Bleaching of paper pulp represents a major industrial use of chlorine and could provide an environmentally significant source of atmospheric halocarbons. The related global production of chloroform is estimated at 3 × 10^5 ton yr^(−1) and there could be additional production associated with atmospheric decomposition of perchloroethylene. Estimates are given for the production of methyl chloride, methyl bromide and methyl iodide, 5.2 × 10^6, 7.7 × 10^4, and 7.4 × 10^5 ton yr^(−1) respectively. The relative yields of CH_3Cl, CH_3Br and CH_3I are consistent with the hypothesis of a marine biological source for these compounds. Concentrations of other halocarbons observed in the atmosphere appear to indicate industrial sources

McElroy, Michael B.

Wofsy, Steven C.

Yung, Yuk Ling

Caltech Authors - Main

Volo 2 No. 9 GEOPHYSICAL RESEARCH LETTERS September 1975 
ATMOSPHERIC HALOCARBONS: 
A DISCUSSION WITH EMPHASIS ON CHLOROFORM 
Yuk Ling Yung, Michael B. McElroy and Steven C. Wofsy 
Center for Earth and Planetary Physics 
Harvard University, Cambridge, Massachusetts 
Abstract. Bleaching of paper pulp represents a major industrial use of 
chlorine and could provide an environmentally significant source of atmos- 
pheric halocarbons. The related global production of chloroform is 
estimated at 3 x 105 ton yr -• and there could be additional production 
associated with atmospheric decomposition of perchloroethylene. Esti- 
mates are given for the production of methyl chloride, methyl bromide 
and methyl iodide, 5.2 x 106, 7.7 x 10 4, and 7.4 x l0 s ton yr 'l respec- 
tively. The relative yields of CH3C1 , CH3Br and CH3I are consistent with 
the hypothesis of a marine biological source for these compounds. Concen- 
trations of other halocarbons observed in the atmosphere appear to 
indicate industrial sources. 
The atmosphere contains mall though detectable quantities of CC12 F2, 
CC13F, CH3C1 , CH3CC13, C2HC13, C2C14, CH3I , CC14, and CHC13, with 
concentrations as summarized in Table 1. The fluorocarbons, CC12F 2and 
CC13F , are industrial in origin, employed primarily as refrigerants and as 
aerosol propellants (Lovelock, 1971; Lovelock et al., 1973; Wilkniss et al., 
1973, 1974; Su and Goldberg, 1973; Rasmussen, 1975). Trichloroethylene 
perchloroethylene, and 1,1,1-trichloroethane are extensively used as sol- 
vents and dry-cleaning agents. Methyl chloride, CH3CI, and methyl iodide, 
CH3I , appear to be produced mainly by marine biological activity (Love- 
lock, 1975; Rasmussen, 1975) and there is evidence for an analogous 
source of methyl bromide, CH3Br (Rasmussen, 1975; Wofsy et al., 1975). 
Carbon tetrachloride, CC14, is used extensively as an intermediate in the 
manufacture of CC12F2 and CC13F. An uncertain fraction of the CC14 is 
inadvertantly released to the atmosphere and, with reasonable assumptions 
(Molina and Rowland, 1974a), can account for the abundance listed in 
Table 1, although there may be additional, natural, sources (Lovelock et 
al., 1973). Chloroform, CHC13, was used estensively as an anesthetic for 
almost 50 years after its discovery in 1831. In recent years it has been used 
mainly as an intermediate in the manufacture of penicillin, and various 
fluorocarbon refrigerants (Freon 22) and resins (notably Teflon). 
This paper is concerned primarily with CHC13. We shall attempt to 
define the magnitude and identify the nature of the source for this com- 
pound. There is an urgent need for a comprehensive study of atmospheric 
halocarbons, in light of the potentially important role which they may 
play in the chemistry of stratospheric ozone (Molina and Rowland, 1974, 
a, b; Wofsy et al., 1975). This paper describes an initial step towards this 
objective. It offers, in addition to the discussion of CHC13, some updated 
