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Aerosol–cloud interactions in a warming Arctic
Stockholm University, Faculty of Science, Department of Environmental Science.ORCID iD: 0000-0003-0885-7650
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Atmospheric aerosol particles are small liquid or solid particles suspended in the air. They are present in the atmosphere all around us and affect the planetary energy balance by scattering and absorbing radiation and by interacting with clouds. In model projections of future climate, aerosol–cloud interactions contribute a lot of uncertainty. Large-scale climate models particularly struggle with simulating low-level clouds in the Arctic, which is a region that is not only warming at twice the global average rate or higher but also where natural aerosol emissions are expected to change most as a result of the warming. The goal of this thesis was to study aerosol–cloud interactions to help improve our understanding of what role clouds play in the Arctic climate and how they will respond to climate change. Specifically, the project focused on studying the microphysical properties of aerosol particles and cloud nucleating particles—the subset of aerosol particles that participate in cloud formation. This was done both through field experiments in the high Arctic over the pack ice and by analysis of an existing two-year data set from an Arctic research station on Svalbard.

The main instrument used in this thesis was a ground-based counterflow virtual impactor (GCVI) inlet, which dries cloud droplets and ice crystals and allows us to characterise the particles that were inside. The Svalbard study is the longest GCVI study to date, and the first to cover more than a full annual cycle. It also involved a detailed evaluation of the GCVI. Using the GCVI inlet and a large array of other instruments, we were able to show that small, so-called Aitken mode particles act as cloud nucleating particles, supporting results from previous studies. However, our measurements showed these particles to be more abundant in the cloud droplets and ice crystals than expected, both over the pack ice and on Svalbard. While some uncertainties remain, these datasets can potentially be used to evaluate and improve model representations of low-level Arctic clouds. In the other parts of this thesis, we found that iodine nucleation and breakup of larger particles are potential formation pathways for Aitken mode particles over the pack ice. However, detailed chemical composition measurements of cloud nucleating particles would be needed to determine whether these formation mechanisms are important for Arctic cloud formation.

Place, publisher, year, edition, pages
Stockholm: Department of Environmental Science, Stockholm University , 2022. , p. 58
Keywords [en]
aerosols, clouds, aerosol-cloud interactions, Arctic, climate, field studies
National Category
Meteorology and Atmospheric Sciences Environmental Sciences
Research subject
Environmental Sciences
Identifiers
URN: urn:nbn:se:su:diva-204133ISBN: 978-91-7911-900-3 (print)ISBN: 978-91-7911-901-0 (electronic)OAI: oai:DiVA.org:su-204133DiVA, id: diva2:1654275
Public defence
2022-06-13, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14 and online via Zoom, public link is available at the department website, Stockholm, 14:00 (English)
Opponent
Supervisors
Projects
Arctic climate across spatial and temporal scales (ACAS)Available from: 2022-05-19 Created: 2022-04-26 Last updated: 2025-02-01Bibliographically approved
List of papers
1. A long-term study of cloud residuals from low-level Arctic clouds
Open this publication in new window or tab >>A long-term study of cloud residuals from low-level Arctic clouds
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2021 (English)In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 21, no 11, p. 8933-8959Article in journal (Refereed) Published
Abstract [en]

To constrain uncertainties in radiative forcings associated with aerosol-cloud interactions, improved understanding of Arctic cloud formation is required, yet long-term measurements of the relevant cloud and aerosol properties remain sparse. We present the first long-term study of cloud residuals, i.e. particles that were involved in cloud formation and cloud processes, in Arctic low-level clouds measured at Zeppelin Observatory, Svalbard. To continuously sample cloud droplets and ice crystals and separate them from non-activated aerosol, a ground-based counter-flow virtual impactor inlet system (GCVI) was used. A detailed evaluation of the GCVI measurements, using concurrent cloud particle size distributions, meteorological parameters, and aerosol measurements, is presented for both warm and cold clouds, and the potential contribution of sampling artefacts is discussed in detail. We find an excellent agreement of the GCVI sampling efficiency of liquid clouds using two independent approaches. The 2-year data set of cloud residual size distributions and number concentrations reveals that the cloud residuals follow the typical seasonal cycle of Arctic aerosol, with a maximum concentration in spring and summer and a minimum concentration in the late autumn and winter months. We observed average activation diameters in the range of 58-78 nm for updraught velocities below 1 m s(-1). A cluster analysis also revealed cloud residual size distributions that were dominated by Aitken mode particles down to around 20-30 nm. During the winter months, some of these small particles may be the result of ice, snow, or ice crystal shattering artefacts in the GCVI inlet; however, cloud residuals down to 20 nm in size were also observed during conditions when artefacts are less likely.

