Dependency of the impacts of geoengineering on the stratospheric sulfur injection strategy – Part 1: Intercomparison of modal and sectional aerosol modules

Injecting sulfur dioxide into the stratosphere with the intent to create an artificial reflective aerosol layer is one of the most studied options for solar radiation management. Previous modelling studies have shown that stratospheric sulfur injections have the potential to compensate for 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 a modal or sectional distribution. Here, we have studied different spatio-temporal injection strategies with different injection magnitudes using the aerosol–climate model ECHAM-HAMMOZ with two aerosol microphysical modules: the sectional module SALSA (Sectional Aerosol module for Large Scale Applications) and the modal module M7. We found significant differences in the model responses depending on the aerosol microphysical module used. In a case where SO2 was injected continuously in the equatorial stratosphere, simulations with SALSA produced an 88 %–154 % higher all-sky net radiative forcing than simulations with M7 for injection rates from 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 injected sulfur tends to produce new particles at the expense of gaseous sulfuric acid condensing on pre-existing particles in the SALSA module, most of the gaseous sulfuric acid partitions to particles via condensation at the expense of new particle formation in the M7 module. Thus, the effective radii of stratospheric aerosols were 10 %–52 % larger in M7 than in SALSA, depending on the 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 the aerosol number size distribution between the accumulation and 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 rates using SALSA, the most efficient studied injection strategy produced a 33 %–42 % radiative forcing compared with the least efficient strategy, whereas simulations with M7 showed an even larger difference of 48 %–116 %. 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 depend on both the injection strategy and the aerosol microphysical model. Overall, these results highlight the crucial impact of aerosol microphysics on the physical properties of stratospheric aerosol which, in turn, causes significant uncertainties in estimating the climate impacts of stratospheric sulfur injections.

Dependency of the impacts of geoengineering on the stratospheric sulfur injection strategy part 1: Intercomparison of modal and sectional aerosol module

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.

Stratospheric aerosol enhancement and decay after Raikoke eruption in July 2019 as observed from Tbilisi, Georgia and Halle, Belgium using ground-based twilight sky brightness spectral measurements.

<p>Twilight sky brightness spectral measurements are an inexpensive and effective way to observe enhancements of stratospheric aerosols. In this work, we present our observations of the volcanic cloud produced by the eruption of Raikoke volcano (Kuril Islands, 48°N, 153°E) above two distinct sites in South Caucasus and Western Europe, respectively: Tbilisi, Georgia (41° 43’ N, 44° 47° E) and Halle, Belgium (50° 44′ N, 4° 14′ E).</p><p>We present our dataset, which describes the evolution of the stratospheric aerosol in the period July 2019-December 2020. Stratospheric aerosol vertical extinction profiles were retrieved at 780 nm from spectral measurements of twilight sky brightness above both sites.</p><p>The first aerosols originating from Raikoke  were observed in the beginning of July above Halle and in August above Georgia. The layer maximum was mostly observed at 17 km above Georgia and at 10-17 km above Belgium until April-May 2020. Later, the volcanic cloud was observed sporadically until the end of 2020.</p>

Retrieval of aerosol particle size distributions in the stratosphere from SCIAMACHY limb observations and comparison to balloon-borne measurements and ECHAM5-HAM simulations

<p>Stratospheric aerosols play an important role in the climate system and the atmospheric chemistry. They alter the radiative budget of the Earth affecting the global temperature and interact with stratospheric trace gases leading to ozone depletion. Effects are most noticeable after vulcanic eruptions enhancing the amount of aerosols in the stratosphere. Thus, vertically and spatially resolved knowledge about stratospheric aerosols, such as the particle size distribution and extinction coefficient, is crucial for the initialization of climate models, investigation of geoengineering, validation of aerosol micro-physical models, and improvement of trace gas retrievals. We present an algorithm to retrieve aerosol particle size distribution parameters (mode radius and distribution width, number density) from limb observations of SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric ChartograpHY) operated aboard Envisat between 2002 and 2012. SCIAMACHY retrieved particle size distribution profiles are compared with in-situ balloon-borne measurements from Laramie, Wyoming. Both data-sets show good agreement. The stratospheric plume evolution after the eruption of Sarychev in the Kuril Islands, Russia, in June 2009 is investigated and compared to the output from the aerosol-climate modelling system ECHAM5-HAM.</p>