Measurements of sub-2.5 &mgr;m aerosol concentration were made from a NASA U-2 aircraft by several experiments before and after the eruptions of El Chichon in March and April of 1982. Concentrations of sub-2.5 &mgr;m diameter particles encountered between 19.6 and 21.6 km altidues were nearly uniform over large distances in regions thought to be unaffected by El Chichon. Comparisons of measurements made with three instruments suggest that particles smaller than 0.1 &mgr;m in diameter contributed significantly to the number distribution of these non-El Chichon aerosols. Measurements of large concentrations of sub 0.1 &mgr;m particles in April and May 1982 imply that new particle formation occured following the eruption. Measurements made in November and December of 1982 showed decreased numbers of sub-0.1&mgr;m particles compared to the non-El Chichon measurements. Simultaneous measurements of SO2 and aerosol volume concentrations made on the lower edge of the volcanic cloud two weeks after the eruption permitted a range of SO2 conversion rates to be estimated.