Three particle size spectrometers were flown aboard the NASA-U2 on numerous flights before and after the eruption of El Chichon to determine the effects of the eruption. Measurements of aerosol particles between 20¿-50¿ north latitude over the North American continent show strong orders of magnitude enhancements in number and mass. Our analyses and supporting measurements by others, indicate that the aerosol between 0.1-2.0 &mgr;m is largely H2SO4 while the aerosol larger than 2 &mgr;m is likely ash. Comparisons of measurements using a spectrometer with an aerodynamically sensitive (particle size sorting) inlet with a second one that operated ''in situ'', and a third one that operated isokinetically, indicate that the larger ash particles are of low bulk density. Observations of particles as large as 30 &mgr;m four months after the eruption are only compatible with very low bulk densities. The H2SO4 droptlet populations show decreasing number density with altitude but increasing mass. The distribution is largely bimodal with 0.4 and 1.2 &mgr;m modes typically observed. The mass is dominated by the 1.2 &mgr;m mode.