²²²Rn and ²¹⁰Pb in the Arctic summer air

As part of an extensive air chemistry programme, during summer 1980, on board the Swedish ice-breaker 'Ymer', levels of ²²²Rn (radon) and its long-lived daughters ²¹⁰Pb and ²¹⁰Po were measured. The radon was trapped on charcoal and the long-lived daugther products sampled on filters on a daily basis. In addition, short-lived progenies were followed continuously on the filters in order to achieve a time resolution of about one hour. The concentrations of radon and ²¹⁰Pb in the Arctic summer air north of latitude 75° N averaged 75 ± 21 (1 sd) and 0·075 ± 0·028 mBqm^-3, r respectively. During a two week period of persistent polar winds, the mean radon concentration was 19 ±5 mBq m^-3. During such 'Arctic background' conditions, radon exhalation from the sea may contribute significantly to the measured radon-in-air concentration. It is shown that steady-state equilibrium models, applied to an air mass over the sea, overstimate the aerosol residence-time calculated from activity ratios. Time-dependent calculations indicate a mean aerosol residence time of 4 to 7 d in Arctic air. Good agreement is observed between radon levels and the time since the air mass left larger areas. Both the ²²²Rn and the long-lived daughter measurements are insensitive to contamination from ship and local settlement.