Using Nanoparticle X-ray Spectroscopy to Probe the Formation of Reactive Chemical Gradients in Diffusion-Limited
Michael I Jacobs1,2, Bo Xu2, Oleg Kostko2
1Department of Chemistry , University of California , Berkeley , California 94720 , United States.
The Journal of Physical Chemistry. A
|July 9, 2019
Summary
Chemical gradients form in viscous aerosol particles during atmospheric aging. Slower diffusion limits reactions, creating oxidized surfaces and impacting particle behavior.
Area of Science:
- Atmospheric Chemistry
- Chemical Kinetics
- Aerosol Science
Background:
- Photochemical aging of atmospheric aerosols can lead to steep chemical gradients due to diffusion limitations.
- Understanding these gradients is crucial for predicting aerosol behavior and atmospheric processes.
Purpose of the Study:
- To investigate the formation and evolution of chemical gradients in aerosol particles.
- To determine how diffusion limitations and phase separation impact particle reactivity during oxidation.
Main Methods:
- Studied heterogeneous reaction of hydroxyl radicals (OH) on squalane and octacosane particles and their mixtures.
- Utilized aerosol mass spectrometry to measure uptake coefficients and bulk oxidation kinetics.
- Employed X-ray photoemission and X-ray absorption spectroscopy to analyze surface elemental composition and oxidation.
Main Results:
- Efficient OH uptake (γeff ∼ 0.3) observed for low-viscosity squalane, limited by OH arrival.
- Mixing-limited regime (γeff ∼ 0.03) observed for high-viscosity octacosane, with reactions significantly slower than diffusion.
- Surface oxidation rates were higher for octacosane (8.6 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹) than squalane (3.0 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹).
- A 1-2 nm oxidized crust formed on octacosane, while squalane showed homogeneous mixing.
Conclusions:
- Diffusion limitations significantly influence aerosol oxidation pathways.
- Highly oxidized particle surfaces can form even at low oxidant exposures.
- These findings impact the understanding of aerosol microphysics and atmospheric chemistry.
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