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. Volcanic eruptions can increase the stratospheric sulfur loading by ordersof magnitude above the background level and are the most important source ofvariability in stratospheric sulfur. We present a set of vertical profilesof sulfate aerosol volume densities and derived liquid-phase H 2 SO 4 (sulfuric acid) mole fractions for 2005–2012, retrieved from infrared limbemission measurements performed with the Michelson Interferometer forPassive Atmospheric Sounding (MIPAS) on board of the Environmental Satellite(Envisat). Relative to balloon-borne in situ measurements of aerosol atLaramie, Wyoming, the MIPAS aerosol data have a positive bias that has beencorrected, based on the observed differences to the in situ data. Weinvestigate the production of stratospheric sulfate aerosol fromvolcanically emitted SO 2 for two case studies: the eruptions ofKasatochi in 2008 and Sarychev in 2009, which both occurred in the NorthernHemisphere midlatitudes during boreal summer. With the help of chemicaltransport model (CTM) simulations for the two volcanic eruptions we showthat the MIPAS sulfate aerosol and SO 2 data are qualitatively andquantitatively consistent with each other. Further, we demonstrate that thelifetime of SO 2 is explained well by its oxidation by hydroxyl radicals(OH). While the sedimentation of sulfate aerosol plays a role, we find that thelong-term decay of stratospheric sulfur after these volcanic eruptions inmidlatitudes is mainly controlled by transport via the Brewer–Dobsoncirculation. Sulfur emitted by the two midlatitude volcanoes resides mostlynorth of 30 ∘  N at altitudes of ∼  10–16 km, whileat higher altitudes ( ∼  18–22 km) part of the volcanic sulfuris transported towards the Equator where it is lifted into the stratospheric“overworld” and can further be transported into both hemispheres.

MIPAS observations of volcanic sulfate aerosol and sulfur dioxide in the stratosphere