National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-195846 (URN)10.5194/acp-21-8933-2021 (DOI)000662205600001 ()
Available from: 2021-08-26 Created: 2021-08-26 Last updated: 2025-02-07Bibliographically approved
2. Physical and Chemical Properties of Cloud Droplet Residuals and Aerosol Particles During the Arctic Ocean 2018 Expedition
Open this publication in new window or tab >>Physical and Chemical Properties of Cloud Droplet Residuals and Aerosol Particles During the Arctic Ocean 2018 Expedition
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2022 (English)In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 127, no 11, article id e2021JD036383Article in journal (Refereed) Published
Abstract [en]

Detailed knowledge of the physical and chemical properties and sources of particles that form clouds is especially important in pristine areas like the Arctic, where particle concentrations are often low and observations are sparse. Here, we present in situ cloud and aerosol measurements from the central Arctic Ocean in August–September 2018 combined with air parcel source analysis. We provide direct experimental evidence that Aitken mode particles (particles with diameters ≲70 nm) significantly contribute to cloud condensation nuclei (CCN) or cloud droplet residuals, especially after the freeze-up of the sea ice in the transition toward fall. These Aitken mode particles were associated with air that spent more time over the pack ice, while size distributions dominated by accumulation mode particles (particles with diameters ≳70 nm) showed a stronger contribution of oceanic air and slightly different source regions. This was accompanied by changes in the average chemical composition of the accumulation mode aerosol with an increased relative contribution of organic material toward fall. Addition of aerosol mass due to aqueous-phase chemistry during in-cloud processing was probably small over the pack ice given the fact that we observed very similar particle size distributions in both the whole-air and cloud droplet residual data. These aerosol–cloud interaction observations provide valuable insight into the origin and physical and chemical properties of CCN over the pristine central Arctic Ocean.

National Category
Meteorology and Atmospheric Sciences Environmental Sciences
Identifiers
urn:nbn:se:su:diva-204135 (URN)10.1029/2021JD036383 (DOI)000806579000001 ()
Available from: 2022-04-26 Created: 2022-04-26 Last updated: 2025-02-01Bibliographically approved
3. Frequent new particle formation over the high Arctic pack ice by enhanced iodine emissions
Open this publication in new window or tab >>Frequent new particle formation over the high Arctic pack ice by enhanced iodine emissions
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2020 (English)In: Nature Communications, E-ISSN 2041-1723, Vol. 11, no 1, article id 4924Article in journal (Refereed) Published
Abstract [en]

In the central Arctic Ocean the formation of clouds and their properties are sensitive to the availability of cloud condensation nuclei (CCN). The vapors responsible for new particle formation (NPF), potentially leading to CCN, have remained unidentified since the first aerosol measurements in 1991. Here, we report that all the observed NPF events from the Arctic Ocean 2018 expedition are driven by iodic acid with little contribution from sulfuric acid. Iodic acid largely explains the growth of ultrafine particles (UFP) in most events. The iodic acid concentration increases significantly from summer towards autumn, possibly linked to the ocean freeze-up and a seasonal rise in ozone. This leads to a one order of magnitude higher UFP concentration in autumn. Measurements of cloud residuals suggest that particles smaller than 30nm in diameter can activate as CCN. Therefore, iodine NPF has the potential to influence cloud properties over the Arctic Ocean. Which vapors are responsible for new particle formation in the Arctic is largely unknown. Here, the authors show that the formation of new particles at the central Arctic Ocean is mainly driven by iodic acid and that particles smaller than 30nm in diameter can activate as cloud condensation nuclei.

National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-187658 (URN)10.1038/s41467-020-18551-0 (DOI)000577113300004 ()33004812 (PubMedID)
Available from: 2021-01-11 Created: 2021-01-11 Last updated: 2025-02-07Bibliographically approved
4. New Insights Into the Composition and Origins of Ultrafine Aerosol in the Summertime High Arctic
Open this publication in new window or tab >>New Insights Into the Composition and Origins of Ultrafine Aerosol in the Summertime High Arctic
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2021 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 48, no 21, article id e2021GL094395Article in journal (Refereed) Published
Abstract [en]

The summertime high Arctic atmosphere is characterized by extremely low aerosol abundance, such that small natural aerosol inputs have a strong influence on cloud formation and surface temperature. The physical sources and the mechanisms responsible for aerosol formation and development in this climate-critical and changing region are still uncertain. We report time-resolved measurements of high Arctic Aitken mode (∼20–60 nm diameter) aerosol composition during August–September 2018. During a significant Aitken mode formation event, the particles were composed of a combination of primary and secondary materials. These results highlight the importance of primary aerosol sources for high Arctic cloud formation, and they imply the action of a poorly understood atmospheric mechanism separating larger particles into multiple sub-particles.

Keywords
ultrafine aerosol, Arctic, marine boundary layer, aerosol composition, nanoparticle, aerosol formation
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-200696 (URN)10.1029/2021GL094395 (DOI)000716768700040 ()
Available from: 2022-01-13 Created: 2022-01-13 Last updated: 2025-02-07Bibliographically approved

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