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Microphysical explanation of the RH-dependent water affinity of biogenic organic aerosol and its importance for climate
Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
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Number of Authors: 292017 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 10, p. 5167-5177Article in journal (Refereed) Published
Abstract [en]

A large fraction of atmospheric organic aerosol (OA) originates from natural emissions that are oxidized in the atmosphere to form secondary organic aerosol (SOA). Isoprene (IP) and monoterpenes (MT) are the most important precursors of SOA originating from forests. The climate impacts from OA are currently estimated through parameterizations of water uptake that drastically simplify the complexity of OA. We combine laboratory experiments, thermodynamic modeling, field observations, and climate modeling to (1) explain the molecular mechanisms behind RH-dependent SOA water-uptake with solubility and phase separation; (2) show that laboratory data on IP- and MT-SOA hygroscopicity are representative of ambient data with corresponding OA source profiles; and (3) demonstrate the sensitivity of the modeled aerosol climate effect to assumed OA water affinity. We conclude that the commonly used single-parameter hygroscopicity framework can introduce significant error when quantifying the climate effects of organic aerosol. The results highlight the need for better constraints on the overall global OA mass loadings and its molecular composition, including currently underexplored anthropogenic and marine OA sources. Plain Language Summary The interaction of airborne particulate matter (aerosols) with water is of critical importance for processes governing climate, precipitation, and public health. It also modulates the delivery and bioavailability of nutrients to terrestrial and oceanic ecosystems. We present a microphysical explanation to the humidity-dependent water uptake behavior of organic aerosol, which challenges the highly simplified theoretical descriptions used in, e.g., present climate models. With the comprehensive analysis of laboratory data using molecular models, we explain the microphysical behavior of the aerosol over the range of humidity observed in the atmosphere, in a way that has never been done before. We also demonstrate the presence of these phenomena in the ambient atmosphere from data collected in the field. We further show, using two state-of-the-art climate models, that misrepresenting the water affinity of atmospheric organic aerosol can lead to significant biases in the estimates of the anthropogenic influence on climate.

Place, publisher, year, edition, pages
2017. Vol. 44, no 10, p. 5167-5177
National Category
Earth and Related Environmental Sciences
Research subject
Environmental Sciences
Identifiers
URN: urn:nbn:se:su:diva-145259DOI: 10.1002/2017GL073056ISI: 000404131900093OAI: oai:DiVA.org:su-145259DiVA, id: diva2:1128441
Available from: 2017-07-25 Created: 2017-07-25 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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