Contributions of Ions and Organics to the Refractive Index of Weakly Absorbing, Aqueous aerosol

Abstract:

In order to accurately simulate aerosol radiative forcing, the optical properties of aerosol must be well characterized. Accuracy in aerosol optical properties requires their characterization over the full range of atmospherically relevant water contents, range of temperatures and over the full Solar spectrum. While optical properties for many types of weakly absorbing aerosol have been investigated, these measurements are often limited to a single wavelength with only a few solutes parameterized over the full visible spectrum for a range of aerosol water contents.

Aerosol optical trapping allows for the precise determination of the refractive index by fitting the morphology dependent resonances in the Mie scattering spectrum. Here, we collect the broadband scattering of single particles to determine the refractive index over a large spectral range. Optical trapping also allows for the characterization of metastable states that particles in the atmosphere would experience, but are not accessible through bulk measurements. By controlling the relative humidity in the trapping cell, we can determine the refractive index over a large range of aerosol water contents.

Recently, we have shown the refractive index of weakly absorbing aerosol can be determined using an effective Lorentzian oscillator to model the far-UV electronic transitions, which give rise to the refractive index in the visible region, for an individual solute. Using mixing rules and the Kramers-Kronig relations, the refractive index of mixtures can then be determined. We fit the refractive index as a function of water content for individual solutes determined from the Mie scattering spectra in order to determine the necessary oscillator parameters. We now extend this model further by introducing effective oscillators for individual ions. The wavelength-dependent refractive index of aerosol with known ion and organic mass fractions as a function of water content can then be accurately calculated using these oscillator parameters. This is demonstrated for salts and strong acids made up of ions with known oscillator parameters as well as mixtures of organics and inorganics. Finally, the effect of temperature on the refractive index in this framework is explored.