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

Volcanic eruptions can increase the stratospheric sulfur loading by orders of magnitude above the background level and are the most important source of variability in stratospheric sulfur. We present a set of vertical profiles of sulfate aerosol volume densities and derived liquid-phase H2SO4 (sulfuric acid) mole fractions for 2005-2012, retrieved from infrared limb emission measurements performed with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board of the Environmental Satellite (Envisat). Relative to balloon-borne in situ measurements of aerosol at Laramie, Wyoming, the MIPAS aerosol data have a positive bias that has been corrected, based on the observed differences to the in situ data. We investigate the production of stratospheric sulfate aerosol from volcanically emitted SO2 for two case studies: the eruptions of Kasatochi in 2008 and Sarychev in 2009, which both occurred in the Northern Hemisphere midlatitudes during boreal summer. With the help of chemical transport model (CTM) simulations for the two volcanic eruptions we show that the MIPAS sulfate aerosol and SO2 data are qualitatively and quantitatively consistent with each other. Further, we demonstrate that the lifetime of SO2 is explained well by its oxidation by hydroxyl radicals (OH). While the sedimentation of sulfate aerosol plays a role, we find that the long-term decay of stratospheric sulfur after these volcanic eruptions in midlatitudes is mainly controlled by transport via the BrewerDobson circulation. Sulfur emitted by the two midlatitude volcanoes resides mostly north of 30 degrees N at altitudes of similar to 10-16 km, while at higher altitudes (similar to 18-22 km) part of the volcanic sulfur is transported towards the Equator where it is lifted into the stratospheric "overworld" and can further be transported into both hemispheres.