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

Abstract

Abstract. The reactive uptake of ozone to deliquesced potassium iodide aerosol particles coated with linear saturated fatty acids (C 9 , C 12 , C 15 , C 18 and C 20 ) 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 (≥C 15 ), while it was only slightly reduced for the short ones (C 9 , C 12 ). 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 C 12 , 85% for C 15 and 50% for C 18 in presence of a monolayer of a fatty acid at the equilibrium spreading pressure at the air/water interface. The variation among C 12 , C 15 and C 18 follows the density of the monolayer at equilibrium spreading pressure, which is highest for the C 15 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 O 3 at the aqueous surface by the two longer chained surfactants pentadecanoic acid (C 15 ) and stearic acid (C 18 ). For these, the probability that a molecule hitting the surface is actually transferred to the aqueous phase underneath was β C 15 = 6.8 × 10 −4 and β C 18 = 3.3 × 10 −4 , respectively. Finally, the effect of twocomponent coatings, consisting of a mixture of long and short chained surfactants, was studied qualitatively.

Evaluated kinetic and photochemical data for atmospheric chemistry: Volume VI – heterogeneous reactions with liquid substrates

“…There is excellent agreement in the retrieved parameters α b and k II of Rouvière et al (2010) and despite the different types of experiments. The liquid phase rate constant k II obtained in both studies agreed with literature values (Garland et al (1980) derive 2 × 10 9 M −1 s −1 at 298 K) and the values reported by are recommended.…”

“…Known bulk aqueous phase kinetics was unable to explain Cl 2 formation in the study by Knipping et al (2000) and later by Knipping and Dabdub (2002). Therefore, an Eley-Rideal type surface reaction was proposed to bring experiment and simulation into agreement, which is recommended in parameterized form as a function of the chloride concentration.…”

“…However, for the atmospherically relevant concentration range (<10 12 molecules cm −3 ), the shape independent surface reaction term would dominate. Br 2 has been observed as gas-phase product resulting from the interaction of O 3 with frozen and aqueous bromide surfaces (Oum et al, 1998;Hunt et al, 2004 …”

“…The datasets of , , Rouvière et al (2010) concur in that the reaction proceeds without a significant surface component and can be described by the following resistor expression:…”

“…Formation of Cl 2 was observed with atmospheric pressure chemical ionization MS in presence of (0.63-3.4) × 10 14 molecule cm −3 of O 3 and 254 nm light in the chamber. Cl 2 formation was simulated with a kinetic model that was extended in the follow up study by Knipping and Dabdub (2002). The uptake coefficient given in the table is the result of fitting the simulation to experimental data based on a parameterization of a surface reaction between OH and Cl − .…”