Estimation of stratospheric input to the Arctic troposphere: 7 Be and 10 Be in aerosols at Alert, Canada

Concentrations of 7 Be and 210 Pb in 2 years of weekly high‐volume aerosol samples collected at Alert, Northwest Territories, Canada, showed pronounced seasonal variations. We observed a broad winter peak in 210 Pb concentration and a spring peak in 7 Be. These peaks were similar in magnitude and duration to previously reported results for a number of stations in the Arctic Basin. Beryllium 10 concentrations (determined only during the first year of this study) were well correlated with those of 7 Be; the atom ratio 10 Be/ 7 Be was nearly constant at 2.2 throughout the year. This relatively high value of 10 Be/ 7 Be indicates that the stratosphere must constitute an important source of both Be isotopes in the Arctic troposphere throughout the year. A simple mixing model based on the small seasonal variations of 10 Be/ 7 Be indicates an approximately twofold increase of stratospheric influence in the free troposphere in late summer. The spring maxima in concentrations of both Be isotopes at the surface apparently reflect vertical mixing in rather than stratospheric injections into the troposphere. We have merged the results of the Be‐based mixing model with weekly O 3 soundings to assess Arctic stratospheric impact on the surface O 3 budget at Alert. The resulting estimates indicate that stratospheric inputs can account for a maximum of 10–15% of the O 3 at the surface in spring and for less during the rest of the year. These estimates are most uncertain during the winter. The combination of Be isotopic measurements and O 3 vertical profiles could allow quantification of the contributions of O 3 from the Arctic stratosphere and lower latitude regions to the O 3 budget in the Arctic troposphere. Although at present the lack of a quantitative understanding of the temporal variation of O 3 lifetime in the Arctic troposphere precludes making definitive calculations, qualitative examples of the power of this approach are given.