Daniel M. Lienhard

Comparing the mechanism of water condensation and evaporation in glassy aerosol

Atmospheric models generally assume that aerosol particles are in equilibrium with the surrounding gas phase. However, recent observations that secondary organic aerosols can exist in a glassy state have highlighted the need to more fully understand the kinetic limitations that may control water partitioning in ambient particles. Here, we explore the influence of slow water diffusion in the condensed aerosol phase on the rates of both condensation and evaporation, demonstrating that significant inhibition in mass transfer occurs for ultraviscous aerosol, not just for glassy aerosol. Using coarse mode (3–4 um radius) ternary sucrose/sodium chloride/aqueous droplets as a proxy for multicomponent ambient aerosol, we demonstrate that the timescale for particle equilibration correlates with bulk viscosity and can be ≫103 s. Extrapolation of these timescales to particle sizes in the accumulation mode (e.g., approximately 100 nm) by applying the Stokes-Einstein equation suggests that the kinetic limitations imposed on mass transfer of water by slow bulk phase diffusion must be more fully investigated for atmospheric aerosol. Measurements have been made on particles covering a range in dynamic viscosity from < 0.1 to > 1013 Pa s. We also retrieve the radial inhomogeneities apparent in particle composition during condensation and evaporation and contrast the dynamics of slow dissolution of a viscous core into a labile shell during condensation with the slow percolation of water during evaporation through a more homogeneous viscous particle bulk.

Morphologies of mixed organic/inorganic/aqueous aerosol droplets

Despite major progress in the understanding of properties of tropospheric aerosol particles, it remains challenging to understand their physical state and morphology. To obtain more detailed knowledge of the phases, phase transitions and morphologies of internally mixed organic/inorganic aerosol particles, we evaluated liquid-liquid phase separation (LLPS), deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) of 33 organic/ammonium sulfate (AS)/H2O systems from our own and literature data. The organic fraction consists of single compounds or mixtures with up to ten aliphatic and/or aromatic components with carboxylic acid, hydroxyl, carbonyl, ether, and ester functionalities, covering O : C ratios between 0.29 and 1.33. Thirteen out of these 33 systems did not show LLPS for any of the studied organic-to-inorganic mixing ratios, sixteen underwent LLPS showing core-shell morphology, and four showed both core-shell and partially engulfed configurations depending on the organic-to-inorganic ratio and RH. In all cases the organic fractions of the systems with partially engulfed configurations consisted of dicarboxylic acids. AS in mixed organic/AS/H2O particles deliquesced between 70 and 84% RH. AS effloresced below 58% RH or remained in a one-liquid-phase state. AS in droplets with LLPS always showed efflorescence with ERH between 30 and 50% RH, providing clear evidence that the presence of LLPS facilitates AS efflorescence. Spreading coefficients of the organic-rich phase on the AS-rich phase for systems containing polyethylene glycol 400 (PEG-400) and a mixture of dicarboxylic acids are in agreement with the optically observed morphologies of droplets deposited on the hydrophobic substrate. Analysis of high resolution elastic Mie resonance spectra allowed the detection of LLPS for single levitated droplets consisting of PEG-400/AS/ H2O, whereas LLPS was difficult to detect in (2-methylglutaric acid + 3-methylglutaric acid + 2,2-dimethylsuccinic acid)/AS/H2O. Measured Mie spectra of PEG-400/AS/H2O at 93.5% and at 80.9% RH agreed with computed Mie spectra for a homogeneous and a core-shell configuration, respectively, confirming the results obtained from droplets deposited on a hydrophobic substrate. Based on the presented evidence, we therefore consider the core-shell morphology to be the prevalent configuration of liquid-liquid-phase-separated tropospheric organic/AS/H2O particles.

Measurements of Thermodynamic and Optical Properties of Selected Aqueous Organic and Organic–Inorganic Mixtures of Atmospheric Relevance

Atmospheric aerosol particles can exhibit liquid solution concentrations supersaturated with respect to the dissolved organic and inorganic species and supercooled with respect to ice. In this study, thermodynamic and optical properties of sub- and supersaturated aqueous solutions of atmospheric interest are presented. The density, refractive index, water activity, ice melting temperatures, and homogeneous ice freezing temperatures of binary aqueous solutions containing L(+)-tartaric acid, tannic acid, and levoglucosan and ternary aqueous solutions containing levoglucosan and one of the salts NH(4)HSO(4), (NH(4))(2)SO(4), and NH(4)NO(3) have been measured in the supersaturated concentration range for the first time. In addition, the density and refractive index of binary aqueous citric acid and raffinose solutions and the glass transition temperatures of binary aqueous L(+)-tartaric acid and levoglucosan solutions have been measured. The data presented here are derived from experiments on single levitated microdroplets and bulk solutions and should find application in thermodynamic and atmospheric aerosol models as well as in food science applications.

Experimental evidence for excess entropy discontinuities in glass-forming solutions

Glass transition temperatures T(g) are investigated in aqueous binary and multi-component solutions consisting of citric acid, calcium nitrate (Ca(NO(3))(2)), malonic acid, raffinose, and ammonium bisulfate (NH(4)HSO(4)) using a differential scanning calorimeter. Based on measured glass transition temperatures of binary aqueous mixtures and fitted binary coefficients, the T(g) of multi-component systems can be predicted using mixing rules. However, the experimentally observed T(g) in multi-component solutions show considerable deviations from two theoretical approaches considered. The deviations from these predictions are explained in terms of the molar excess mixing entropy difference between the supercooled liquid and glassy state at T(g). The multi-component mixtures involve contributions to these excess mixing entropies that the mixing rules do not take into account.

Response to “Comment on ‘Experimental evidence for excess entropy discontinuities in glass-forming solutions”’ [J. Chem. Phys. 139, 047101 (2013)]

In their comment, Bogdan and Loerting1 (hereafter called BoLo) question the validity of the experimental data of Lienhard et al. (hereafter called LZZKP) concerning the glass transition temperatures (T_g) of binary aqueous citric acid and aqueous malonic acid solutions. BoLo present own measurements and find disagreements between their results and the results published by LZZKP. In this reply, we show calorimetric thermograms from which the results published by LZZKP are derived and discuss why BoLo’s criticisms are unjustified. Below, we address each of the four claims.