troposphere through reaction with OH gradient inferior to 2 K at 220 K between two radicals, but accurate assessment of mean tropo- thermic resistors placed on the inside walls, at spheric residence time is still in dispute. the ends of the absorption tube.

Depending on the values adopted for the Gas pressure is initially measured, and its experimental rate constant of hydroxyl attack and decrease followed during a refrigeration process, time average hydroxyl concentration, the proposed by a capacitance manometer MKS Baratron directly residence time ranges from 3 to 7 years (Chang coupled to the absorption cell. The actual gas and Kaufman, 1977; Derwent and Eggleton, 1978). temperature is assessed by considering both the HoweVer, simultaneous consideration of global conditions prevailing at the cell walls, and the emissions figures for methylchloroform (Neel¾ and value deduced from the pressure decrease, Plonka, 1978) and recent monitoring data suggests according to the perfect gases law.

a value as high as 8-11 years (Singh, 1977a,b, Determination of the absorption cross section Chang and Penner, 1978). is made after several sequential recordings of

I n such conditions, a 10-15% penetration of the incident and absorbed fluxes measured in the ground released methylchloroform is to be same temperature conditions.

expected at the tropopause level, and significant MEthylchloroform was kindly provided by Solvay steady state ozone depletion values could be due and Co., with a gas chromatography certified to photodissociation of this compound (McConnell purity of better than 99%, and was used after ß and Schiff, 1978). thorough degassing, without further purification.

The reliability of such predictions is

strongly dependent on the photodissociation Results and discussion

pattern adopted for methylchloroform. Until now,

only two sets of measurements of absorption cross At ambient temperature, measurements have been sections were available (Robbins, 1976; F.S. performed at working pressures ranging from 0.1 Rowland, unpublished data, 1977) : the proposed to 30 torr, and Beer's law was found to hold in values disagree, in the whole wavelength range of all cases. In such conditions, appreciable absorp- stratospheric interest, by a factor close to two tion (more than 10%) is easily observed for beyond 200 nm. wavelengths between 182 and 240 nm, so that We report here a new investigation of the absorption cross sections can be determined with ultraviolet absorption spectrum of methyl- good accuracy (ñ 2%).

chloroform between 180-240 nm. MEasurements have As can be seen on fig. 1, our results are in been completed down to 210 K, as it is now fair agreement with those reported by Robbins clearly established that in the case of halo- (1976), but significantly lower than the values carbons, a significant decrease of absorption proposed by Rowland (unpublished data, 1977).

Determinations have been extended to low

Copyright 1979 by the American Geophysical Union. temperature conditions (272 K, 252 K, 220 K) : as

Paper number 9L0389.

0094-8276/79/069L-0389501. O0

451

452 Vanlaethem-Meur•e et al.: Methylchloroform in the Vapor Phase

_17

• 10 -18

z 0

u') 0-19

o 1

z o

n- 20 o 10-

10 -21 ₁

I I I I

.i- 4.

.i.

ROBBINS 1976 ] 295K

ROWLAND 1977 THIS WORK

,,i.

,,i-

I

CH3CCI 3

2521"

272K

80 190 200 210 220 230 240

WAVELENGTH (nm)

Fig. 1. Absorption cross sections of CH3CC13 vs. wavelength, as a function of temperature.

already observed in the case of other halocarbons (Chou et al, 1977; Wisemberg et al., 1977) absorp-

tion cross sections decrease with decreasing temperature, the largest effect being detected

near the absorption threshold. As temperature is 182 3150 given with a + 2 K accuracy, the overall o(T) 184 2800 value is to be considered with a + 4% confidence 186 2500 interval. At temperatures lower than 250 K, vapor 188 2200 pressure limitations considerably lower the _{190 1920} maximum working pressure and prohibit direct _{192 1635} determinations in the region of low absorption.

However, the analysis of the absorption cross 194 14OO section vs. temperature relationship at a given 196 1180 wavelength shows an exponential decrease, so that 198

interpolation for other temperatures and extra- 200 polation down to 210 K can be reasonably made to 202 determine numerical values at given temperatures. 204

Table 1 presents absorption cross-sections for 206 270, 250, 230 and 210 K and table 2 gives inter- 208 polated values at wavelengths corresponding to 210 wavenumber intervals generally used in strato- 212 spheric photodissociation calculations. In both 214 tables, blank space means that values at lower 216 temperature are the same as those at 295 K. 218 At long wavelengths (230-240 nm) and low

temperatures (210 K), absorption cross sections

are reduced down to 20-30% of their 295 K values.

