Satellite-based global volcanic SO2 emissions and sulfate direct radiative forcing during 2005-2012

An 8year volcanic SO2 emission inventory for 2005-2012 is obtained based on satellite measurements of SO2 from OMI (Ozone Monitoring Instrument) and ancillary information from the Global Volcanism Program. It includes contributions from global volcanic eruptions and from eight persistently degassing volcanoes in the tropics. It shows significant differences in the estimate of SO2 amount and injection height for medium to large volcanic eruptions as compared to the counterparts in the existing volcanic SO2 database. Emissions from Nyamuragira (DR Congo) in November 2006 and Grimsvotn (Iceland) in May 2011 that were not included in the Intergovernmental Panel on Climate Change 5 (IPCC) inventory are included here. Using the updated emissions, the volcanic sulfate (SO42-) distribution is simulated with the global transport model Goddard Earth Observing System (GEOS)-Chem. The simulated time series of sulfate aerosol optical depth (AOD) above 10km captures every eruptive volcanic sulfate perturbation with a similar magnitude to that measured by Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). The 8year average contribution of eruptive SO42- to total SO42- loading above 10km is similar to 10% over most areas of the Northern Hemisphere, with a maxima of 30% in the tropics where the anthropogenic emissions are relatively smaller. The persistently degassing volcanic SO42- in the tropics barely reaches above 10km, but in the lower atmosphere it is regionally dominant (60%+ in terms of mass) over Hawaii and other oceanic areas northeast of Australia. Although the 7year average (2005-2011) of eruptive volcanic sulfate forcing of -0.10Wm(-2) in this study is comparable to that in the 2013 IPCC report (-0.09Wm(-2)), significant discrepancies exist for each year. Our simulations also imply that the radiative forcing per unit AOD for volcanic eruptions can vary from -40 to -80Wm(-2), much higher than the -25Wm(-2) implied in the IPCC calculations. In terms of sulfate forcing efficiency with respect to SO2 emission, eruptive volcanic sulfate is 5 times larger than anthropogenic sulfate. The sulfate forcing efficiency from degassing volcanic sources is close to that of anthropogenic sources. This study highlights the importance of characterizing both volcanic emission amount and injection altitude as well as the key role of satellite observations in maintaining accurate volcanic emissions inventories.