Optical Interrogation of Single Levitated Droplets in a Linear Quadrupole Trap by Cavity Ring-Down Spectroscopy

Area of Science:

• Atmospheric science
• Optical physics
• Aerosol science

Background:

• Optical trapping is limited to spherical, non-absorbing particles.
• A universal trap for diverse aerosol morphology and absorption is needed.
• Electrodynamic traps offer new possibilities for aerosol levitation.

Purpose of the Study:

• To develop and demonstrate a novel electrodynamic trap combined with cavity ring-down spectroscopy.
• To enable simultaneous measurement of extinction cross sections and elastic scattering phase functions for single aerosol particles.
• To characterize optical properties of diverse aerosol types, including nonspherical and crystallized particles.

Main Methods:

• Utilized a combined electrodynamic linear quadrupole trap and cavity ring-down spectrometer.
• Investigated evaporation of 1,2,6-hexanetriol and evolution of (NH4)2SO4 and NaCl droplets.
• Measured extinction cross sections of crystallized inorganic salt particles.
• Compared experimental data with Mie theory and T-matrix/EBCM light scattering models.

Main Results:

• Successfully trapped and measured optical properties of single aerosol droplets and crystallized particles.
• Found dry (NH4)2SO4 particles are well-represented by spheres (Mie theory).
• Determined dry NaCl particles require nonspherical models for accurate cross-section reconciliation.
• Demonstrated capability for characterizing nonspherical and light-absorbing aerosols.

Conclusions:

• The novel electrodynamic trap platform enables precise optical property measurements of diverse single aerosol particles.
• Results impact remote sensing and radiative forcing calculations by improving aerosol optical property parameterizations.
• This technique advances accurate characterization of nonspherical and absorbing aerosols for atmospheric science applications.