sulfate to total sulfur is increased only slightly in the plume compared to the flue-line samples. Dimethyl sulfate was not seen in the flue-line samples from these two power plants. This suggests that dimethyl sulfate either is not present in the flue line at 160 "C or is present only in the gas phase. In summary, the presence of strong mineral acidity in collected particulate samples will result in artifact formation of dimethyl sulfate in methanol extracts. The artifact can be avoided by neutralization of the sample with trimethylamine followed by GC-MS analysis of a methanol extract, by reaction of the sample with gas-phase ammonia followed by analysis of aqueous extracts of the sample for monomethyl sulfate and methylamine, or by determination of monomethyl sulfate in aqueous extracts before and after exposure of the sample to NH3(g). Neutralization of the sample by trimethylamine followed by analysis of methanolic extracts allows a direct determination of dimethyl sulfate. On the other hand, (CH30)2S02 is not stable in environmental samples unless stored at very low temperatures. Exposure of the sample to ammonia or a suitable primary amine immediately after collecton is a convenient field method which converts the unstable (CH30)2S02 to products which are more stable and allows determination of (CH30)2S02 in environmental samples which cannot be analyzed immediately or kept very cold after collection. Finally, the data in Table IV show that both (CH30)2S02 and CH3OSO3- can be determined by three independent methods with good agreement among the various analyses. This agreement provides strong evidence that the procedures for the determination of (CH30)&02 and CH30S03- are accurate. The data in Table V show that both (CH30)2S02 and CHBOSO3- are present in the plumes of the large oil- and coal-fired power plants studied. Dimethyl sulfate is a pollutant of potential importance from combustion of sulfur-containing fossil fuels. We are currently
studying the stability and formation of dimethyl sulfate in plumes. These studies should indicate the rate at which the carcinogen dimethyl sulfate is both formed and hydrolyzed in the environment. These data should assist in the assessment of the possible impact of dimethyl sulfate pollution on human health. Acknowledgment
Appreciation is expressed to P. T. Cunningham, Argonne National Laboratory, for providing the IR and acidity data and to Wayne Wells for technical assistance in the study. Literature Cited (1) Lee, M. L.; Later, D. W.; Rollins, D. K.; Eatough, D. J.; Hansen, L. D. Science 1980,207,186. (2) Hoffmann, G. F. Mutat. Res 1980,75,63. (3) Mackle, H.; Steele, W. V. Trans. Faraday Soc 1969,65,2053. ( 4 ) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Halow, I.; Bailey, S. M.; Schumn, R. H. NBS Techn Note (U.S ) 1968, No. 270-3. (5) Hansen, L. D.; Fisher, G. L. Enuiron Sei Technol. 1980, 14, 1111. (6) Cunningham, P. T.; Johnson, S. A. Science 1976,191,77. (7) Cunningham, P. T., Argonne National Laboratory, personal communication, 1981. (8) Eatough, D. J.; Richter, B. E.; Eatough, N. L.; Hansen, L. D. Atmos Enuiron , in press. (9) Maddalone, R. F.; Newton, S. F.; Rhudy, R. G.; Statnrok, R. M. J Air Pollut. Control Assoc 1979,29,626.
Receiued for reuiew March 6,1981. Reuised Manuscript Receiued J u l y 31,1981. Accepted August 31,1981 This work was supported by the USDOE (Contract DE-AC02-80EVl0405), by the Electric Power Research Institute (Contract RDlI54-1), by the Utah Division o j the American Cancer Society, and by a n American Chemical Society Diuision of Analytical Chemistry Fellowship to B.E.R. sponsored by the Society of Analytical Chemists of Pittsburgh.
