Complexity of Measuring and Representing the Hygroscopicity of Mixed Component Aerosol.

The validation of approaches to predict the hygroscopicity of complex mixtures of organic components in aerosol is important for understanding the hygroscopic response of organic aerosol in the atmosphere. We report new measurements of the hygroscopicity of mixtures of dicarboxylic acids and amino acids using a comparative kinetic electrodynamic balance (CK-EDB) approach, inferring the equilibrium water content of the aerosol from close to a saturation relative humidity (100%) down to 80%. We show that the solution densities and refractive indices of the mixtures can be estimated with an accuracy of better than ±2% using the molar refractive index mixing rule and densities and refractive indices for the individual binary organic-aqueous solutions. Further, we show that the often-used mass-, volume-, and mole-weighted mixing rules to estimate the hygroscopicity parameter κ can overestimate the hygroscopic parameter by a factor of as much as 3, highlighting the need to understand the specific nonideal interactions that may arise synergistically in mixtures and cannot be represented by simple models. Indeed, in some extreme cases the hygroscopicity of a multicomponent mixture can be very close to that for the least hygroscopic component. For mixtures of similar components for which no additional synergistic interactions need be considered, the hygroscopicity of the mixed component aerosol can be estimated with high accuracy from the hygroscopic response of the binary aqueous-organic aerosol. In conclusion, we suggest that the hygroscopicity of multicomponent organic aerosol can be highly nonadditive and that simple correlations of hygroscopicity with composition may often misrepresent the level of complexity essential to interpreting aerosol hygroscopicity.

Condensation Kinetics of Water on Amorphous Aerosol Particles.

Responding to changes in the surrounding environment, aerosol particles can grow by water condensation changing rapidly in composition and changing dramatically in viscosity. The timescale for growth is important to establish for particles undergoing hydration processes in the atmosphere or during inhalation. Using an electrodynamic balance, we report direct measurements at -7.5, 0, and 20 °C of timescales for hygroscopic condensational growth on a range of model hygroscopic aerosol systems. These extend from viscous aerosol particles containing a single saccharide solute (sucrose, glucose, raffinose, or trehalose) and a starting viscosity equivalent to a glass of ∼1012 Pa·s, to nonviscous (∼10-2 Pa·s) tetraethylene glycol particles. The condensation timescales observed in this work indicate that water condensation occurs rapidly at all temperatures examined (<10 s) and for particles of all initial viscosities spanning 10-2 to 1012 Pa·s. Only a marginal delay (<1 order of magnitude) is observed for particles starting as a glass.

Amorphous phase state diagrams and viscosity of ternary aqueous organic/organic and inorganic/organic mixtures.

A Dimer Coagulation, Isolation and Coalescence (DCIC) technique is used to probe the phase behaviour and glass transition temperatures of ternary aerosol mixtures. The DCIC technique is used to perform temperature and relative humidity dependent viscosity measurements at viscosities near 5 × 106 Pa s. Measurements include organic-organic and organic-inorganic mixtures composed of sucrose-citric acid and sucrose-sodium nitrate. The data reported here add additional insight into the wide discrepancies in glass transition temperatures reported for pure sodium nitrate. The phase diagram model used in the work of Rothfuss and Petters (Phys. Chem. Chem. Phys., 2017, 19, 6532-6545) is expanded to include multiple solute components. Data and model predictions of the mixtures are in good agreement with the modified model. These measurements are compared with values from Holographic Optical Tweezer (HOT) measurements taken at room temperature. Overall, the viscosities determined from the DCIC and HOT techniques are in good agreement.

Group Contribution Approach To Predict the Refractive Index of Pure Organic Components in Ambient Organic Aerosol.

We introduce and assess a group contribution scheme by which the refractive index (RI) (λ = 589 nm) of nonabsorbing components common to secondary organic aerosols can be predicted from the molecular formula and chemical functionality. The group contribution method is based on representative values of ratios of the molecular polarizability and molar volume of different functional groups derived from data for a training set of 234 compounds. The training set consists of 106 nonaromatic compounds common to atmospheric aerosols, 64 aromatic compounds, and 64 compounds containing halogens; a separate group contribution model is provided for each of these three classes of compound. The resulting predictive model reproduces the RIs of compounds in the training set with mean errors of ±0.58, ±0.36, and ±0.30% for the nonaromatic, aromatic, and halogen-containing compounds, respectively. We then evaluate predictions from the group contribution model for compounds with no previously reported RI, comparing values with predictions from previous treatments and with measurements from single aerosol particle experiments. We illustrate how such comparisons can be used to further refine the predictive model. We suggest that the accuracy of this model is already sufficient to better constrain the optical properties of organic aerosol of known composition.

Accurate representations of the physicochemical properties of atmospheric aerosols: when are laboratory measurements of value?

Laboratory studies can provide important insights into the processes that occur at the scale of individual particles in ambient aerosol. We examine the accuracies of measurements of core physicochemical properties of aerosols that can be made in single particle studies and explore the impact of these properties on the microscopic processes that occur in ambient aerosol. Presenting new measurements, we examine here the refinements in our understanding of aerosol hygroscopicity, surface tension, viscosity and optical properties that can be gained from detailed laboratory measurements for complex mixtures through to surrogates for secondary organic atmospheric aerosols.

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

Representing 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 tricarboxylic acids, alcohols, diols, nitriles, sulfoxides, amides, ethers, sugars, amino acids, aminium sulfates, 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 the density and refractive index predictions depend on the range of subsaturated 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.