Exploring accumulation-mode-H 2 SO 4 versus SO 2 stratospheric sulfate geoengineering in a sectional aerosol-chemistry-climate model

Stratospheric sulfate geoengineering (SSG) could contribute to avoiding some of the adverse impacts of climate change. We used the global aerosol-chemistry-climate model SOCOL-AER to investigate 21 different SSG scenarios, each with 1.83 Mt S yr injected either in form of accumulation-mode-H2SO4 droplets (AM-H2SO4), gas-phase SO2, or as combinations of both. For most scenarios, the sulfur was continuously emitted at 50 hPa (≈ 20 km) altitude in the tropics and 10 subtropics. We assumed emissions to be zonally and latitudinally symmetric about the equator. The spread of emissions ranges from 3.75° S-3.75° N to 30° S-30° N. In the SO2 emission scenarios, continuous production of tiny nucleation mode particles results in increased coagulation, which together with gaseous H2SO4 condensation produces coarse mode particles. These large particles are less effective for backscattering solar radiation and have a shorter stratospheric residence time than AM-H2SO4 particles. On average, the stratospheric aerosol burden and corresponding all sky short-wave radiative forcing for 15 the AM-H2SO4 scenarios are about 37 % larger than for the SO2 scenarios. Simulated stratospheric aerosol burdens show a weak dependence on the latitudinal spread of emissions. Emitting 30° N-30° S instead of 10°N-10°S decreases stratospheric burdens only by about 10 %. This is because a decrease in coagulation and the resulting smaller particle size is roughly balanced by faster removal through stratosphere-to-troposphere transport via tropopause folds. Increasing the injection altitude is also ineffective, although it generates a larger stratospheric burden, because enhanced condensation and/or 20 coagulation leads to larger particles, which are less effective scatterers. In the case of gaseous SO2 emissions, limiting the sulfur injections spatially and temporally in the form of point and pulsed emissions reduces the total global annual nucleation, leading to less coagulation and thus smaller particles with increased stratospheric residence times. Pulse or point emissions of AM-H2SO4 have the opposite effect: they decrease stratospheric aerosol burden by increasing coagulation and only slightly decreased clear sky radiative forcing. This study shows that direct emission of AM-H2SO4 results in higher 25 radiative forcing for the same sulfur equivalent mass injection strength than SO2 emissions, and that the sensitivity to different injection strategies vary for different forms of injected sulfur.


Introduction
Driven by human emissions, long-lived atmospheric greenhouse gas (GHG) concentrations now exceed levels ever experienced by Homo sapiens. The effects of these GHGs-as written by the Intergovernmental Panel on Climate Change in 2014-"have been detected throughout the climate system and are extremely likely to have been the dominant cause of the observed warming since the mid-20 th century" (IPCC, 2014). Emissions of CO2 and other GHGs must be curbed to reduce 5 the impacts of climate change. Yet, the long lifetime of CO2 and some other GHGs suggest that even if emissions were eliminated today, climate change and resulting human and environmental risks would persist for centuries.
We might bring global warming to a halt or reduce its rate of growth by combining emissions cuts with other interventions, such as a deliberate increase in the Earth's stratospheric aerosol burden, which would enhance the albedo of the stratospheric aerosol layer and reduces solar climate forcing. This idea, now often called "solar geoengineering", "solar 10 climate engineering" or "solar radiation management", was first proposed by Budyko (1977) who suggested injecting sulfate aerosols into the stratosphere to increase Earth's albedo. Research on this topic became tabooed because of the entailed risks.
However, efforts were renewed after Crutzen (2006) suggested that solar radiation management might be explored as a useful climate change mitigation tool, since adequate emission reductions were becoming increasingly unlikely.
Most research on solar geoengineering has focused on stratospheric sulfate geoengineering (SSG) through SO2 15 injection, in part due to its volcanic analogues such as the 1991 eruption of Mt. Pinatubo. Studies of SSG, however, have found limitations with SO2 injection as a method of producing a radiative forcing (RF) perturbation. Limitations include: (1) reduced efficacy at higher loading, limiting the achievable short-wave radiative forcing (Heckendorn et al., 2009;Niemeier et al., 2011;English et al., 2012;Kleinschmitt et al., 2018), (2) depletion of stratospheric ozone (Tilmes et al., 2008;Heckendorn et al., 2009), (3) stratospheric heating (Heckendorn et al., 2009;Ferraro et al., 2011), especially in the tropical 20 lower stratosphere, which would modify the Brewer-Dobson circulation and increase stratospheric water vapor, and (4) enhanced diffuse light at the Earth's surface with repercussions for ecosystems (Kravitz et al., 2012). Limitation (1) is primarily a function of the sulfate particle size distribution, determining their gravitational removal, whereas (2) to (4) are primarily dependent on chemical and radiative particle properties.
The size distribution problem with SO2 injection arises after oxidation of SO2 to H2SO4 when aerosol particles are 25 formed through nucleation and condensation. Condensation onto existing particles increases their average size. In addition, the continuous flow of freshly nucleated particles leads to coagulation, both through self-coagulation of the many small new particles and-more importantly-coagulation with already existing bigger particles from the background aerosol layer.
These particles then grow further through coagulation and condensation, which increases the average sedimentation velocity of the aerosol population (Heckendorn et al., 2009). Mean particle sizes tend to increase with the SO2 injection rate, reducing 30 the stratospheric aerosol residence time and, hence, their radiative forcing efficacy (e.g., W m -2 (Mt S yr -1 ) -1 ). This problem could be reduced-and the radiative efficacy increased-if there was a way to produce additional accumulation mode (0.1-1.0 µm radius) sulfate particles (AM-H2SO4). Such particles are sufficiently large to decrease their mobility and hence their Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1070 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 15 November 2018 c Author(s) 2018. CC BY 4.0 License.
coagulation. Furthermore, such particles are close to the radius of maximum mass specific up-scattering of solar radiation on sulfate particles, which is ~0.3 µm . One proposed method of doing this is to directly inject H2SO4 vapor into a rapidly expanding aircraft plume during stratospheric flight, which would be expected to lead to the formation of accumulation mode particles with a size distribution that depends on the injection rate and the expansion characteristics of the plume (Pierce et al., 2010). Two theoretical studies, Pierce et al. (2010) and Benduhn et al. (2016), suggest that 5 appropriate size distributions could be produced in aircraft plumes using this method.