estimates for the magnitude of the surface source required in order to 
account for the measurements of CH3C1 and CH3I given in Table 1. 
Chloroform will be removed from the atmosphere primarily by reaction 
with OH, and similar remarks apply to CH3C1. Rate constants for the 
relevent reactions were measured recently at 296øK by Howard and Even- 
son (1975) and Davis e[ al, (1975). Their data, together with estimates for 
the relevent activation energies, were used to construct the rate constants 
listed in Table 2. The OH profile summarized in Figure 1 was computed in 
the manner described by McEltoy et al., (1974). The lifetime for CH3I, 
determined mainly by photolysis, was computed using cross sections meas- 
ured by Porret and Goodeve [i938). The lifetimes given i Table 2 
represent upper limits. The actual values could be somewhat less if there 
were important loss processes in addition to photolysis and reaction with 
OH. In like manner the sources for CH3C1 , CH3I and CHC13 given in Table 
2 represent lower limits and should be adjusted upwards if additional 
atmospheric sinks can be identified for these species. The sources for 
CH3C1 and CH3I given here may be compared with the source of7.7 x 104 
metric tons yr q derived earlier for CH3Br (Wofsy et al., 1975). 
Table 2 includes two estimates for the global production of CHC13. The 
Copyright 1975 by the American Geophysical Union 
Table 1 
Measurements of Halomethanes 
Species Mixing Ratio (v/v) Reference 
(10 '12 ) 
CC12F2 102 - 115 Lovelock (1974) 
CC13F 60 - 80 Will(niss eta!. (1973) 80 - 90 Lovelo•k (1974) 
CH3C1 400 Lovelock (1975) 550 + 50 Rasmussen (1975) 
CH3I 
CC14 
1.2 Lovelock (1973) 
71 + 7 Lovelock et al. (1973) 
75 + 8 Wilkniss et al. (1973) 
111 - 138 Lovelock (1974) 
CHC13 19 - 27 Lovelock (1974) 20 Rasmussen (1975) 
1.5 + 1.2 
c2 HC13 (Southern Hemisphere) 15 + 12 Lovelock (1975) 
(Northern Hemisphere) 
<1.5 
C2C14 (Southern Hemisphere) Lovelock (1975) 28 +9 
(Northern Hemisphere) 
24 +3 
CH3CC13 (Southern Hemisphere) Lovelock (1975) 65 +17 
(Northern Hemisphere) 
larger value would be appropriate if there were globally dispersed sources 
for the gas, i.e. if the abundance listed in Table 1 were repr, esentative of 
mean atmospheric onditions. The smaller value assumes that CHCI 3 is 
anthropogenic n origin and supposes that the source is restricted to popu- 
lated areas of the northern hemisphere. Wemay note that eWn•the smaller 
of the sources in Table 2 is significantly larger than the U.S. Tariff Com- 
mission's e timate for the total U.S. production of CHC13 in 1972, I x l0 s 
tons. The discrepancy is particularly serious if we recall that most CHC13 
produced industrially is consumed in the manufacture of other chemicals. 
If we adopt McCarthy's (1975• estimate for the inadvertent release rate, 
about 50%, we estimate a global source of atmospheric CHC13 of 1 x 10 s 
tons yr q , significantly less than either of the values in Table 2. It is hard 
to escape the conclusion that there must be sources of atmospheric CHC13 
in addition to those listed by the Tariff Commission. A summary of data 
on major industrial uses of chlorine is given in Table 3. As we may see 
below, the use of •hlorine in the bleaching of paper pulp shows most 
promise as a possibie source for the chloroform listed in Table 1. The 
mechanism suggested here, the haloform reaction, was suggested by Morris 
(1975) as the source of CHC13 and other haloforms produced uring the 
chlorination of municipal water supplies (Rook, 1974; Bellar et al., 1974). 
397 
398 Yung: Atmospheric Halocarbons 
Table 2 
Lifetimes and Sources of Halocarbons 
CHC13 
Mixing ratio (v/v) 
(10 -12 ) 
Major sink 
Rate constant (a) 
Lifetime (s) 
Source (if globally 
distributed) 
(106 metric ton/yr) 
Source (if spatially 
restricted, see 
text) 
