Dr. Fleming: 1 would like to confine my remarks largely to an assessment of the four methods we have heard described in the last day or two. steady altitude. One, the advantage of balloons, it seems to me, is that they can offer samples at a very It's the only system, apparently, which is able to do that. samples with it, and, of course, it is a cheap method. In principle, we can get large The disadvantage apparently is the ceiling. beginning to wonder whether we can get samples by filtering much above 125,000 feet. on this in any scientific way. sample. Also, Iam I can't really comment Two, the diffusive collection system has an advantage in that it gets a very large Again, there is the question of whether it will work above 125,000 or 150,000 feet. limitation of approximately 200, 000 feet, presumably. It has a ceiling You can't get a gas sample with it, and there is also a question of the efficiency of the sample gotten by diffusion techniques. Three, cryogenics really has no upper ceiling limit, but there may be trouble at lower altitudes, of the order of 100, 000 to 125,000 feet, but it gets everything with the exception of helium and tritium, if it is tritium gas. is taken. If it is tritium in water, of course, it This includes all particle sizes even down to the molecular size, which I think is very important. This is one area, of course, where diffusion is lacking. Four, impaction seems to look good in principle: again gets all particle sizes, and I think this could turn out to be very important. problem in efficiency. it But again, it may have a My own feeling about this is that all these methods ought to be carried forward, as one man says,''with vigor" in the 1960's. prove radiochemical techniques. I have heard several commentsin the last two days about the need to im- JI am not denying this, but I'd like to point out that great strides have been made, even in the last few years, in this area, and I don"t think it should be taken as an excuse for not going ahead on these various sampling systems. Dr. Magar: I don't really know what I can say in way of a summary. methods mentioned here has its own areas of application. I would like to say that each one of the sampling Perhaps the thing which I should stress is that it is possible to combine some of these various sampling methods to make an even more efficient sampler. example, a combination of filtering technique and cryogenics could be used. For Another thing that I would like to mention is that we have concentrated here on certain special problems of sampling, namely, those concerning radioactivity. There are other problems of sampling concerning chemical reactions in the upper atmosphere, for example, problems concerning detection of organic matter, and some of these techniques, particularly the eryogenic techniques, I think could handle these very well. Let me give an example. However, again a new host of problemsis created. The problem of ceilings will be an extremely important one when we try to sample for chemical reactions which occur in the upper atmosphere. I think that the new problems and the new technology more or less by themselves generate new solutions, and the cryogenic sampling is one example of such a solution. I think we need to revise our thinking when we talk about these new solutions. For example, I have heard here during this meeting a considerable amount of concern regarding the size of the sample required. In view of some of these new techniques, this concern is not quite real to me because, if you have positively a given certain amount of material, you know how much you have of it and you can use very careful diagnostic techniques in your laboratory. No matter how large a sample or how small a sample you have, the diagnostic technique in the laboratory is at slow time. You can take your time to analyze it, and, if the diagnosis is good enough, you will be able to determine what you have in it. 161