Radon in atmospheric studies: a review

The distribution of the isotopes of radon in space and time, their physical characteristics, and their behavior in the dynamics of the atmosphere have presented challenges for many decades. 220Rn, 222Rn and their daughters furnish a unique set of tracers for the study of transport and mixing processes in the atmosphere. Appropriate applications of turbulent diffusion theory yield general agreement with measured profiles. Diurnal and seasonal variations follow patterns set by consideration of atmospheric stability. 222Rn has been used successfully in recent studies of nocturnal drainage winds and cumulus convection. Good results have been obtained using 222Rn and its long-lived 210Pb daughter as tracers in the study of continent-to-ocean and ocean-to-continent air mass trajectories, 220Rn (thoron) because of its short half-life of only 55 seconds has been used to measure turbulent diffusion within the first few meters of the earth's surface and to study the influence of meteorological variables on the rate of exhalation from the ground. Radon daughters attach readily to atmospheric particulate matter which makes it possible to study these aerosols with respect to size spectra, attachment characteristics, removal by gravitation and precipitation, and residence times in the troposphere. The importance of ionization by radon and its daughters in the lower atmosphere and its effect on atmospheric electrical parameters is well known. Knowledge of the mobility and other characteristics of radon daughter ions has led to applications in the study of atmospheric electrical environments under fair weather and thunderstorm conditions and in the formation of condensation nuclei. The availability of increasingly sophisticated analytical tools and atmospheric measurement systems can be expected to add much to our understanding of radon and its daughters as trace components of the atmospheric environment in the years ahead