The effect of fatty acid surfactants on the uptake of ozone to aqueous halogenide particles

Abstract. The reactive uptake of ozone to deliquesced potassium iodide aerosol particles coated with linear saturated fatty acids (C₉, C₁₂, C₁₅, C₁₈ and C₂₀) was studied. The experiments were performed in an aerosol flow tube at 293 K and atmospheric pressure. The uptake coefficient on pure deliquesced KI aerosol was γ = (1.10±0.20)×10⁻² at 72–75% relative humidity. In presence of organic coatings, the uptake coefficient decreased significantly for long straight chain surfactants (≥C₁₅), while it was only slightly reduced for the short ones (C₉, C₁₂). We linked the kinetic results to the monolayer properties of the surfactants, and specifically to the expected phase state of the monolayer formed (liquid expanded or liquid condensed state). The results showed a decrease of the uptake coefficient by 30% for C₁₂, 85% for C₁₅ and 50% for C₁₈ in presence of a monolayer of a fatty acid at the equilibrium spreading pressure at the air/water interface. The variation among C₁₂, C₁₅ and C₁₈ follows the density of the monolayer at equilibrium spreading pressure, which is highest for the C₁₅ fatty acid. We also investigated the effect of organic films to mixed deliquesced aerosol composed of a variable mixture of KI and NaCl, which allowed determining the resistance exerted to O₃ at the aqueous surface by the two longer chained surfactants pentadecanoic acid (C₁₅) and stearic acid (C₁₈). For these, the probability that a molecule hitting the surface is actually transferred to the aqueous phase underneath was β_C15=6.8×10⁻⁴ and β_C18 = 3.3×10⁻⁴, respectively. Finally, the effect of two-component coatings, consisting of a mixture of long and short chained surfactants, was studied qualitatively.