Uptake of Hg on TiO, AlO, and FeO Nanoparticles: Importance in Atmospheric Chemical and Physical Processes.

Mineral dust aerosols play an important role in tropospheric chemistry and aerosol-cloud interaction processes. Yet, their interactions with gaseous elemental mercury (Hg) are not currently well understood. Using a coated-wall flow tube (CWFT) reactor, we measured the uptake of Hg on some common components of mineral dust aerosols, including TiO, AlO, and FeO, and the effects of irradiation (dark, visible and UV-A) and relative humidity (<2% to 60%) on the uptake kinetics. Under UV-A irradiation (320-400 nm) in dry air, we measured uptake coefficients (γ) equal to >1 × 10 and (3 ± 1) × 10 on TiO and AlO, respectively. Under visible light irradiation (380-700 nm), Hg uptake was only observed on TiO, with γ = (4 ± 3) × 10. Raising the relative humidity inhibited the uptake on both TiO and AlO, and the uptake coefficient at 60% RH for TiO under UV-A irradiation was lower by ca. 3 orders of magnitude than dry conditions. Furthermore, we observed that water vapor induced the desorption of two distinct fractions from Hg-exposed surfaces via the displacement of weakly, physisorbed Hg and the photocatalyzed reduction of chemisorbed Hg. Based on the uptake coefficients from this report, we estimate that heterogeneous interactions with mineral dust may be significant under conditions with low relative humidity (<30%) and high dust loading masses. We herein discuss the implication of this study on understanding the life cycle analysis of atmospheric mercury in nature.