Comparison of Methods for Predicting the Compositional Dependence of the Density and Refractive Index of Organic–Aqueous Aerosols

Abstract

General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms AbstractRepresenting the physicochemical properties of aerosol particles of complex composition is of crucial importance for understanding and predicting aerosol thermodynamic, kinetic and optical properties and processes, and for interpreting and comparing analysis methods. Here, we consider the representations of the density and refractive index of aqueous-organic aerosol with a particular focus on the dependence of these properties on relative humidity and water content, including an examination of the properties of solution aerosol droplets existing at supersaturated solute concentrations. Using bulk phase measurements of density and refractive index for typical organic aerosol components, we provide robust approaches for the estimation of these properties for aerosol at any intermediate composition between pure water and pure solute.Approximately 70 compounds are considered, including mono-, di-and tri-carboxylic acids, alcohols, diols, nitriles, sulphoxides, amides, ethers, sugars, amino acids, aminium sulphates, and polyols. We conclude that the molar refraction mixing rule should be used to predict the refractive index of the solution using a density treatment that assumes ideal mixing or, preferably, a polynomial dependence on the square root of the mass fraction of solute, depending on the solubility limit of the organic component. Although the uncertainties in 2 the density and refractive index predictions depend on the range of sub-saturated compositional data available for each compound, typical errors for estimating the solution density and refractive index are less than ±0.1 % and ±0.05 %, respectively. Owing to the direct connection between molar refraction and the molecular polarizability, along with the availability of group contribution models for predicting molecular polarizability for organic species, our rigorous testing of the molar refraction mixing rule provides a route to predicting refractive indices for aqueous solutions containing organic molecules of arbitrary structure.

Complexity of Measuring and Representing the Hygroscopicity of Mixed Component Aerosol

“…32,37 The application of the geometric optics approximation relies on the knowledge of the refractive index of a droplet, which is calculated using the molar refraction mixing rule. 38,39 The application of the molar refraction mixing rule for complex mixtures such as those considered in this work is discussed in section 3.1.…”

“…In the work of Cai et al, bulk measurements of density and refractive index up to the solubility limit were taken for ~ 70 organic compounds in aqueous solutions. 39 These bulk data were used to parametrise density and refractive index across the entire mass fraction range (i.e. beyond the solubility limit).…”

“…1. From the data for the binary aqueous mixtures of organics collected by Cai et al, 39 An ideal mixing treatment is used to parametrise the density of the aqueous solution. The fitting is performed using the same method as for binary solutions described by Cai et al 39 The notation used is:…”

“…Recognising that the comparison of the accuracies of the parameterisations can only be made up to the solubility limits of the bulk measurements, we adopt the same cautious approach as assessed in our previous study of binary solutions. 39 Using polynomial representations of the mixture density can lead to deviations and unphysical behaviour, particularly if the solubilities of the individual components are low. In our previous work, 39 optical tweezers measurements were used to assess the accuracy of different polynomial treatments of density to very low water activity/high solute saturation.…”

“…39 Using polynomial representations of the mixture density can lead to deviations and unphysical behaviour, particularly if the solubilities of the individual components are low. In our previous work, 39 optical tweezers measurements were used to assess the accuracy of different polynomial treatments of density to very low water activity/high solute saturation. For consistency with this, we base our treatments of the solution density and refractive index on the use of the molar refraction model and simple binary solution density data.…”