Vertical mixing above Summit, Greenland: Insights into seasonal and high frequency variability from the radionuclide tracers 7Be and 210Pb

The activity of the natural radionuclide tracers 7Be and 210Pb has been determined in bulk aerosol samples collected over 2-day intervals for nearly five full years at Summit, Greenland. Year-round sampling was conducted in three campaigns; summer 1997 to summer 1998, summer 2000 to summer 2002, and summer 2003 to present. As in previous summer campaigns at Summit, and a year-round investigation at Dye 3, variations in the activities of the tracers on short time scales were strongly correlated despite the upper troposphere/lower stratosphere source of 7Be and the continental surface source of 222Rn (precursor of 210Pb). This behavior is attributed to boundary layer dynamics exerting the dominant control on activities in air just above the ice sheet. Aerosols and associated species are depleted from the boundary layer above the snow when a strong inversion limits exchange with the free troposphere. Episodic weakening of the inversion allows ventilation of the boundary layer. This cycle drives simultaneous decreases and increases in the radionuclide tracers. The correlation between 7Be and 210Pb on seasonal and annual bases was found to be stronger than at Dye 3, and the average activity of 7Be was lower at Summit despite the higher elevation (3.0 versus 2.5 km). These observations indicate that the boundary layer at Summit is more effectively isolated than at Dye 3. The activity of 7Be at Summit peaked in June or July all 5 years, closely following the seasonality of stratospheric injection of 7Be into the Arctic troposphere (based on seasonality of the 10Be/7Be ratio previously measured at Alert, NWT). This suggests that when the boundary layer at Summit is replenished by ventilation, it receives air reflecting the composition of the mid and upper troposphere.