Effects of different stratospheric SO2 injection altitudes on stratospheric chemistry and dynamics

Strategically applied geoengineering is proposed to reduce some of the known side effects of stratospheric aerosol modifications. Specific climate goals could be reached depending on design choices of stratospheric sulfur injections by latitude, altitude, and magnitude. Here we explore in detail the stratospheric chemical and dynamical responses to injections at different altitudes using a fully coupled Earth System Model. Two different scenarios are explored that produce approximately the same global cooling of 2 degrees C over the period 2042-2049, a high-altitude injection case using 24TgSO(2)/year at 30hPa (approximate to 25-km altitude) and a low-altitude injection case using 32TgSO(2)/year injections at 70hPa (between 19- and 20-km altitude), with annual injections divided equally between 15 degrees N and 15 degrees S. Both cases result in a warming of the lower tropical stratosphere up to 10 and 15 degrees C for the high- and low-altitude injection case and in substantial increases of stratospheric water vapor of up to 2 and 4ppm, respectively, compared to no geoengineering conditions. Polar column ozone in the Northern Hemisphere is reduced by up to 18% in March for the high-altitude injection case and up to 8% for the low-altitude injection case. However, for winter middle and high northern latitudes, low-altitude injections result in greater column ozone values than without geoengineering. These changes are mostly driven by dynamics and advection. Antarctic column ozone in 2042-2049 does not recover from present-day (2002-2009) values for both cases.