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The influence of biogenic organic compounds on cloud formation
Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Aerosols and clouds provide the largest uncertainty in the atmospheric radiation budget. The main focus of this thesis was to investigate the ability of organic compounds in aerosol particles to form clouds, and more specifically those emitted by living organisms.

The cloud forming properties of the highly water-soluble methyltetrols and polyols, which are compounds produced by plants and fungi that are common in aerosol, were studied. All compounds and their salt mixtures have a moderate potential to serve as cloud condensation nuclei (CCN). They are thus not likely to have a significant global impact on cloudiness.

The potential presence of surfactants released by microorganisms was investigated for aerosols sampled at different locations. Very low surface tension values were measured for these aerosol extracts (30 mN/m), which implies that these aerosols have good CCN properties and indicate the presence of biosurfactants. Their occurrence in aerosols still needs to be confirmed directly by chemical identification.

Reactions of organic compounds in sulfate salt solutions exposed to UV-light were studied and found to produce surface active compounds. Thus, mixed sulfate/organic aerosol could have more favourable CCN properties after exposure to light than when kept in the dark. The surface active compounds were proposed to be long-chained organosulfates with hydrophilic and hydrophobic parts, similar to other amphiphilic surfactants.

Mixtures of salt and strong surfactants formed by bacteria were studied using two different techniques for determining their CCN properties. There were inconsistencies between the two methods which could be accounted for by surface partitioning. The studied mixtures were determined to be good potential CCN material in both techniques.

All these aspects require further investigation, but if the impact of strong biogenic surfactants on cloud formation is confirmed, a new link between living organisms and climate would be identified.

Place, publisher, year, edition, pages
Stockholm: Department of Applied Environmental Science (ITM), Stockholm University , 2010. , 43 p.
Keyword [en]
biogenic, aerosol, CCN, water-soluble, microorganisms, surfactant, organosulfate
National Category
Natural Sciences
Research subject
Applied Environmental Science
Identifiers
URN: urn:nbn:se:su:diva-45714ISBN: 978-91-7447-175-5 (print)OAI: oai:DiVA.org:su-45714DiVA: diva2:369398
Public defence
2010-12-17, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Manuscript. Available from: 2010-11-25 Created: 2010-11-10 Last updated: 2010-11-30Bibliographically approved
List of papers
1. The Cloud Condensation Nuclei (CCN) properties of 2-methyltetrols and C3-C6 polyols from osmolality and surface tension measurements
Open this publication in new window or tab >>The Cloud Condensation Nuclei (CCN) properties of 2-methyltetrols and C3-C6 polyols from osmolality and surface tension measurements
2009 (English)In: Atmospheric Chemistry And Physics, ISSN 1680-7316, Vol. 9, no 3, 973-980 p.Article in journal (Refereed) Published
Abstract [en]

A significant fraction of the organic material in aerosols is made of highly soluble compounds such as sugars (mono-and polysaccharides) and polyols such as the 2-methyltetrols, methylerythritol and methyltreitol. Because of their high solubility these compounds are considered as potentially efficient CCN material. For the 2-methyltetrols, this would have important implications for cloud formation at global scale because they are thought to be produced by the atmospheric oxidation of isoprene. To investigate this question, the complete Kohler curves for C3-C6 polyols and the 2-methyltetrols have been determined experimentally from osmolality and surface tension measurements. Contrary to what was expected, none of these compounds displayed a higher CCN efficiency than organic acids. Their Raoult terms show that this limited CCN efficiency is due to their absence of dissociation in water, this in spite of slight surface-tension effects for the 2-methyltetrols. Thus, compounds such as saccharides and polyols would not contribute more to cloud formation than other organic compounds studied so far. In particular, the presence of 2-methyltetrols in aerosols would not particularly enhance cloud formation in the atmosphere, in contrary to recently suggested

Keyword
ACTIVATION, AEROSOL, AMAZON BASIN, ATMOSPHERE, ISOPRENE, MASS-SPECTROMETRY, ORGANIC AEROSOL FRACTION, ORGANIC-COMPOUNDS, PARTICLE PHASE, PHOTOOXIDATION, PRODUCTS, SOLUBLE ORGANICS, SULFATE, SURFACE, WATER
National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-34964 (URN)10.5194/acp-9-973-2009 (DOI)000263325900016 ()1680-7316 (ISBN)
Available from: 2010-01-13 Created: 2010-01-13 Last updated: 2012-03-27Bibliographically approved
2. A possible role of ground-based microorganisms on cloud formation in the atmosphere
Open this publication in new window or tab >>A possible role of ground-based microorganisms on cloud formation in the atmosphere
Show others...
2010 (English)In: Biogeosciences, ISSN 1726-4170, Vol. 7, no 1, 387-394 p.Article in journal (Refereed) Published
Abstract [en]

