Hi, the paper on Mie scattering is very interesting and it is an important addition to the small number of experimental studies for alternate materials for stratospheric aerosol injection.
It is worth noting that the sulfuric acid coatings on silica are however very large in the study. The thinnest coating, after one collision, was achieved via collision with a 4 um^3 sulfuric acid particle, much larger than found in the stratosphere with average sulfuric acid aerosol radii of 0.08 um (~10/cm3). In the paper, after this first collision, already ~50% of the volume consist of sulfuric acid. Given, that the total stratospheric load of sulfuric acid aerosol is around 0.46 Mt H2SO4 even in a moderate scenario of 1MT Al2O3 injection/year there is not enough sulfuric acid to form such a thick coating. In fact, a fully coupled model we are currently running with 5MT Al2O3 injections/year shows coating thicknesses of <1 - 25 nm on alumina in the stratosphere depending on the radius of the emitted particles (80-320nm), with the majority of particles having thicknesses below 10nm. This is for particles at high latitudes (far from the injection latitude) which have the largest coating thicknesses. My point mainly is that, yes, if a large fraction or majority of the volume consists of sulfuric acid, the scattering properties are close to those of sulfuric acid. However, for scenarios that would be relevant for potential stratospheric aerosol injection with alternate materials that is not the case. Lastly, from my perspective, the main reason for exploring alternate materials is not tied to scattering efficiency, but rather to potentially reducing risk from the stratospheric heating and perhaps ozone destruction. It is definitely great to see more laboratory studies of alternate materials for SAI! All the best, Frank ___________________________________________________________________________________________ Frank N. Keutsch Stonington Professor of Engineering and Atmospheric Science Harvard John A. Paulson School of Engineering and Applied Sciences Department of Chemistry and Chemical Biology Department of Earth and Planetary Sciences Harvard University 12 Oxford Street Cambridge, MA 02138 USA E-mail: [email protected]<mailto:[email protected]> Tel:+1-617-495-1878 ___________________________________________________________________________________________ From: <[email protected]> on behalf of Francis Pope <[email protected]> Reply-To: "[email protected]" <[email protected]> Date: Tuesday, March 1, 2022 at 11:59 AM To: geoengineering <[email protected]> Subject: [geo] Mie scattering from optically levitated mixed sulfuric acid–silica core shell aerosols Hi All, A new paper of potential interest: Mie scattering from optically levitated mixed sulfuric acid–silica core–shell aerosols: observation of core–shell morphology for atmospheric science https://pubs.rsc.org/en/content/articlelanding/2022/CP/D1CP04068E<https://urldefense.proofpoint.com/v2/url?u=https-3A__pubs.rsc.org_en_content_articlelanding_2022_CP_D1CP04068E&d=DwMFaQ&c=WO-RGvefibhHBZq3fL85hQ&r=SxgtIVByLvhD8QiO8rgwrvm9f9GML9Drha7439cd21U&m=ouQdAL9_Hinqa8MYv8CnkUl8Ufmbp0NR7G5txWzSQzjsk5mw9xW77OHfx4iSQ5gr&s=4KRdQPlWpnlvoJrgigjGqu4upxd7o-Mn8Otmo2CHKU8&e=> Potential Stratospheric Aerosol Injection (SAI) implications There's been various papers (my own included) that have suggested that materials other than sulphate would be optimal for light scattering and weight lifting considerations for SAI, e.g. alumina, titania, diamond etc... This paper suggests that the collision and coating of sulphuric acid on solid aerosol particles would cause the benefits of the solid particle scattering to diminish rapidly, with the scattering properties being driven by sulphuric acid coating. So you might as well use sulphuric acid / sulphate aerosols in the first place. Note - useful further work would repeat the experiments at stratospheric temperatures (this lab work was conducted at room temperature) and the coagulation dynamics needs to be explored to see how long it would take for a sufficient sulphuric acid coating to be generated upon the injected particles. Abstract Sulfuric acid is shown to form a core–shell particle on a micron-sized, optically-trapped spherical silica bead. The refractive indices of the silica and sulfuric acid, along with the shell thickness and bead radius were determined by reproducing Mie scattered optical white light as a function of wavelength in Mie spectroscopy. Micron-sized silica aerosols (silica beads were used as a proxy for atmospheric silica minerals) were levitated in a mist of sulfuric acid particles; continuous collection of Mie spectra throughout the collision of sulfuric acid aerosols with the optically trapped silica aerosol demonstrated that the resulting aerosol particle had a core–shell morphology. Contrastingly, the collision of aqueous sulfuric acid aerosols with optically trapped polystyrene aerosol resulted in a partially coated system. The light scattering from the optically levitated aerosols was successfully modelled to determine the diameter of the core aerosol (±0.003 μm), the shell thickness (±0.0003 μm) and the refractive index (±0.007). The experiment demonstrated that the presence of a thin film rapidly changed the light scattering of the original aerosol. When a 1.964 μm diameter silica aerosol was covered with a film of sulfuric acid 0.287 μm thick, the wavelength dependent Mie peak positions resembled sulfuric acid. Thus mineral aerosol advected into the stratosphere would likely be coated with sulfuric acid, with a core–shell morphology, and its light scattering properties would be effectively indistinguishable from a homogenous sulfuric acid aerosol if the film thickness was greater than a few 100 s of nm for UV-visible wavelengths. 4 Conclusions The study presented here demonstrates that sulfuric acid successfully forms a core–shell geometry aerosol upon collision with silica. Through application of optical trapping techniques alongside Mie spectroscopy, it was observed that when a sulfuric acid aerosol collides with a silica aerosol, the system would begin to resemble a sulfuric acid aerosol of similar diameter to the combined aerosol. Secondly, the study experimentally demonstrates that mineral aerosol emitted to the stratosphere will soon adopt the light scattering patterns associated with a pure sulfuric acid aerosol. The implication of the study to stratospheric science is that hydrophilic stratospheric mineral aerosol will rapidly resemble the optical properties of sulfuric acid through natural collision processes and the formation of core–shell morphology. -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]<mailto:[email protected]>. To view this discussion on the web visit https://groups.google.com/d/msgid/geoengineering/7e9931c6-c62e-463a-96b0-3e48a7647143n%40googlegroups.com<https://urldefense.proofpoint.com/v2/url?u=https-3A__groups.google.com_d_msgid_geoengineering_7e9931c6-2Dc62e-2D463a-2D96b0-2D3e48a7647143n-2540googlegroups.com-3Futm-5Fmedium-3Demail-26utm-5Fsource-3Dfooter&d=DwMFaQ&c=WO-RGvefibhHBZq3fL85hQ&r=SxgtIVByLvhD8QiO8rgwrvm9f9GML9Drha7439cd21U&m=ouQdAL9_Hinqa8MYv8CnkUl8Ufmbp0NR7G5txWzSQzjsk5mw9xW77OHfx4iSQ5gr&s=qX3B0Jx04aAVBpcvqZ_WE8jQwUHU1ZA3B6G8Ni5Efwc&e=>. -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/geoengineering/6BC2E637-2515-471D-B152-E2C5424B113B%40seas.harvard.edu.
