Upper tropospheric aerosols observed in spring 1991 over Laramie, Wyoming, were sampled using balloon-borne cascade impactors. Three impactor samples were collected; two were in upper tropospheric aerosol layers and one was collected at the same altitude in cleaner, ''background'' upper tropospheric air. Optical particle counters measured concentrations of particles with radii ≥0.15 &mgr;m in the layers which were increased 5--10 times over what is normally observed at these altitudes. Electron microscope analyses showed acidic and neutralized sulfate particles to be the dominant aerosol constituents in these layers, although carbonaceous soot aggregates and crustal dust particles were also found. The morphology and elemental composition of these particles closely resembled particles collected in the large mixed smoke plume of the Kuwaiti oil fires. Meteorological analyses showed favorable transport conditions from the Middle East to the continental U.S. when layers were present over Wyoming, and less than ideal conditions when the layers were absent. Based on these microanalytical results and the corroborating meteorological and air trajectory analyses, the most likely source of these aerosol layers is the oil fires in Kuwait. ¿ American Geophysical Union 1992