The biogenic role in cloud formation and aerosol

The biogenic role in cloud formation and aerosol chemistry Sanna Ekström Stockholm University, Sweden

My Ph. D-thesis The aim is to investigate how biogenic compounds that are emitted to the atmosphere affect it. It is also to study how microorganisms can change the chemical composition in the atmosphere by breaking down compounds in situ. My first project – CCN properties of polyols and methyltetrols.

Why methyltetrols? Very common in aerosol Thought to be produced by the oxidation of isoprene (Claeys et al, 2004) An impact of phytoplankton on clouds have been found and proposed to be caused by oxidation products from isoprene (Meskhizde and Nenes, 2006). Isoprene Methyltetr ol

Why polyols? Fig: Arabitol and mannitol Natural compounds mainly emitted by fungi. Have been found in both fine and coarse mode aerosol globally. (Carvalho et al. , 2003, Graham et al. , 2003). Very high solubility (Saxena and Hildemann, 1996)

Solubility Cohen 1993, Saxena and Hildemann 1996, Hu 1998

Köhler theory The simplified version of the Köhler equation has previously often been used for measurements of CCN properties of organic compounds and it results in errors: Ø Significant error in the calculation of S when the droplet is small as a consequence of mathematical simplifications. Ø Significant error in the calculation of S when the droplet is small as a consequence of physico-chemical facts (g ≠ 1). Ø Additional error of unknown magnitude in the calculation of S in the entire droplet size range when organics are also considered as a consequence of lack of information on the properties of organic components (m, MW, i).

Köhler theory A new technique (Kiss and Hansson, 2004) was used to obtain the Köhler curves. § All three errors are eliminated if the original Köhler equation is used! S = p/p 0− 1 = aw exp[2 solution. Mw/(r water. RT)]− 1 p = water vapor pressure over the droplet solution p 0 = water vapor pressure over a flat water surface aw = water activity in the droplet solution= surface tension of the droplet solution Mw = molecular weight of water r = density of the droplet solution (water) R = universal gas constant T = temperature G. Kiss and H-C. Hansson,

The Köhler theory § Water activity can be measured with vapor pressure osmometry: aw = 1000/MWwater/(1000/MWwater + Cosm) § Osmolality is determined through measuring the change in water vapour pressure § The osmometer accounts for all interactions in the solution (dissociation, formation of aggregates, co-ordination, etc. )

Experimental method Surface tension and osmolality measurements were performed on solutions with different concentrations, building curve point by point. Easy technique which provides accurate results

Comparison of results for organic acids Experimental measurements can give Sc for any Ddry. Excellent agreement with other techniques for known compounds and theoretical predications from the Köhler equation (a).

Results: Köhler curves Ddry = 60 nm

Results: Osmometry and tensiometry High CCN efficiency is controlled by the Raoult effect for compounds that dissociate. The methyltetrols had a small effect on the surface tension

Effect of salt on CCN efficiency Addition of salt has a major impact on slightly soluble organic compounds (Bilde and Svenningson, 2004) It can remove the ”branch” on the Köhler curve caused by the insoluble part. What impact would it have on more soluble compounds?

Results – salt solutions Pure sodium chloride Mannitol - lower Sc Adipic acid - lower Sc Methylthreitol - same Sc Methylerythritol - higher Sc! 2 -methylerythritol is: less soluble than methylthreitol and mannitol liquid in contrast to slightly soluble adipic acid could form a film on the surface, which prevents growth.

Conclusions of first project Polyols and methyltetrols are not good CCN. Contrary to expected, methyltetrols are not important for cloud formation. Dissociation has a much stronger control of the Raoult effect than solubility. However, solubility can result in cloud activation of smaller particle sizes than slightly insoluble compounds. Addition of salt has a small effect, except for erythritol, which might form a film on the droplet surface

My current project – Other biogenic compounds If water solubility isn’t the key to high CCN efficiency, are there other natural compounds that could be interesting? Questions to answer… Which natural compounds are common or in aerosol? In which concentration? How can we find the answers?

Aerosol sampling Aerosol samples from remote boreal forest, marine, amazonian rainforest, which are known to have mainly natural sources is needed. Aerosol sampling from forests wewas carreid out in the ”traditional” way with quartz filters. Sampling marine aerosol required another technique.

Marine aerosol generation Air in Water in Air out Carbon filter Aerosol sampling Pressure regulation Bubble tank Water was sampled and aerosol was generated from it in the bubble tank. • The technique ensures that the aerosol comes solely from the water Water out • Small scale – easier than a campaign

HPLC-MSMS analysis The detection of complicated biogenic compounds in aerosol is not straight forward… Filter extraction with water yields the WSOC fraction of the aerosol. Further extraction with an absorbant in order to concentrate the desired organic compounds. Analysis of standards to optimize instrument tuning.

…and more work will come. Thank you for your attention!