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Connecting the solubility and CCN activation of complex organic aerosols: a theoretical study using solubility distributions
Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Carnegie Mellon University, USA.
Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
Number of Authors: 3
2015 (English)In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 15, no 11, p. 6305-6322Article in journal (Refereed) Published
Abstract [en]

We present a theoretical study investigating the cloud activation of multicomponent organic particles. We modeled these complex mixtures using solubility distributions (analogous to volatility distributions in the VBS, i.e., volatility basis set, approach), describing the mixture as a set of surrogate compounds with varying water solubilities in a given range. We conducted Khler theory calculations for 144 different mixtures with varying solubility range, number of components, assumption about the organic mixture thermodynamics and the shape of the solubility distribution, yielding approximately 6000 unique cloud condensation nucleus (CCN)-activation points. The results from these comprehensive calculations were compared to three simplifying assumptions about organic aerosol solubility: (1) complete dissolution at the point of activation; (2) combining the aerosol solubility with the molar mass and density into a single effective hygroscopicity parameter kappa; and (3) assuming a fixed water-soluble fraction eff. The complete dissolution was able to reproduce the activation points with a reasonable accuracy only when the majority (70-80 %) of the material was dissolved at the point of activation. The single-parameter representations of complex mixture solubility were confirmed to be powerful semi-empirical tools for representing the CCN activation of organic aerosol, predicting the activation diameter within 10% in most of the studied supersaturations. Depending mostly on the condensedphase interactions between the organic molecules, material with solubilities larger than about 0.1-100 g L-1 could be treated as soluble in the CCN activation process over atmospherically relevant particle dry diameters and supersaturations. Our results indicate that understanding the details of the solubility distribution in the range of 0.1-100 g L-1 is thus critical for capturing the CCN activation, while resolution outside this solubility range will probably not add much information except in some special cases. The connections of these results to the previous observations of the CCN activation and the molecular properties of complex organic mixture aerosols are discussed. The presented results help unravel the mechanistic reasons behind observations of hygroscopic growth and CCN activation of atmospheric secondary organic aerosol (SOA) particles. The proposed solubility distribution framework is a promising tool for modeling the interlinkages between atmospheric aging, volatility and water uptake of atmospheric organic aerosol.

Place, publisher, year, edition, pages
2015. Vol. 15, no 11, p. 6305-6322
National Category
Earth and Related Environmental Sciences
Research subject
Environmental Sciences
Identifiers
URN: urn:nbn:se:su:diva-119022DOI: 10.5194/acp-15-6305-2015ISI: 000356180900019OAI: oai:DiVA.org:su-119022DiVA, id: diva2:843259
Available from: 2015-07-28 Created: 2015-07-24 Last updated: 2018-04-11Bibliographically approved
In thesis
1. Aerosol-water interaction at sub and super-saturated regimes: From small scale molecular mechanisms to large scale atmospheric models
Open this publication in new window or tab >>Aerosol-water interaction at sub and super-saturated regimes: From small scale molecular mechanisms to large scale atmospheric models
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The term “atmospheric aerosol” refers to solid or liquid particles suspended in the atmosphere. Atmospheric aerosols influence the Earth’s energy budget directly by scattering and absorbing radiation (known as the direct aerosol effect) and indirectly by acting as cloud condensation nuclei (CCN) and ice nucleating particles and thereby modifying cloud properties (known as the indirect aerosol effect). The water-affinity of aerosols plays an important role on one hand in defining the aerosol water-content and optical properties, and on the other hand in determining the conditions at which the aerosols can act as CCN. Aerosol-water interactions thus affect both the direct as well as the indirect aerosol effects, leading to impacts on the Earth’s energy budget and ultimately climate. The role of aerosols and clouds in determining the radiative balance of the Earth is one of the largest sources of uncertainty in understanding climate change. Therefore, the main goal of this thesis was to improve the knowledge of aerosol-water interactions. In this thesis, we investigated the links between aerosol molecular composition, hygroscopic growth and CCN activation, with a focus on organic compounds. Specifically, we tested several commonly-used simplifying approaches for describing water uptake, CCN activation and their impact on aerosol radiative properties.

The traditional Köhler theory that describes the equilibrium between droplet and vapor phase along with modifications of these theory were used to investigate the water affinity of aerosol particles. The modifications to this theory used in this study are as follows: complete dissolution, hygroscopicity parameter (κ), soluble fraction (ε), treatment of adsorption, counting for gas-particle partitioning of volatile organic compounds. Also a Solubility Basis Set (SBS) model was developed to investigate the CCN activation behavior of complex organic aerosols accounting for the distribution of solubilities present in these mixtures.  Based on the theoretical approaches, a coupled hygroscopicity and radiative transfer model was developed to investigate the effect of hygroscopic growth and CCN activation of aerosol particles on radiative properties in Arctic and boreal forest environments. Finally on the global scale, we used two climate models (NorESM and ECHAM6-HAM2) to investigate the sensitivity of climate models to treatment of water uptake of organics.

By using different thermodynamic modelling approaches it was found that an approach using assumptions of limited solubility of the SOA components and solubility distributions cannot alone explain the hygroscopic behavior of SOA at subsaturation, while they can explain the CCN activation behaviour of organic mixtures. Quantifying the hygroscopic behavior of SOA compounds below 90% Relative Humidity (RH) requires consideration of processes such as adsorptive water uptake, bulk to surface partitioning, gas-particle partitioning of the semivolatile vapors and non ideality of the liquid phases with decreasing relative humidity (RH). On the other hand, at supersaturation most SOA behave as nearly completely soluble in water. We found that the differences in water-affinity of SOA at sub- and supersaturated conditions can be explained by Liquid-Liquid Phase Separation (LLPS) effects. By using the coupled hygroscopicity and radiative transfer model, a great impact of water uptake of aerosol particles on direct radiative effect was found in Arctic and boreal forest environment. The climate impacts resulting from OA are currently estimated using model parameterizations of water uptake that drastically simplify this complexity of OA. We found that the single-parameter hygroscopicity framework commonly used in climate models, can introduce significant errors when quantifying the climate effects of OA. The results highlight the need for better constraints on the interactions between water vapor and OA and its molecular composition, as well as overall global OA mass loadings, including currently under-explored anthropogenic and marine OA sources.

Place, publisher, year, edition, pages
Stockholm: Department of Environmental Science and Analytical Chemistry, Stockholm University, 2018. p. 58
National Category
Environmental Sciences
Research subject
Environmental Sciences
Identifiers
urn:nbn:se:su:diva-154856 (URN)978-91-7797-282-2 (ISBN)978-91-7797-283-9 (ISBN)
Public defence
2018-05-25, Ahlmannsalen,Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
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Available from: 2018-05-02 Created: 2018-04-06 Last updated: 2018-04-19Bibliographically approved

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