Twenty-three filter sampling flights of the NASA Lewis F-106 aircraft, were calculated in the Great Lakes region between June 4 and December 23, 1980, following the major eruption of Mount St. Helens (Washington State) on May 18. The IPC-1478 filters were exposed over an altitude range spanning the local tropopause. A filter sample exposed above the tropopause on June 5 indicated a sulfate level of 50 times the baseline measurements, which is consistent with the trajectory predictions of the leading edge of the cloud on its second transit around the earth. Subsequent measurements over a period of 7 months revealed the existence of a layer of sulfate above the tropopause that decayed to a level of about 4 times previously measured background levels by the beginning of August. Concentration of nitrate above the tropopause exhibited considerable variability and showed some enhancement compared with previously measured concentration levels. On the basis of the null results of X ray fluorescence measurements, there is no evidence of ash particle concentrations >3.4 μm/m3 persisting in the layer above the tropopause following the second transit of the cloud.