Temporal variation of thoron decay product concentration in the atmosphere and comparison with radon decay product concentration

Abstract

Seasonal and long-term variation of the airborne 212Pb concentration, representative of the equilibrium equivalent concentration of thoron decay products (EECRn220), was investigated from 1989 through 1996 at a semi-natural location in southern Germany. Continuous measurement yielded a long-term average concentration of 0.082 Bq m-3, while daily mean concentrations varied from < or = 0.01 to 0.34 Bq m-3. An average annual effective dose of 1.4 mSv due to outdoor thoron progeny concentration was estimated. This is about 2% of the dose due to the average short-lived radon progeny concentration (EECRn222) of 8.4 Bq m-3 measured for this location in the same period. In most years the seasonal pattern of 212Pb activity concentration in the atmosphere is characterized by two maxima: the first in May and the second one in September. Low concentrations are observed from November through February of each year. This is in contrast to the behaviour of the short-lived 222Rn progeny which exhibit enhanced concentrations exactly during these months. The most probable reason for the different temporal behaviour of 212Pb is the extremely reduced flux of thoron gas from the ground during the winter months.

Radon and thoron progeny concentration variability in relation to meteorological conditions at Bucharest (Romania)

“…The lowest radon progeny concentrations are recorded during the springesummer months when atmospheric turbulence is high and soil humidity is also high. The data presented in this paper are in good agreement with literature data (Moses et al, 1963;Duen˜as et al, 1994;Eisenbud and Gesell, 1997;Winkler and Aehlig, 1998;El-Hussein et al, 2001;Winkler et al, 2001;Sesana et al, 2003). The lowest thoron progeny concentrations were recorded in the winter months characterised by significant snow coverage of the soil for many days.…”

“…The lowest thoron progeny concentrations were recorded in the winter months characterised by significant snow coverage of the soil for many days. Similar thoron progeny concentration variations are reported in Winkler and Aehlig (1998) for a semi-natural location in southern Germany.…”

“…Radon concentrations are less influenced by snow cover (Moses et al, 1963;Winkler and Aehlig, 1998). In most situations, 10 cm of snow or less did not influence the radon progeny concentrations while 10e20 cm of snow coverage reduced the radon progeny concentrations by a factor of 1.2e2.3 as compared to the multiannual averages.…”

“…Few correlations with atmospheric pressure are available in literature. Winkler and Aehlig (1998) reported some slight tendency of the radon concentration to decrease as the atmospheric pressure increases.…”

“…Thoron concentrations are much more sensitive to the snow coverage as compared with the radon concentrations, due to its fast radioactive decay (Winkler and Aehlig, 1998). Snow coverages over 10 cm lowered the thoron progeny concentrations by an order of magnitude as compared with the multiannual averages (down to 18 times for night aspirations in blizzard conditions).…”