The Impacts of Phase Separation and Particle Shape on Aerosol Optical Properties Measured using Single Particle Cavity Ring-Down Spectroscopy

Atmospheric aerosol particles regulate Earth's climate, yet their description represents one of the
largest uncertainties in climate models. In particular, the interactions of light with atmospheric
particles of varying shape and internal structure are understood poorly. The optical properties of
morphologically complex aerosols are also important in understanding disease transmission. For
example, pathogen-containing droplets emitted during human exhalation are thought to exhibit
multiple phases,1,2 and understanding their optical properties is central to designing sterilisation
devices that could improve public health. Liquid droplets adopt spherical shapes for which the
calculation of optical properties are straightforward. However,solid particles (such as those formed
through drying of aqueous droplets that contain dissolved salts) adopt a wide range of shapes that
scatter and absorb light in complex ways. Particles also adopt a wide range of internal structures.

For example, an aqueous particle comprised of a mixture of organic and inorganic species may
undergo liquid-liquid phase separation (LLPS), the optical properties for which will differ from those
expected of a homogeneous particle.

Few studies have measured the impacts of shape or LLPS on
aerosol optical properties with sufficient accuracy or sensitivity to challenge optical models that
describe morphologically complex particles.