(106 metric ton/yr) 
Global industrial 
source 
(106 metric ton/yr) 
20 
CHCI 3 + OH 
k= l•0x 10 r12 
e'630/T (b) 
1.0x 107 
0.99 
0.23 
see text 
CH 3 CI CH 31 C2 HC13 C2 CI 4 C 2 H 3 C13 
500 
CH3C1 + OH 
k= 1.69x 10 '12 
e-1066/T (c) 
2.8x 107 
5.2 
1.2 
CH3I + hp 
J= 6.0x 10 '6 
1.7x I0 s 
.74 
15 
C 2 HC13 + OH 
k=3x 10 '12(d) 
3.7x 10 s 
31 
1.o (h) 
.68 (f) 
1.0 (g) 
28 
C 2 C14 + OH 
k= 1.3 x 10 '12 (d) 
8.5 x 105 
31 
1.o (h) 
.61 (f) 
1.1 (g) 
45 
C2H3CI 3 + OH 
k= 8.2x 10 '13 
e-12 oo/T (e) 
9.5 x 107 
0.4 
.28 (f) 
.48 (g) 
CHF2C1 
CHF 2 C1 + OH 
k= 1.2x 10 '12 
e-1640/T (i) 
3.0x 108 
.036 (f) 
(a) Bimolecular rate constants, k, and dissociation rate, J, are in units cm 3s -1 and s -t respectively. (b) Howard and Evenson (1975) (at 296øK) combined with an estimate of activation e ergies. (c) Davis et al., (1975). 
(d) AdjUsted to the industrial source strength. These rates are consistent with those measured by Howard nd Evensen (1975) for other haloethylenes. (e) Estimate by C. Howard, private communication, 1975. 
(f) McCarthy, 1975. 
(g) McConnell, 1975. 
(h) Sources restricted to populated area nd planetary boundary layer (% ! km thick). 
(i) R. Atkinson, D.A. Hansen and J.N. Pitts, J. Chem. Phys. (in press, 1975). 
The reaction sequence begins with 
C12 + HCO3---> HOC1 + CI-+ CO2 (1) 
followed by 
I•OCl •_ i-I + + OCl- (2) 
in which C12 is hydrolyzed to HOC1 and OC1-. It proceeds through a series 
of complicated reactions involving dissolved organic material. These reac- 
tions may be written symbolically in the form 
CH3COR + 3HOC1 --> CC13COR + 3H20 (3) 
followed by 
CC13COR + H20--> CHC13 + RCOOH (4) 
Other reactions may lead to formation of additional halogenated species 
and it is interesting to note that a recent survey by the EPA (Symons, 
1975) found evidence for some 35 compounds in the mdnicipal waters of 
Miami, Florida. Thest compounds included CHC13, CHC12Br , CHC1Br2, 
20 
18 
16 
14• 
• 12.-- 
õ •o- 
6-- 
4• 
!x!O 5 
TEMPERATURE (OK) 
200 220 240 260 280 300 320 
-- 
2 4 6 8 !xlO s 2 
NUMBER DENSITY (crn '3) 
Figure 1' Model atmosphere and OH profile (see text). 
and CHBr3, and there are reasons to believe that a similarly rich suite of 
halogenated compounds would be f'orrrl•d uring the bleaching of paper 
pulp. The bleaching process, combining an abundant source of organic 
material with an ample supply of chlorine at high pH, would appear to 
provide an ideal medium for the production of the halocarbons of interest 
here. The yield of CHC13 in chlorination f water is about 1-3% by weight 
according to Symons' data (1975). This result represents a lower limit to 
the potential conversion efficiency since the production f haloforms in 
the treatment of municipal water supplies appears to be limited by the 
availability ofdissolved organic material. Aconversion efficiency as low as 
6% in the bleaching process would supply a global source of CHC13 of 
magnitude 3 x 10 $ tons year '1 and could account for the concentration 
of the gas given in Table 1. 
Decomposition f perchloroethylene, C12C = CC12, could provide an 
additional source for atmospheric CHC13. Perchlotoethylene is widely used 
as a solvent, with annual rates for release inexcess of106 tons (U.S. Tariff 
Commission). Howard and Evenson (1975) found that halogenated ethyl- 
enes react with OH at rates faster than 10'12cm3sec'1 to form the OH 
addition compound. The addition reaction should be faster than photolysis 
Table 3 
Production and major industrial uses of chlorine 
for the United States in 1974 (Klingnan, 1975). 
Major Uses Quantity (106 metric tons) 
Solvents 2.68 
Plastics and synthetics 2.19 
Paper products I 1.78 
Automobile fluids 2 .96 
Water and sanitation .59 
Agriculture .49 
Miscellaneous 1.1 8 
TOTAL 9.88 
