We report measurements of the subsaturated hygroscopic growth of aerosol particles composed of single organic components of varying oxygen-to-carbon ratio up to relative humidities approaching saturation using the techniques of aerosol optical tweezers and an electrodynamic balance. The variation in the hygroscopicity parameter κ between compounds of even the same O/C ratio is found to be significant with, for example, a range in κ values from 0.12 to 0.38 for compounds with an O/C of 1. The measurements are compared with a review of all of the available literature data for which both the κ value and O/C ratio are reported, and a new parametrization is determined. Critical supersaturations predicted using this parametrization yield values that have associated uncertainties that are comparable to typical uncertainties in experimental measurements of critical supersaturations. However, the systematic variability between κ parametrizations determined from different studies remains large, consistent with the O/C ratio providing only an approximate guide to aerosol hygroscopicity and reflecting significant variations for aerosols of different chemical functionality, composition, and oxidation history.

Direct comparison of diffusion coefficients and viscosities of ternary component single aerosol particles levitated using optical tweezers.

Mass transfer between the gas and condensed phases in aerosols can be limited by slow bulk diffusion within viscous particles. During the heterogeneous and multiphase reactions of viscous organic aerosol particles, it is necessary to consider the interplay of numerous mass transfer processes and how they are impacted by viscosity, including the partitioning kinetics of semi-volatile organic reactants, water and oxidants. To constrain kinetic models of the heterogeneous chemistry, measurements must provide information on as many observables as possible. Here, the ozonolysis of maleic acid (MA) in ternary aerosol particles containing water and sucrose is used as a model system. By varying the mass ratio of sucrose to MA and by performing reactions over a wide range of relative humidity, direct measurements show that the viscosity of the particle can be varied over 7 orders of magnitude. Measurements of the volatilisation kinetics of MA show that this range in viscosity leads to a suppression in the effective vapour pressure of MA of 3-4 orders of magnitude. The inferred values of the diffusion coefficient of MA in the particle phase closely mirror the expected change in diffusion coefficient from the Stokes-Einstein equation and the change in viscosity. The kinetics of ozonolysis show a similar dependence on particle viscosity that can be further investigated using the kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB). Two scenarios, one constraining the diffusion coefficients for MA to those expected based on the Stokes-Einstein equation and the other including the diffusion coefficients as a fit parameter, yield similarly adequate representations of the ozonolysis kinetics, as inferred from the experimental decay in the signature of the vinylic C-H stretching vibration of MA. However, these two scenarios provide very different parameterisations of the compositional dependence of the diffusion coefficients of ozone within the condensed phase, yielding qualitatively different time-dependent internal concentration profiles. We suggest that this highlights the importance of providing additional experimental observables (e.g. particle size, heterogeneity in composition) if measurements and models are to be universally reconciled.

The microphysical structure and heterogeneous oxidation by ozone of single aerosol particles containing maleic acid (MA) has been studied using aerosol optical tweezers and cavity enhanced Raman spectroscopy. The evaporation rate of MA from aqueous droplets has been measured over a range of relative humidities and the pure component vapor pressure determined to be (1.7 ± 0.2) × 10(-3) Pa. Variation in the refractive index (RI) of an aqueous MA droplet with relative humidity (RH) allowed the subcooled liquid RI of MA to be estimated as 1.481 ± 0.001. Measurements of the hygroscopic growth are shown to be consistent with equilibrium model predictions from previous studies. Simultaneous measurements of the droplet composition, size, and refractive index have been made during ozonolysis at RHs in the range 50-80%, providing insight into the volatility of organic products, changes in the droplet hygroscopicity, and optical properties. Exposure of the aqueous droplets to ozone leads to the formation of products with a wide range of volatilities spanning from involatile to volatile. Reactive uptake coefficients show a weak dependence on ozone concentration, but no dependence on RH or salt concentration. The time evolving RI depends significantly on the RH at which the oxidation proceeds and can even show opposing trends; while the RI increases with ozone exposure at low relative humidity, the RI decreases when the oxidation proceeds at high relative humidity. The variations in RI are broadly consistent with a framework for predicting RIs for organic components published by Cappa et al. ( J. Geophys. Res. 2011 , 116 , D15204 ). Once oxidized, particles are shown to form amorphous phases on drying rather than crystallization, with slow evaporation kinetics of residual water.

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