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.
Highlights
IntroductionSulfuric acid aerosols are relatively abundant in the stratosphere:[12–14] the number density of sulfuric acid in the stratosphere has been determined from balloon-borne mass spectrometer experiments to be 104–105 molecules cmÀ3 below an altitude of 30 km and 106–107 molecules cmÀ3 between 30 to 35 km.[15,16]
Sulfuric acid is shown to form a core–shell particle on a micron-sized, optically-trapped spherical silica bead
Micron-sized silica aerosols 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
Summary
Sulfuric acid aerosols are relatively abundant in the stratosphere:[12–14] the number density of sulfuric acid in the stratosphere has been determined from balloon-borne mass spectrometer experiments to be 104–105 molecules cmÀ3 below an altitude of 30 km and 106–107 molecules cmÀ3 between 30 to 35 km.[15,16]. The light scattering from an optically trapped silica bead coated in a thin film of sulfuric acid has been measured. The study will (a) determine that aqueous sulfuric acid can wet and uniformly coat a mineral aerosol particle to form an aerosol with core–shell morphology and (b) record the back-scattered, visible Mie the scattered light to size the aerosol and determine the refractive index and shell thickness. The Mie scattering as sulfuric acid collides with polystyrene aerosol was monitored: in the presented study it was observed that sulfuric acid did not wet polystyrene aerosols, demonstrating the lack of core–shell geometry. Jones et al.[25] determined the refractive index change as a film of oleic acid developed on a silica aerosol. Owing to the composition and size of the acid aerosols,[51] sulfuric acid does not have the desirable characteristics of a highly reflective aerosol and recent studies have begun to explore other, non-sulfate possibilities such as silica or titania.[27,51] Understanding how a thin film of sulfuric acid alters the scattering properties of the mineral aerosol is crucial to estimate how effective mineral aerosols are at scattering solar radiation
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