Wet deposition efficiency of short-lived radon-222 progeny in central Finland

The washout efficiency of 214Pb, a short-lived daughter nuclide of 222Rn, was studied by measuring external gamma radiation at Tikkakoski, Central Finland, in 1995. A mean washout ratio (volume per volume) of 1.2 x 106 was obtained in this study. According to the results, rain removes airborne 214Pb more efficiently than snow, which is in agreement with earlier studies. Higher washout ratio values were encountered during afternoon hours than during early morning hours, a result which is associated with the better vertical mixing of the lower troposphere during afternoon hours. The observed inverse correlation between washout ratio and rain intensity supports the earlier results that below-cloud scavenging processes are less efficient removal mechanisms of airborne 214Pb compared to in-cloud processes. The highest washout ratios were measured during westerly winds, which are usually associated with cyclones travelling from the North Atlantic Ocean in an easterly direction. The results obtained in this study can also be applied to fallout estimates of wet-depositing chemical substances, e.g. airborne sulphate or heavy metals, for which equivalent remote-sensing methods, such as that involving gamma radiation in this case, may not be available.