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Renbaum-Wolff L.,University of British Columbia | Renbaum-Wolff L.,Aerodyne Research, Inc. | Renbaum-Wolff L.,Boston College | Song M.,University of British Columbia | And 11 more authors.
Atmospheric Chemistry and Physics | Year: 2016

Particles consisting of secondary organic material (SOM) are abundant in the atmosphere. To predict the role of these particles in climate, visibility and atmospheric chemistry, information on particle phase state (i.e., single liquid, two liquids and solid) is needed. This paper focuses on the phase state of SOM particles free of inorganic salts produced by the ozonolysis of α-pinene. Phase transitions were investigated in the laboratory using optical microscopy and theoretically using a thermodynamic model at 290 K and for relative humidities ranging from < 0.5 to 100 %. In the laboratory studies, a single phase was observed from 0 to 95 % relative humidity (RH) while two liquid phases were observed above 95 % RH. For increasing RH, the mechanism of liquid-liquid phase separation (LLPS) was spinodal decomposition. The RH range over which two liquid phases were observed did not depend on the direction of RH change. In the modeling studies, the SOM took up very little water and was a single organic-rich phase at low RH values. At high RH, the SOM underwent LLPS to form an organic-rich phase and a water-rich phase, consistent with the laboratory studies. The presence of LLPS at high RH values can have consequences for the cloud condensation nuclei (CCN) activity of SOM particles. In the simulated Köhler curves for SOM particles, two local maxima were observed. Depending on the composition of the SOM, the first or second maximum can determine the critical supersaturation for activation. Recently researchers have observed inconsistencies between measured CCN properties of SOM particles and hygroscopic growth measured below water saturation (i.e., hygroscopic parameters measured below water saturation were inconsistent with hygroscopic parameters measured above water saturation). The work presented here illustrates that such inconsistencies are expected for systems with LLPS when the water uptake at subsaturated conditions represents the hygroscopicity of an organic-rich phase while the barrier for CCN activation can be determined by the second maximum in the Köhler curve when the particles are water rich. © Author(s) 2016. Source

Stewart D.J.,University of Bristol | Cai C.,University of Bristol | Cai C.,Beijing Institute of Technology | Nayler J.,University of Bristol | And 6 more authors.
Journal of Physical Chemistry A | Year: 2015

Direct measurements of the phase separation relative humidity (RH) and morphology of aerosol particles consisting of liquid organic and aqueous inorganic domains are presented. Single droplets of mixed phase composition are captured in a gradient force optical trap, and the evolving size, refractive index (RI), and morphology are characterized by cavity-enhanced Raman spectroscopy. Starting at a RH above the phase separation RH, the trapped particle is dried to lower RH and the transition to a phase-separated structure is inferred from distinct changes in the spectroscopic fingerprint. In particular, the phase separation RHs of droplets composed of aqueous solutions of polyethylene glycol (PEG-400)/ammonium sulfate and a mixture of C6-diacids/ammonium sulfate are probed, inferring the RH from the RI of the droplet immediately prior to phase separation. The observed phase separation RHs occur at RH marginally higher (at most 4%) than reported in previous measurements made from studies of particles deposited on hydrophobic surfaces by brightfield imaging. Clear evidence for the formation of phase-separated droplets of core-shell morphology is observed, although partially engulfed structures can also be inferred to form. Transitions between the different spectroscopic signatures of phase separation suggest that fluctuations in morphology can occur. For droplets that are repeatedly cycled through the phase separation RH, the water activity at phase separation is found to be remarkably reproducible (within ±0.0013) and is the same for the 1-phase to 2-phase transition and the 2-phase to 1-phase transition. By contrast, larger variation between the water activities at phase separation is observed for different droplets (typically ±0.02). © 2015 American Chemical Society. Source

Ganbavale G.,ETH Zurich | Zuend A.,ETH Zurich | Zuend A.,California Institute of Technology | Zuend A.,McGill University | And 3 more authors.
Atmospheric Chemistry and Physics | Year: 2015

