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

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 of sulfur deposition deserves additional investigation. In this study we present results from a composition-climate coupled model (ULAQ-CCM) and a chemistry-transport model (GEOS-Chem), assuming a sustained lower stratospheric equatorial injection of 8 Tg-SO2/yr. 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 NH and SH, due to +2.2 Tg-S/yr and +1.8 Tg-S/yr, respectively). The two models show good consistency, both globally and on regional scale under background and geoengineering conditions, except for S-deposition changes over Africa and the Arctic. The consistency is on time averaged values, but also on monthly and inter-annual deposition changes. The latter is driven essentially by the variability of stratospheric large-scale transport associated to 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 of their effective radius (+13% with respect to W-shear conditions). The net result is an increase of the downward cross-tropopause S-flux over the tropics with dominant E-shear conditions respect to W-shear periods (+0.61 Tg-S/yr, +42%, mostly due to enhanced aerosol gravitational settling) and a decrease over the extratropics (-0.86 Tg-S/yr, -35%, mostly due to decreased large scale strat-trop 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 Tg-S/yr in the tropics and -0.67 Tg-S/yr in the extra-tropics. Most online QBO schemes predict a significant change of the zonal wind periodicity, up to a blocked E-shear condition for large enough injections, so that our results indicate an upper limit tropical increase of S-deposition by 16.5% relative to average conditions of unperturbed QBO periodicity and a correspondent extratropical S-deposition decrease by 16%.