The effect of fatty acid surfactants on the uptake of nitric acid to deliquesced NaCl aerosol

Abstract

Abstract. Surface active organic compounds have been observed in marine boundary layer aerosol. Here, we investigate the effect such surfactants have on the uptake of nitric acid (HNO 3 ), an important removal reaction of nitrogen oxides in the marine boundary layer. The uptake of gaseous HNO 3 on deliquesced NaCl aerosol was measured in a flow reactor using HNO 3 labelled with the short-lived radioactive isotope 13 N. The uptake coefficient γ on pure deliquesced NaCl aerosol was γ =0.5±0.2 at 60% relative humidity and 30 ppb HNO 3 (g). The uptake coefficient was reduced by a factor of 5-50 when the aerosol was coated with saturated linear fatty acids with carbon chain lengths of 18 and 15 atoms in monolayer quantities. In contrast, neither shorter saturated linear fatty acids with 12 and 9 carbon atoms, nor coatings with the unsaturated oleic acid (C18, cisdouble bond) had a detectable effect on the rate of HNO 3 uptake. It is concluded that it is the structure of the monolayers formed, which determines their resistance towards HNO 3 uptake. Fatty acids (C18 and C15), which form a highly ordered film in the so-called liquid condensed state, represent a significant barrier towards HNO 3 uptake, while monolayers of shorter-chain fatty acids (C9, C12) and of the unsaturated oleic acid form a less ordered film in the liquid expanded state and do not hinder the uptake. Similarly, high contents of humic acids in the aerosol, a structurally inhomogeneous, quite water soluble mixture of oxidised high molecular weight organic compounds did not affect HNO 3 uptake. As surfactant films on naturally occurring aerosol Correspondence to: M. Ammann (markus.ammann@psi.ch) are expected to be less structured due to their chemical inhomogeneity, it is likely that their inhibitory effect on HNO 3 uptake is smaller than that observed here for the C15 and C18 fatty acid monolayers.

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

“…For the C 9 acid, which exhibits the highest water solubility among the fatty acids used in the present investigation, less than 1% of the number of molecules in a monolayer at ESP could be dissolved in a pure aqueous particle of the size as used in our experiments. For C 9 , we may also suspect some losses due to evaporation as already discussed by Stemmler et al (2008). However, as presented in detail below, it is more likely the monolayer properties that determine the higher permeability of the C 9 and C 12 films.…”

“…This figure should demonstrate that for higher mass fractions, the monolayer density and structure remained constant, while the excess material formed a separate phase increasing the overall particle surface area but not affecting the monolayer properties above the aqueous solution. As already pointed out by Stemmler et al (2008), we do not expect formation of micelles for these fatty acids. We are aware that many amphiphilic organic compounds may form micelles above the so called critical micelle concentration, where monolayers at the ESP and micelles may coexist.…”

“…The effectiveness of the surfactant coatings will be related to their properties, as provided in Gabler and Heumann (1993). The procedure to get a reproducible amount of fatty acid on the particles has already been described by Stemmler et al (2008). An organic vapor was generated by heating 0.5 g of surfactant in a temperature controlled reservoir.…”

“…McNeill et al (2006) found that the presence of sodium dodecyl sulfate (SDS) decreases the N 2 O 5 reaction probability on deliquesced NaCl particles. Stemmler et al (2008) studied the effect of surfactants on the uptake of nitric acid HNO 3 to deliquesced NaCl aerosol. They showed that the uptake coefficient was reduced by a factor of 5-50 when the aerosol was coated with fatty acids.…”

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

“…From the very high solubility of HNO 3 , the very fast rates for its dissociation in the aqueous phase, and the very fast protonation of chloride, we conclude that solubility, reaction or diffusion cannot limit the rate of uptake of HNO 3 , which was suspected to be responsible for the size and humidity dependence by Saul et al, Liu et al and also earlier by ten Brink (1998). We therefore recommend the bulk accommodation coefficient observed by Guimbaud et al (2002) and Stemmler et al (2008) as a lower limit. With NaCl solution in excess, HNO 3 -protons are nearly completely displaced into the gas phase as HCl at equilibrium due to the divergent behaviour of the activity coefficients of chloride and nitrate at high ionic strength Cl − and RSS (pH ≈ −0.7 to −1, RH 75 %-85 %)…”