Airborne measurements of aerosols around convective clouds over the tropical Pacific Ocean show enhanced concentrations of small (Aitken) particles in the vicinity of some of the clouds. In the same regions there were increases in the concentrations of internally mixed particles with diameters around 0.3 μm that contained acidic sulfate. The acidic sulfate is attributed to heterogeneous oxidation of SO2 in cloud droplets. There was also some evidence of nitrate in the internally mixed particles. The enhancement in the concentrations of ~0.3 μm diameter particles produced smoother (i.e., more Junge-type) particle size spectra. ¿ 2001 American Geophysical Union