Laboratory studies of heterogeneous reactions of sulfur dioxide and ozone on mineral dust and mineral dust proxies

Increasing evidence that mineral dust particles have a profound effect on the Earth's troposphere has motivated field and modeling studies as well as laboratory investigations. Reactions of mineral dust particles with trace gases in the troposphere have been postulated as important processes that can affect the chemical balance of the troposphere. In this thesis, the reactions of gas-phase sulfur dioxide and ozone, two important constituents of the troposphere, with mineral dust and mineral dust proxies are investigated. Using a Knudsen cell reactor, kinetic studies were performed in which the heterogeneous uptake of SO₂ and O₃ onto mineral dust and mineral oxide powders was quantified. Uptake coefficients obtained in these studies show that the removal of SO₂ to particle surfaces is competitive with homogeneous loss routes in regions of high particulate concentration. Measurements made for the uptake of ozone onto dust surfaces show that the reaction is catalytic in nature, as the uptake coefficient does not diminish to zero with extended reaction times. The catalytic destruction of ozone on dust may increase the importance of this reaction in atmospheric chemistry despite relatively low uptake coefficients. The uptake of ozone onto particles that have been previously treated to mimic coatings found on particles that have been transported large distances in the troposphere was shown to be altered with respect to particles that were not treated. The results of the studies for both ozone and sulfur dioxide show that chemical specificity of dust particles, as well as transport history, are necessary inclusions in atmospheric modeling studies in order to yield the most accurate results. Using FT-IR it was shown that the major adsorption product with SO₂ onto dust is sulfite, which is oxidized to sulfate when exposed to ozone. 13C NMR showed that particles that have organic coatings containing double bonds are reactive to ozone, with loss of the double bond and formation of surface alkane. Mechanisms are proposed that may help explain observations made in field studies with regard to particulate sulfate formation, as well as provide insight as to the nature of ozone uptake on mineral dust.