Sulfur deposition changes under sulfate geoengineering conditions: Quasi-biennial oscillation effects on the transport and lifetime of stratospheric aerosols

Abstract

Sustained injection of sulfur dioxide (SO2) in the tropical lower stratosphere has been proposed as a climate engineering technique for the coming decades. Among several possible environmental side effects, the increase in sulfur deposition deserves additional investigation. In this study we present results from a composition-climate coupled model (University of L'Aquila Composition-Chemistry Model, ULAQ-CCM) and a chemistry-transport model (Goddard Earth Observing System Chemistry-Transport Model, GEOS-Chem), assuming a sustained lower-stratospheric equatorial injection of 8TgSO2yr1. Total S deposition is found to globally increase by 5.2% when sulfate geoengineering is deployed, with a clear interhemispheric asymmetry (+3.8 and +10.3% in the Northern Hemisphere (NH) and the Southern Hemisphere (SH), due to +2.2 and +1.8TgSyr1, respectively). The two models show good consistency, both globally and on a regional scale under background and geoengineering conditions, except for S-deposition changes over Africa and the Arctic. The consistency exists with regard to time-averaged values but also with regard to monthly and interannual deposition changes. The latter is driven essentially by the variability in stratospheric large-scale transport associated with the quasi-biennial oscillation (QBO). Using an externally nudged QBO, it is shown how a zonal wind E shear favors aerosol confinement in the tropical pipe and a significant increase in their effective radius (+13% with respect to W shear conditions). The net result is an increase in the downward cross-tropopause S flux over the tropics with dominant E shear conditions with respect to W shear periods (+0.61TgSyr1, +42%, mostly due to enhanced aerosol gravitational settling) and a decrease over the extratropics (0.86TgSyr1, 35%, mostly due to decreased large-scale stratosphere-troposphere exchange of geoengineering sulfate). This translates into S-deposition changes that are significantly different under opposite QBO wind shears, with an E-W anomaly of +0.32 in the tropics and 0.67TgSyr1 in the extratropics. Most online QBO schemes predict a significant change in the zonal wind periodicity, up to a blocked E shear condition for large enough injections, so that our results indicate an upper limit for the tropical increase in S deposition of 16.5% relative to average conditions of unperturbed QBO periodicity and a correspondent extratropical S deposition decrease of 16%.