NOTES
Methylchloroform. Cycles of Global Emissions M. A. K. Khalil" and R. A. Rasmussen Department of Environmental Science, Oregon Graduate Center, 19600 N.W. Walker Road, Beaverton, Oregon 97006
The global emissions of methylchloroform (CH3CC13),in addition to increasing steadily over the years, have also undergone substantial cyclic fluctuations (f-20% peak to peak). When these cycles are taken into account, the atmospheric observations of the rate of increase of CHjCC13 and the changes in the interhemispheric ratio of CH3CC13 concentrations come into close agreement with theoretically expected values. Introduction
The current concentrations of methylchloroform (CH3CCl3) exceed 100 pptv (10-l2) everywhere in the earth's lower atmosphere (troposphere). Increasing industrial production, use, and consequent direct emissions are the only known sources of CH3CCI3, which, coupled to a relatively long atmospheric lifetime of between 7 and 10 yr ( 1 , 2 ) ,account for 1506
Environmental Science & Technology
all of the CH3CC13 present in the earth's atmosphere. The rapid increases in the global emissions have recently led to concerns that the accumulation of CH3CC13 in the atmosphere would adversely affect the future global environment by causing a reduction of the stratospheric ozone layer which shields the earth's surface from biologically effective ultraviolet radiation. Large accumulations of CH3CC13 could also warm the earth's surface (enhanced greenhouse effect) by absorbing outgoing infrared radiation. In these environmental roles, CH3CC13 is similar to the well-known, man-made fluorocarbons CC13F (F-11) and CCl2F2 (F-12) which are still continuing to increase in the earth's atmosphere. One factor which makes CH3CC13 safer (per molecule emitted) than the fluorocarbons (F-11and F-12) is that it reacts with hydroxyl (HO) radicals, which exist naturally in the atmosphere, so that some of the CH3CC13 molecules are removed in the troposphere, thus never reaching the stratosphere where the ozone
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Chemical Society
layer is. Nevertheless, CH3CC13 is emitted in sufficient quantities (1.1 X 109 lb in 1978, for example ( 1 ) ) that it is currently being eyed with growing concern (3-5). The invention of the electron-capture detector by Lovelock made it possible to measure certain halocarbons (including F-11, F-12, and methylchloroform) by gas chromatography, at the low atmospheric concentrations (6).Measurements of CH3CCl3 have been made by Rasmussen (7), using the same reference standards, since 1975. These measurements show that the ratio ( R )of the CH3CC13 concentration at high (2.45') northern-hemisphere (NH) latitudes to the concentration a t high southern-hemisphere (SH) latitudes has undergone a curious, almost cyclical variation. Carefully compiled estimates of the global industrial emissions (8)of CH3CCl3 since 1951 have always been represented by an exponential of the form S = a exp(bt) (9, 10). When we looked at the relative residual, [S(estimated) - a exp(bt)]/[a exp(bt)], we found that the emissions also have a nearly cyclical variation superimposed on the exponential rise. Mass-balance calculations confirmed that the rather sizable fluctuations of the emissions (amplitude, -20%) do indeed agree quantitatively with the observed variation of the NH/SH ratio R , indicating that this long (observed) time series of CH3CCl3 concentrations spanning 5 yr is relatively accurate with regard to information on the response of the atmosphere to the changing patterns of human activities. Cycles of Global Emissions Consider first the estimates of past emissions (8) which we have divided into two periods: before 1963 and since 1963. The total global emissions between 1951 and 1963 are small compared to the amounts emitted every year since then. Emissions after 1963 are shown in Figure 1. We obtained the leastsquares estimate of the exponential function describing the yearly emissions (SO= aebt).The residual, A1 = (S So)/So, was found to be as shown in Figure 2. After further mathematical considerations, we derived the following expression for the emissions function (1963-1978) (11):
-
S
a exp(bt)[l
+ A1 + A,] + 4)
A, = 01 cos(&
+
A2 = 010 C O S ( U O ~ #Jo)H(t- T)
plicated to be of interest here, but, if A, is neglected, being a small effect, then R , is
+ +
+ + +
+ +
