Ozonolysis of fatty acid monolayers at the air–water interface: organic films may persist at the surface of atmospheric aerosols

Benjamin Woden,Christian Pfrang,James Tellam,Adam Milsom,Curtis Gubb,Armando Maestro,Maximilian W A Skoda

doi:10.5194/acp-21-1325-2021

Abstract

Abstract. Ozonolysis of fatty acid monolayers was studied to understand the fate of organic-coated aerosols under realistic atmospheric conditions. Specifically, we investigated the effects of temperature and salinity on the degradation of oleic acid at the air–water interface and the persistence of the aged surfactant film at the surface. The presence of a residual film is of atmospheric importance, as surface monolayers affect the physical properties of the droplets and because of the role they play in cloud formation. This occurs via several effects, most notably via surface tension reduction. The interplay between atmospheric aerosol loading and the formation, nature, and persistence of clouds is a key uncertainty in climate modelling. Our data show that a residual surface film, which we suspect to be formed of nonanoic acid and a mixture of azelaic and 9-oxononanoic acids, is retained at the interface after ozonolysis at near-zero temperatures but not at room temperature. Given the low-temperature conditions used here are atmospherically realistic, the persistence of a product film must be considered when assessing the impact of unsaturated fatty acid partitioned to the air–water interface. The presence of stable (non-oxidisable) reaction products also opens the possibility of build-up of inert monolayers during the aerosol life cycle with potential implications for cloud formation. Furthermore, we measured the kinetic behaviour of these films and found that the reactions are not significantly affected by the shift to a lower temperature with rate coefficients determined to be (2.2 ± 0.4) × 10−10 cm2 s−1 at 21 ± 1 ∘C and (2.2 ± 0.2) × 10−10 cm2 s−1 at 2 ± 1 ∘C.

Highlights

The present study focuses on monolayers at the air–water interface as would be seen on aqueous aerosol droplets, but the concept is broadly transferrable to any aerosol particle with an organic film coating
This research was performed on the specular neutron reflectometry instruments INTER at the ISIS Neutron and Muon Source and FIGARO at Institut Laue–Langevin (ILL) and builds on previous work by this research group on the oxidation of floating monolayers at the air–water interface performed at these facilities (Pfrang et al, 2014; Sebastiani et al, 2015; Skoda et al, 2017; Sebastiani et al, 2018; Woden et al, 2018)
Prior to the ozonolysis studies we characterised the stability of the oleic acid monolayers at room and reduced temperatures

Summary

Introduction

Organic films are formed at the surfaces of aerosol particles in the atmosphere (Gill et al, 1983; Ellison et al, 1999; Sareen et al, 2013; Nozière et al, 2014 Kroflicet al., 2018; Gerard et al, 2019), and the partitioning of organic components in this manner changes the physical properties of the aerosol particle and its chemical reactivity (Rudich, 2003; Ruehl et al, 2016; Ovadnevaite et al, 2017). An important area of interest regarding these films concerns the interactions between atmospheric aerosol particles and clouds. This relationship is complex and difficult to measure or predict (Stevens and Feingold, 2009), and resolving the role played by organic monolayers at the surface of aerosol particles is part of solving this puzzle. The organic species that are contained in these surface films oxidise in the atmosphere and may produce low-volatility products that form secondary organic aerosols. If the oxidation of these species is prevented, accelerated, or otherwise modified by the partitioning of the reactant into a surface monolayer, this will have impli-

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