Uptake and surface chemistry of SO2 on natural Icelandic volcanic dusts under simulated atmospheric conditions
Résumé
<p>Volcanic dust (v-dust) is a highly variable source of natural particles in the atmosphere, and during the period of high volcanic activity it can provide a large surface for heterogeneous interactions with other atmospheric compounds. With an area of 103,000 km<sup>2</sup>, Iceland is the biggest volcanic desert on earth. It was chosen as a case study due to frequency of volcanic eruptions and high aeolian activity in the area. This is a comprehensive study of the heterogeneous reactivity of Icelandic volcanic dust with sulfur dioxide (SO<sub>2</sub>) gas. First, we focused on the kinetics of the reaction of SO<sub>2 </sub>with natural v-dust samples under atmospheric conditions using coated wall flow tube reactor. Steady-state uptake coefficients were measured to represent the long-term phenomena of the processing of aerosols in the atmosphere and the values obtained can be directly incorporated in chemical transport modeling. Second, the mechanism of the reaction of SO<sub>2</sub> with natural v-dust samples was studied using infrared Fourier transform spectroscopy (DRIFTS). Both sulfites and sulfates were observed on the surface of v-dust, with sulfates being the final oxidation product, attesting to SO<sub>2</sub> heterogeneous reactivity. Surface hydroxyl groups were found to play a crucial role in the conversion of SO<sub>2</sub> to sulfates as evidenced from both flow tube and DRIFTS experiments. Based on these experimental results, a mechanism for SO<sub>2</sub> interaction with different surface sites of v-dust was proposed and discussed. Third, in order to monitor the amount of sulfites and sulfates formed on the surface of mineral dusts of different origins a simple, accurate and precise reversed-phase liquid chromatography method was developed and validated to stabilize and analyze sulfites and sulfates in the extract of dusts exposed to SO<sub>2</sub>. Besides SO<sub>2</sub> gas, v-dust reacts with other atmospheric pollutants, such as NO<sub>2</sub> and O<sub>3</sub>, proving that heterogeneous processes play an important role in the atmospheric chemistry. One must keep in mind that as a result of such transformations, such properties as ice nucleation and optical properties might change as well soliciting further investigation of heterogeneous reactivity of Icelandic v-dusts.</p>