To evaluate a geoengineering approach with AM-H2SO4 one needs to study the evolution of aerosol particles after injection of H2SO4 vapor into an aircraft wake and the subsequent transport and evolution of the aerosol plume around the globe. This is a problem with temporal scales ranging from milliseconds to years, and spatial scales from millimeters to thousands of kilometers. At present there is no model that could seamlessly handle the entire range. However, the problem 10 can be divided into two separate domains: (a) from injection to plume dispersal, and (b) from plume dispersal to global scale distribution. Each domain has associated uncertainties, but these can be studied separately with different modeling tools: plume dispersion models for (a), and general circulation models (GCMs) or chemistry-climate models (CCMs) for (b).
(a) Plume modeling. The integration of the plume model starts with the production of small particles in a plume from the exit point of the injection nozzle, and ends when the plume has expanded sufficiently so that the loss of particles by 15 coagulation with ambient particles dominates the self-coagulation, whereupon the GCM or CCM becomes the appropriate tool (Pierce et al., 2010). The plume model needs to account for the initial formation of nucleation mode particles by homogeneous nucleation of H2SO4 and H2O vapor and the subsequent evolution of the particle size distribution by coagulation of the nucleation mode, as well as by condensation of H2SO4 vapor on already existing particles. In an expanding aircraft plume, these processes occur on timescales from milliseconds to hours, and length scales from millimeters 20 to kilometers. This was addressed by Pierce et al. (2010) and then by Benduhn et al. (2016). There is rough agreement that particles of 0.095-0.15 µm radius could be produced after the initial plume processing, but these results are subject to uncertainties and need further investigation.
(b) General circulation modeling. The second part of the problem can be analyzed by a GCM or a CCM, starting from the release of sulfate particles of the size distribution calculated by the plume model into the grid of the GCM, all the 25 way to implications on aerosol burden, radiative forcing, ozone, stratospheric temperature and circulation. To this end, the GCM must be coupled to chemistry and aerosol modules. The GCM then provides solutions on how the new accumulation mode particles change the large-scale size distribution, and thus the overall radiative and dynamical response to sulfate aerosol injection. Missing in this methodology are processes smaller than the grid size of the GCM, which may involve filaments of injected material being transported in thin layers. Consideration of these sub-grid scale processes remains an 30 uncertainty of our study, but might be handled by a Lagrangian transport model in a future study.
A sectional (i.e. size-bin resolved) aerosol module is important for a mechanistic understanding of the factors that determine the size distributions of the aerosols. Sectional aerosol models handle the aerosols in different size bins (40 in Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1070 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 15 November 2018 c Author(s) 2018. CC BY 4.0 License. SOCOL-AER) whereas modal models usually only apply 3 modes (e.g. Niemeier et al., 2011;Tilmes et al., 2017), each with different rm and σ, to describe the aerosol distribution. Therefore, the degrees of freedom among modal models usually vary between 4 and 8, whereas there are 40 for a sectional model like SOCOL-AER. Thus, sectional aerosol models represent aerosol distributions with better accuracy, though numerical diffusion does result from the discretization in size space. Two earlier studies of SSG modelling, Heckendorn et al. (2009) and Pierce et al. (2010), used the AER-2D chemistry-transport-5 aerosol model with sectional microphysics (Weisenstein et al., 1997(Weisenstein et al., , 2007. Although the sectional aerosol module within has high size resolution, this 2D model only has a limited spatial resolution with simplified dynamical processes. Two GCM model studies of SSG have used sectional aerosol modules, namely English et al. (2012) and Kleinschmitt et al. (2018). English et al. (2012) used the GCM WACCM (Garcia et al., 2007) coupled to the sectional aerosol module CARMA (Toon et al., 1988) to simulate various SSG scenarios with sulfur emissions in the form of SO2-gas, H2SO4-gas and AM-H2SO4, but 10 without treatment of the quasi biennial oscillation (QBO) and without online interaction between aerosols, chemistry and radiation. Kleinschmitt et al. (2018) used the GCM LMDZ (Hourdin et al., 2006(Hourdin et al., , 2013 which was coupled to the sectional aerosol module S3A (Kleinschmitt et al., 2017) to simulate various SO2 emission scenarios, but without injection of AM-H2SO4. In their model setup, the aerosols were fully interactive with the radiative scheme, but the model included only simplified chemistry and a prescribed SO2-to-H2SO4 conversion rate. 15 There have been prior studies with other advanced interactive GCMs, but using modal aerosol schemes. Niemeier et al. (2011) looked at SO2 and H2SO4 gas injection by using the GCM MA-ECHAM coupled to the modal aerosol module HAM (Stier et al., 2005). However, the radiation scheme did not interact with the aerosol module and the chemistry was simplified using prescribed OH concentrations and without ozone chemistry. Mills et al. (2017) and Tilmes et al., (2017) used the fully coupled global chemistry-climate model CESM1 (Hurrell et al., 2013) to simulate SO2 emission scenarios. In 20 their model setup, they applied higher horizontal and vertical resolutions compared to SOCOL-AER as well as a fully coupled ocean module and more complex chemistry. However, they also relied on a modal aerosol module, which in turn was coupled to cloud microphysics.
In this study we investigate different SO2 and AM-H2SO4 emission scenarios by using the sectional global 3D aerosol-chemistry-climate model SOCOL-AER (Sheng et al., 2015), which treats prognostic transport as well as radiative 25 and chemical feedbacks of the aerosols online in one models. As described above, GCMs are not yet able to interactively couple plume dispersion models. Hence, we follow Pierce et al., 2010 and use a log-normal distribution for the injected aerosols in the AM-H2SO4 emission scenarios. SO2 emission scenarios are performed as a reference, and to gain insight into aerosol formation processes on a global scale. We perform a number of sensitivity studies with both SO2 and AM-H2SO4 that highlight differences between the two injection strategies and indicate future research needs. 30 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1070 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 15 November 2018 c Author(s) 2018. CC BY 4.0 License.

