Space-borne estimation of volcanic sulfate aerosol lifetime

Abstract:

Aerosol are a major component of the climate system. While observations have dramatically improved our knowledge of the amount and location of aerosol, processes that govern aerosol removal and lifetime remain especially poorly constrained. The removal rate and lifetime of gas phase species has been quantified using satellite observations based on fitting to exponentially modified gaussian (EMG) functions. However, application of such methodology to aerosols is challenging as they typically have a longer lifetime of days to weeks, corresponding to spatial scales of thousands instead of hundreds of kilometers. Here, we present the first direct inference of aerosol lifetime from space based observations.

The Kīlauea volcano on Hawaii is a remote and persistent source of sulfur dioxide (SO₂) and sulfate aerosols, where steady trades winds carry degassed SO₂ downwind to form continental-scale aerosol plumes in summer. We combined observations of aerosol optical depth (AOD) from MODIS, space-borne lidar retrieved aerosol extinction profiles from CALIOP, SO₂ column density from OMI, as well as meteorological information from reanalyses to estimate the sulfate abundance and lifetime resulting from Kīlauea emissions during May-September, 2008. A chemical transport model (GEOS-Chem) is also used to support our interpretation.

Application of EMG fitting to the SO₂ plume from OMI yields monthly emission flux of 200-430 Gg and SO₂ decay lifetime of 40-60 hours. These SO₂ lifetimes agree closely with the GEOS-Chem model for SO₂ above the planetary boundary layer (PBL) where the simulated SO₂ lifetime is driven by slow chemical decay in clouds, as opposed to faster removal rates by dry deposition (~25 hours) near the surface. We use these observed SO₂ decay rates as prescribed sulfate formation rates for the sulfate lifetime fitting. Combined with observations of sulfate mass loading above the PBL as estimated based on vertically resolved AOD (from MODIS and CALIOP), we derive a lifetime for sulfate aerosol of 40-60 hours. Examination of predictions by GEOS-Chem suggests that these short lifetimes of sulfate above the PBL are dominated by wet scavenging, consistent with the vertical location of precipitation. Loss to dry deposition in the PBL is simulated to be much slower, ~200 hours.