Investigating the zero transmission problem in satellite solar occultation measurements in the context of possible stratospheric aerosol injections

Abstract. Stratospheric aerosol injections have been proposed to mitigate the effects of global warming. The injection of sulphur dioxide into the stratosphere is one possible idea. However, depending on the latitude, high emission rates can lead to very low transmissions from the perspective of a typical satellite solar occultation instrument, leading to the so-called zero transmission problem. Consequently, it is highly unlikely that a physically meaningful retrieval of the stratospheric aerosol extinction profiles is possible, depending on the latitude and wavelength. The current study analyses, using MAECHAM5-HAM and SCIATRAN, continuous injections of 30 Tg S/y as a hypothetical large-scale stratospheric aerosol injection scenario. For this purpose, sulphur dioxide was continuously injected at an altitude of 60 hPa (about 19 km) into one grid box (2.8°x 2.8°) centred on the Equator at 121°E. Specifically, it is investigated which wavelengths, depending on the latitude, are necessary for plausible aerosol extinction profile retrievals. While a wavelength of 520 nm is insufficient for the retrieval for 5°N, the opposite can be concluded for 75°N and 75°S. For the latitudes 45°N and 45°S, a wavelength of at least 1543 nm is necessary. In contrast, 1900 nm is sufficient for 15°N and 15°S, as well as 5°N. Simulation results for an emission rate of 10 Tg S/y show that a minimum wavelength of 1543 nm is already sufficient for 5°N. The results emphasize that the zero transmission problem does not mean that solar occultation measurements are entirely useless. Consistent with expectations, a longer wavelength is required for the latitude range of and near the injection. These findings are therefore also relevant for satellite solar occultation measurements after major volcanic eruptions.