This study presents a new, improved parameterisation of the temperature dependence of activity coefficients in the AIOMFAC (Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficients) model applicable for aqueous as well as water-free organic solutions. For electrolyte-free organic and organic-water mixtures the AIOMFAC model uses a group-contribution approach based on UNIFAC (UNIversal quasi-chemical Functional-group Activity Coefficients). This group-contribution approach explicitly accounts for interactions among organic functional groups and between organic functional groups and water. The previous AIOMFAC version uses a simple parameterisation of the temperature dependence of activity coefficients, aimed to be applicable in the temperature range from ∼ 275 to ∼ 400 K. With the goal to improve the description of a wide variety of organic compounds found in atmospheric aerosols, we extend the AIOMFAC parameterisation for the functional groups carboxyl, hydroxyl, ketone, aldehyde, ether, ester, alkyl, aromatic carbon-alcohol, and aromatic hydrocarbon to atmospherically relevant low temperatures. To this end we introduce a new parameterisation for the temperature dependence. The improved temperature dependence parameterisation is derived from classical thermodynamic theory by describing effects from changes in molar enthalpy and heat capacity of a multi-component system. Thermodynamic equilibrium data of aqueous organic and water-free organic mixtures from the literature are carefully assessed and complemented with new measurements to establish a comprehensive database, covering a wide temperature range (∼ 190 to ∼ 440 K) for many of the functional group combinations considered. Different experimental data types and their processing for the estimation of AIOMFAC model parameters are discussed. The new AIOMFAC parameterisation for the temperature dependence of activity coefficients from low to high temperatures shows an overall improvement of 28% in comparison to the previous model version, when both versions are compared to our database of experimentally determined activity coefficients and related thermodynamic data. When comparing the previous and new AIOMFAC model parameterisations to the subsets of experimental data with all temperatures below 274 K or all temperatures above 322 K (i.e. outside a 25 K margin of the reference temperature of 298 K), applying the new parameterisation leads to 37% improvement in each of the two temperature ranges considered. The new parameterisation of AIOMFAC agrees well with a large number of experimental data sets. Larger model-measurement discrepancies were found particularly for some of the systems containing multi-functional organic compounds. The affected systems were typically also poorly represented at room temperature and further improvements will be necessary to achieve better performance of AIOMFAC in these cases (assuming the experimental data are reliable). The performance of the AIOMFAC parameterisation is typically better for systems containing relatively small organic compounds and larger deviations may occur in mixtures where molecules of high structural complexity such as highly oxygenated compounds or molecules of high molecular mass (e.g. oligomers) prevail. Nevertheless, the new parameterisation enables the calculation of activity coefficients for a wide variety of different aqueous/water-free organic solutions down to the low temperatures present in the upper troposphere. © Author(s) 2015. Source

Ganbavale G.,ETH Zurich | Marcolli C.,ETH Zurich | Marcolli C.,Marcolli Chemistry and Physics Consulting GmbH | Krieger U.K.,ETH Zurich | And 4 more authors.
Atmospheric Chemistry and Physics | Year: 2014

This work presents experimental data of the temperature dependence of water activity in aqueous organic solutions relevant for tropospheric conditions (200-273 K). Water activity (w) at low temperatures (T) is a crucial parameter for predicting homogeneous ice nucleation. We investigated temperature-dependent water activities, ice freezing and melting temperatures of solutions, and vapour pressures of a selection of atmospherically relevant aqueous organic systems. To measure w over a wide composition range and with a focus on low temperatures, we use various w measurement techniques and instruments: a dew point water activity meter, an electrodynamic balance (EDB), differential scanning calorimetry (DSC), and a setup to measure the total gas phase pressure at equilibrium over aqueous solutions. Water activity measurements were performed for aqueous multicomponent and multifunctional organic mixtures containing the functional groups typically found in atmospheric organic aerosols, such as hydroxyl, carboxyl, ketone, ether, ester, and aromatic groups. The aqueous organic systems studied at several fixed compositions over a considerable temperature range differ significantly in their temperature dependence. Aqueous organic systems of 1,4-butanediol and methoxyacetic acid show a moderate decrease in w with decreasing temperature. The aqueous M5 system (a multicomponent system containing five different dicarboxylic acids) and aqueous 2-(2-ethoxyethoxy)ethanol solutions both show a strong increase of water activity with decreasing temperature at high solute concentrations for T < 270 K and T < 260 K, respectively. These measurements show that the temperature trend of w can be reversed at low temperatures and that linear extrapolations of high-temperature data may lead to erroneous predictions. To avoid this, experimentally determined w at low temperature are needed to improve thermodynamic models towards lower temperatures and for improved predictions of the ice nucleation ability of organic-water systems. Source

Bilde M.,University of Aarhus | Barsanti K.,Portland State University | Booth M.,University of Manchester | Cappa C.D.,University of California at Davis | And 34 more authors.
Chemical Reviews | Year: 2015

There are a number of techniques that can be used that differ in terms of whether they fundamentally probe the equilibrium and the temperature range over which they can be applied. The series of homologous, straight-chain dicarboxylic acids have received much attention over the past decade given their atmospheric relevance, commercial availability, and low saturation vapor pressures, thus making them ideal test compounds. Uncertainties in the solid-state saturation vapor pressures obtained from individual methodologies are typically on the order of 50-100%, but the differences between saturation vapor pressures obtained with different methods are approximately 1-4 orders of magnitude, with the spread tending to increase as the saturation vapor pressure decreases. Some of the dicarboxylic acids can exist with multiple solid-state structures that have distinct saturation vapor pressures. Furthermore, the samples on which measurements are performed may actually exist as amorphous subcooled liquids rather than solid crystalline compounds, again with consequences for the measured saturation vapor pressures, since the subcooled liquid phase will have a higher saturation vapor pressure than the crystalline solid phase. Compounds with equilibrium vapor pressures in this range will exhibit the greatest sensitivities in terms of their gas to particle partitioning to uncertainties in their saturation vapor pressures, with consequent impacts on the ability of explicit and semiexplicit chemical models to simulate secondary organic aerosol formation. Source

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