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.
Although organosulfates
(ROSO3
–) comprise
a significant component of secondary organic aerosol (SOA) mass, their
atmospheric formation mechanisms are not fully understood. Here, using
methacrolein as a model organosulfate precursor, we present a new,
mineral-mediated photochemical pathway for organosulfate formation.
First, we describe studies of TiO2-catalyzed formation
of the atmospherically important organosulfate hydroxyacetone sulfate
from methacrolein as a function of illumination time, catalyst loading,
sulfate concentration, counterion identity, and methacrolein concentration.
Then, we propose a sulfate radical-mediated mechanism for organosulfate
formation consistent with these observations. Finally, we show that
natural Ti-containing minerals and road dust not only catalyze the
formation of comparable amounts of hydroxyacetone sulfate to those
formed in the presence of commercial TiO2 but also facilitate
the production of additional organosulfate species. These results
highlight the complex nature of photochemistry at the surface of natural
mineral samples and underscore the need for further study of the role
of mineral–organic interactions in atmospheric organosulfate
formation.
Volcanic
particulate matter (PM), whether emitted directly as ash
or indirectly via suspension of glaciogenic sediments, comprises a
large fraction of atmospheric PM in Iceland, a major high-latitude
dust source area. This PM leads to direct reductions in air quality
and health; in addition, because it provides a surface for reactions
with trace pollutant gases, it also has the potential to indirectly
influence the chemical composition of the troposphere. Here, we investigate
the reaction of gas-phase ozone with a volcanic dust sample obtained
from the Mýrdalssandur source region in southern Iceland. We
find that the steady-state surface area-scaled ozone uptake coefficient
(γBET) for this sample decreases with increasing
ozone mixing ratio and relative humidity, which implies that the reaction
proceeds via a Langmuir–Hinshelwood mechanism with water vapor
as competitive adsorbate. Using the γBET values we
obtain here, we conclude that the ozone flux to volcanic PM would
be <10% of its flux to the ground surface under typical Icelandic
weather conditions, even during major dust events. Interestingly,
although the Mýrdalssandur dust sample is high in elemental
Ti, which in its anatase and rutile forms is a powerful semiconductor
photocatalyst, its photochemistry is relatively modest. We use electron
microprobe analysis to help resolve this apparent contradiction: in
particular, we show that the bulk of the Ti in this sample is present
in its glass fraction, with the remainder present not as anatase or
rutile but rather in other predicted mineral phases (pyroxene, plagioclase,
ilmenite, titanomagnetite, and olivine). These results highlight the
advantages of using elemental speciation analysis to understand the
atmospheric reactivity of volcanic PM.
Non-exhaust emissions have surpassed exhaust emissions in their contribution to traffic particulate matter (PM). One major type is brake wear and, although its physical and toxicological properties have been studied,...
Biomass burning is a significant contributor to atmospheric pollution, its emissions have been found to have adverse impacts on climate and human health. Largely, these impacts are dictated by how...
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