Model Description
We use the global 3D aerosol-chemistry-climate model SOCOL-AER (Sheng et al., 2015) in this study. An earlier geoengineering study by Heckendorn et al. (2009) used SOCOLv2 (Egorova et al., 2005Schraner et al., 2008) to study the stratospheric response to SO2 injections. In that case, the AER-2D model (Weisenstein et al., 1997(Weisenstein et al., , 2007 was used to calculate global aerosol properties, which were prescribed in the 3D chemistry-climate model SOCOLv2. The SOCOL-AER 5 (Sheng et al., 2015;Sukhodolov et al., 2018) model is based on SOCOLv3 (Stenke et al., 2013), and improves on the earlier versions of SOCOL by incorporating a sectional aerosol module based on the AER-2D model. SOCOL-AER includes the chemistry of the sulfate precursors H2S, CS2, dimethyl sulfide (C2H6S, DMS), OCS, methanesulfonic acid (CH4O3S, MSA), SO2, SO3, and H2SO4, as well as the formation and evolution of particulates via particle size resolving microphysical processes such as homogeneous bimolecular nucleation of H2SO4 and H2O, 10 condensation and evaporation of H2SO4 and H2O, coagulation and sedimentation. As opposed to an earlier version of SOCOL-AER that used a wet radius binning scheme (Sheng et al., 2015), this model version separates the aerosol according to H2SO4 mass in 40 bins, with H2SO4 mass doubling between neighbouring bins. The new binning approach allows for more accurate consideration of size distribution changes caused by evaporation and condensation of H2O on the sulfate aerosols.
Depending on the grid-box temperature and relative humidity, wet aerosol radii in the new scheme can range from 0.4 nm to 15 7 µm. Nevertheless, to simplify post-processing of results the sulfate aerosols are rebinned into the original wet size bins of Sheng et al. (2015). SOCOL-AER interactively couples the aerosol module AER with the chemistry module MEZON (Rozanov et al., 1999Egorova et al., 2001Egorova et al., , 2003 via photochemistry of the sulfate precursor gases as well as heterogeneous chemistry on the particle surfaces. In SOCOL-AER, MEZON treats 56 chemical species of oxygen, nitrogen, hydrogen, carbon, chlorine, bromine and sulfur families with 160 gas phase reactions, 58 photolysis reactions, and 16 20 heterogeneous reactions, representing the most relevant aspects of stratospheric chemistry. SOCOL-AER also treats tropospheric chemistry, though with a reduced set of organic chemistry (isoprene as most complex organic species), and prescribed aerosols (other than sulfate aerosols, which are fully coupled). SOCOL-AER also interactively couples AER with the general circulation model ECHAM5.4 (Manzini et al., 1997;Roeckner et al., 2003Roeckner et al., , 2006 of SOCOLv3 via the radiation scheme. SOCOL-AER treats 6 short-wave (SW) radiation bands (185 nm-4 µm) and 16 long-wave radiation bands (4 µm-1 25 mm) which respond to the aerosol module via the aerosol radiative feedbacks. The extinction coefficients, which are required for each of the 22 wavelengths, as well as the single scattering albedo and the asymmetry factors, which are only taken into account for the 6 short-wave bands, are calculated from the particle size distribution of the 40 size bins according to Mie scattering theory (Biermann et al., 2000), with radiative indexes from Yue et al. (1994). The transport of the sulfur gas species and the aerosol bins is integrated into the advection scheme of ECHAM5 (Lin and Rood, 1996). MEZON is 30 interactively coupled to ECHAM5 using the three-dimensional fields of temperature, wind and radiative forcing of water vapor, methane, ozone, nitrous oxide and chlorofluorocarbons. Operator splitting is used, whereupon transport is calculated every 15 minutes, and chemistry, microphysics and radiation are calculated every 2 hours, with 3-minute sub-timesteps for microphysical processes. We used T31 horizontal truncation (i.e. 3.75° resolution in longitude and latitude) with a vertical resolution of 39 hybrid sigma-p levels from the surface up to 0.01 hPa (i.e. about 80 km altitude). This results in a vertical resolution of about 1.5 km in the lower stratosphere. 5 This study is among the first modelling studies on SSG which couple a size-resolved sectional aerosol module interactively to well-described stratospheric chemistry and radiation schemes in a global three-dimensional chemistryclimate model. Furthermore, this study explores the injection of AM-H2SO4 in detail and contrasts the resulting atmospheric effects and sensitivities with those of gaseous SO2 injections.

Experimental Setup 10
In this study, 21 different injection scenarios with annual emissions of 1.83 Mt of sulfur per year (Mt S yr -1 ) in the form of AM-H2SO4 (sulfate aerosols) or gaseous SO2 were performed, as well as runs with mixtures of both species. Per year, this correspond to about one quarter of the sulfur emitted during the Mt. Pinatubo eruption. See Table 1 for a complete list of scenarios. Additionally, a reference run (termed "BACKGROUND") without artificial sulfur emissions was conducted to enable comparison with background conditions. Natural and anthropogenic emissions of chemical species were treated as 15 described by Sheng et al. (2015). Each simulation was performed for 20 years representing atmospheric conditions of the years 2030-2049 for ozone depleting substances (2.3 ppb ClY and 18 ppt BrY above 50 km altitude, WMO, (2008)) and greenhouse gas concentrations following the representative concentration pathway 6.0 scenario (RCP6.0), with the first 10 years used as spin-up and the last 10 years used for analysis. Sea surface temperatures (SST) and sea ice coverage (SIC) were prescribed as a repetition of monthly means of the year 2001 from the Hadley Centre Sea Ice and Sea Surface 20 Temperature 1 data set (hadISST1) by the UK Met Office Hadley Centre (Rayner et al., 2003). The QBO was nudged by a linear relaxation of the simulated zonal winds in the equatorial stratosphere to observed wind profiles of the years 1999 and 2000 in a repeating two-year cycle. The geoengineering emissions were injected at 50 hPa altitude (≈ 20 km) except for runs number 19 (termed "GEO_AERO_25km_15") and 20 (termed "GEO_SO2_25km_15") which emitted AM-H2SO4 and SO2, respectively, at about 24 hPa altitude (≈ 25 km) to investigate the sensitivity to the emission altitude. 25 AM-H2SO4 emissions were parameterized as a log-normal distribution with a dry mode radius (rm) of 0.095 µm and a distribution width () of 1.5. This is the resulting size distribution determined by Pierce et al. (2010) from a plume model, derived at the point when coagulation with the larger background sulfate particles became dominant over self-coagulation.
