STATEMENT OF DR. MALCOLM CAMPBELL

Malcolm J. Campbell

doi:10.7273/000007809

This presentation is a report of work carried out by myself, John Sheppard, Murray McEwan, and Brian Lamb, with funding from the National Science Foundation and the Environmental Protection Agency. It describes an attempt to estimate the rate of methyl chloroform removal from the atmosphere. Si nee the current consensus is that the methyl chloroform removal process involves oxidation by the hydroxyl radical, we started by using measured hydroxyl concentrations. Specifically, we utilize our groundlevel hydroxyl determinations at several sites, in conjunction with Fishman and Crutzen 1 s (1977) model of the variation of hydroxyl concentrations with altitude and latitude, to estimate the methyl chloroform lifetime. This process shows that a major part of the methyl chloro form oxidation probably occurs in the tropics, and at low altitudes. In these calculations we tacitly assume that the methyl chloroform that has escaped oxidation by the hydroxyl radical has been transferred to the stratosphere. The NASA (1977) rate constant for the hydroxyl radical-methyl chloroform reaction was used. Experimentally determined hydroxyl radical con centrations provided an initial basis for estimation of the prevailing concen trations; these individual OH concentrations are for a particular site and time and are not themselves global, diurnal or seasonal averages. Much larger numbers of hydroxyl radical concentrations will need to be measured at a variety of latitudes, altitudes and seasons before an estimate of the methyl chloroform 1 ifetime can be confidently made. Our hydroxyl measurements involve the local oxidation of 14co and have been described elsewhere (Campbell et al., 1979). The current accuracy is believed to be about +40%, 1 imited mostly by wall effects. Some of our earlier data have larger error bands. The measurement is inherently an absolute measurement,

STATEMENT OF DR. MALCOLM CAMPBELL

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