However, in the region of high absorption, the 224 temperature effect vanishes progressively and 226 falls within the limits of experimental errors. 228 In this case, absorption cross-sections have been 230 deduced by linear extrapolation from values 232 directly observed at longer wavelengths in order 234 to provide a smooth transition with the tem- 236 perature independent part of the absorption 238

spectrum. 240

TABLE 1. Absorption (1021 cm2.molecule cross section -1) of CH3CC13

i(nm) 295 K 270 K 250 K 230 K 210 K

99O

810 810 810 810 810

658 655 650 645 640

520 510 502 495 487

400 387 379 370 360

308 295 284 274 265

240 227 216 207 198

168 156 147 139 132

120.O 110.O 102.O 95.0 88.5

86.0 77.5 71.5 66.0 61.O

60.O 54.O 50. O 46.O 42.5

41.5 37.4 34.2 31.2 29.0

29.5 25.7 23.O 20.6 18.6

20.5 17.4 15.3 13.5 11.8

14.8 12.2 10.4 8.90 7.60

10.2 8.35 7.10 6.05 5.15

7.OO 5.60 4.70 3.95 3.30

4.90 3.65 2.90 2.30 1.82

3.35 2.42 1.87 1.45 1.12

2.30 1.58 1.17 0.87 0.64

1.53 1.O1 0.73 O.51 0.36

1.02 O. 64 O. 48 O. 34 O. 24

Vanlaethem-Meur•e et al.: Methylchloroform in the Vapor Phase 453

TABLE 2. Absorption (1021 cm2.molecule-1). cross sections of CH3CC13

k(nm) 295 K 270 K 250 K 230 K 210 K

182.6 3050 184.3 2780 186. O 2500 187.8 2250 189.6 2000 191.4 1750 193.2 1520 195.1 1290 197. O 1080 199.O 880 201.O 725 203.0 590 205.1 460 207.3 355 355 209.4 2 58 2 58 211.6 190 187 213.9 128 123 216.2 84.0 78.1 218.6 54.O 48.6 221.O 35.8 31.1 223.5 23.3 19.6 226.0 14.8 12.O

228.6 9.OO 7.02

231.2 5.60 4.20 233.9 3.30 2.38

236.7 1.96 1.33

239.5 1.15 O.75

355 355

258 257

184 179

119 114

74.8 45.6 28.6 17.7 10.5 5.94 3.44 1.86 1.O2 O. 48

TABLE 4. Destruction rates of CH3CC1

radicals vs. altitude. 3 by OH

-3 -1 -1

Z(km) T(K) [OH] cm kl[0H] s k2[0H] s

given altitude, have been computed by the product of the absorption cross section with the relevant solar flux based on the discussion published by Simon (1978) and integrated over the 180-240 nm wavelength interval.

355 Table 3 lists the values obtained for 251 different altitudes, either for temperature 174 independent or dependent absorption cross 109 sections. Due to the significant increase of the 70.6 66.8 solar flux absorption at longer wavelengths, the 42.7 39.7 amplitude of the overall photodissociation coeffi- 26.3 24.2 cients between 20 and 35 km is mainly influenced

15.8 14.2 by the 200-210 nm interval contribution. Con- 9.18 8.14 sequently, consideration of temperature dependent

cross sections only results, in the case of 5.04 4.32

2.86 2.30 methylchloroform, _{of the} photodissociation in a reduction coefficients. of a few percent More

1.48 1.14 significant is the reduction of the values by a 0.75 0.53 factor close to two, if the absorption cross

0.37 0.23 sections obtained in this work are used instead of those proposed by Rowland (unpublished data, 1977).

It is currently admitted that in the wave- As mentionned earlier, the only chemical sink length and pressure ranges of stratospheric for methylchloroform is due to reaction with OH interest, the only significant primary photolytic radicals :

path for all halocarbons is the release of one C1

atom with a unitary quantum yield (Majer and CH3CCl 3 + OH CH2CCl 3 + H20 (2)

Simons 1964)

Two different expressions of the temperature-

CH3CC13 + hv CH3CC12 + C1 (1) been proposed until dependent rate constant now ß for this reaction have

The immediate reaction of the CH3CC13 radical k = 1 95 10 12 - with 02 releases another C1 atom leaving CH^CClO '

which zn turn can be subject to photolysis• The (•hang and Kaufman, 1977)

ultimate conditions fate has not yet been considered in detail of this radical in stratospheric k = 3 72 10 12 exp [- 1627/T] ' -

but it can be assumed that photodecomposition of (•atson et al, 1977)

methylchloroform results in the average produc-

tion of more than two, if not three, C1 entities. 50 • • •

Primary photodissociation coefficients J for a

TABLE 3. Photodissociation coefficient of CH3CC1

vs. altitude.

