Dependency of the impacts of geoengineering on the stratospheric sulfur injection strategy part 1: Intercomparison of modal and sectional aerosol module
Authors: Laakso Anton , Niemeier Ulrike , Visioni Daniele , Tilmes Simone , Kokkola Harri 
Published: July 2021

Abstract. Injecting sulfur dioxide into the stratosphere with the intent to create an artificial reflective aerosol layer is one of the most studied option for solar radiation management. Previous modelling studies have shown that stratospheric sulfur injections have the potential to compensate the greenhouse gas induced warming at the global scale. However, there is significant diversity in the modelled radiative forcing from stratospheric aerosols depending on the model and on which strategy is used to inject sulfur into the stratosphere. Until now it has not been clear how the evolution of the aerosols and their resulting radiative forcing depends on the aerosol microphysical scheme used, that is, if aerosols are represented by modal or sectional distribution. Here, we have studied different spatio-temporal injections strategies with different injection magnitudes by using the aerosol-climate model ECHAM-HAMMOZ with two aerosol microphysical modules: the sectional module SALSA and the modal module M7. We found significant differences in model responses depending on the used aerosol microphysical module. In a case where SO2 was injected continuously in the equatorial stratosphere, simulations with SALSA produced 88 %–154 % higher all-sky net radiative forcing than simulations with M7 for injection rates from 1 to 1 to 100 Tg(S) yr−1. These large differences are identified to be caused by two main factors. First, the competition between nucleation and condensation: while in SALSA injected sulfur tends to produce new particles at the expense of gaseous sulfuric acid condensing on pre-existing particles, in M7 most of the gaseous sulfuric acid partitions to particles via condensation at the expense of new particle formation. Thus, the effective radii of stratospheric aerosols were 10–52% larger in M7 than in SALSA, depending on injection rate and strategy. Second, the treatment of the modal size distribution in M7 limits the growth of the accumulation mode which results in a local minimum in aerosol number size distribution between the accumulation and the coarse modes. This local minimum is in the size range where the scattering of solar radiation is most efficient. We also found that different spatial-temporal injection strategies have a significant impact on the magnitude and zonal distribution of radiative forcing. Based on simulations with various injection rate using SALSA, the most efficient studied injection strategy produced 33–42 % radiative forcing compared to the least efficient strategy while simulations with M7 showed even larger difference of 48–76 %. Differences in zonal mean radiative forcing were even larger than that. We also show that a consequent stratospheric heating and its impact on the quasi-biennial oscillation depends both on the injection strategy and the aerosol microphysical model. Overall, these results highlight a crucial role of aerosol microphysics on the physical properties of stratospheric aerosol which in turn causes significant uncertainties in estimating climate impacts of stratospheric sulfur injections.