The aerosol size distributions measured during the 1969 Pasadena Smog Study and those obtained later in laboratory smog-simulation experiments are analyzed. Emphasis is on the identification of physical mechanisms and parameters which are responsible for the daily aerosol concentration changes. The role of relative humidity, solar radiation intensity, coagulation, and condensation is discussed. Data from field measurements and artificial humidification experiments indicate that by changing the relative humidity, the submicron aerosol volume concentration, as well as the total light scattering, may be changed by at least a factor of two. The effect of solar radiation on the photochemical gas-particle conversion rate was investigated by inflating a plastic bag with particulate-free ambient air and exposing it to solar radiation. The observed nuclei-generation rates in Pasadena were on the order of 10 5 nuclei/sec. The solar radiation was found to be necessary for the nuclei production. Laboratory experiments performed in Minnesota suggest that the growth of photochemical nuclei after formation is governed by simultaneous coagulation and condensation. During late night hours, the aerosol was found to decay according to the laws of coagulation. The size distributions were observed to approach a universal form which could be simulated by laboratory aging experiments and by numerical (Monte Carlo) simulation. The mean coagulation coefficient for the smog decay was between 2 × 10 −9 and 10 −8 cm 3/sec. During the daytime, coagulation was found to limit the total number concentration to 2 × 10 5/cm 3 by rapid removal of small particles (0.01 μm) by the larger ones (0.1 μm). It appears that the important particle diameter range 0.1 < D p < 1.0 μm, which on the average contains about 60% of the total aerosol mass fraction, was not affected significantly by coagulation. A comparison of field data with laboratory experiments and numerical calculations suggests that the noontime accumulation of aerosol mass in the 0.1 < D p < 1.0 μm subrange is primarily due to condensation of photochemically produced supersaturated vapors on the existing particles.