The uptake of O₃by myristic acid–oleic acid mixed particles: evidence for solid surface layers

Abstract

The oleic acid ozonolysis in mixed oleic and myristic acid particles was studied in a flow tube reactor using single particle mass spectrometry. The change in reactivity was investigated as a function of the myristic acid concentration in these 2 micron particles. For pure oleic acid aerosol, the reactive ozone uptake coefficient, gamma, was found to be 3.4 (+/-0.3) x 10(-4) after taking secondary reactions into account. At the myristic acid crystallization point, where only 2.5% of the particle is in the solid phase, the uptake coefficient was reduced to 9.7 (+/-1.0) x 10(-5). This dramatic drop in the uptake coefficient is explained by the presence of a crystalline monolayer of myristic acid, through which ozone diffusion is reduced by several orders of magnitude, relative to liquid oleic acid. Scanning electron microscope images of the mixed particles confirm that the particle surface is crystalline when the myristic acid mole fraction exceeds 0.125. The findings of these experiments illustrate that particle morphology is important to understanding the reactivity of species in a mixed particle. The decay of myristic acid during the course of ozonolysis is explained in terms of a reaction with stabilized Criegee intermediates, which attack the acidic groups of the oleic and myristic acids with equal rate constants.

The ozonolysis of primary aliphatic amines in fine particles

“…This may impede the diffusion of ozone into the particle causing retention of OL at these very high exposures, where it would be totally consumed in single component particles of OL. This observation and similar ones by others (Nash et al, 2006) in fatty acid rich particulate, along with the retention of OL even at high exposures, may help elucidate the disparity that exists between the lifetime of OL measured in the field vs. in the laboratory (Zahardis and Petrucci, 2007). The formation of high molecular weight amides is likely to occur in meat-cooking aerosols that are rich in both fatty acids and amines and that are formed under conditions of high temperature (facilitating Route 1 amide formation).…”

“…This suggests that at a critical level of ozone exposure, the particles develop a solid or highly viscous liquid surface layer that impedes the diffusion of ozone into the particle. Similar effects have been observed in the formation of solid surface layers with ozonized myristic acid + OL particles (Nash et al, 2006). Figure 8 compares the PERCI ion signal of OL (281 m/z) to the integrated ion signal of the 5 high molecular weight products (422, 438, 576, 592 and 608 m/z).…”

The oleic acid-ozone heterogeneous reaction system: products, kinetics, secondary chemistry, and atmospheric implications of a model system – a review

“…As shall be discussed, the heterogeneous ozonolysis of OL and related compounds results in the formation of a medium rich in peroxides, including SO (LaFranchi et al, 2004;Zahardis et al, 2005;Ziemann, 2005;Mochida et al, 2006) and other peroxidic products (Hung et al, 2005;Zahardis et al, 2005Zahardis et al, , 2006aZiemann, 2005;Mochida et al, 2006;Nash et al, 2006). As will be explained, these products arise primarily from the reactivity of stabilized CI.…”

“…This method has recently been applied to the ozonolysis of mixed particles of OL and C14, revealing the presence of a solid surface layer and showing the effects of this surface layer on the reactive uptake of ozone (Nash et al, 2006). In these experiments, particles pass through an adjustable injector into a flow tube where they are reacted with ozone.…”

“…To date, aerosol-ozone studies of OL that have focused on kinetics, mechanism and/or product evolution have utilized pure OL particles Hearn and Smith, 2004b;Morris et al, 2002;Smith et al, 2002;Zahardis et al, 2005Zahardis et al, , 2006aZiemann, 2005), internally mixed particles containing OL Smith, 2004b, 2005;Katrib et al, 2005a;Ziemann, 2005;Mochida et al, 2006;Nash et al, 2006), and OL coatings on inert latex spheres (Katrib et al, 2004(Katrib et al, , 2005b. Very recently, complex OA such as meat-cooking (Hearn and Smith, 2006) and olive oil aerosols (Zahardis et al, 2006c), both with high OL content, have been evaluated.…”