The mass accommodation coefficient of ozone on an aqueous surface

Abstract

A wetted-wall cylindrical flow reactor was used to measure uptake coefficients, γ, of O₃ on aqueous surfaces at 293 K. The loss of O₃ from the gas-phase following contact with varying areas of aqueous surface was determined by UV absorption. The use of Na₂S₂O₃ as an aqueous-phase scavenger for O₃ ensured that uptake coefficients were in a reaction-controlled rather than mass accommodation-controlled regime. Observed uptake coefficients were corrected for radial gas-diffusion to yield values of γ^corr. From extrapolation of a plot of 1/γ^corr against the inverse square-root of the Na₂S₂O₃ activity, a value of α = 4 × 10⁻² was derived for the true mass accommodation coefficient of O₃. Evaluation of uncertainties indicate a conservative lower limit of 10⁻² for α. The data do not rule out that the upper limit approaches unity. However, it is shown that the measured value of α is sufficiently high that mass accommodation does not limit heterogeneous processing of O₃ in the atmosphere for droplets of diameter >10 μm. A value of 3.7^+0.7_−0.6 × 10⁸ L mol⁻¹ s⁻¹ is derived for the aqueous-phase reaction rate coefficient between O₃ and Na₂S₂O₃ at 293 K.