Nighttime oxidation of surfactants at the air–water interface: effects of chain length, head group and saturation

Abstract. Reactions of the key atmospheric nighttime oxidant NO₃ with organic monolayers at the air–water interface are used as proxies for the ageing of organic-coated aqueous aerosols. The surfactant molecules chosen for this study are oleic acid (OA), palmitoleic acid (POA), methyl oleate (MO) and stearic acid (SA) to investigate the effects of chain length, head group and degree of unsaturation on the reaction kinetics and products formed. Fully and partially deuterated surfactants were studied using neutron reflectometry (NR) to determine the reaction kinetics of organic monolayers with NO₃ at the air–water interface for the first time. Kinetic modelling allowed us to determine the rate coefficients for the oxidation of OA, POA and MO monolayers to be (2.8±0.7) × 10⁻⁸, (2.4±0.5) × 10⁻⁸and (3.3±0.6) × 10⁻⁸ cm² molecule⁻¹ s⁻¹ for fitted initial desorption lifetimes of NO₃ at the closely packed organic monolayers, τ_{d, NO3, 1}, of 8.1±4.0, 16±4.0 and 8.1±3.0 ns, respectively. The approximately doubled desorption lifetime found in the best fit for POA compared to OA and MO is consistent with a more accessible double bond associated with the shorter alkyl chain of POA facilitating initial NO₃ attack at the double bond in a closely packed monolayer. The corresponding uptake coefficients for OA, POA and MO were found to be (2.1±0.5) × 10⁻³, (1.7±0.3) × 10⁻³ and (2.1±0.4) × 10⁻³, respectively. For the much slower NO₃-initiated oxidation of the saturated surfactant SA we estimated a loss rate of approximately (5±1) × 10⁻¹² cm² molecule⁻¹ s⁻¹, which we consider to be an upper limit for the reactive loss, and estimated an uptake coefficient of ca. (5±1) × 10⁻⁷. Our investigations demonstrate that NO₃ will contribute substantially to the processing of unsaturated surfactants at the air–water interface during nighttime given its reactivity is ca. 2 orders of magnitude higher than that of O₃. Furthermore, the relative contributions of NO₃ and O₃ to the oxidative losses vary massively between species that are closely related in structure: NO₃ reacts ca. 400 times faster than O₃ with the common model surfactant oleic acid, but only ca. 60 times faster with its methyl ester MO. It is therefore necessary to perform a case-by-case assessment of the relative contributions of the different degradation routes for any specific surfactant. The overall impact of NO₃ on the fate of saturated surfactants is slightly less clear given the lack of prior kinetic data for comparison, but NO₃ is likely to contribute significantly to the loss of saturated species and dominate their loss during nighttime. The retention of the organic character at the air–water interface differs fundamentally between the different surfactant species: the fatty acids studied (OA and POA) form products with a yield of  ∼ 20 % that are stable at the interface while NO₃-initiated oxidation of the methyl ester MO rapidly and effectively removes the organic character ( ≤ 3 % surface-active products). The film-forming potential of reaction products in real aerosol is thus likely to depend on the relative proportions of saturated and unsaturated surfactants as well as the head group properties. Atmospheric lifetimes of unsaturated species are much longer than those determined with respect to their reactions at the air–water interface, so they must be protected from oxidative attack, for example, by incorporation into a complex aerosol matrix or in mixed surface films with yet unexplored kinetic behaviour.