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Liquid-liquid phase separation in aerosol particles: imaging at the nanometer scale
Rachel E O'Brien1, Bingbing Wang2, Stephen T Kelly1
1†Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720-8198, United States.
Environmental Science & Technology
|April 9, 2015
Summary
Atmospheric aerosols exhibit liquid-liquid phase separation (LLPS) influenced by relative humidity (RH). Chemical imaging revealed organic components predominantly in the outer phase of ammonium sulfate mixtures, indicating complex aerosol behavior.
Area of Science:
- Atmospheric Chemistry
- Materials Science
- Chemical Physics
Background:
- Atmospheric aerosols are crucial for climate and air quality.
- Aerosol particles undergo phase transitions, including liquid-liquid phase separation (LLPS), with changing relative humidity (RH).
- Understanding LLPS is vital for accurate atmospheric modeling.
Purpose of the Study:
- To investigate the LLPS behavior of internally mixed aerosol particles.
- To analyze the influence of different organic compounds on LLPS.
- To characterize the phase separation dynamics during hydration-dehydration cycles.
Main Methods:
- Utilized environmental scanning electron microscopy (ESEM) and scanning transmission X-ray microscopy (STXM) for chemical imaging.
- Studied micrometer-sized particles composed of ammonium sulfate (AS) mixed with limonene secondary organic carbon (LSOC), HMMA, or PEG-400.
- Performed in situ analysis during a full hydration-dehydration cycle.
Main Results:
- Observed LLPS events in all studied LSOC/AS, HMMA/AS, and PEG-400/AS particles using both ESEM and STXM.
- STXM revealed that LSOC/AS and HMMA/AS particles were never fully homogeneous above the deliquescence point, with organics primarily in the outer phase.
- Estimated outer phase compositions: LSOC/AS (65:35 organic:inorganic), HMMA/AS (50:50), and PEG-400/AS (70:30) below 89-92% RH.
Conclusions:
- ESEM and STXM are effective techniques for in situ analysis of aerosol hygroscopic behavior and phase separation.
- The organic component's location and phase separation dynamics are dependent on the specific organic compound and RH.
- Findings provide insights into aerosol particle evolution and its impact on atmospheric processes.
