Enhanced ion production in convective storms by transpired radon isotopes and their decay products

Abstract

The contribution of radioisotopes of continental surface origin to the production of ions in convective storms has not been adequately assessed. The average 222Rn flux from bare soils and average distributions of 222Rn versus altitude in the troposphere for the normal range of vertical eddy mixing rates provide an inadequate basis for such an assessment. The influence of the biosphere, particularly the substantially higher fluxes for transpired 222Rn and 220Rn, and enhanced ion production by high concentrations of 222Rn and 220Rn decay products on aerosols and vegetation surfaces, have not been considered. Effective transpiration of 222Rn and 220Rn by vegetation is due to their high concentrations in soil gas and their solubility in soil moisture. Substantially enhanced fluxes of 222Rn over a mature corn field (6 times higher than the average flux over bare soils), and of 220Rn over a wheat field, illustrate the effectiveness of plant transpiration. Conditions favoring effective transpiration include mature vegetation, excess soil moisture throughout the root zone, strong radiative heating of the leaf canopy, and moderate surface winds. Rising air parcels with transpired water vapor, 222Rn, 220Rn, and their decay products may contribute importantly to the development of cumulus clouds and air mass storms. The dynamic lift of surface air, which feeds updrafts of convective storms, transports radioisotopes and ions from several sources within the vegetation canopy: transpired 222Rn, 220Rn, and their decay products; ions produced by the decay products of 220Rn and 222Rn on aerosol and vegetation surfaces; and biological aerosols and plant debris with high surface concentrations of radioisotopes. Observed frequencies of lightning flashes are highly correlated with areas of effective transpiration and their variations with season, latitude, and time of day.