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

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 Mkrdalssandur source region in southern Iceland. We find that the steady-state surface area-scaled ozone uptake coefficient (gamma(BET)) 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 gamma(BET) values we obtain here, we condude 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 Myrdalssandur 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.