We also performed one run (number 21, termed "GEO_AERO_radii_00") with a mean radius of 0.15 µm to investigate the sensitivity of aerosol burden and radiative forcing to the initial AM-H2SO4 size distribution. 30 Runs 1 to 6 injected AM-H2SO4 while runs 7 to 12 injected SO2 at the equator (i.e. ±3.75° N and S), from 5° N to 5° S, 10° N to 10° S, 15° N to 15° S, 20° N to 20° S and from 30° N to 30° S, each uniformly spread over all longitudes. Emission into the tropical and subtropical stratosphere was chosen to achieve global spreading via the Brewer-Dobson circulation, and various latitudinal spreads were chosen to investigate sensitivity of emitting partly into the surf zone and not only into the tropical pipe. We assumed emission continuous in time, and injected into one vertical model level and the indicated emission area for all the scenarios, except for scenarios 13 and 14 which emitted AM-H2SO4 and SO2, respectively, in two pulses per year (January 1 st -2 nd and July 1 st -2 nd of every modelled year) between 10° N and 10° S. Run 15 and 16 are 5 scenarios with emissions into a single equatorial grid box (3.75° x 3.75° in longitude and latitude), whereas all the other scenarios emitted equally at all longitudes around the globe. With these scenarios, we investigated differences between a point source emission (e.g. emissions resulting from a tethered balloon), and equally spread emissions (e.g. emissions from continuously flying planes or a dense grid of continuously operating balloons).
We assume that AM-H2SO4 is produced in situ in the plumes behind planes which generate SO3 or H2SO4 from 10 burning elemental sulfur. As a 100 % conversion rate is unlikely to be achieved (Smith et al., 2018), we also performed two runs emitting mixtures of SO2 and AM-H2SO4 with only 30 % or 70 % in the form of AM-H2SO4 and the rest in the form of SO2 (runs 17 and 18, respectively).
Finally, we note that a fully coupled ocean would be desirable to study impacts on tropospheric climate such as surface temperature change. For computational efficiency, we chose not to couple the deep ocean module of SOCOL-AER 15 in the present study. Therefore, we focus on changes in stratospheric aerosol microphysics, chemistry, and changes in surface radiation. We also note particular sensitivities to which horizontal and vertical resolution may play an important role.

The stratospheric sulfur cycle under SSG conditions
A simplified representation of the modelled stratospheric sulfur cycle is shown for background conditions and for two 20 geoengineering scenarios in Figure 1. Under background conditions (black numbers in figure), the stratospheric sulfur burden arises primarily from cross-tropopause net fluxes of SO2 and SO2 precursor species (OCS, DMS, CS2 and H2S) as well as primary tropospheric sulfate aerosols in the rising air masses in the tropics. OCS contributes to the SO2 burden in the middle stratosphere, while the direct cross-tropopause flux of SO2 influences mainly the lower stratosphere (Sheng et al., 2015). In the tropical stratosphere, the mean stratospheric winds disperse sulfur species readily in east-west directions, 25 whereas meridional transport is determined by large-scale stirring and mixing through the edge of the tropical pipe at typical latitudes of 15°-20° N and S (Plumb, 1996). Transport into higher latitudes, which occurs mainly via the Brewer-Dobson circulation (Brewer, 1949;Dobson, 1956), results in a stratospheric aerosol residence time (= stratospheric burden divided by net flux to the troposphere) of about 13 months for background conditions in SOCOL-AER (see Table 2). After decomposition of sulfate precursors to SO2 and subsequent oxidation to H2SO4 vapor, sulfate aerosol particles are forming 30 via bimolecular nucleation with H2O or grow through condensation onto already existing aerosol particles. Condensation is proportional to the available surface area of pre-existing aerosols and nucleation is mainly a function of temperature and H2SO4 partial pressure. The resulting stratospheric aerosol burden differs in some details (~16 % larger total stratospheric aerosol mass) from the one simulated in Sheng et al. (2015) due to different temporal sampling of model output as well as subsequent model updates and development.
The aerosol burden resulting from geoengineering AM-H2SO4 injection for scenario GEO_AERO_15 (blue) is 5 about 41.4 % larger than the aerosol burden resulting from the equivalent SO2 injection for scenario GEO_SO2_15 (red).
The chemical lifetime of SO2 in the lower stratosphere varies between 39 to 46 days among our SO2 emission scenarios (about 35 days for background conditions). The SO2 injection from GEO_SO2_15 results in an averaged stratospheric SO2 burden of 222.4 Gg S for steady state conditions, which is 12.8 % of the combined stratospheric SO2 and aerosol burden. This is a significant fraction compared to the 0.6 % in AM-H2SO4 emission scenarios. Especially when considering that the 10 sulfur becomes only "useful" for SSG after transformation to sulfate aerosols. In SO2 emission scenarios only 4.7 % of the total emitted SO2 is transported back to the troposphere unprocessed via diffusion and mixing due to tropopause folds at the edges of the tropical pipe. The other 95.3 % of the annually emitted SO2 is subsequently oxidized to H2SO4 of which 37 % nucleates to form new particles and 63 % condenses onto already existing particles. Compared to background conditions, GEO_AERO_15 shows a shift in the processing from nucleation to condensation due to the increased surface area 15 availability. Both SSG scenarios show an increased OCS flux across the tropopause (about +7 % among all scenarios) when comparing to the background run, which indicates that upward mass fluxes across the tropical tropopause are enhanced under SSG conditions. Table 2 lists averaged values of aerosol burden, short-wave radiative forcing, and other quantities for all scenarios modelled, while Table 3 provides the quantities normalized by the corresponding all sky short-wave radiative forcing. 