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The atmospheric layer close to the earth's surface carries extensive geochemical information relating to the composition of the underlying terrain. During active mixing, there is an upward flux of both gaseous and particulate material from the surface. Gaseous forms of atmospheric geochemical interest include mercury vapour, halogen vapours. sulphur compounds, and radon. These gases diffuse rapidly and need to be measured very close to the surface to be of value. In general, experiments on the atmospheric geochemical measurement of trace gases have been mainly confined to mercury and radon, and have not led to the development of techniques that have been widely applied. This is due to the inherent problems relating to sensitivity requirements and the effects of rapid dilution. In the case of atmospheric particulates, comparatively large fluxes of material rise into the atmosphere when miXing conditions are good and with appropriate instrumentation it has been shown feasible to carry out atmospheric geochemical surveys using this material. If it is desired. however, to collect particulate material that relates closely to the underlying terrain, it is important to separate material carried in parcels of rapidly I'ising air from particulates associated with neutral or sinking conditions. It is also advantageous to use coarse particulate fractions in size ranges larger than 30 microns to minimize the effects of lateral migration and the re-entrainment of particles that have been previously translocated by wind. Atmospheric particles can be of both inorganic and organic orgin, much of the latter arising from vegetation. It has been established both in the laboratory and in the field that there can be movement of elements through vegetation to leaf surfaces followed by dispersion of these elements into the atmosphere as particulate material. Particulates derived {rom vegetation are, therefore, related geochemically to the composition of the underlying soils. In practice, material arising from both vegetation and residual soil sur{aces can be utilized for geochemical exploration purposes. Several types of airborne equipment have been developed for carrying out systematic atmospheric geochemical surveys. The spatial resolution of these systems varies according to design between 100 metres and several kilometres. Analytical methods employed have included conventional emission spectroscopy, laser vapourization coupled with emission spectroscopy. x-ray fluorescence sp(-'ct"oscopy. and {ission trace-etch counting. The spectroscopic measurements prOVide analyses for 20 or more elements, including all o{ the base metals while fission track-etch methods give exceptionally high sensivity and specificity for uranium alone. and are insensitive to radon and bismuth 214. Optical monitoring and fluidic switching devices have been used to provide for separation of upwelling particulates from material in stagnant and sinking air. Atmospheric airborne geochemical prospecting appears to offer important potential as a complementary tool to a variety of airborne geophysical methods. In the case of uranium exploration. it can prOVide information that is unaffected by sur{ace disequilibrium effects and the fine size aerosol interference that can considerably mOdify airborne gamma-ray spectrometer results.

The application of atmospheric particulate geochemistry in mineral exploration