Oxidation of oleic acid at the air–water interface and its potential effects on cloud critical supersaturations

Abstract

The oxidation of organic films on cloud condensation nuclei has the potential to affect climate and precipitation events. In this work we present a study of the oxidation of a monolayer of deuterated oleic acid (cis-9-octadecenoic acid) at the air-water interface by ozone to determine if oxidation removes the organic film or replaces it with a product film. A range of different aqueous sub-phases were studied. The surface excess of deuterated material was followed by neutron reflection whilst the surface pressure was followed using a Wilhelmy plate. The neutron reflection data reveal that approximately half the organic material remains at the air-water interface following the oxidation of oleic acid by ozone, thus cleavage of the double bond by ozone creates one surface active species and one species that partitions to the bulk (or gas) phase. The most probable products, produced with a yield of approximately (87 +/- 14)%, are nonanoic acid, which remains at the interface, and azelaic acid (nonanedioic acid), which dissolves into the bulk solution. We also report a surface bimolecular rate constant for the reaction between ozone and oleic acid of (7.3 +/- 0.9) x 10(-11) cm2 molecule s(-1). The rate constant and product yield are not affected by the solution sub-phase. An uptake coefficient of ozone on the oleic acid monolayer of approximately 4 x 10(-6) is estimated from our results. A simple Kohler analysis demonstrates that the oxidation of oleic acid by ozone on an atmospheric aerosol will lower the critical supersaturation needed for cloud droplet formation. We calculate an atmospheric chemical lifetime of oleic acid of 1.3 hours, significantly longer than laboratory studies on pure oleic acid particles suggest, but more consistent with field studies reporting oleic acid present in aged atmospheric aerosol.

Kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB): the influence of interfacial transport and bulk diffusion on the oxidation of oleic acid by ozone

“…Our model results suggest that the transformation of oleic acid particles will occur on a timescale of ∼1 h depending on atmospheric O 3 concentration and particle size. This is in good agreement with the characteristic time of 1.3 h reported by King et al (2009).…”

“…The surface reaction rate coefficient k SLR,X,Y = 6 × 10 −12 cm 2 s −1 was adopted from Pfrang et al (2009). Note that this value is an order of magnitude lower than reported value by other studies (Gonzalez-Labrada et al, 2007;King et al, 2009). Bulk diffusion coefficients were adopted from earlier studies (D b,X = 10 −5 cm 2 s −1 , D b,Y = 10 −10 cm 2 s −1 ) (Smith et al, 2002(Smith et al, , 2003, and the parameters of reversible adsorption were adjusted to match the experimental data of oleic acid decay (α s,0,X = 4.2 × 10 −4 and τ d,X = 0.01 s).…”

“…The ozonolysis of oleic acid leads to an increase of oxygen content and hygroscopicity (Asad et al, 2004), and it can convert CCN-inactive particles into CCN (King et al, 2009). Our model results suggest that the transformation of oleic acid particles will occur on a timescale of ∼1 h depending on atmospheric O 3 concentration and particle size.…”

“…We demonstrate the applicability of KM-SUB for a common model system of organic aerosol chemistry, the ozonolysis of oleic acid droplets (e.g., Moise and Rudich, 2002;Katrib et al, 2004Katrib et al, , 2005King et al, 2004King et al, , 2009Knopf et al, 2005;Ziemann, 2005;Hearn and Smith, 2007;Pfrang et al, 2009;Sage et al, 2009;Vesna et al, 2009), and we compare the results of our numerical simulations with the results of earlier experimental and theoretical studies of this system.…”

“…It is often difficult to discriminate surface and bulk reactions, and the relative importance of surface and bulk processes is not well understood (e.g., Moise and Rudich, 2000;Hearn et al, 2005;Pfrang et al, 2009). Resistor model formulations are widely used to describe and investigate heterogeneous reactions and multiphase processes in laboratory, field and model studies of atmospheric chemistry (Hanson, 1997;Finlayson-Pitts and Pitts, 2000;Worsnop et al, 2002;Anttila et al, 2006;King et al, 2008King et al, , 2009and references therein). The traditional resistor models, however, are usually based on simplifying assumptions such as steady-state conditions, homogeneous mixing, and limited numbers of non-interacting species and processes.…”