First X-ray Spectroscopic Observations of Atmospheric Titanium Species: Size Dependence and the Emission Source

Titanium dioxide (TiO₂) in mineral dust is considered as one of the driving forces of photocatalytic reaction at the aerosol surface in the atmosphere. As a precursor of mineral dust, soil contains ilmenite (FeTiO₃) and titanite (CaSiTiO₅), which have lower photochemical reactivities than TiO₂. However, Ti species other than TiO₂ in aerosol particles are not well recognized due to the lack of observation in ambient samples. In this study, Ti species in size-fractionated aerosol samples collected in the Noto Peninsula, Japan, were determined by macroscopic and semi-microscopic X-ray absorption fine structure spectroscopy. Regardless of aerosol particle size, Ti species were primarily composed of rutile, anatase, ilmenite, and titanite. Semi-microscopic Ti speciation showed that Ti-poor spots associated with mineral dust were composed of a mixture of rutile, anatase, ilmenite, and titanite, and Ti-rich spots were primarily composed of TiO₂ (rutile or anatase) derived from authigenic minerals or anthropogenic materials. Thus, the Ti species in aerosol particles, especially mineral dust, were not composed solely of TiO₂ polymorphs. Therefore, the photochemical reactivities of Ti in aerosol particles may be overestimated when laboratory experiments or model studies employ TiO₂ as the representative Ti species.