1 a0 sin(wt 4) a1 cos(ot 4) 1 bo sin(& i-+) b l cos(& 4) where ao, a l , bo, bl, and& are all_constantsdetermined by CY and the transport parameters in Q ( 1 1 ) (see also supplementary material). The a's and b's are 0 if the emissions are described by a simple exponential ( S O= a exp(bt); 01 0 in eq 2). Thus, the ratio R is expected to be a constant over the period of observations (1/1975-1/1980) if we ignore the fluctuations of the emissions. We have included eq 5 to point out that a source with a simple cyclical behavior is manifested in a more complicated variation of the observable concentrations or the ratio R. In Figure 3 we summarize our discussion and show the observed values of R since 1975 as well as the calculated values of R ( R,) over the same period. The transport times and lifetimes used to obtain the solution shown in Figure 3 are as follows: T~ = r2= 8 yr (see ref 12) (lifetime in regions 1and 2); 7 3 = 74 = 12 yr (lifetime in regions 3 and 4); 7 / 1 2 = 12 months (average transport time between regions 1and 2); 7 / 2 4 - 7I 13 N 3 months (transport within southern and northern hemispheres, respectively). Details of these calculations are given in the supplementary material. In 1979 we were able to obtain several thousand measurements of CH3CC13 at 42"s and 45"N which are included as monthly averages in Figure 3. During 1979 the average change
=Wo
R,
-
6.5
(1)
(2) (3)
where in eq 3 H(t - T ) = 0 if t < T , and 1if t 2 T . The values of these parameters were found to be the following: t = 0 at 1951,t = T a t 1972, a = 121.2 pptv/yr-l (number of molecules emitted per year divided by number of molecules in the entire atmosphere), b = 0.153 yr-l, (Y = 0.22, o = 0.94 rad yr-l, 4 = 1.91 rad, 010 = 0.12, wo = s/2 rad yr-l, and 40 = 4.3 rad. Global Observations and Theory
/-
4
4
0
CH3CC13 r e l e o s e d t o the atmosphere
"
to3
The global mass balance was written as where C, (concentrations) and S (sources) are vectors and 6 is a 4 X 4 matrix which describes the losses and transport of CH3CC13 in each of four regions of the atmosphere. Details are given as supplementary material. (See paragraph at end of text regarding supplementary material.) Region 1 consists of the atmosphere between the equator and 30°N latitude. Similarly, regions 2-4 represent the atmosphere for 0-30°S, 30-90°N, and 3O-9O0S, respectively. Most of the emissions of CH3CC13 take place in region 3, which is the "upper" half of the northern hemisphere. Equation 4 was solved by using eq 1, and R , = C3*/C4* was calculated for comparison with the measured values of R found by Rasmussen (7) at 45ON (region 3) and at 42"s or 90"s (region 4). The star designates theoretical estimates of the ratio or concentrations based on the emissions estimates. The final expression for R ,is too com-
s in m i l l i o n s of p o u n d s of
,P..J
:
-031
8
1
'
I
r
1
I
j
'
1
1
bd
A I(1)
c---.o
0 2 1 C o s ( O 9 4 t + 2 38)
1
I
1963
1965
0
2
1967 4
I
I
1969
6
I
1
1971 8
I
I
I
I
1973 10
/
I
I
1975 12
'
/
I
I
I 9 7 7 I978 14 I S
TIME (years)
Figure 2. The At cycle in the global emissions of methylchloroform.
Volume 15, Number 12, December 1981 ,1507
1.9 1
1
1
I
Conclusions
I
We have shown that, in addition to yearly increases, the global emissions of CH:ICCI:~ have undergone cyclical fluctuations. These fluctuations explain both the observed changes in the interhemispheric ratio ( K )of methylchloroform concentrations and the observed global rate of increase, whereas assuming the global emissions to be simply increasing exponentially is a t odds with observations. It is also noteworthy that the two observational variables which we chose for our study, namely, the ratio R and the rate of increase fl, are both insensitive to errors in absolute accuracy of measurements. Theoretical and experimental work is currently underway to study further the behavior and the balance of CH:$2Cl:{in the earth’s atmosphere. Acknowledgment
We thank Professor W. Zoller, R. Dalluge, D. Pierotti, J. Allwine, and R. Stordeur for their contributions to this project. Literature Cited (1) Jesson, J. P. I n “Proceedings of the NATO Advanced Study In-
I .3
I
1
1
1
TIME Figure 3. Change in the northern to southern hemisphere ratio of
CH~CCIJconcentrations: (0)theoretical, (A)experimental from 45’N and 90’s latitudes, and ( 0 )experimental from 45’N and 42’s latitudes (1979); solid line through the 1979 data is the least-squares estimate.
