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 NO 3 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 NO 3 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 −8 and (3.3 ± 0.6) × 10 −8 cm 2 molecule −1 s −1 for fitted initial desorption lifetimes of NO 3 at the closely packed organic monolayers, τ d,NO 3 ,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 NO 3 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 NO 3 -initiated oxidation of the saturated surfactant SA we estimated a loss rate of approximately (5 ± 1) × 10 −12 cm 2 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 NO 3 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 O 3 . Furthermore, the relative contributions of NO 3 and O 3 to the oxidative losses vary massively between species that are closely related in structure: NO 3 reacts ca. 400 times faster than O 3 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 NO 3 on the fate of saturated surfactants is slightly less clear given the lack of prior kinetic data for comparison, but NO 3 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 NO 3 -initiated oxidation of the methyl ester MO rapidly and effectively removes the organic character (≤ 3 % surface-active products). The film-forming potential of r...

Night-Time Oxidation of a Monolayer Model for the Air–Water Interface of Marine Aerosols—A Study by Simultaneous Neutron Reflectometry and in Situ Infra-Red Reflection Absorption Spectroscopy (IRRAS)

“…Figure 7 shows the neutron reflectometry data for this oxidation (black circles and left ordinate), and no significant reduction in the NR signal is observed. PA does not have a double bond and so is not susceptible to rapid attack by NO3• as are unsaturated species, therefore it would not be expected to react quickly on the timescales of our experiment (compare the stability of stearic acid monolayers to NO3• recently reported by Sebastiani et al [39]). The lack of any change in neutron reflectometry signal indicates that, as in the case of the single component GCB monolayer, the deuterated (saturated) chain remains at the surface.…”

“…NO 3 • was produced as described previously by Sebastiani et al [39] through a gas flow system by the reaction of an excess of NO 2 (Scientific and Technical Gases Ltd.; 1000 ppm in air ± 2%) with O 3 in a 5 L reaction bulb. O 3 was generated from O 2 (BOC; 99.999%) using a pen-ray ozoniser (model: SOG-2; Ultra-Violet Products Ltd., Cambridge, UK).…”

“…Understanding the night-time atmospheric ageing by NO 3 •, in addition to the chemistry of the dominant day-time oxidant OH• (e.g., [36]) and photochemical reactions that have recently received lots of attention (e.g., [37]) is therefore an important focus of current research. It is thus essential to investigate the NO 3 • chemistry at the air-water interface given the scarcity of previous studies [38,39].…”

“…A mixed monolayer of GCB and the unsaturated analogue of PA, palmitoleic acid (POA), was also oxidised in order to test how the introduction of a double bond affects the interaction between GCB and the fatty acid. Unlike saturated PA that is only subject to slow hydrogen abstraction [23,39], the double bond in POA reacts quickly with NO 3 • [39], so this system should behave rather differently.…”

Photochemical aging of atmospherically reactive organic compounds involving brown carbon at the air-aqueous interface

“…Langmuir monolayers at the air-aqueous interface were employed to mimic the organic films that coat the aqueous aerosols (Sobanska et al, 2015;Ruehl and Wilson, 2014;King et al, 2009;Sebastiani et al, 2018;Adams et al, 2016). The stability behaviour of these organic films under irradiation and the impact of photosensitizers on processing the aging of the aerosol are presented and discussed.…”