Seasonal and long-term variation of ²¹⁰Pb concentration in air, atmospheric deposition rate and total deposition velocity in south Germany

The activity concentration in air and atmospheric deposition rate of the long-lived radon progeny 210Pb has been investigated at Munich-Neuherberg, south Germany, from 1972 (activity concentration) and from 1981 (atmospheric flux) to 1999. For these periods, the continuous measurements yielded an average 210Pb activity concentration at ground level of 0.57 mBq m-3, and an average total 210Pb deposition rate to ground of 180 Bq m-2 year-1. The average total deposition velocity, which relates the total 210Pb deposition rate to the 210Pb activity concentration was calculated to be 1.0 cm s-1. The variation of the data with time was studied by time-series analysis and distinct seasonal patterns were identified. Maximum 210Pb activity concentrations in air are observed in the autumn and winter months (October through February) of each year. By contrast, the maximum 210Pb deposition rate is observed during summer (June-August), i.e. in the months with the highest amount of rainfall at this site. Like the 210Pb deposition rate, the total deposition velocity exhibits a seasonal pattern with maximum values in summer. Due to the long observation period of 18 years, it was possible to observe for the first time a strong positive relationship between 210Pb deposition and precipitation, especially for the months May and June and to a smaller extent for several other months. In the long-term, variations of approximately a factor of 2 were observed in the annual average 210Pb activity concentrations, the annual deposition sums and the annual average deposition velocities. Since around 1981 210Pb concentrations in air steadily decreased while 210Pb depositions increased. As a consequence of these significant trends, the time series of the total deposition velocity exhibits a trend of the data from approximately 0.7 cm s-1 in 1981 to 1.7 cm s-1 in 1999