stitute on Atmospheric Ozone”; Aikin, A. C., Ed.; U.S. Department of Transportation: Washington, DC, 1980. (2) Crutzen, P. J.; Fishman, J . Geophy.~.Krs. h t t . 1977,4, 321-4. (3) McConnell, J. C.; Schiff, H. I. Science 1978, 199, 174. (4) Crutzen, 1’. .I.; Isaksen, I. S. A,; McAfee, J. R. J . Caophys. Rcs. 1978,83, 345-63. ( 5 ) Natinnal Academy of Sciences. “Stratospheric Ozone Depletion by Halocarbons: Chemistry and Transport”; NAS: Washington, DC, 1979. (6) Lovelock, J. E.;Maggs, R. cJ.; Wade, H.J. Nature (London) 1973, 241, 194. (7) Itasmussen, R. A,; Khalil, M. A. K.; Dalluge, R. W. Science 1981, 21 1 285-7. (8) Neely, W. B.; I’lonka, J. H. Enuiron. Sci. Techno/. 1978, 12, 317--21. (9) Singh, H. R. Grophys. Res. Lett. 1977,4, 101-4. (IO) Chang, tJ. S.; I’enner, J. E. Atmos. Enuiron. 1978, 12, 186773. (11) Khalil, M. A. K. Ph.11. Dissertation, Oregon Graduate Center, Reaverton, OR, 1979. (12) Khalil, M. A. K.; Rasmussen, It. A. Chemosphcre, in press. (13) Newell, R. E.; Vincent, D. G.; Kidson, ,J. W. ?’ellus 1969, 21, 641--7. ~
in R obtained by considering all of the monthly data (0.12 yr-l) is in agreement with the change obtained by considering only the January data at the south pole and 45’N (0.11 yr-l). The fluctuations of R during 1979 are probably related to seasonal variations in interhemispheric transport as found by Newell e t al. (13). Additional evidence for the significance of the source fluctuations (eq 1) comes from the observation that the global rate of increase of CH3CCI:I determined by (l/C)(dC/dt) = P
is on the average 13
* 1%yr-l (1/1975-1/1980)
(6)
based on the data shown in Figure 3 (6).T h e fl% reflects the estimate of the 90% confidence limit for the average 0. If one assumes that S = So = a exp(bt), the theoretically expected rate of increase is -15% yr-], which is outside the 90% confidence limits of the observed rate of increase. When the main cycle A1 is added, the theoretically expected rate of increase is -12.8% yr-’ between 1975 and 1980, in remarkable agreement with observations.
Received for rcjuic~u:February 23, 1981. Accepted August 17, 1981. This u’ork was supported in part by grants from the National Science Foundation (A TM78-09711, DI’P77-23468).
Supplementary Material Available: Three sections describing (I) cycles of glohal emissions, (11) details of calculations of R,, and (111) details of calculations of /3 and a diagram illustrating the parameters of the global model ( 1 3 pages) will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper or microfiche (105 X 148 mm, 24X reduction, negatives) may he obtained from Business Operations, Books and Journals Division, American Chemical Society, 1155 16th St., N.W., Washington, D.C. 20036. Full bibliographic citation (journal, title of‘ article, author) and prepayment, check o r money order for $13.00 for photocopy ($14.50 foreign) o r $4.00 for microfiche ($5.00 foreign), are required.
CORRESPONDENCE
SIR: I am concerned by several statements in the article by Smith and Shapiro (1).The authors seem to have misinterpreted the purpose and rationale behind the “threshold approach” to estimating the effects of phosphorus reduction on chlorophyll levels presented in an EPA report (2).In addition to a number of inaccurate statements about the EPA report, 1508
Environmental Science 8 Technology
the authors draw conclusions which are not consistent with the data presented. Smith and Shapiro state that the EPA model “assumes that the best predictive equation is that for the line chlorophyll a = 1.0 total P.” This is clearly not true. The EPA model assumes that the line provides an upper limit o r boundary. It was
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@ 1981 American Chemical Society
