CORRESPONDENCE
Volcanic CFCs Dear Editors: In the recent ES&T feature article on "The Natural Production of Chlorinated Compounds" by Gordon W. Gribble (July 1994, p. 310A), the author includes volcanoes as a natural source of chlorofluorocarbons (CFCs), citing two reports of measurements taken downwind in volcanic gas plumes {1,2). Readers of ES&T should be aware that the data reported in these two publications are sampling artifacts. Volcanic gases sampled in the cited studies were mixed with the surrounding tropospheric air, which contains CFCs. The data reported clearly show lower levels of the CFCs in the volcanic gas plume at the same relative mixing ratios as in the surrounding tropospheric air. Thus, volcanoes may not be a "real" source of CFCs. In fact, a more reasonable conclusion is that the CFCs in the tropospheric air found in the volcanic plumes are a good measure of the mixing in the plume and the plume dilution; these CFC levels could be useful in determining volcanic emissions more accurately. Physical mixing and contamination from outside air can explain all of the volcanic data and indeed are probably also the explanation for CFCs observed in drill wells. The biogenic sources of chlorinated, brominated, and iodinated natural organic compounds described by Gribble are all quite reasonable and are becoming well established. Moreover, a better understanding of natural products chemistry may improve our capabilities to model the role the biosphere plays with regard to natural trace levels of halogenated compounds in the atmosphere. However, the production of CFCs by volcanos is suspect. All of the natural chlorinated and brominated organic compounds will have much shorter lifetimes in the troposphere than the CFCs and thus will not have the same effect on stratospheric ozone depletion as the CFCs (3).
It is also important that we remember that CFCs are greenhouse gases that affect the tropospheric radiative balance. The role of natural chlorinated compounds in the chemistry of the troposphere certainly needs to be understood, but it should not be confused with mixing phenomena. In addition, we should not ignore the fact that man-made CFCs are affecting our atmosphere. References (1) Isidorov, V. A. Organic Chemistry of the Earth's Atmosphere; Springer-Verlag: Berlin, 1990. (2) Stoiber, R. E. et al. Geol. Soc. Am. Bull. 1971, 82, 2299. (3) Hurtak, J. J.; Bailey, P. Proceedings 29th Intersociety Energy Conversion Engineering Conference, 1994; Part 3, AIAA94-4152-CR p. 1139. JEFFREY S. GAFFNEY
Argonne National Laboratory Argonne, IL 60439 Author's response Jeffrey S. Gaffney is mistaken. The Isidorov measurements were not of samples taken "downwind in volcanic gas plumes" but, rather, of gas samples taken directly from solfataric vents where mixing with tropospheric air is expected to be minimal or nonexistent (i, 2). Moreover, the concentrations of the CFCs (e.g., CF2C12) in some cases were found to exceed background levels by 400 times, reaching concentrations of 160 ppbv. Furthermore, high concentrations of CFCs were also discovered in gas bubbles rising from the bottom of hydrothermal sources. Likewise, the Stoiber measurements, which were taken directly from a fumarole, revealed CFC levels "well above unpolluted atmospheric levels." In addition, not only have CFCs and other chloroalkanes (chloroform, carbon tetrachloride) been detected in deep drill wells, but a number of minerals, such as silvinites, halites, carnallites, and apatites, have been found to contain these chemicals (3). As difficult as it may be for some to absorb, the assertion that CFCs
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are exclusively anthropogenic is no longer correct. We must now accept the fact that there are natural sources of CFCs. The key question is: What is the overall contribution of these natural CFCs to the global environment? References (1) Isidorov, V A. et al. /. Atmos. Chem. 1990, 10, 329. (2) Isidorov, V. A. et al. /. Ecol Chem. 1993, 19. (3) Isidorov, V. A. et al./. Ecol. Chem. 1993, 201. GORDON GRIBBLE
Dartmouth College Hanover, NH 03755 Isidorov's response The publications by Stoiber et al. and myself cited by Gaffney were based on volcanic gas samples collected using tubes introduced directly into solfataras; the sampling scheme is presented in Reference 1. Although this procedure does not rule out the possibility of dilution of volcanic gases by surrounding tropospheric air, the dilution is substantially less pronounced than that which could take place if sampling were carried out "downwind in volcanic gas plumes," as Gaffney erroneously ascribes to these authors. The concentrations of CFCs reported in solfataric gases from the Kurile Islands and Kamchatka were about 160 ppbv, which is 3-4 orders of magnitude higher than the background concentrations and exceeds by 1-3 orders of magnitude the value typical of urban air. The "dilution with the surrounding air" evidently could not result in increasing CFCs concentration of solfataric gases. Reference (1) Isidorov, V. A. et al. /. Atmos. Chem. 1990, 10, 329. VALERY A. ISIDOROV St. Petersburg University 198904 St. Petersburg, Russia
Zero emissions? Dear Editors: There is no such thing as "zero emissions" or a "zero emis-
