The photochemistry of sulfuric acid aerosols. (2001)
AuthorsRowland, Glenn Anthonyshow all
We report here the experimental investigation of several photochemical reactions that may occur in concentrated sulfuric acid droplets in the clouds of the planet Venus. It was found that the irradiation of 80-100wt.% sulfuric acid aerosol with 193nm light in the presence of carbon monoxide resulted in the formation of carbon dioxide and either the formation or destruction of sulfur dioxide. The measured quantum yield for carbon dioxide formation is 0.011±0.010. Various instrumental techniques for determining the size and number density of aerosol droplets were utilised in an attempt to generate a stable, monodisperse sulphuric acid aerosol. Concentrated sulfuric acid solutions were irradiated with 178.3nm, 184.9nm, 193.3nm and 253.7nm light in order to investigate the UV photochemistry of the bulk liquid. No significant photochemistry was observed. The reactions occurring in sulfuric acid aerosol at 193nm were attributed to multiphoton processes, permitted by the optical focussing effects observed in small droplets. Irradiation of solutions of Fe(II) in 98wt.% sulfuric acid solution with 193nm or 248nm light results in the formation of Fe(III). Under the same conditions, the reduction of Fe(III) is also observed. The presence of sulfur dioxide increases the initial rate of the reaction, and dissolved oxygen is consumed in the initial stages. A qualitative mechanism for the photochemical interconversion of Fe(II) and Fe(III) in concentrated sulfuric acid is proposed, which incorporates the effect of sulfur dioxide and oxygen on the reaction.