Ozonolysis of methyl oleate monolayers at the air-water interface: oxidation kinetics, reaction products and atmospheric implicationsElectronic supplementary information (ESI) available: Detailed information about the neutron data analysis can be found in part A of the ESI; the kinetic analysis of all the oxidative decays recorded is presented in part B; part C presents a comparison of the behaviour of oleic acid and its methyl ester at the air-water interface and part D discusses linear vs. exp

• Main Authors: Pfrang, Christian, Sebastiani, Federica, Lucas, Claire O. M, King, Martin D, Hoare, Ioan D, Chang, Debby, Campbell, Richard A
• Format: Journal Article
• Language: English
• Published: 11.06.2014
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c4cp00775a

Ozonolysis of methyl oleate monolayers at the air-water interface results in surprisingly rapid loss of material through cleavage of the C&z.dbd;C bond and evaporation/dissolution of reaction products. We determine using neutron reflectometry a rate coefficient of (5.7 ± 0.9) × 10 −10 cm 2 molecule −1 s −1 and an uptake coefficient of ∼3 × 10 −5 for the oxidation of a methyl ester monolayer: the atmospheric lifetime is ∼10 min. We obtained direct experimental evidence that <2% of organic material remains at the surface on atmospheric timescales. Therefore known long atmospheric residence times of unsaturated fatty acids suggest that these molecules cannot be present at the interface throughout their ageing cycle, i.e. the reported atmospheric longevity is likely to be attributed to presence in the bulk and viscosity-limited reactive loss. Possible reaction products were characterized by ellipsometry and uncertainties in the atmospheric fate of organic surfactants such as oleic acid and its methyl ester are discussed. Our results suggest that a minor change to the structure of the molecule (fatty acid vs. its methyl ester) considerably impacts on reactivity and fate of the organic film. Ozonolysis of a methyl oleate monolayer results in its surprisingly rapid loss suggesting atmospheric lifetimes of organic material at the air-water interface of only ∼10 min.