IAccording to B. Slating of American Paper Institute, N.Y. this chlorine s used for 
bleaching paper pulp. 
2Mainly used to produce automobile anti-freeze ( thylene glycol) and anti-knock 
compounds (ethylene chloride). 
Yung: Atmospheric Halocarbons 399 
(Tyerman, 1969) and should be more efficient also than possible com- 
peting reactions with ozone. The fate of the radical CC12-CC12 OH has not 
been studied in any detail. A number of experiments however (Dahlberg 
et al., 1970; Dahlberg, 1969; Mathias et al., 1974; Haszeldine and Nyman, 
1959) suggest that dichloroacetyl hloride, HC12C-•'Oc1, should be a 
major product. Photolysis of dichloroactyl chloride 
hl• + HC12C-C•cO1 --> C  + CHC13 
should take place at wavelengths as long as 3600• (cf. Capey et al., 1968) 
and could pr,•vide a significant source of atmospheric CHC13 . A quantita- 
tive estimate for this source would however equire a careful analysis of 
competing loss processes for HC12 C-Cxci. The source strength estimates in 
Table 2 are consistent with industrial origin for C2C14, C2HC12, and 
CH3CC13 in the atmosphere. 
It is of interest to examine possible natural sources of halogenated 
compounds. The oceans contain 2 x 10 '2 , 7 x 10 's and 3-7 x 10 '8 parts by 
weight of chlorine, bromine and iodine respectively. Seaweeds have long 
been known for their ability to concentrate bromine and iodine. The 
enrichment factors, defined with respect o the concentrations in sea water, 
are typically in the range 2-10 for bromine and 103 - 3 x 104 for iodine 
(Hoppe, 1969). It appears that the concentration of chlorine in sea weed is 
similar to that in the overall marine environment (Shaw, 1962). Wofsy et al. 
(1975) estimated a marine production of methyl bromide of magnitude 
7.7 x 104 ton yr 'l and combining their data with the results of Table 2 
we find production rates by weight for CH3C1 , CH3Br and CH3I in the 
ratio 1:.015:.14, within the present limits of uncertainty, consistent with 
the halogen composition falgae.? There are no known natural sources for 
chloroform, although we may note that CHBr 3 is a major constituent in 
the essential oil of the red alga Asparagopsis taxiformis, which includes in 
addition, detectable amounts of CHC1Br 2, CHC1BrI, CHBr 2 I, CHBrI 2 , and 
CHI 3 (Burreson et al., 1975). The abundance of CHC13 listed in Table 1 
would supply a source of stratospheric chlorine which could account for a 
global reduction i  ozone by as much as 0.04%. Corresponding values for 
the reduction in ozone due to marine biological production of methyl 
chloride and methyl bromide are 0.8% and 0.5% respectively and the 
present day reduction due to CFC13 and CF2C12 is 0.6%. These considera- 
tions serve to emphasize the need for a comprehensive d finition of the 
atmospheric halogen budget. It is clear that the reservoir includes contribu- 
tions from a variety of natural and anthropogenic activities ranging from 
certain functions of marine algae to the preparation of paper, the fumiga- 
tion of agricultural soil and the large scale commercial application of 
aerosol sprays and refrigerants. 
Acknowledgement 
We are indebted particularly to J.C. Morris who shared readily with us 
his remarkable insights into the chemistry of chlorinated water. We thank 
C.L. Klingman for providing us with the contents of Table 3, and R. 
Rasmussen for making available his unpublished measurements. The work 
was supported in part by the Atmospheric Sciences Division of the 
National Science Foundation under Grant GA33990X and by the National 
Aeronautics and Space Administration under Grant NASA NSG 2031 to 
Harvard University. 
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accepted August 7, 1975o) 


Atmospheric halocarbons: A discussion with emphasis on chloroform

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