20 The SO2 injection case produces both more nucleation mode (i.e. < 0.01 µm radius) particles and more coarse mode (>1 µm radius) particles than the AM-H2SO4 case (see Figure 2). GEO_SO2_15 shows about 3 orders of magnitude higher number concentrations of large particles in the coarse mode compared to GEO_AERO_15. The high concentration of tiny nucleation mode particles (about 4 orders of magnitudes higher compared to BACKGROUND) is due to the large nucleation rate driven by H2SO4 gas formed from SO2 oxidation in GEO_SO2_15. The increase in coarse mode particles is partly due to 25 H2SO4 condensing onto existing aerosols, and partly due to increased continuous coagulation of freshly formed nucleation mode particles with larger particles. The larger concentration of coarse mode particles in SO2 emission scenarios leads to increased aerosol sedimentation rates and thus to about 23.3 % shorter stratospheric aerosol residence times compared to AM-H2SO4 emission scenarios. We also show the 5 th moment of the aerosol size distribution (see Figure 2, c), which gives an estimate of the downward mass flux due to aerosol sedimentation. This shows that particles in the size range 0.4-1.5 µm 30 are contributing the most to sedimentation in the GEO_SO2_15 scenario. Together with the SO2 that gets lost to the troposphere unprocessed (4.7 %), the difference in the stratospheric aerosol residence time (32.0 %) explains the biggest part of the total difference of 39.6 % in stratospheric aerosol burden between SO2 and AM-H2SO4 emission scenarios. In the AM-H2SO4 case, the number concentration of nucleation mode particles decreases below background conditions due to the increased surface area for condensation (see Figure 2). For particles larger than 10 nm radius, the simulated distribution in the tropics is similar to the injected aerosol distribution with a peak of about 100 particles per cm 3 at about 0.1 µm radius. In Figure 2, the size range between 0.12 µm and 0.40 µm is highlighted in green as the range in which the efficacy of backscattering solar radiation on sulfate aerosols is at least 70 % of its peak value at 0.3 µm (solid 5 green line, Dykema et al., 2016). In the tropics, the mass fraction of particles in the 0.12 to 0.40 µm size range is 0.79 of the total tropical aerosol mass for GEO_AERO_15 and 0.60 for GEO_SO2_15 (see also Table 3). Coagulation and sedimentation during transport to higher latitudes reduces the overall particle concentration in higher latitudes (dashed curves in Fig. 2) while increasing the mean particle size. Subsequently, the peak at about 0.1 µm in the tropics in GEO_AERO_15 becomes less pronounced and shifts slightly towards larger particles, which is closer to the radius of 10 maximal backscattering of solar radiation on sulfate aerosols. Among all scenarios, the mass fraction in the optimal size range between 0.12 µm and 0.40 µm radius increases with transport to higher latitudes (e.g. 0.86 for GEO_AERO_15 and 0.67 for GEO_SO2_15 between 40° N and 60° N), which results in larger radiative forcing efficiency per stratospheric aerosol burden (see Fig. 2 and Table 3). Overall, AM-H2SO4 emission scenarios result in a more favourable aerosol size distributions, with more particles in the optimal size range for backscattering solar radiation compared to SO2 emission 15 scenarios.
The more favourable size distribution in AM-H2SO4 emission scenarios is also illustrated by values of effective radius (reff) for different scenarios ( Fig. 3 and Table 2). The effective radius is the ratio of the 3 rd moment to the 2 nd moment of the size distribution. The size range between 0.24 µm and 0.36 µm, which is the range at which mass specific up-scatter is at least 90% of its peak backscattering efficiency at 0.3 µm is marked stippled in Figure 3. In GEO_AERO_15 (Fig. 3, a)

Sensitivity simulations 25
Latitudinal spread of emissions: Previous studies found that the latitude range of emissions was important in determining size distribution and aerosol burden, and thus the resulting short-wave radiative forcing. English et al. (2012) found a 40 % larger aerosol burden when emitting AM-H2SO4 between 32° N and 32° S when compared to emitting between 4° N and 4° S. They suggested that this is partly due to reduced aerosol concentrations and thus less coagulation as a result of a more dilute aerosol plume. However, they simultaneously increased the emission altitude, and thus they also state that the 30 increased aerosol burden could partly be due to the increased stratospheric aerosol residence time at higher emission altitude.
In contrast, Niemeier and Timmreck (2015) found small decreases in aerosol burden when emitting between 30° N and 30° S relative to emitting from 5° N to 5° S. They found greater coagulation with more diffuse emissions, and that emission into the surf zone increased cross-tropopause transport of aerosols and SO2, thus resulting in a reduced stratospheric aerosol burden compared to scenarios that emitted SO2 only into the tropical pipe.
We find a small reduction in aerosol burden (<10 %) and short-wave radiative forcing for both AM-H2SO4 and SO2 emission scenarios with increased latitudinal spread of the emissions (see Fig. 4). We assume that increased loss of 5 stratospheric aerosols through tropopause folds in the surf zone with broadly spread emissions (>15° N-15° S) is compensated by increased coagulation and sedimentation of aerosols in scenarios which emit only into the tropical pipe.
Sensitivity to emission altitude: The stratospheric aerosol residence time when emitting at 24 hPa (~25 km) is increased 29.1 % and 40.8 % when emitting AM-H2SO4 and SO2 respectively, relative to emitting at 50 hPa (~20 km).
Therefore, GEO_AERO_25km_15 and GEO_SO2_25km_15 result in stratospheric aerosol burden of 2761 and 2190 Gg S 10 respectively. In GEO_SO2_25km_15 the loss of unprocessed SO2 to the tropopause is reduced to 2.3 % which is due to the greater distance of emissions from the tropopause and higher OH concentrations with increasing altitude in the stratosphere.
However, because of warmer temperatures at higher altitudes in the stratosphere, the nucleation rate decreases and a larger fraction of the H2SO4 gas condenses onto already existing particles (12 % nucleation, 88 % condensation). Therefore, particles tend to grow to even larger sizes and more accumulate in the coarse mode. The mass fraction of particles between 15 0.12 µm and 0.40 µm is reduced to only 0.41 and the all sky short-wave radiative forcing is only increased by 27 % in magnitude compared to GEO_SO2_15, which is not proportional to the increase in stratospheric aerosol burden (44.3 %).
For GEO_AERO_25km_15 the fraction between 0.12 µm and 0.40 µm is only reduced from 0.79 to 0.77. This results in an all sky short-wave radiative forcing increase of 23 % compared to GEO_AERO_15, which is close to the increase in stratospheric aerosol burden (28.6 %). The latitudinal and vertical distribution of the aerosol mass density and the larger 20 aerosol sizes are also seen in values of reff ( Fig. 3c and e). When emitting at 24 hPa, reff in the tropical lower stratosphere increases from 0.35 µm to 0.52 µm in the SO2 emission case and from 0.23 µm to 0.39 µm in the AM-H2SO4 emission case.
Thus, the stippled area in Figure 3c and e (reff of 0.24 to 0.36 µm) is below and above the lower stratosphere where most aerosol mass is accumulated. Therefore, emitting at 25 km results in less short-wave radiative forcing per resulting stratospheric aerosol burden. Furthermore, aerosol density remains more concentrated in the tropics when emitting at 25 km 25 (see also Fig. 7a and b), due to a less leaky tropical pipe at higher stratospheric altitudes. However, when looking at the resulting clear sky short-wave radiative forcing ( Fig. 7c and d), the peak in the tropics is only slightly increased in the SO2 emission case and is even lower than the equivalent emission scenario at 20 km in the AM-H2SO4 emission case.
