This paper supports other measurements at Idaho Hill by describing measurements of the aerosol surface area, volume, and size distribution. The aerosol size distributions at the Idaho Hill site showed nuclei and accumulation modes similar to other clean continental sites. However, there were fewer large aerosols. Principal components analysis verifies that the modes present in the size distribution also had distinct time behavior. The coarse aerosol mode had two components with distinctly different time behavior: aerosols larger than 2 μm displayed a great deal of short-term variability not present in smaller aerosols. Accumulation mode aerosols had separate correlations with fresh and aged pollution, whereas smaller aerosols were correlated only with fresh pollution. Aerosols larger than 15 nm showed no diurnal variation in downslope conditions and an afternoon maximum in upslope conditions. Aerosol surface areas were too small to directly affect OH chemistry even if OH or HO2 had fast surface losses. Aerosol volumes indicate that aerosol nitrate is not a likely candidate to balance the NOy shortfall.¿ 1997 American Geophysical Union