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Appendix 3: CD4 as an Air-Mass Tracer l Under proper conditions, CD4 can be used as a reliable tracer of an air mass. It is the deuterated analogue to the common atmospheric trace gas methane (CH4), and it is present in the unpolluted atmosphere at very low concentrations. The natural abundance of the 2H isotope of hydrogen (deute- rium) is only 0.0155to of the total hydrogen (iH + 2H ~ 3H), and the proba- bility of four deuterium atoms combining in natural-formation processes with the same carbon atom to form CD4 is very low. If a random association of the isotopes occurred in methane formation, only about 5 molecules in 10~6 molecules of CH4 would be expected to be CD4. Background levels (2.7 x 10~ ppt) reported in the NPS-WHITEX report are considerably higher than that expected from natural sources, but they are sufficiently low to make the use of CD4 attractive as a tracer in field studies. The deposition velocity of CD4 is expected to be near zero. The tropo- spheric lifetime of CD4 is determined by its rate of reaction with OH radicals. This rate coefficient is less than that of CH4 at atmospheric temperatures, and hence, it is not destroyed measurably by chemistry in the troposphere during a tracer experiment, which lasts only a few days. The CD4 can be isolated from other non-CH4 atmospheric trace gases using CH4-enriched gas samples coupled with gas chromatographic techniques. The CD4: CH4 ratio can be readily determined from the resulting fraction containing the separated CH4 gases using mass spectrometric procedures. From this measured ratio and the known amount of CH4 added to the mix- ture, the concentration of CD4 in the original air sample can be estimated. 95

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