Daily and seasonal variation of short-lived radon decay product concentrations in ground-level air at Munich-Neuherberg - a long-term study

Daily and seasonal variation of the equilibrium equivalent radon concentration (EECRn-222) and of the concentration of the short-lived radon decay product 214Pb in ground-level air was investigated at Munich-Neuherberg from 1982 to 2000, and from 1989 to 2000, respectively. For this, the EEC was measured continuously with an alpha/beta aerosol monitor at 2.5 m above ground, and 214Pb was determined by online gamma spectrometry at about 12 m above ground. The resulting time series were analysed statistically. For the respective periods, the measurements yielded a long-term average concentration of 7.8 Bq m-3 (geometric mean: 6.1 Bq m-3) for the EEC, and 6.0 Bq m-3 (geom. mean: 5.0 Bq m-3) for 214Pb. In these periods, daily averages ranged from 0.6 to 49 Bq m-3 (EEC), and from 0.6 to 43 Bq m-3 (214Pb). Monthly mean concentrations varied between 3.0 Bq m-3 in March 1988 and 20 Bq m-3 in October 1985 (EEC), and from 3.4 Bq m-3 in April 1992 to 11 Bq m-3 in October 1995 (214Pb), respectively. The annual average concentrations were from 5.9 Bq m-3 in 1999 to 10.6 Bq m-3 in 1985 (EEC), and from 5.3 Bq m-3 in 1999 to 6.8 Bq m-3 in 1991 (214Pb). From the long-term average equilibrium equivalent radon concentration (EEC), an average annual effective dose of 0.13 mSv due to the short-lived radon progeny outdoors was estimated. The time series showed distinct daily and seasonal variations. The variations from day to day are mainly caused by the actual weather conditions (wind, rain, etc.), i.e. the short-term turbulent conditions. The seasonal pattern is characterised by an autumn to winter maximum and an early summer minimum, and reflects the prevailing turbulent conditions at the respective seasons. As known, at Munich-Neuherberg during autumn and winter months (October/November to February) inversion weather conditions frequently occur, while the other months are characterised by more turbulent conditions. (orig.)