The chemical compositions of single aerosol particles larger than 0.3 μm optical diameter were measured at Idaho Hill using a laser ionization mass spectrometer. The mass spectrometer can analyze either positive or negative ions; this paper covers the negative ion results. Sulfate, nitrate, organics, O-, and OH- were the most common peaks observed in the negative ion spectra. Other species identified in the negative ion spectra include silicon, chlorine, iodine, organic acids, and elemental carbon. Neither external nor internal mixing completely describes the particles: most of the particles were in various classes that were distinctly different from each other. All air masses had more than one class of particle present. However, most of the particle classes were in themselves mixtures. Sulfate was the most common negative ion in smaller particles whereas nitrate and complicated mixtures were more common in particles larger than about 0.7 μm. However, nearly all types of particles were observed in all size ranges and wind conditions. Aerosol chemistry was not well correlated with gas phase chemistry. Certain combinations, such as nitrate and silicon, were frequently observed together in single particles. Sulfate and chlorine did not coexist in the same particles, nor did silicon and elemental carbon. Small amounts of organics were present in most particles. Sulfate and nitrate were usually, but not always, in different particles. The nitrate/sulfate ratio estimated with the mass spectrometer data correlated well with filter measurements.¿ 1997 American Geophysical Union