Efficient formation of stratospheric aerosol for geoengineering by emission of condensable vapour from aircraft

JEFFREY PIERCE (1), Debra Weisenstein (2), Patricia Heckendorn (3), Thomas Peter (3), David Keith (4)

(1) Dalhousie University, Halifax, Canada, (2) Atmospheric and Environmental Research, Inc., Lexington, MA, (3) ETH Zurich, Switzerland, (4) Energy and Environmental Systems Group, Calgary, Canada

     Abstract Number: 120
     Last modified: April 19, 2010

Abstract
It may be possible to engineer an increase in the Earth's albedo by increasing the stratospheric aerosol burden. While such geoengineering entails novel environmental and security risks and cannot fully compensate for CO2-driven warming, it may nevertheless provide an important tool for managing climate risk. Analysis of stratospheric geoengineering has focused almost exclusively on the injection of non-condensable sulphur species such as SO2 into the stratosphere where they are slowly (~1 month) converted to low-volatility H2SO4, adding to the aerosol mass. Recent analysis suggests that the effectiveness of this method is limited because it tends to produce aerosols that are larger than optimal and thus are inefficient scatterers that settle quickly out of the stratosphere; indeed, these results suggest that it may be difficult to counteract more than half of the radiative forcing due to a CO2 doubling with continuous injection of SO2 in a narrow region (Heckendorn et al., 2009). Here we describe an alternate method in which aerosol is formed following injection of H2SO4 vapour from an aircraft, and show that this method gives better control of aerosol size, increases the cooling per mass of sulphur added to the stratosphere and reduces adverse effects due to heating of the lower stratosphere relative to SO2. This approach may be generalized to other aerosols that can form by fast reaction/condensation from the vapor phase. The method may, however, introduce unknown risks since it does not replicate volcanic aerosol formation.