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The evolution of the high Arctic summer aerosol over the pack ice
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Ambient aerosol samples were collected over the pack ice north of 80° N during the summer of 2008 during the course of the ASCOS campaign. Aerosol particles were collected during nine sampling events and were subsequently sorted into five groups according to the number of days the air spent over the pack ice since last contact with the ice edge. As a tracer for marine sources, the molar ratio of methane sulfonic acid/non-sea-salt-sulfate was used. Scanning electron microscopy allowed size resolved identification of three types of aerosol particles, single particles, gel particles and halo particles within each sample group. Between the five groups we found significant differences in aerosol morphology, largely dependent on the time of advection over the pack ice (days over ice, DOI) and the synoptic weather encountered. The most obvious differences were observed for marine gel particles. The fraction of these particles in the lower accumulation mode, ≤ 100 nm, increased from 15% (DOI = 1) over 20% (DOI = 3.2) and 27% (DOI= 6.7) to 30% (DOI = 8.9). In parallel the gel particle morphology changed with increasing DOI value, from a widely outspread and weakly contrasting morphology (DOI = 1) over a more distinct and better contrasting appearance (DOI = 3.2) to sharply and well contoured particles (DOI = 6.7). The gel particles with the highest DOI value (DOI = 8.9), however, showed a branched and widely outspread morphology that indicated a relatively recent emission of these particles into the submicrometer size range, either from sources over the pack ice or through fragmentation of supermicrometer particles.

Keyword [en]
Arctic, aerosol, morphology, size distribution
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-136962OAI: oai:DiVA.org:su-136962DiVA: diva2:1064789
Available from: 2017-01-13 Created: 2016-12-19 Last updated: 2017-01-16Bibliographically approved
In thesis
1. The high Arctic summer aerosol: Size, chemical composition, morphology and evolution over the pack-ice
Open this publication in new window or tab >>The high Arctic summer aerosol: Size, chemical composition, morphology and evolution over the pack-ice
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Aerosol particles, especially in the high Arctic are still not very well represented in climate models. Particle size and number concentrations are strongly under-predicted and temporal variations of aerosol composition and size are still not very well understood, mainly due to the sparsity of observations.

The main objective of this thesis is the characterization of the high Arctic summer aerosol by means of electron microscopy in order to extend the existing data set from previous expeditions by size resolved data on aerosol number, morphology and chemical composition and to gain a better understanding of the evolution of the aerosol in the atmosphere.

Ambient aerosol was collected over the pack ice during the Arctic Summer Cloud and Ocean (ASCOS) campaign to the high Arctic in summer 2008. Aerosol particles were evaluated with scanning electron microscopy and subsequent digital image processing to assess particle size and morphology. More than 3900 aerosol particles from 9 sampling events were imaged with scanning electron microscopy and merged into groups of similar morphology which contributed to different degrees to the total aerosol: single particles (82%), gel particles (11%) and halo particles (7%). Single particles were observed over the whole size range with a maximum at 64 nm in diameter, gel particles appeared > 45 nm with a maximum in number at 174 nm, halo particles appeared > 75 nm with a maximum in number at 161 nm. The majority of particles showed the morphology of marine gels, no sea salt or otherwise crystalline particles were observed. Transmission electron microscopy enabled more subtle insights into particle morphology and allowed further subdivision of gel particles into aggregates, aggregates with film and mucus-like particles. Energy dispersive X-ray spectroscopy of individual particles revealed a gradual transition in the content of Na+/K+ and Ca2+/Mg2+ between particle morphologies. Single particles and aggregate particles preferentially contained Na+/K+ whereas aggregate with film particles and mucus-like particles mainly contained Ca2+/Mg2+ suggesting a connection between particle morphology and ion content. Back-trajectory analysis was used to identify aerosol sources and to understand the evolution of the aerosol as a function of the synoptic weather situation. Particle numbers, size and morphology changed with the days the air mass spent over the pack-ice. A morphological descriptor applied to gel particles showed a clear trend suggesting that the contour of the particles becomes sharper and more distinct with increased time spent over the pack-ice. For a very long time over the pack-ice, however, we observed a morphology comparable to freshly emitted particles suggesting aerosol sources over the inner pack-ice.

Size resolved aerosol chemical composition measurements were utilized to investigate the inorganic composition of laboratory generated nascent sea spray aerosol particles and ambient aerosol samples collected during ASCOS. A significant enrichment of Ca2+ was observed in submicrometer particles in either case with a tendency for increasing Ca2+ enrichment with decreasing particle size. This has strong implications for the alkalinity of sea spray aerosol particles with consequences for the sulfur chemistry in the marine boundary layer, the hygroscopicity and thus the potential of sea spray aerosol particles to act as cloud condensation nuclei.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University, 2017. 42 p.
Keyword
Arctic, aerosol, marine gel, electron microscopy, EDX-spectroscopy, chemical composition, morphology, size distribution
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-136211 (URN)978-91-7649-624-4 (ISBN)978-91-7649-625-1 (ISBN)
Public defence
2017-02-24, De Geersalen, Geovetenskapens hus, Svante Arrheniusväg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.

Available from: 2017-02-01 Created: 2016-12-01 Last updated: 2017-01-25Bibliographically approved

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