Sulfuric acid aerosol formation by the reaction of HO radicals with SO₂

In order to investigate sulfuric acid formation by the reaction of HO radicals with SO₂, photoirradiation experiments were carried out by means of a 4‐m³ chamber, in which humidified air containing SO₂, CH₃ONO, n‐C₄H₁₀ and NO with concentrations in ppm range was exposed to simulated solar radiation at 303 ± 1K and under atmospheric pressures. The HO radicals were generated by a photolysis of CH₃ONO and their concentration was estimated from the decay rate of n‐C₄H₁₀ concentration. The amount of SO₂ lost by the reaction was evaluated through time integration of the instantaneous reaction rate k ₁ [HO] [SO₂] by using the rate constant k ₁ = 1.22 × 10^‐12 cm³ molecule^‐1 s^‐1 which we reported previously for the HO + SO₂ reaction. The yield of sulfuric acid was determined, on the other hand, by the filter collection and the subsequent ion chromatographic analysis of the aerosol product. From the agreement between the yield and the reactant loss, the accuracy of the k ₁ value was proved and simultaneously the stoichiometry of the reaction path from HO + SO₂ to H₂SO₄ was established. Particle size distributions of the aerosol were measured and found to be characterized by the self‐preserving distribution. Effects of relative humidity on the size and the acidity of the aerosol particles were also investigated.

Key words:

• Sulfuric acid
• Aerosol
• HO radical
• SO₂