Optical extinction of highly porous aerosol following atmospheric freeze drying

Abstract

Porous glassy particles are a potentially significant but unexplored component of atmospheric aerosol that can form by aerosol processing through the ice phase of high convective clouds. The optical properties of porous glassy aerosols formed from a freeze-dry cycle simulating freezing and sublimation of ice particles were measured using a cavity ring down aerosol spectrometer (CRD-AS) at 532 nm and 355 nm wavelength. The measured extinction efficiency was significantly reduced for porous organic and mixed organic-ammonium sulfate particles as compared to the extinction efficiency of the homogeneous aerosol of the same composition prior to the freeze-drying process. A number of theoretical approaches for modeling the optical extinction of porous aerosols were explored. These include effective medium approximations, extended effective medium approximations, multilayer concentric sphere models, Rayleigh-Debye-Gans theory, and the discrete dipole approximation. Though such approaches are commonly used to describe porous particles in astrophysical and atmospheric contexts, in the current study, these approaches predicted an even lower extinction than the measured one. Rather, the best representation of the measured extinction was obtained with an effective refractive index retrieved from a fit to Mie scattering theory assuming spherical particles with a fixed void content. The single-scattering albedo of the porous glassy aerosols was derived using this effective refractive index and was found to be lower than that of the corresponding homogeneous aerosol, indicating stronger relative absorption at the wavelengths measured. The reduced extinction and increased absorption may be of significance in assessing direct, indirect, and semidirect forcing in regions where porous aerosols are expected to be prevalent.

Calculations of the mass absorption cross sections for carbonaceous nanoparticles modeling soot

“…More generally, it is interesting to point out that, in a recent work aiming at quantifying the optical extinction of highly porous aerosols, the results coming from various theoretical approaches including the DDA method were found to largely underestimate those of the corresponding experimental measurements [58]. Although the reasons of this discrepancy were not fully elucidated, it has been suggested that the DDA method cannot account for the large porosity leading to a high number of boundary subvolumes.…”

“…Indeed, PDI calculations, based on the exact positions of the atoms in the porous structure, would not depend on any assumptions on the porous subvolumes as in the DDA approach. However, because PDI calculations are time consuming, considering very large porous nanoparticles like those usually encountered in atmospheric aerosol [58] could be not straightforward.…”

Porous aerosol in degassing plumes of Mt. Etna and Mt. Stromboli

“…Jeong and Nousiainen (2014) demonstrated, using transmission electron microscopy analysis, that the internal structures of individual Asian dust particles were formed by the patterned arrangement of nano-to-micron-sized mineral grains and pores. Highly porous aerosol particles, obtained from proxies for organic compounds, were investigated by Adler et al (2013Adler et al ( , 2014 with focused ion beam -scanning electron microscopy techniques and a cavity ring-down system. Most of the information on volcanic vesicular, i.e., porous ash, particles was obtained from samples collected at the ground level.…”

“…Examples for which the refractive index was retrieved along with a size distribution can be found in the works by Toledano et al (2012), Waquet et al (2014), and references therein. Effective medium approximations were used in inverse problems by Abo Riziq et al (2007) and Adler et al (2013Adler et al ( , 2014 where refractive indices of laboratory-generated aerosols were retrieved by comparing the measured size-resolved extinction efficiency with Mie theory calculations.…”

The efficiency of secondary organic aerosol particles acting as ice-nucleating particles under mixed-phase cloud conditions

“…Adler et al, 2014): so-called cloud processing. Thus, such a freezedrying cycle might increase their ice-nucleating abilities.…”