Abstract
Abstract. Solid aerosol particles have long been proposed as an alternative to sulfate aerosols for solar geoengineering. Any solid aerosol introduced into the stratosphere would be subject to coagulation with itself, producing fractal aggregates, and with the natural sulfate aerosol, producing liquid-coated solids. Solid aerosols that are coated with sulfate and/or have formed aggregates may have very different scattering properties and chemical behavior than uncoated non-aggregated monomers do. We use a two-dimensional (2-D) chemistry–transport–aerosol model to capture the dynamics of interacting solid and liquid aerosols in the stratosphere. As an example, we apply the model to the possible use of alumina and diamond particles for solar geoengineering. For 240 nm radius alumina particles, for example, an injection rate of 4 Tg yr−1 produces a global-average shortwave radiative forcing of −1.2 W m−2 and minimal self-coagulation of alumina although almost all alumina outside the tropics is coated with sulfate. For the same radiative forcing, these solid aerosols can produce less ozone loss, less stratospheric heating, and less forward scattering than sulfate aerosols do. Our results suggest that appropriately sized alumina, diamond or similar high-index particles may have less severe technology-specific risks than sulfate aerosols do. These results, particularly the ozone response, are subject to large uncertainties due to the limited data on the rate constants of reactions on the dry surfaces.
Highlights
Solar geoengineering, or solar radiation management (SRM) has the possibility of deliberately introducing changes to the Earth’s radiative balance to partially offset the radiative forcing of accumulating greenhouse gases and so lessen the risks of climate change
Most research on SRM has concentrated on the possibility of adding aerosols to the stratosphere, and essentially all atmospheric modeling of stratospheric aerosol injection has focused on increasing the loading of aqueous sulfuric acid aerosols (Rasch et al, 2008; Heckendorn et al, 2009; Niemeier et al, 2011; Pitari et al, 2014)
The lifetime and scattering properties of a solid aerosol are strongly dependent on these dynamical interactions, and the chemical properties of the aerosol depend on the extent to which it becomes coated by the ambient sulfate
Summary
Solar radiation management (SRM) has the possibility of deliberately introducing changes to the Earth’s radiative balance to partially offset the radiative forcing of accumulating greenhouse gases and so lessen the risks of climate change. Our motivation for studying solid particles is the possibility that they enable a decrease in the risks of SRM (e.g., ozone loss) or an increase in its efficacy such as the ability to produce larger radiative forcings, or an improved ability to “tune” the spectral or spatial characteristics of the radiative forcing (Blackstock et al, 2009; Keith, 2010). This is in contrast to much of the prior literature that has focused on the potential of solid particles to deliver higher mass-specific scattering efficiency, reducing the amount of material needed to produce a given radiative forcing.
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