Water uptake and optical properties of mixed organic-inorganic particles

Atmospheric aerosol particles are frequently mixtures of inorganic and organic species, both of which can contribute to aerosol water uptake and determine the particles’ ability to scatter and absorb light. While water uptake of purely inorganic aerosol is well represented in current regional and global chemical transport models, it is challenging to represent it for particles that are mixtures of organic and inorganic species. Here we quantified the growth factor for aerosols that consist of mixed organic-inorganic particles using an accurate lattice-based adsorption isotherm model (Ad-iso) as a benchmark. We then determined the error in the growth factor and resulting optical properties for simplifying assumptions that are commonly made in current chemical transport models. The systems studied here are representative of ambient atmospheric aerosols, consisting of model water-soluble inorganic-organic mixtures, with and without a core of absorbing black carbon, under conditions of relative humidity larger than 85%. The assumption of completely neglecting the water uptake by organic components, for particles with an organic mass fraction of 50%, led to errors of up to 7% in growth factor and up to 3.5% in single scattering albedo. Larger errors occurred for larger organic mass fractions. Approximating the organic water uptake with a constant hygroscopicity parameter, for organic mass fractions between 45 and 65%, the errors remained within 3% for the growth factor and 0.6% for the single scattering albedo. For organic mass fractions smaller than 45% or larger than 65%, the errors increased up to 6% for the single scattering albedo. The magnitudes of these errors underscore the importance of considering organic/inorganic mixtures for estimating direct aerosol radiative forcing.