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The high Arctic summer aerosol: Size, chemical composition, morphology and evolution over the pack-ice
Stockholm University, Faculty of Science, Department of Meteorology .ORCID iD: 0000-0002-0299-5510
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. , p. 42
Keyword [en]
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: urn:nbn:se:su:diva-136211ISBN: 978-91-7649-624-4 (print)ISBN: 978-91-7649-625-1 (print)OAI: oai:DiVA.org:su-136211DiVA: diva2:1064801
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
List of papers
1. A method for sizing submicrometer particles in air collected on Formvar films and imaged by scanning electron microscopy
Open this publication in new window or tab >>A method for sizing submicrometer particles in air collected on Formvar films and imaged by scanning electron microscopy
2013 (English)In: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 6, no 12, p. 3459-3475Article in journal (Refereed) Published
Abstract [en]

A method was developed to systematically investigate individual aerosol particles collected onto a polyvinyl formal (Formvar)-coated copper grid with scanning electron microscopy. At very mild conditions with a low accelerating voltage of 2 kV and Gentle Beam mode aerosol particles down to 20 nm in diameter can be observed. Subsequent processing of the images with digital image analysis provides size resolved and morphological information (elongation, circularity) on the aerosol particle population. Polystyrene nanospheres in the expected size range of the ambient aerosol particles (20–900 nm in diameter) were used to confirm the accuracy of sizing and determination of morphological parameters. The relative standard deviation of the diameters of the spheres was better than ±10% for sizes larger than 40 nm and ±18% for 21 nm particles compared to the manufacturer's certificate. Atmospheric particles were collected during an icebreaker expedition to the high Arctic (north of 80°) in the summer of 2008. Two samples collected during two different meteorological regimes were analyzed. Their size distributions were compared with simultaneously collected size distributions from a Twin Differential Mobility Particle Sizer, which confirmed that a representative fraction of the aerosol particles was imaged under the electron microscope. The size distributions obtained by scanning electron microscopy showed good agreement with the Twin Differential Mobility Sizer in the Aitken mode, whereas in the accumulation mode the size determination was critically dependent on the contrast of the aerosol with the Formvar-coated copper grid. The morphological properties (elongation, circularity) changed with the number of days the air masses spent over the pack-ice area north of 80° before the aerosol particles were collected at the position of the icebreaker and are thus an appropriate measure to characterize transformation processes of ambient aerosol particles.

Keyword
Aerosol, EPS, Electron Microscopy
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-97768 (URN)10.5194/amt-6-3459-2013 (DOI)000328263800009 ()
Available from: 2013-12-17 Created: 2013-12-17 Last updated: 2017-12-06Bibliographically approved
2. Size-resolved morphological properties of the high Arctic summer aerosol during ASCOS-2008
Open this publication in new window or tab >>Size-resolved morphological properties of the high Arctic summer aerosol during ASCOS-2008
2016 (English)In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 16, no 10, p. 6577-6593Article in journal (Refereed) Published
Abstract [en]

The representation of aerosol properties and processes in climate models is fraught with large uncertainties. Especially at high northern latitudes a strong underprediction of aerosol concentrations and nucleation events is observed and can only be constrained by in situ observations based on the analysis of individual aerosol particles. To further reduce the uncertainties surrounding aerosol properties and their potential role as cloud condensation nuclei this study provides observational data resolved over size on morphological and chemical properties of aerosol particles collected in the summer high Arctic, north of 80A degrees aEuro-N. Aerosol particles were imaged with scanning and transmission electron microscopy and further evaluated with digital image analysis. In total, 3909 aerosol particles were imaged and categorized according to morphological similarities into three gross morphological groups: single particles, gel particles, and halo particles. Single particles were observed between 15 and 800aEuro-nm in diameter and represent the dominating type of particles (82aEuro-%). The majority of particles appeared to be marine gels with a broad Aitken mode peaking at 70aEuro-nm and accompanied by a minor fraction of ammonium (bi)sulfate with a maximum at 170aEuro-nm in number concentration. Gel particles (11aEuro-% of all particles) were observed between 45 and 800aEuro-nm with a maximum at 154aEuro-nm in diameter. Imaging with transmission electron microscopy allowed further morphological discrimination of gel particles in 'aggregate' particles, 'aggregate with film' particles, and 'mucus-like' particles. Halo particles were observed above 75aEuro-nm and appeared to be ammonium (bi)sulfate (59aEuro-% of halo particles), gel matter (19aEuro-%), or decomposed gel matter (22aEuro-%), which were internally mixed with sulfuric acid, methane sulfonic acid, or ammonium (bi)sulfate with a maximum at 161aEuro-nm in diameter. Elemental dispersive X-ray spectroscopy analysis of individual particles revealed a prevalence of the monovalent ions Na+/K+ for single particles and aggregate particles and of the divalent ions Ca2+/Mg2+ for aggregate with film particles and mucus-like particles. According to these results and other model studies, we propose a relationship between the availability of Na+/K+ and Ca2+/Mg2+ and the length of the biopolymer molecules participating in the formation of the three-dimensional gel networks.

National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-132618 (URN)10.5194/acp-16-6577-2016 (DOI)000378354100033 ()
Available from: 2016-08-25 Created: 2016-08-17 Last updated: 2017-11-28Bibliographically approved
3. Calcium enrichment in sea spray aerosol particles
Open this publication in new window or tab >>Calcium enrichment in sea spray aerosol particles
Show others...
2016 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, no 15, p. 8277-8285Article in journal (Refereed) Published
Abstract [en]

Sea spray aerosol particles are an integral part of the Earth's radiation budget. To date, the inorganic composition of nascent sea spray aerosol particles has widely been assumed to be equivalent to the inorganic composition of seawater. Here we challenge this assumption using a laboratory sea spray chamber containing both natural and artificial seawater, as well as with ambient aerosol samples collected over the central Arctic Ocean during summer. We observe significant enrichment of calcium in submicrometer (<1m in diameter) sea spray aerosol particles when particles are generated from both seawater sources in the laboratory as well as in the ambient aerosols samples. We also observe a tendency for increasing calcium enrichment with decreasing particle size. Our results suggest that calcium enrichment in sea spray aerosol particles may be environmentally significant with implications for our understanding of sea spray aerosol, its impact on Earth's climate, as well as the chemistry of the marine atmosphere.

Keyword
sea spray aerosol, inorganic ions, calcium, air-sea interaction
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-135114 (URN)10.1002/2016GL070275 (DOI)000383290300057 ()
Available from: 2016-11-14 Created: 2016-10-31 Last updated: 2017-11-29Bibliographically approved
4. The evolution of the high Arctic summer aerosol over the pack ice
Open this publication in new window or tab >>The evolution of the high Arctic summer aerosol over the pack ice
(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
Arctic, aerosol, morphology, size distribution
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-136962 (URN)
Available from: 2017-01-13 Created: 2016-12-19 Last updated: 2017-01-16Bibliographically approved

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