Sensitivity to the injection mode radius: GEO_AERO_radii_15 released AM-H2SO4 with mean radii of 0.15 µm instead of 0.095 µm. This results in a slightly fewer but larger particles in the emitted aerosol plume. The increased injection 30 radius resulted in slightly reduced aerosol burden due to either different coagulation and condensation regimes or faster sedimentation of the slightly larger particles. However, due to the increase of particles in the optimal size range for backscattering solar radiation (+11 %), the all sky short-wave radiative forcing increased about 20 % compared to GEO_AERO_15 (see Figure 4). When looking at Tables 2 and 3, there is only a small difference in reff between the AM-H2SO4 emission scenarios with rm of 0.95 µm and 0.15 µm, indicating only minor dependence of small changes in rm on the resulting distribution of accumulation mode particles.

Mixtures of AM-H2SO4 and SO2:
We also performed calculations to explore the utility of emitting SO2 and AM-H2SO4 together (see Fig. 5). Some studies have suggested that planes carrying elemental sulfur and burning it in situ to 5 directly emit H2SO4-gas or AM-H2SO4 would be the most effective way to deliver sulfate to the stratosphere (Benduhn et al. 2016, Smith et al. 2018. Burning elemental sulfur would also reduce the freight to be transported to the stratosphere (i.e. 32 g mol -1 for sulfur rather than 98 g mol -1 for H2SO4 or 64 g mol -1 for SO2). However, 100 % conversion to H2SO4 is unlikely, with the remainder emitted as SO2. Our results show that a small portion (30 %) of SO2 in the emission mixture (i.e. GEO_70%AERO_15) leads only to a slight reduction in the resulting stratospheric aerosol burden (-7 %) as well as in the 10 mass fraction in the optimal size range for backscattering solar radiation (see Table 2 and 3). And that a small portion (30 %) of AM-H2SO4 in an emission mixture dominated by SO2 (i.e. GEO_30%AERO_15) can increase stratospheric aerosol burden and short-wave radiative forcing significantly with respect to a pure SO2 emission scenario (GEO_SO2_15). As the decrease in efficiency with increasing SO2 portion in the emission mixture is strongly non-linear in our simulations, we do not expect significant efficiency loss for small portions of SO2 in the emissions after in situ burning of elemental sulfur in 15 planes.

Spatio-temporal spread of emissions: Emitting AM-H2SO4 in two pulses per year as assumed in
GEO_AERO_pulsed_10 decreases the stratospheric aerosol burden by about 5 % compared to the continuous emission scenario, GEO_AERO_10. This is due to the ~91 times larger aerosol mass concentration in the emission plume compared to GEO_AERO_10 (emission during 4 days per year instead of 365), and thus more coagulation and faster sedimentation. 20 Similar but much smaller effects were observed for GEO_AERO_point_00, which emitted at one equatorial grid box only (3.75° x 3.75°), and thus with 96 times increased mass concentration in the emission region compared to GEO_AERO_00 (emissions spread over 3.75° in longitude instead of 360°). This indicates fast zonal mixing which reduces the effect of the higher initial mass concentration compared to the pulsed emissions.
However, looking at GEO_SO2_pulsed_10 and GEO_SO2_point_00, which are the equivalent SO2 emission 25 scenarios, the stratospheric aerosol burden increased compared to the continuous emission scenarios. This is mainly due to a reduction in the total globally averaged aerosol nucleation rate (23 % nucleation, 77 % condensation) and the subsequent reduction in total coagulation. However, locally for GEO_SO2_point_00 and spatially for GEO_SO2_pulsed_10 the nucleation rates are increased in the emission area due to the greatly increased H2SO4 concentration in the emission region in these scenarios. These two scenarios produce spatially/temporally large amounts of nucleation mode particles which then 30 quickly coagulate to produce accumulation mode particles. After dilution when nucleation is small again, the continuous flow of tiny, freshly nucleated particles is disrupted, and coagulation is reduced. Thus, the mean particle diameter is reduced

Temperature and H2O at the tropical cold point tropopause
Increased aerosol burden in the stratosphere also leads to heating of the lower stratosphere, mainly due to absorption of longwave radiation. The longwave surface anomaly is about 0.1 W m -2 among all scenarios. Heating of the cold point 5 tropopause results in temperature increases of 0.7 to 1.2 K among all scenarios associated, with an H2O entry value increase of about 0.30 to 0.55 ppmv (i.e. 9-17 %, see Fig. 6). The increased stratospheric H2O volume mixing ratio results in an increase of OH volume mixing ratio (H2O+O( 1 D) => 2 OH), which additionally increases the HOX-ozone depletion cycle. Figure 6 (c and d) shows how H2O volume mixing ratios increase above 200 hPa. The increase is slightly larger for the AM-H2SO4 scenarios due to the higher aerosol load and more pronounced heating of the lower stratosphere. However, 10 when comparing the vertical OH profiles (Fig. 4, a and b), the difference in OH between the SO2 and the AM-H2SO4 scenarios increases to about 4 % at about 50 hPa, which is caused by the depletion of OH due to SO2 oxidation (SO2+OH=>SO3 +HO2). Kleinschmitt et al. (2018) applied constant climatological SO2 with a lifetime of 41 days in their study and found a SO2-to-H2SO4 conversion rate of 96 %. They expected a slightly lower concentration in oxidants and therefore a longer SO2 15 lifetime when taking chemical interactions into account. However, they did not consider the increased stratospheric H2O volume mixing ratio under SSG conditions which causes an OH concentration increase of up to 8 %, making stratospheric SO2 lifetime shorter and not longer.

