Low pressure impactor measurements show two distinct types of fine particle sulfur size distributions in Los Angeles. These types have mass median diameters of 0.54 + or - 0.07 micrometers and 0.20 + or - 0.02 micrometers, respectively. Factors which may account for the two distribution types including effects of relative humidity, coagulation, fogs and formation mechanisms are discussed. Calculations show that the 0.5 micrometers particles are consistent with chemical reactions in the aerosol phase, whereas the smaller particles result from gas phase SO2 oxidation. While the growth of the total aerosol volume distribution is not too sensitive to the mechanism, the chemical species distribution strongly depends on the growth law and can be used to establish its form. (Copyright (c) 1982 Pergamon Press Ltd.)