A review of the physical characteristics of sulfur-containing aerosols, with respect to size distribution of the physical distributions, sulfur distributions, distribution modal characteristics, nuclei formation rates, aerosol growth characteristics, and in situ measurement, has been made. Physical size distributions can be characterized well by a trimodal model consisting of three additive lognormal distributions. When atmospheric physical aerosol size distributions are characterized by the trimodal model, the following typical modal parameters are observed: 1. 1. Nuclei mode — geometric mean size by volume, DGV a from 0.015 to 0.04 μm, σ gn , = 1.6, nuclei mode volumes from 0.0005 over the remote oceans to 9μm 3 cm −3 on an urban freeway. 2. 2. Accumulation mode — geometric mean size by volume, DGV a, from 0.15 to 0.5μm, σ gn = 1.6–2.2 and mode volume concentrations from 1 for very clean marine or continental backgrounds to as high as 300μm 3cm −3 under very polluted conditions in urban areas. 3. 3. Coarse panicle mode — geometric mean size by volume, DGV a, from 5 to 30 μm, σ gn , = 2–3. and mode volume concentrations from 2 to 1000 μm 3 cm −3. It has also been concluded that the fine particles ( D p < 2 μm) are essentially independent in formation, transformation and removal from the coarse particles ( D p > 2 μm). Modal characterization of impactor-measured sulfate size distributions from the literature shows that the sulfate is nearly all in the accumulation mode and has the same size distribution as the physical accumulation mode distribution. Average sulfate aerodynamic geometric mean dia. was found to be 0.48 ± 0.1 μm (0.37 ± 0.1 μm vol. dia). and ν = 2.00 ± 0.29. Concentrations range from a low of about 0.04 μg m −3 over the remote oceans to over 80μm −3 under polluted conditions over the continents. Review of the data on nucleation in smog chambers and in the atmosphere suggests that when SO 2 is present, SO 2-to-aerosol conversion dominates the Aitken nuclei count and, indirectly, through coagulation and condensation, the accumulation mode size and concentration. There are indications that nucleation is ubiquitous in the atmosphere, ranging from values as low as 2 cm −3 h −1' over the clean remote oceans to a high of 6 × 10 6 cm −3 h −1 ' in a power plant plume under sunny conditions. There is considerable theoretical and experimental evidence that even if most of the mass for the condensational growth of the accumulation mode comes from hydrocarbon conversion, sulfur conversion provides most of the nuclei.