The reaction of SO2 and NH3 in humid air to produce an aerosol of ammonium sulfate is studied in a photolytic laboratory reactor. The reaction is shown to be controlled by ultraviolet irradiation in the wavelength range of 240-330 nm and is initiated by excitation of SO2. It is found that NH3 has a strong catalytic effect on the photolytic oxidation of SO2 and surprisingly high values of the quantum yield are observed with NH3 concentration in the range of 20-400 ppm. The reaction rate increases with decreasing temperature. An overall mechanism for the formation of aerosol particles is proposed. It is concluded that aerosol formation due to sunlight photolysis by this mechanism explains the high-opacity smoke formed in industrial stack plumes with small amounts of NH3 and SO2.