Correlation of Optical Properties with Particle Size, Morphology, and Polymorph for Multiple Sources of Submicron Titanium Dioxide Particles

SCHUYLER LOCKWOOD, Zezhen Cheng, Valentina Sola, Nurun Nahar Lata, Tanya Myers, Timothy Johnson, Alla Zelenyuk, Pacific Northwest National Laboratory

     Abstract Number: 188
     Working Group: Aerosol Physics

Abstract
Titanium dioxide (TiO₂₎ particulates are known to exhibit different visible and infrared optical properties vs. the bulk material. Such properties are further known to depend strongly on particle size. For example, ultrafine particles (<100 nm primary particle diameter) of the rutile polymorph are used in sunscreen and appear transparent, while larger particles (200-300 nm) are used as pigments or reflective coatings, appearing bright white. Larger particles are also of interest for geoengineering (stratospheric aerosol injection) due to (1) the high refractive index of TiO₂ across much of the UV, visible, and infrared wavelength regions and (2) the large scattering cross sections of TiO₂ nanoparticles, particularly at UV wavelengths. TiO₂ also exhibits high photocatalytic activity in reactions involving many organic materials (and even water), potentially complicating its effects on cloud/aerosol chemistry in these applications. In this study, we compare the optical properties, sizes and morphologies of TiO₂ particles from several sources with nominally different primary particle sizes and various mixtures of crystal polymorphs using a combination of methods including single particle mass spectrometry, optical spectroscopy, and several submicron aerosol characterization instruments.