| Summary: | <p>Volcanic emissions make an important contribution to the atmospheric sulphur budget. The volcanic contribution of sulphur to the atmosphere is poorly defined due to uncertainties in the amount of sulphur ejected, its distribution by sulphur species and the height of injection. Also there is little or no ground-based monitoring of many volcanoes, and little knowledge of sulphur emitted by degassing and other non-explosive volcanic processes. The altitude of volcanic injections of SO2 is therefore poorly understood, but of great importance when considering the chemistry and movement of the SO<sub>2</sub>. SO<sub>2</sub> reaction rates vary with height and atmospheric composition: SO<sub>2</sub> injected into the troposphere will relatively quickly leave the atmosphere as acidification of rainfall; in the stratosphere, however, SO<sub>2</sub> is oxidised to H<sub>2</sub>SO<sub>4</sub>, which in aerosol form can remain and affect climate processes for many years. Sulphate aerosols affect the Earth’s radiative balance through scattering of sunlight, and are therefore an important factor when considering climate change.</p> <p>Atmospheric sulphur compounds derive from a variety of sources. The largest direct source of SO<sub>2</sub> is from volcanic emission, though it also indirectly stems from conversion from carbonyl sulphide (OCS), the most abundant atmospheric sulphur species.</p> <p>Satellite-based instrumentation is a vital tool for the analysis of volcanic eruptions. Here the IASI instruments, Fourier transform spectrometers on the MetOp platforms, are used to study medium to large eruptions between 2008-2012. The vertical distribution of SO<sub>2</sub> is investigated with respect to the local height of the tropopause, which is then further used to estimate the total atmospheric mass of SO<sub>2</sub> as a result of these eruptions. These eruptions are also plotted in terms of their latitude variation, and a large difference between the hemisphere loadings is shown.</p> <p>The trajectory model HYSPLIT is used in conjunction with the IASI instru- ment to investigate both the injection height of SO<sub>2</sub> plumes and their chemistry. A number of forward trajectories are generated for the duration of the eruption at a series of initial altitudes and the final positions a given time later are compared against the IASI retrieval. It is found that the injection height determined is on average lower than those presented in literature, but that the principle of the analysis is sound. The time frame of the investigation is found to be too short to draw any meaningful conclusions about the plume chemistry.</p>
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