An organic crystalline state in ageing atmospheric aerosol proxies: spatially resolved structural changes in levitated fatty acid particles

DOI: 10.5194/acp-21-15003-2021

Authors: Adam Milsom (University of Birmingham) , Adam M. Squires (University of Reading) , Jacob A. Boswell (University of Bath) , Nicholas J. Terrill (Diamond Light Source) , Andrew D. Ward (Central Laser Facility) , Christian Pfrang (University of Birmingham; University of Reading)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Atmospheric Chemistry And Physics , VOL 21 , PAGES 15003 - 15021

State: Published (Approved)
Published: October 2021
Diamond Proposal Number(s): 20541 , 21663

Abstract: Organic aerosols are key components of the Earth's atmospheric system. The phase state of organic aerosols is known to be a significant factor in determining aerosol reactivity, water uptake and atmospheric lifetime – with wide implications for cloud formation, climate, air quality and human health. Unsaturated fatty acids contribute to urban cooking emissions and sea spray aerosols. These compounds, exemplified by oleic acid and its sodium salt, are surface-active and have been shown to self-assemble into a variety of liquid-crystalline phases upon addition of water. Here we observe a crystalline acid–soap complex in acoustically levitated oleic acid–sodium oleate particles. We developed a synchrotron-based simultaneous small-angle and wide-angle X-ray scattering (SAXS and WAXS)–Raman microscopy system to probe physical and chemical changes in the proxy during exposure to humidity and the atmospheric oxidant ozone. We present a spatially resolved structural picture of a levitated particle during humidification, revealing a phase gradient consisting of a disordered liquid crystalline shell and crystalline core. Ozonolysis is significantly slower in the crystalline phase compared with the liquid phase, and a significant portion (34 ± 8 %) of unreacted material remains after extensive oxidation. We present experimental evidence of inert surface layer formation during ozonolysis, taking advantage of spatially resolved simultaneous SAXS–WAXS experiments. These observations suggest that atmospheric lifetimes of surface-active organic species in aerosols are highly phase-dependent, potentially impacting climate, urban air quality and long-range transport of pollutants such as polycyclic aromatic hydrocarbons (PAHs).

Journal Keywords: atmospheric chemistry; aerosols; atmosphere; environment

Subject Areas: Chemistry, Environment


Instruments: I22-Small angle scattering & Diffraction

Added On: 14/10/2021 10:38

Documents:
acp-21-15003-2021.pdf

Discipline Tags:

Desertification & Pollution Earth Sciences & Environment Atmospheric Processes Chemistry Organic Chemistry

Technical Tags:

Scattering Small Angle X-ray Scattering (SAXS)