Significant Variability in the Photocatalytic Activity of Natural Titanium-Containing Minerals: Implications for Understanding and Predicting Atmospheric Mineral Dust Photochemistry

Abstract

The billions of tons of mineral dust released into the atmosphere each year provide an important surface for reaction with gas-phase pollutants. These reactions, which are often enhanced in the presence of light, can change both the gas-phase composition of the atmosphere and the composition and properties of the dust itself. Because dust contains titanium-rich grains, studies of dust photochemistry have largely employed commercial titanium dioxide as a proxy for its photochemically active fraction; to date, however, the validity of this model system has not been empirically determined. Here, for the first time, we directly investigate the photochemistry of the complement of natural titanium-containing minerals most relevant to mineral dust, including anatase, rutile, ilmenite, titanite, and several titanium-bearing species. Using ozone as a model gas-phase pollutant, we show that titaniumcontaining minerals other than titanium dioxide can also photocatalyze trace gas uptake, that samples of the same mineral phase can display very different reactivity, and that prediction of dust photoreactivity based on elemental/mineralogical analysis and/or lightabsorbing properties is challenging. Together, these results show that the photochemistry of atmospheric dust is both richer and more complex than previously considered, and imply that a full understanding of the scope and impact of dust-mediated processes will require the community to engage with this complexity via the study of ambient mineral dust samples from diverse source regions.

Ozone Chemistry and Photochemistry at the Surface of Icelandic Volcanic Dust: Insights from Elemental Speciation Analysis

“…Although previous investigations of water uptake by clays, 57,63 dust, 51,64 and volcanic ash 51 have demonstrated the equivalent of monolayer water coverage at <30% RH, we find that the reactivity of our sample toward ozone persists even at 50% RH. This observation, which again agrees with previous studies of ozone uptake by mineral dust and mineral dust proxies, 31,42,48,57,61 may reflect uneven surface water coverage, which would leave surface sites available for ozone uptake at RH values higher than those associated with apparent monolayer coverage. 65 Alternatively, it may reflect the reaction of ozone with soluble species (e.g., chloride and bromide) present in a surface water film under elevated RH conditions.…”

“…As shown in Figure 3, the highest γ BET values (50 ppb ozone) were found under dry conditions, whereas the lowest value was observed at 50% RH. This trend, previously observed for clays 42,57 and mineral dust proxies, 31,48,61 has been attributed to competition by water vapor for reactive surface sites. This explanation is consistent with our results, and in particular our observation that γ BET values for ozone determined under dry conditions were highly variable, as under these conditions, any variation in the amount of sorbed water remaining in the dust films after overnight drying would not be mitigated by contributions from the humidified air flow.…”

“…Similar sustained ozone uptake has previously been observed for metal oxides, 59−62 clays, 42 and natural desert dusts. 31,32 Although the mechanism of ozone loss on the surface of volcanic dust is not currently known, ozone uptake by metal oxides has been suggested to proceed via the following set of reactions, where SS represents a Lewis acid surface site needed for the decomposition of ozone 62,67…”

“…Interestingly, these values are also at the lower end of those reported for mineral phases commonly found in volcanic PM, including ilmenite, augite (a type of pyroxene), and phlogopite. 31,71 However, Myŕdalssandur dust is predominantly composed of glass, with only minor amounts of ilmenite and pyroxene (see Section 3.3), and the majority of the mineral components that are present are engulfed by glass (Figure 5). Since ozone uptake by both SiO 2 powder 31,73 and planar glass surfaces 74 is minimal, this may explain the modest reactivity observed here.…”

Ozone uptake by commercial brake pads and brake pad components: assessing the potential indirect air quality impacts of non-exhaust emissions

“…We quantified ozone uptake by our brake material sample set using an atmospheric pressure coated-wall flow tube photoreactor, which our group has previously used to investigate the photochemical reactivity of titanium-containing minerals relevant to desert dust. 23 The reactor consists of a sample-coated Pyrex tube inside a water-chilled jacket and surrounded by four UV-A lamps (300-410 nm, λ max : 356 nm); the exposure of coatings to ozone is controlled using a movable injector. Experiments were performed under environmentally relevant conditions (295 K, 25% relative humidity [RH]) and at an ozone mixing ratio of ~50 ppb, which in Canada is the threshold that triggers "actions to prevent air quality deterioration".…”