The formation of clouds is an important process for the atmosphere, the hydrological cycle, and climate, but some aspects of it are not completely understood. In this work, we show that microorganisms might affect cloud formation without leaving the Earth’s surface by releasing biological surfactants (or biosurfactants) in the environment, that make their way into atmospheric aerosols and could significantly enhance their activation into cloud droplets. In the first part of this work, the cloud-nucleating efficiency of standard biosurfactants was characterized and found to be better than that of any aerosol material studied so far, including inorganic salts. These results identify molecular structures that give organic compounds exceptional cloud-nucleating properties. In the second part, atmospheric aerosols were sampled at different locations: a temperate coastal site, a marine site, a temperate forest, and a tropical forest. Their surface tension was measured and found to be below 30 mN/m, the lowest reported for aerosols, to our knowledge. This very low surface tension was attributed to the presence of biosurfactants, the only natural substances able to reach to such low values. The presence of strong microbial surfactants in aerosols would be consistent with the organic fractions of exceptional cloud-nucleating efficiency recently found in aerosols, and with the correlations between algae bloom and cloud cover reported in the Southern Ocean. The results of this work also suggest that biosurfactants might be common in aerosols and thus of global relevance. If this is confirmed, a new role for microorganisms on the atmosphere and climate could be identified.

National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-45694 (URN)10.5194/bg-7-387-2010 (DOI)000274058100030 ()
Note
authorCount :7Available from: 2010-11-10 Created: 2010-11-10 Last updated: 2011-02-03Bibliographically approved
3. Radical-initiated formation of organosulfates and surfactants in atmospheric aerosols
Open this publication in new window or tab >>Radical-initiated formation of organosulfates and surfactants in atmospheric aerosols
2010 (English)In: Geophysical Research Letters, ISSN 0094-8276, Vol. 37, no L05806Article in journal (Refereed) Published
Abstract [en]

Many atmospheric aerosols contain both organic compounds and inorganic material, such as sulfate salts. In this work, we show that these sulfates could trigger some chemical transformations of the organic compounds by producing sulfate radicals, SO4, when exposed to UV light (280–320 nm). In particular, we show by mass spectrometry (LC/ESI-MSMS) that isoprene, methyl vinyl ketone, methacrolein, and α-pinene in irradiated sulfate solutions (ammonium and sodium sulfate) produce the same organosulfates as previously identified in aerosols, and even some that had remained unidentified until now. With a typical time constant of 9 h instead of 4600 days for esterifications, these radical reactions would be a plausible origin for the atmospheric organosulfates. These reactions also produced efficient surfactants, possibly resembling the long-chain organosulfates found in the experiments. Thus, photochemistry in mixed sulfate/organic aerosols could increase cloud condensation nuclei (CCN) numbers, which would be supported by previous atmospheric observations

Keyword
atmospheric aerosol, sulfate material
National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-44825 (URN)10.1029/2009GL041683 (DOI)
Available from: 2010-11-09 Created: 2010-11-09 Last updated: 2010-12-30Bibliographically approved
4. Biosurfactants as CCN: comparison between on-line and off-line measurements
Open this publication in new window or tab >>Biosurfactants as CCN: comparison between on-line and off-line measurements
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We are presenting the CCN properties for the bacterial compounds rhamnolipid and surfactin, which are extremely strong surfactants. Three organic:sodium chloride mixtures with mass percentages of 80:20, 50:50 and 20:80 were measured for each biosurfactant. Both on-line Cloud Condensation Nuclei Counter (CCNC) and off-line osmolality combined with surface tension measurements were performed to obtain two sets of critical supersaturations for various dry particle diameters. The critical supersaturations measured by the CCNC were systematically higher than the corresponding supersaturations derived from osmolality/surface tension measurements. A simple surface partitioning-adaption was applied to the off-line data and resulted in a correlation with the results from CCNC measurements for both mixtures with 20 wt% biosurfactant and the 50 wt% rhamnolipid mixture but not for the mixtures with 80 wt% biosurfactant and the 50 wt% surfactin mixture. An explanation can be unreliable CCNC results from the surfactin mixtures as we suspect poor dissolvement of the organic crystals. The choice of the assumed biosurfactant density also has an effect which should not be ignored. However, this indicate that the experimental method using osmolality and surface tension measurements together with a simple surface partitioning model can be used for strongly surfactant compounds as long as they do not dominate the particle mass. We also conclude that biosurfactants in mixed potential CCN particles can activate at relatively low supersaturation compared to other organic mixtures. Still, the critical supersaturation increases with increasing surfactant fraction.

National Category
Natural Sciences
Research subject
Applied Environmental Science
Identifiers
urn:nbn:se:su:diva-45695 (URN)
Available from: 2010-11-10 Created: 2010-11-10 Last updated: 2010-11-11Bibliographically approved

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