Abstract
Various methods of determining vertical eddy diffusion coefficients in the planetary boundary layer from two natural radioactive tracers, radon (Rn-222) and ThB (Pb-212), are presented and discussed. In all cases, it is more pertinent to measure vertical exchanges in the lower atmosphere using ThB (10.6 hr half-life) than radon (3.8 days half-life).
About 30 vertical profiles of Rn and ThB in the lower troposphere (100–3000 m) are determined over a 12-month period. Vertical eddy coefficient values are calculated according to the various methods discussed. The variation range, between 10−1 and several 100 m2 sec−1, is large and closely related to the vertical temperature gradient variation.
Low values (10−2 to 10−1 m2 sec−1) of the exchange coefficients determined in the inversion layer which frequently exists at the top of the planetary boundary layer show that vertical turbulent fluxes toward the free atmosphere are relatively small.
