Abstract
This paper describes experiments on the ageing of a monolayer model for the air–water interface of marine aerosols composed of a typical glycolipid, galactocerebroside (GCB). Lipopolysaccharides have been observed in marine aerosols, and GCB is used as a proxy for these more complex lipopolysaccharides. GCB monolayers are investigated as pure films, as mixed films with palmitic acid, which is abundant in marine aerosols and forms a stable attractively mixed film with GCB, particularly with divalent salts present in the subphase, and as mixed films with palmitoleic acid, an unsaturated analogue of palmitic acid. Such mixed films are more realistic models of atmospheric aerosols than simpler single-component systems. Neutron reflectometry (NR) has been combined in situ with Fourier transform infra-red reflection absorption spectroscopy (IRRAS) in a pioneering analysis and reaction setup designed by us specifically to study mixed organic monolayers at the air–water interface. The two techniques in combination allow for more sophisticated observation of multi-component monolayers than has previously been possible. The structure at the air–water interface was also investigated by complementary Brewster angle microscopy (BAM). This study looks specifically at the oxidation of the organic films by nitrate radicals (NO3•), the key atmospheric oxidant present at night. We conclude that NO3• oxidation cannot fully remove a cerebroside monolayer from the surface on atmospherically relevant timescales, leaving its saturated tail at the interface. This is true for pure and salt water subphases, as well as for single- and two-component films. The behaviour of the unsaturated tail section of the molecule is more variable and is affected by interactions with co-deposited species. Most surprisingly, we found that the presence of CaCl2 in the subphase extends the lifetime of the unsaturated tail substantially—a new explanation for longer residence times of materials in the atmosphere compared to lifetimes based on laboratory studies of simplified model systems. It is thus likely that aerosols produced from the sea-surface microlayer at night will remain covered in surfactant molecules on atmospherically relevant timescales with impact on the droplet’s surface tension and on the transport of chemical species across the air–water interface.
Highlights
In papers published in 1983 [1] and 1999 [2], it was noted that organic films formed on the surface of atmospheric aerosol particles and that the reactivity both of the aerosol particles underneath and of the species that make up the organic films themselves were different from what would be expected if each were encountered individually in the atmosphere
The films’ effects on the physical nature of the water droplets onto which they partition is of vital importance, as the surface tension of the droplets is lowered by the presence of these films [3], meaning that the removal of such films by oxidation can cause an increase in surface tension
The present study investigates the oxidation of model marine aerosol systems via reaction with nitrate radicals (NO3 ), the key atmospheric oxidant present at night [23,24]
Summary
In papers published in 1983 [1] and 1999 [2], it was noted that organic films formed on the surface of atmospheric aerosol particles and that the reactivity both of the aerosol particles underneath and of the species that make up the organic films themselves were different from what would be expected if each were encountered individually in the atmosphere. The relationship between aerosol concentrations and the radiative forcing attributable to clouds is exceptionally complex, poorly understood, and difficult to measure [4] The study of these organic films at the air–water interface may form a key part of solving this puzzle. Forestieri et al [19] studied the surface tension effects on sea spray aerosol mimics (oleic, palmitic, myristic acids, and their mixtures) using a surface film model to establish the properties that surface-active organic molecules must have to substantially impact on the aerosol’s activation efficiency They found that the properties of surface-active marine-derived organic molecules must differ substantially from the simple fatty acids they studied e.g., by having larger molecular areas
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