Long period study of outdoor radon concentration in Milan and correlation between its temporal variations and dispersion properties of atmosphere.

The results of a survey of outdoor radon concentrations in Milan are reported. Measurements were performed hourly over a continuous four year period from January 1997 to December 2000. Radon concentration was obtained by two means: both direct measurement of radon; and measurement of its decay products. The average daily pattern of radon concentration featured a minimum in the late afternoon and a maximum in the early hours of the morning. A seasonal pattern with higher concentrations in winter than in summer (from around 15 Bq m(-3) in winter to around 5 Bq m(-3) in summer) was also observed. Similar average annual values of around 10 Bq m(-3) were obtained. The annual effective outdoor radon dose was found to be 0.12 mSv. The variation from minimum in the afternoon to maximum the following morning was found to be a good indicator of the height of the nocturnal mixing layer. The variation between maximum and minimum levels on the same day is an index of the maximum height of the mixing layer. Furthermore, our long term measurements of radon have permitted us to examine the dispersion characteristics of the atmosphere over Milan, and to establish the frequency of conditions unfavourable to the dispersion of atmospheric pollutants.