ESTIMATION OF STRATOSPHERIC INPUT TO THE ARCTIC TROPOSPHERE - BE-7 AND BE-10 IN AEROSOLS AT ALERT, CANADA

Concentrations of Be-7 and Pb-210 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 Pb-210 concentration and a spring peak in Be-7. 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 Be-7; the atom ratio Be-10/Be-7 was nearly constant at 2.2 throughout the year. This relatively high value of Be-10/Be-7 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 Be-10/Be-7 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 O3 soundings to assess Arctic stratospheric impact on the surface O3 budget at Alert. The resulting estimates indicate that stratospheric inputs can account for a maximum of 10-15% of the O3 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 O3 vertical profiles could allow quantification of the contributions Of O3 from the Arctic stratosphere and lower latitude regions to the O3 budget in the Arctic troposphere. Although at present the lack of a quantitative understanding of the temporal variation Of O3 lifetime in the Arctic troposphere precludes making definitive calculations, qualitative examples of the power of this approach are given,