Variation of short-lived beta radionuclide (radon progeny) concentrations and the mixing processes in the atmospheric boundary layer.

Radon is emitted to the atmosphere with quasi constant emission rates depending on the radium concentration in the earth's crust and soil physical properties. In this way, the 222Rn and 220Rn concentration in air reflects significantly the thickness of the atmospheric boundary layer (ABL). The aerosol-associated, beta-emitting progeny nuclides of 222Rn were measured daily in the framework of the atmospheric radioactivity monitoring program of NIMH at Sofia. The 214Pb concentration was estimated from the measured short-lived beta activity of 24-h filter samples, changed daily at 6:00 GMT. The impact of some meteorological factors such as wind direction, wind velocity, humidity, and temperature on short-lived beta radionuclides is estimated, and the results show no simple statistical relationship. A seasonal pattern was observed with winter minima and late summer-early autumn maxima. High variability in daily morning concentrations and mean monthly values was observed. There were well pronounced differences between years. The height of the convective ABL was estimated from daily radio-soundings at 12:00 GMT for the period 2001-2006 and from seven soundings per day during the experimental campaign in Sofia in October 2003. In general, concentrations of short-lived 222Rn progeny nuclides decreased with increased convective ABL height.