-1 -1 -1

Z(km) J (s ) 3 (s ) 3 (s ) c• (295K) a c• = f(T) c• Rowland

-10 -10

20 3.26 10 -10 3.17 10_8 5.69 10_8 25 3 85 10 -8 3.70 10 6.68 10_6 30 6 50 10 -7 6 21 10 -7 1 12 10_6

35 3 12 10 -6 -6 3.00 10_6 -6 5.37 10_5 ' 40 7 42 10_5 7.32 10_5 1.28 10_5

45 1.14 10 1.13 10 1.98 10

a

Temperature independant cross section.

3

'- 30

2O

exp [ 1331/T]

cm3.mol '1 -1

3 -1 -1

cm .mol .s

... k T(OH) / I / /

.... k'• (OH) I i /.'

- --.---- J (or(T)) / / / / -

-- -- J (Or ROWLAND) /' / //

_ /,:i/'/

••'- CH3CCI 3 SINK _

I I I I I

•o -s •o -8 lo -? lo -o •o -s LOSS RATE (s 4)

Fig. 2. Contribution of photodissociation and reaction with OH to stratospheric destruction

of CH3CC13 .

454 Vanlaethem-Meur•e et al.: Methylchloroform in the Vapor Phase

Destruction rates of CH3CC13 due to this reaction in the troposphere, Environmental Science and

have b•en evaluated for stratospheric conditions, Technology, 12, 317, 1978.

on the basis of currently admitted concentration Robbins, D.E. ß UV photoabsorption cross sections

profiles of OH (•fasseur, 1976) and listed as k 1 for halocarbons, Int. Conf. on the Strato- [OH] and k 2 [OH] in table 4. sphere and related problems, Logan (USA),

As can be seen on fig. 2, this mechanism 1976.

provides an efficient sink for CH•CCi• up to at Simon, P.C. ß Irradiation solar flux measurements

least 25 km, while at higher altitudes, photo- between 120 and 400 nm. Current position and dissociation - and consequently, C1 release- is future needs, Planet. Space Science, 26, 355,

the dominant process. 1978.

Singh, H.B. ß Atmospheric halocarbons ß Evidence

References in favor of reduced average hydroxyl radical

concentration in the troposphere, Geophys.

Res. Letters, 4, 101, 1977a.

Brasseur, G. ß L'action des oxydes d'azote sur

l'ozone dans la stratosphere, Th•se de Singh, H.B. ß Preliminary tropospheric HO concentrations estimation in the northern of average

doctorat, Universit• Libre de Bruxelles,

Aeronomica Acta A, 173, 1976. and southern hemispheres, Geophys. Res.

Chang J.S. and F. Kaufman, ß Kinetics of the Letters, •, 453, 1977b.

reactions of hydroxyl radicals with some Watson, R.T., G. Machado, B. Conaway,S. Wagner,

and Davis, D ß A temperature dependent kine- halocarbons : CB]FCi•, CB]F•C1, CH•CCi•, C•HCi^,

C•C1., J. Chem. PhyS. 66,z4989, •977• z •

Chang, J.S., and J.E. •nner, : Analysis of global budgets of halocarbons, Atm. Envir- onment, 12, 1867, 1978.

Chou, C.C., W.S. Smith, H. Vera Ruiz, K. Moe, G.

Crescentini, M. Molina and F.S. Rowland : The temperature dependence of the UV absorption

cross sections of CC12F 2 and CCi•F and their stratospheric significance, J. •hys. Chem.,

tics study of the reaction of OH with CH•C1F,

CHC1 F, CHC1F CH CC1 CH CF C1, CF C1C•C1

J. P•ys. Chem•i 81• 25•i 19•7. 2 2 2'

Wisemberg, J., N. Vanlaethem-Meurke, and P.C.

Simon : Ultraviolet absorption measurements of

halocarbons and other minor constituents of

stratospheric interest : About the signif- icance and consequences of temperature effects, IIId Conf. IAGA/IAMP, Seattle (USA),

81, 286, 1977. 1977.

Derwent, R.G., and A.E.J. Eggleton ß Halocarbon Wisemberg, J., and N. Vanlaethem-Meurke ß Mesure lifetimes and concentration distributions des sections efficaces d' absorption de calculated using a 2-D tropospheric model,

Atm. Environ., 12, 1261, 1978.

McConnell, J.C., and H.I. Schiff ß Methyl- chloroform ß Impact on stratospheric ozone, Science, 199, 174, 1978.

Neely, W.B., and J.H. Plonka ß Estimation of time-averaged hydroxyl radical concentration

constituants atmosphkriques dans 1 'ultra- violet ß description du syst•me experimental, Bull. Acad. Roy. Bel•ique• C1. Sci., 64, 31,

1978.

(Received February 27, 1979;

accepted March 9, 1979.)