Ion concentration variations at short distances downwind of continuous and quasi-instantaneous point sources

An improved experimental technique is described which enables the fine structure of the ion concentration variations downwind of both continuous and instantaneous point sources to be examined. The method, which involves the use of ionised air, permits resolution of features within diffusing clouds of the order of centimetres since the detector response time is about ten milliseconds. The success of the method can be attributed to the fact that the measurements are entirely electrical in nature, thereby eliminating certain defects inherent in most other sensing techniques. However, the method does possess disadvantages and these are critically examined. Experimental results are presented which show clearly the presence of considerable fine structure hitherto suspected, but not quantitatively observed, in diffusing plumes, i.e. from continuous sources, and puffs, i.e. from instantaneous sources. The implications of these data are considered briefly.