Aerosol and gas measurements were made with the University of Minnesota Mobile Laboratory (UMML) during the VISTTA program (26 June, 1979 to 13 July, 1979) near Page, Arizona. The UMML was stationed on Zilnez Mesa (36.79°N, 110.63°W, elevation 2,200 m) approx. 65 km east of the Navajo coal-hred power plant. Measurements were made both in and out of the power plant plume and were made during the day and night. Measured parameters included aerosol size distributions, Aitken nuclei count, ozone and sulfur dioxide concentrations, the aerosol light scattering coefficient and meteorological parameters including ultraviolet radiation intensity. Concentrations of NO and NOx were occasionally measured. Data show clear evidence of gas-to-particle conversion in the plume with aerosol volume being added in the 0.01 μm-0.32 μm dia. range and new particles being formed. The observed excess aerosol volume depends strongly upon the SO2 concentrations and the time of day with none observed in plume parcels which were not exposed to sunlight. This implies that the excess aerosol in the plume was secondary and was not due to primary particulate emissions from the stack. The fraction of sulfur appearing in the aerosol was inferred from the measurements of SO2 concentrations and excess aerosol volume concentrations in the plume and was found to correlate with the time integral of the UV flux density received by the plume parcels since pollutant emission. The observed rate of SO2 conversion was found to be 1.9 ± 0.8% h-1 at noon, with a diurnal average of 0.9 ± 0.4 % h-1.