AM-H2SO4 vs. SO2 emissions
All sky (clear sky) short-wave radiative forcing from AM-H2SO4 emission scenarios 1-6 result in an average of 1.31 W m -2 20 (2.24 W m -2 ) and are 36.4 % (50.3 %) larger compared to the average of the equivalent SO2 emission scenarios (7-12) which result in 0.96 W m -2 (1.49 W m -2 ). Table 2 summarizes globally averaged quantities of all modelled SSG scenarios. The fifth column shows the ratios between surface clear sky and surface all sky short-wave radiative forcing. These values for the AM-H2SO4 emission scenarios are about 10 % larger than for the SO2 emission scenarios, indicating a larger reduction of short-wave radiative forcing due to clouds and/or chemical interactions among these scenarios. The stratospheric aerosol 25 burdens for the AM-H2SO4 scenarios are more concentrated within the tropical pipe between 15° N and 15° S compared to the equivalent SO2 emission scenarios that are more evenly distributed across all latitudes, as shown in Figure 7a and b.
When looking at the clear sky short-wave radiative forcing (Fig. 7 c and d), the tropical peak flattens out compared to the peak in aerosol burden (Fig. 7 a and b) because the up-scattered fraction of solar radiation increases with increasing solar zenith angle. For the SO2 emission scenarios, the clear sky short-wave radiative forcing is almost equally distributed between 30 60° N and 60° S, whereas for the AM-H2SO4 scenarios, a pronounced peak can still be observed in the tropics where cloud cover is larger on average. Thus, the higher aerosol mass fraction in the cloudy tropics makes SSG less efficient in these regions. We assume the difference in surface clear sky to surface all sky short-wave radiative forcing ratios to be a result of a more favourable global spread of the resulting stratospheric aerosol burden for SO2 emission scenarios. However, this indicates that emitting only within the tropical pipe region might not be the optimal setup of SSG studies. Emitting at the edges of the tropical pipe at 15° N and 15° S like that investigated by Tilmes et al. (2017) might be a more efficient way to 5 achieve higher short-wave radiative forcing with fewer negative side effects.
When looking at the depletion of the total ozone column (Fig. 7, e and f), we find larger depletion among the AM-H2SO4 emission scenarios. This is mainly due to the larger surface area densities for AM-H2SO4 scenarios in the emission region in the tropics where ozone is produced. This leads to larger ozone depletion and therefore less ozone transport to higher latitudes. Therefore, among the AM-H2SO4 emissions scenarios, the vertical ozone column is depleted up to 7.5 % 10 (for GEO_AERO_15 and GEO_AERO_00) at the south polar region in a 10-years average, whereas for the SO2 emission scenarios, it is only up to 6 % (for GEO_SO2_10). The ozone depletion arises through formation of the reservoir species HNO3 through N2O5 hydrolysis on aerosol surfaces which indirectly enhances the ClOx ozone depletion cycle. Furthermore, chlorine gets activated through the heterogenous reaction of ClONO2 with HCl, which contributes the most to the ozone depletion due to SSG. 15 Table 3 shows values from Table 2  respectively, for the AM-H2SO4 emission scenarios than for SO2 emission scenarios when normalizing to the emission rate of 1.83 Mt S yr -1 . However, when normalized by the resulting all sky short-wave radiative forcing, the increase in efficiency among the AM-H2SO4 emission scenarios outweighs the worse side effects in the AM-H2SO4 scenarios.

Discussion
In this study, the three-dimensional global aerosol-chemistry-climate model SOCOL-AER was used to investigate AM-25 H2SO4 and SO2 emission scenarios for the purpose of SSG. We analysed the stratospheric sulfur cycle, aerosol burden, short- emission and not only after more than one month of transport and photochemistry like that of SO2 emissions (lifetime of SO2 of 39 to 46 days through oxidation). Thus, direct AM-H2SO4 emission can create an immediate, targeted effect over the area of emission, whereas in SO2 emission scenarios about 12.4 % of the annually emitted sulfur is present in form of SO2 on average. (2) The size distribution of SO2 emission scenarios shows coarse mode particle concentrations which are about two orders of magnitudes larger compared to AM-H2SO4 emission scenarios. These particles sediment faster and reduce the 5 average stratospheric residence time of the aerosols compared to AM-H2SO4 scenarios. In addition, the short-wave radiative forcing in SO2 emission scenarios is influenced by the smaller mass fraction of particles in the optimal size range for backscattering solar radiation (i.e. 0.3 µm radius). The unfavourable size distribution for sulfate aerosols resulting from SO2 emissions is largely due to condensation onto existing particles and the pronounced formation of tiny nucleation mode particles which subsequent coagulation with larger aerosols to create coarse mode particles. 10 Stratospheric aerosol burden and clear sky short-wave radiative forcing are about 10 % higher for scenarios which avoid emitting into the surf zone, i.e. outside 15° N and 15° S. A higher mass fraction of stratospheric aerosol burden in the tropics for AM-H2SO4 emission scenarios reduces all sky radiative forcing efficiency due to the higher cloud fraction in the tropics. AM-H2SO4 emission scenarios also result in slightly more stratospheric ozone depletion and stratospheric warming.
However, due to the larger absolute short-wave radiative forcing, the negative side effects are smaller for AM-H2SO4 15 emission scenarios compared to SO2 emission scenarios when normalizing to the surface all sky short-wave radiative forcing.
On the one hand, for AM-H2SO4 emission scenarios, temporally and spatially increasing the mass density in the emission region leads to slightly shorter aerosol residence times through more coagulation, and thus reduced short-wave radiative forcing. On the other hand, for SO2 emission scenarios, a larger stratospheric aerosol burden for a fixed sulfur emission rate can be achieved by temporally or spatially increasing the SO2 mass density. This strategy increases nucleation 20 and coagulation rates in the emission region while minimizing nucleation and coagulation on a global scale. The optimal frequency of the pulses as well as optimal spatial extent of the emissions requires further investigation. However, these results indicate a strong dependence on small-scale processes such as locally changing SO2 and OH mass concentrations (e.g. emissions along the trajectory of an aircraft), which cannot be resolved in GCMs. Furthermore, they underline the importance of interactions between chemistry and aerosols when modelling SSG scenarios. 25 We found that in the lower stratosphere OH concentrations are increased up to 9 % for AM-H2SO4 emission scenarios compared to the background run due to increased temperatures at the tropical cold point tropopause and thus higher water volume mixing ratios in the stratosphere. However, due to oxidation of SO2 in SO2 emission scenarios, the OH concentrations are only increased by about 4 % in the lower stratosphere compared to the background run in these scenarios.
