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Aerosol microdroplets exhibit a stable pH gradient.

Haoran Wei1,2,3, Eric P Vejerano1,2,3,4, Weinan Leng1,2,3

  • 1Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA 24061.

Proceedings of the National Academy of Sciences of the United States of America
|June 27, 2018
PubMed
Summary
This summary is machine-generated.

Measuring aerosol droplet pH is crucial for atmospheric chemistry. Nanoparticle probes revealed core pH is higher than bulk, impacting reactions.

Keywords:
SERSaerosoldropletinterfacepH

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Area of Science:

  • Atmospheric chemistry
  • Environmental science
  • Nanotechnology

Background:

  • Aqueous aerosol droplets (<50 µm) act as microreactors for atmospheric reactions.
  • Droplet pH is a critical factor influencing chemical and biological processes.
  • Measuring individual droplet pH is challenging due to probe inaccessibility.

Purpose of the Study:

  • To develop and utilize a novel method for quantifying pH distribution within individual aerosol droplets.
  • To investigate the spatial pH gradient within aerosol droplets and its relationship to the air/water interface.
  • To assess the impact of this pH gradient on atmospheric chemical reactions.

Main Methods:

  • Utilized 4-mercaptobenzoic acid-functionalized gold nanoparticle pH nanoprobes.
  • Employed 2D and 3D laser confocal Raman microscopy for droplet scanning.
  • Applied surface-enhanced Raman scattering (SERS) for pH quantification.

Main Results:

  • Determined pH distribution within approximately 20-µm-diameter phosphate-buffered aerosol droplets.
  • Observed a significant pH increase (up to 3.6 units) in the droplet core compared to bulk solution.
  • Confirmed the pH shift by observing a base-catalyzed reaction occurring in the droplet core but not in bulk solution.

Conclusions:

  • A significant pH gradient exists within aerosol droplets, with a higher pH in the core.
  • Proton accumulation at the air/water interface likely causes this observed pH shift.
  • This spatial pH gradient has critical implications for understanding acid-base-catalyzed atmospheric chemistry.