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Cloud droplet activation of black carbon particles coated with organic compounds of varying solubility
Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.ORCID iD: 0000-0002-9283-5747
Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
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2017 (English)In: Atmospheric Chemistry and Physics Discussions, ISSN 1680-7367, E-ISSN 1680-7375Article in journal (Refereed) Epub ahead of print
Abstract [en]

Atmospheric black carbon (BC) particles are a concern due to their impact on air quality and climate. Their net climate effect 15 is, however, still uncertain. This uncertainty is partly related to the contribution of coated BC-particles to the global CCN budgets. In this study, laboratory measurements were performed to investigate cloud condensation nuclei (CCN) activity of BC (Regal black) particles, in pure state or coated through evaporating and subsequent condensation of glutaric acid, levoglucosan (both water-soluble organics) or oleic acid (an organic compound with low solubility). A combination of Soot Particle Aerosol Mass Spectrometer (SP-AMS) measurements and size distribution measurements with Scanning Mobility 20 Particle Sizer (SMPS) showed that the studied BC particles were nearly spherical agglomerates with a fractal dimension of 2.79 and that they were coated evenly by the organic species. The CCN activity of BC particles increased after coating with all the studied compounds and was governed by the fraction of organic material. The CCN activation of the BC particles coated by glutaric acid and levoglucosan were in good agreement with the theoretical calculations using shell-and-core model, which is based on a combination of the CCN activities of the pure compounds. The oleic acid coating enhanced the CCN 25 activity of the BC particles, even though the pure oleic acid particles were CCN inactive. The surprising effect of oleic acid might be related to the arrangement of the oleic acid molecules on the surface of the BC cores or other surface phenomena facilitating water condensation onto the coated particles. Our results show potential in accurately predicting the CCN activity of atmospheric BC coated with organic species by present theories, given that the identities and amount of the coating species are known. Furthermore, our results suggest that even relatively thin soluble coatings (around 2 nm for the compounds studied here) are enough to make the insoluble BC particles CCN active at typical atmospheric supersaturations and thus be efficiently taken up by cloud droplets. This highlights the need of an accurate description of the composition of atmospheric particles containing BC to unravel their net impact on climate.

Place, publisher, year, edition, pages
2017.
Keyword [en]
Black carbon, CCN activation, coated aerosols
National Category
Meteorology and Atmospheric Sciences
Research subject
Applied Environmental Science
Identifiers
URN: urn:nbn:se:su:diva-149334DOI: 10.5194/acp-2017-1084OAI: oai:DiVA.org:su-149334DiVA, id: diva2:1160878
Available from: 2017-11-28 Created: 2017-11-28 Last updated: 2018-03-21
In thesis
1. Investigating parameters governing liquid-phase cloud activation of atmospheric particles
Open this publication in new window or tab >>Investigating parameters governing liquid-phase cloud activation of atmospheric particles
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Aerosol-cloud interactions are one of the main sources of uncertainties in modeling and predicting the Earth’s climate. To overcome this uncertainty, we need to improve the understanding about the processes and parameters defining how aerosol particles turn into cloud condensation nuclei (CCN) or ice nuclei (IN) to produce cloud droplets or ice crystals. The focus of this dissertation is on liquid phase cloud droplets. The thesis investigates the effect of water solubility and surface tension on the CCN activity of atmospheric aerosol particles. These parameters are among the key properties defining how an aerosol particle can turn into a cloud droplet. The main goals of this thesis are to investigate 1) the CCN activity of aerosol particles containing both water soluble and insoluble substances and 2) the contribution of molecular-scale surface structure to the surface tension and CCN activity of atmospherically relevant aqueous mixtures.

In the first part of this thesis, the CCN activity of water-insoluble aerosol constituents coated by water-soluble or sparingly soluble species was investigated. The results showed that the CCN activity of the insoluble silica and black carbon particles, with sizes between 100 and 300 nm, increased with the amount of the coating on the insoluble cores and at thick enough coating approached the CCN activity of the soluble species. Moreover, controlled dry coating of the insoluble BC cores yielded a size-independent distribution of the coating material on the insoluble cores, which was not achieved by wet coating of the silica particles. The results also confirmed that by knowing the fraction of soluble material (coating thicknesses), the existing theories gave a reasonable estimate of the CCN activity for the mixed soluble-insoluble particles. Finally, the results highlight the need for including the impacts of co-emitted or later condensed compounds in estimates of the climate impacts of atmospheric insoluble aerosol species.

In the second part of the thesis, surface propensity of succinic acid, pure or mixed with soluble inorganic salts in the aqueous droplets, were quantified via molecular-level surface composition measurement by X-ray Photoelectron Spectroscopy (XPS). The XPS and molecular dynamic (MD) simulations of succinic acid aqueous solutions showed strong enrichment of the succinic acid at the surface of the liquid droplets compared to the bulk solution. This effect was more pronounced in the presence of the highly soluble inorganic salts like NaCl and (NH4)2SO4 in the system. The modeled surface tension of the pure organic or mixture of organic and inorganic substances, using surface enrichment factors derived from the XPS experiments were in good agreement with the experimental surface tension data. This demonstrates the high potential of XPS for direct measurements of the surface composition of atmospherically relevant aqueous mixtures. The results suggest that for modeling the phase-state and water content of the atmospheric particles, the contribution by the surface layer needs to be considered, because aqueous droplet can contain larger amounts of organic compounds than the bulk solubility limit of the solutions. However, the effect of the aqueous surface composition on the CCN activation of particles consisting of the studied mixtures was estimated to be very small.

The results presented in this thesis provide new insights into the relationship between aerosol particle composition and cloud condensation nuclei activity. However, the effect of more realistic complex mixtures will require more research. The results showed that for modeling semi-volatile species, the partitioning between the gas and condensed phase needs to be considered. In addition, along with the liquid-phase cloud activation, the ice nucleation ability of the particles made of soluble and insoluble species requires to be further investigated.

Place, publisher, year, edition, pages
Stockholm: Department of Environmental Science and Analytical Chemistry, Stockholm University, 2018. p. 50
Keyword
CCN activation, surface tension, coated aerosols, black carbon
National Category
Climate Research Meteorology and Atmospheric Sciences Environmental Sciences
Research subject
Applied Environmental Science
Identifiers
urn:nbn:se:su:diva-152128 (URN)978-91-7797-105-4 (ISBN)978-91-7797-106-1 (ISBN)
Public defence
2018-03-15, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
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Available from: 2018-02-20 Created: 2018-01-26 Last updated: 2018-02-12Bibliographically approved

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