We also examined scenarios in which mixtures of SO2 and AM-H2SO4 were emitted. Pure AM-H2SO4 emission 30 scenarios resulted in the largest stratospheric aerosol burdens as well as the largest clear sky and all sky short-wave radiative forcing. While the short-wave radiative forcing decreases with increasing fractions of SO2, that increase is non-linear. A small fraction of SO2 within emissions of AM-H2SO4 results in only slightly smaller radiative forcing efficiency, whereas a small fraction of AM-H2SO4 within emissions of SO2 increases radiative forcing efficiency significantly. In situ burning of elemental sulfur in planes with subsequent conversion to SO3 and, in the plume to AM-H2SO4 particles might therefore be effective in controlling the aerosol size distribution, even if conversion efficiency were significantly less than unity.
We found clear sky short-wave radiative forcing efficiencies of -1.22 W m -2 and -0.82 W m -2 per emitted Mt of sulfur equivalent injection rate (W m -2 (Mt S yr -1 ) -1 ) for AM-H2SO4 and SO2 emissions at 20 km respectively. For all sky 5 radiative forcing, the efficiencies were about -0.72 W m -2 and -0.52 W m -2 per emitted Mt of sulfur respectively. These values are smaller compared to other models (i.e. ~-0.83 W m -2 (Mt S yr -1 ) -1 for MAECHAM5 emitted as SO2 in Niemeier et al., (2011), ~-0.70 W m -2 (Mt S yr -1 ) -1 for LMDZ-S3A emitted as SO2 in Kleinschmitt et al., (2018)), likely due to differences in transport processes as well as lower stratospheric aerosol burdens in our model. When emitting SO2 at 25 km altitude the efficiency is -0.56 W m -2 (Mt S yr -1 ) -1 in SOCOL-AER and -0.61 W m -2 (Mt S yr -1 ) -1 for MAECHAM5 10 (Niemeier et al., 2011). On the one hand, this could be partially a result of different sedimentation, coagulation and aerosol binning schemes compared to other aerosol modules. On the other hand, SOCOL-AER slightly overestimates the Brewer-Dobson circulation (Dietmüller et al., 2018), which would result in larger aerosol burdens compared to other models.
However, as many processes are non-linear with increasing injection rates, the efficiencies might be different for other injection rates. Due to the lack of atmospheric observations or small-scale field experiments, modelling studies are currently 15 the only method to estimate efficiency and the possible adverse effects of SSG. Therefore, model intercomparison studies should further identify strengths and weaknesses among different models to reduce uncertainty. Furthermore, atmospheric field studies such as the Stratospheric Controlled Perturbation Experiment (SCoPEx, Dykema et al., 2014) could give further insight into stratospheric aerosol formation and plume evolution.
Additional uncertainties arise from the rather low resolution applied in this study. In particular, an increase in the 20 vertical resolution as well as treatment of an interactive QBO could further increase the explanatory power of SSG studies with SOCOL-AER. To study tropospheric climate change, ocean feedback would have to be taken into account with the deep ocean module of SOCOL-AER. Furthermore, SOCOL-AER does not treat cloud interactions-which is likely one of the major uncertainties of the model as aerosols may have large impacts on clouds. We only performed scenarios with emission regions limited to the tropics and subtropics centered around the equator. Nevertheless, SOCOL-AER is one of the 25 first models that interactively couples a sectional aerosol module to the well-described chemistry and radiation schemes of a CCM.
This study shows that direct emission of aerosols can give better control of the resulting size distribution, which subsequently results in more effective radiative forcing. Therefore, the SSG modelling community should increase their focus on direct AM-H2SO4 emission as well as on the emission of solid particles , such as calcite 30 particles . Further investigations using SOCOL-AER to understand adverse effects of SSG, such as a closer look on ozone depletion, impacts on precipitation patterns, or on stratospheric dynamics will be conducted in future Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1070 Manuscript under review for journal Atmos. Chem. Phys. studies. These studies will also include coupling of the deep ocean module of SOCOL-AER to investigate impacts on the tropospheric climate.
Furthermore, we show that interactive coupling of aerosols, chemistry and radiation schemes are essential features for modelling SSG emission scenarios with GCMs. For SO2 emission scenarios, local depletion of oxidants was found, particularly with large SO2 mass concentrations in the emission region. Accurate modelling of these scenarios might require 5 higher temporal and spatial resolution than has been achieved among current GCMs. Therefore, coupling of small-scale Lagrangian plume dispersion models which simulate the first few days of aerosol-chemistry interactions and aerosol microphysics in evolving emission plumes from airplanes might be a desirable tool to improve future SSG modelling studies.
This would appropriately account for the problem of connecting small scale temporal and spatial processes-such as aerosol formation, growth and evolution in an aircraft wakes-to the larger grid of GCMs, which has been neglected in past SSG 10 studies.

Author Contributions
This paper is a result of the master thesis of Sandro Vattioni to gain his master's degree in atmospheric and climate science at ETH Zürich. The thesis was conducted at Harvard University as an exchange research project with ETH Zürich.   Table 1 for scenario definitions). Net fluxes (positive in pointing direction of arrow) are given in Gg S yr -1 and burden (in boxes) are given in Gg S. SO2 precursor species include OCS, DMS, H2S, CS2, and H2S. SO3 as an intermediate step between SO2 and H2SO4 is modeled but for simplicity omitted from the diagram.       shows results for injections at the equator. For equatorial injection (right column), we also show results averaged from 0° E to 3.75° E and 5 from 180° E to 176.25° W. For the 10° injection case (left column), we also show zonal averages of January (emissions during January 1 st and 2 nd ) and June (month before emission) from the pulsed simulation. Note how SO2 injection scenarios tend to reduce OH concentrations around 50 hPa compared with AM-H2SO4 injection scenarios (due to the reaction SO2+OH -> SO3+HO2), while both increase stratospheric H2O concentrations (due to warming).    Table 3. Comparison of important impacts normalized to the all sky short-wave radiative forcing. The smaller the absolute value of the ratio, the smaller the injection rate or impact to achieve a given level of radiative forcing. For each column, the smallest/largest three absolute values are marked in green/red. Columns show globally averaged values of sulfur injection rate, resulting stratospheric aerosol burden and ozone depletion, as well as H2O volume mixing ratio increase, and temperature increases at the tropical cold point tropopause 5 (i.e. at 90 hPa) normalized to the resulting all sky short-wave radiative forcing. Last two columns show the wet aerosol mass fraction in the range between 0.12 and 0.40 µm of the resulting size distribution. The three largest mass fractions are marked in green and the three smallest mass fractions are marked in red.