Exploring the nanostructures accessible to an organic surfactant atmospheric aerosol proxy

DOI: 10.1021/acs.jpca.2c04611

Authors: Adam Milsom (University of Birmingham) , Adam M. Squires (University of Bath) , Isabel Quant (University of Bristol) , Nicholas J. Terrill (Diamond Light Source) , Steven Huband (University of Warwick) , Ben Woden (University of Reading) , Edna Cabrera-Martinez (University of Reading) , Christian Pfrang (University of Reading)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: The Journal Of Physical Chemistry A , VOL 2

State: Published (Approved)
Published: September 2022
Diamond Proposal Number(s): 28020 , 15121

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.

Journal Keywords: Aerosols; Carbohydrates; Mixtures; Nanostructures; Phase transitions

Subject Areas: Chemistry, Materials, Environment


Instruments: I22-Small angle scattering & Diffraction

Added On: 29/09/2022 14:26

Documents:
acs.jpca.2c04611.pdf

Discipline Tags:

Desertification & Pollution Earth Sciences & Environment Climate Change Atmospheric Processes Chemistry Materials Science Organic Chemistry

Technical Tags:

Scattering Small Angle X-ray Scattering (SAXS) Wide Angle X-ray Scattering (WAXS)