Concentrations of the natural radionuclides 7Be and 210Pb were determined in aerosol samples collected in the western hemisphere Arctic during the recent NOAA Arctic Gas and Aerosol Sampling Program (AGASP 3) and NASA Global Tropospheric Experiment/Arctic Boundary Layer Expeditions (GTE/ABLE 3A and ABLE 3B) missions. Beryllium 7 showed a free tropospheric concentration maximum between 4 and 5 km in the summer of 1990. Previous 7Be data obtained in the late 1950s and early 1960s also indicated a similar vertical distribution of 7Be near 70¿N. Injection of stratospheric air through tropopause folds associated with the Arctic jet near 70¿N appears to explain the presence of a layer of air near 4--5 km in the high Arctic free troposphere with elevated 7Be concentrations. The vertical distribution of 210Pb showed a distinct difference between the high-Arctic and sub-Arctic in the summer of 1988.

At latitudes greater than 65 ¿. 210Pb concentrations at 3--6 km were elevated compared to those below 1 km. The reverse of this trend was observed near 60 ¿. These same vertical distributions were also apparent in aerosol SO24-, determined in samples collected on the same flights (Talbot et al., this issue). The results for 210Pb suggest that some of the difference between the summer troposphere in the high- and sub-Artic is also due to enhanced stratosphere-troposphere exchange in the vicinity of the Arctic jet. These observations, and other findings from ABLE 3A presented in this issue, suggest that for some species the stratosphere may be a principal source influencing their distribution in the Arctic summer troposphere. For example, intrusions of stratospheric air constitute the dominant source term for tropospheric budgets of 7Be and ozone, and may be important in the 210Pb, SO24-, and NOy budgets. Further investigation, including determination of detailed 7Be and 210Pb distributions, is needed to quantify the stratospheric impact on the chemistry of the Arctic troposphere during the summer. ¿American Geophysical Union