By using a smog chamber with a long-path infrared spectrometer, a photochemical reaction for C₃H₆-NO-SO₂-dry air system was investigated by varying the concentration of each reactant. When NO₂ formation rate was plotted as a function of initial C³H₆/NO ratio ([C₃H₆] ₀/[NO] ₀), two straight lines were obtained. In the range of [C₃H₆] ₀/[NO] ₀>6, the slope, indicating the photochemical reactivity of C₃H₆, greatly decreased due to the presence of SO₂, while the decrease in the slope was relatively small in the range of [C₃H₆] ₀/[NO] ₀<6. O₃ maximum concentrations observed were not much affected by SO₂ and were not related to [C₃H₆] ₀/[NO] ₀ in the same manner as to the NO₂ formation rate.
SO₂ depletion (oxidation) rate was found to be first order in SO₂ initial concentration. The rates increased linearly with the increase in [C₃H₆]/[NO] 0 in the range of smaller than 5 and became less dependent on the ratio. It was also confirmed that the rate was related to the NO₂ formation rate all over the [C₃H₆] ₀/[NO] ₀ range studied (2-20). OH radical concentrations were calculated from observed C₃H₆ depletion rate and O₃ concentration and the change of the OH concentration with [C₃H₆] ₀/[NO]₀ was relatively small.It was found that the SO₂ depletion rates calculated from the OH concentrations did not exceed 20% of the SO₂ depletion rates observed all over the [C₃H₆] ₀/[NO] ₀. It was clear the oxidation of SO₂ with Criegee intermediates was predominant under the present experimental conditions.