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-20th century” (IPCC, 2014). Emissions of CO2 and other GHGs must be curbed to reduce 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 emission 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 reduce solar climate forcing. This idea, now often called “solar geoengineering”, “solar 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 risks entailed. However, efforts were renewed after Crutzen (2006) suggested that solar radiation management might be explored as a useful climate change mitigation tool, as adequate emission reductions were becoming increasingly unlikely.
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Most research on solar geoengineering has focused on stratospheric sulfate geoengineering (SSG) via SO2 injection, in part due to its volcanic analogues such as the 1991 eruption of Mt. Pinatubo. However, studies of SSG have found limitations regarding SO2 injection as a method of producing a radiative forcing (RF) perturbation. These limitations include the following: (1) reduced efficacy at higher loading, limiting the achievable shortwave (SW) radiative forcing (Heckendorn et al., 2009; Niemeier et al., 2011; English et al., 2012; Niemeier and Timmreck, 2015; Kleinschmitt et al., 2018); (2) increased lifetimes of methane and other GHGs (Visioni et al., 2017; Tilmes et al., 2018); (3) impacts on upper tropospheric ice clouds (Kuebbeler et al., 2012; Visioni et al., 2018b); and (4) stratospheric heating (Heckendorn et al., 2009; Ferraro et al., 2011), especially in the tropical lower stratosphere, which would modify the Brewer-Dobson circulation (Brewer, 1949; Dobson, 1956) and increase stratospheric water vapor. 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 regarding SO2 injection arises after oxidation of SO2 to H2SO4 when aerosol particles are 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 via the self-coagulation of the many small new particles and – more importantly – coagulation with preexisting bigger particles from the background aerosol layer. These particles then grow further via 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 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 large enough to decrease their mobility and, in turn, their 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 (Dykema et al., 2016). 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 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 the 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 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 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 below a radius of 0.01 µm 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 existing particles. In an expanding aircraft plume, these processes occur on timescales from milliseconds to hours and length scales from millimeters to kilometers. This was addressed by Pierce et al. (2010) and then by Benduhn et al. (2016). There is rough agreement that particles between a radius of 0.095 and 0.15 µm could be produced after the initial plume processing, but these results are subject to uncertainties and need further investigation.
- (b)
Xem thêm : Na2SO3 + H2SO4 → Na2SO4 + SO2 + H2O
General circulation modeling. The second part of the problem can be analyzed using 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 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 uncertainty in our study, but might be handled by a Lagrangian transport model in a future study.
A sectional (or 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 SOCOL-AER), whereas modal models usually only apply three modes (e.g., Niemeier et al., 2011; Tilmes et al., 2017), each with different mode radius (rm) and fixed distribution widths (σ), to describe the aerosol distribution. Therefore, the degrees of freedom among modal models usually is 3, whereas there are 40 for a sectional model such as SOCOL-AER. Thus, sectional aerosol models represent aerosol distributions with better accuracy, although numerical diffusion does result from the discretization in size space. Two earlier studies of SSG modeling, Heckendorn et al. (2009) and Pierce et al. (2010), used the AER-2D chemistry-transport-aerosol model with sectional microphysics (Weisenstein et al., 1997, 2007). Although the sectional aerosol module within has high size resolution, this 2-D model only has a limited spatial resolution with simplified dynamical processes. So far, various studies have used four different GCM models to investigate SSG with sectional aerosol modules, namely English et al. (2012), Laakso et al. (2016, 2017), Visioni et al. (2017, 2018a, b) and Kleinschmitt et al. (2018). English et al. (2012) used the WACCM GCM (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 without treatment of the quasi-biennial oscillation (QBO) and without online interaction between aerosols, chemistry and radiation. The three other studies (Laakso et al., 2016, 2017; Visioni et al., 2017, 2018a, b; Kleinschmitt et al., 2018) only performed SO2 emission scenarios, but no AM H2SO4 emission scenarios. Laakso et al. (2016, 2017) used the MA-ECHAM5 GCM interactively coupled to the sectional aerosol module HAM-SALSA (Kokkola et al., 2008; Bergman et al., 2012); however, in both studies stratospheric chemistry was simplified using prescribed monthly mean OH and ozone concentrations. The ULAQ-CCM, which was used in Visioni et al. (2017, 2018a, b), includes an interactive sectional aerosol module and additionally treats detailed stratospheric chemistry. Kleinschmitt et al. (2018) used the LMDZ GCM (Hourdin et al., 2006, 2013) which was coupled to the S3A sectional aerosol module (Kleinschmitt et al., 2017). 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.
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 MA-ECHAM GCM interactively coupled to the HAM modal aerosol module (Stier et al., 2005). Chemistry was simplified in a similar fashion to Laakso et al. (2016, 2017) using prescribed OH and ozone concentrations. Kravitz et al. (2017), MacMartin et al. (2017), Richter et al. (2017) and Tilmes et al. (2017) used the CESM1 fully coupled global chemistry-climate model (Hurrell et al., 2013; Mills et al., 2017) to simulate SO2 emission scenarios. In their model setup, they applied higher horizontal and vertical resolutions compared with 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 using the SOCOL-AER sectional global 3-D aerosol-chemistry-climate model (Sheng et al., 2015), which treats prognostic transport as well as radiative and chemical feedbacks of the aerosols online in one model. 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, assuming that a certain size distribution can be created in an emission plume (Pierce et al., 2010; Benduhn et al., 2016). 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 emissions that highlight differences between the two injection strategies and indicate future research needs.
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