Imaging of pH distribution inside individual microdroplet by stimulated Raman microscopy

Area of Science:

• Environmental Chemistry
• Atmospheric Science
• Physical Chemistry

Background:

• Atmospheric microdroplets act as crucial microreactors for numerous chemical reactions.
• The spatial distribution of pH and chemical species within these microdroplets is poorly understood.
• Accurate measurement of pH distribution without disturbing chemical species is challenging.

Purpose of the Study:

• To develop and demonstrate a non-contact method for visualizing 3D pH distribution in single atmospheric microdroplets.
• To investigate the influence of microdroplet size on spatial pH distribution.
• To provide insights into the chemical processes occurring within atmospheric aerosols.

Main Methods:

• Utilized stimulated Raman scattering microscopy for non-contact pH measurement.
• Visualized the three-dimensional pH distribution within individual microdroplets.
• Employed molecular dynamics simulations to support experimental findings.

Main Results:

• Observed that the surface of all microdroplets is more acidic than the interior.
• Found a monotonic decrease in pH from the center to the edge in a 2.9-μm aerosol microdroplet.
• Identified size-dependent pH distribution differences between small aerosol and larger cloud microdroplets, linked to surface-to-volume ratio.

Conclusions:

• The study successfully demonstrated a novel non-contact method for mapping 3D pH distribution in microdroplets.
• Spatial pH distribution within microdroplets is size-dependent, influenced by surface-to-volume effects.
• This research enhances the understanding of chemical heterogeneity in atmospheric aerosols and their reactions.