Exploring the Nanostructures Accessible to an Organic Surfactant Atmospheric Aerosol Proxy

Abstract

The composition of atmospheric aerosols varies with time, season, location, and environment. This affects key aerosol properties such as hygroscopicity and reactivity, influencing the aerosol’s impact on the climate and air quality. The organic fraction of atmospheric aerosol emissions often contains surfactant material, such as fatty acids. These molecules are known to form three-dimensional nanostructures in contact with water. Different nanostructures have marked differences in viscosity and diffusivity that are properties whose understanding is essential when considering an aerosol’s atmospheric impact. We have explored a range of nanostructures accessible to the organic surfactant oleic acid (commonly found in cooking emissions), simulating variation that is likely to happen in the atmosphere. This was achieved by changing the amount of water, aqueous phase salinity and by addition of other commonly coemitted compounds: sugars and stearic acid (the saturated analogue of oleic acid). The nanostructure was observed by both synchrotron and laboratory small/wide angle X-ray scattering (SAXS/WAXS) and found to be sensitive to the proxy composition. Additionally, the spacing between repeat units in these nanostructures was water content dependent (i.e., an increase from 41 to 54 Å in inverse hexagonal phase d-spacing when increasing the water content from 30 to 50 wt %), suggesting incorporation of water within the nanostructure. A significant decrease in mixture viscosity was also observed with increasing water content from ∼104 to ∼102 Pa s when increasing the water content from 30 to 60 wt %. Time-resolved SAXS experiments on levitated droplets of this proxy confirm the phase changes observed in bulk phase mixtures and demonstrate that coexistent nanostructures can form in droplets. Aerosol compositional and subsequent nanostructural changes could affect aerosol processes, leading to an impact on the climate and urban air pollution.

Molecular Self-Organization in Surfactant Atmospheric Aerosol Proxies

“…We carried out a systematic study of the phase-composition relationship for the oleic acid–sodium oleate system in both bulk mixtures and levitated droplets . The addition of compounds commonly co-emitted with oleic acid such as sugars (glucose, fructose, and sucrose) and a saturated fatty acid (stearic acid) altered the observed LLC phase. , In some instances, a difference in the sugar added meant the difference between an inverse hexagonal phase (opaque and directionally dependent diffusion) and a close-packed inverse micellar phase (viscous and translucent). More complex mixtures informed by atmospheric measurements and with up to 6 components mostly returned the inverse hexagonal and close-packed inverse micellar LLC phases.…”

“…34−36 Recent fieldwork has demonstrated that the trans form of oleic acid, elaidic acid, authors suggested that the steric arrangement for elaidic acid would be similar to that for saturated fatty acids, which are known to crystallize to form solid or semisolid phases. 3 The formation of viscous phases in aerosol particles may provide an explanation for this observation. The oleic acid−ozone reaction is therefore an ideal candidate for the studies we present here due to this discrepancy and the abundant literature for comparison.…”

“…Films made of deposited aerosol particles and condensed semivolatile species can form on indoor surfaces such as windows and furniture. , Indoor surface chemistry is particularly important due to the higher surface-to-volume ratio indoors compared to in an urban environment, increasing the likelihood of film–pollutant interactions. Laboratory experiments on films deposited on solid substrates, such as those presented here, , are needed to help us understand how these films are likely to age indoors.…”

“…There remains a discrepancy between the longer measured atmospheric lifetime of oleic acid and the chemical lifetime measured in the laboratory. − Recent fieldwork has demonstrated that the trans form of oleic acid, elaidic acid, reacts 38 ± 5% slower with ozone than oleic acid . The authors suggested that the steric arrangement for elaidic acid would be similar to that for saturated fatty acids, which are known to crystallize to form solid or semisolid phases . The formation of viscous phases in aerosol particles may provide an explanation for this observation.…”

Advances in Atmospheric Chemical and Physical Processes

“…Laboratory studies of the air–water interface demonstrate surface adsorption of H 2 O 2 , as well as determining the structure of hydroxy organic acids of varying size at the interface . Additional work addresses adsorption–desorption of semivolatile VOCs on mineral surfaces, and reactive uptake of HgO and a recently discovered dimethyl sulfide oxidation product (hydroperoxymethyl thioformate) on aerosols of various compositions. , The effects of aerosol properties on ice nucleation and on the partitioning of semivolatiles, , the effects of pH on organosulfate structures in the condensed phase, and the accessibility of various nanostructures for surfactants on atmospheric aerosol proxies are also reported. Wilson, Willis, and co-workers , provide model frameworks for trace gas uptake and reaction in aerosol or microdroplets, and for multiphase ozonolysis reactions.…”