Internally mixed atmospheric aerosol particles: Hygroscopic growth and light scattering

Internally mixed aerosol particles consisting of one or more hygroscopic compounds can contain both solid and liquid parts, the details depending on the relative humidity (RH). The solid-to-liquid transition of such particles as the RH increases influences their light-scattering properties through changes in particle shape, size, and refractive index. Most techniques used to analyze ambient aerosol particles do not have the ability to view both solid and liquid phases within individual particles. Using a transmission electron microscope fitted with an environmental cell (ETEM), we analyzed laboratory-prepared and ambient aerosol samples. Our results suggest that solid inclusions inside aqueous droplets at high RH values (e.g., >65%) are likely to be common in atmospheric particles. In order to assess the effects of such inclusions, we calculated their combined light-scattering efficiencies using a discrete dipole approximation (DDA). The results show differences compared to those from a core-shell model, with an average increase in light scattering of ∼20%. The results demonstrate that the combination of ETEM measurements of ambient particles with DDA calculations yields new insights into the effects of inclusions on the light-scattering properties of internally mixed particles.