Phase Behavior of Polluted and Clean Urban Winter PM_2.5 from Optical Observations and Thermodynamic Equilibrium...

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Physical Chemistry

Background:

• The phase state of fine particles (PM$_{2.5}$) is crucial for understanding atmospheric chemistry.
• Real-world constraints on PM$_{2.5}$ phase behavior remain poorly understood.

Purpose of the Study:

• To investigate the phase behavior of urban PM$_{2.5}$ under ambient conditions.
• To determine how humidity and composition affect PM$_{2.5}$ phase transitions.

Main Methods:

• Combined optical microscopy, poke-and-flow experiments, and thermodynamic modeling.
• Analyzed wintertime urban PM$_{2.5}$ collected in Ansan, South Korea.

Main Results:

• Observed humidity- and composition-dependent phase transitions, including liquid-liquid and liquid-liquid-solid states.
• Nitrate-rich aerosols showed liquid-like behavior near equilibrium.
• Organic-rich aerosols exhibited higher viscosity and deviations from equilibrium.

Conclusions:

• Ambient PM$_{2.5}$ displays significant phase complexity.
• Particle phase state critically influences thermodynamic behavior and atmospheric reactivity.
• Relative humidity and composition are key modulators of PM$_{2.5}$ phase behavior.