The influence of biogenic organic compounds on cloud formation

THE INFLUENCE OF BIOGENIC ORGANIC COMPOUNDS ON CLOUD FORMATION

Sanna Ekström

The influence of biogenic organic compounds on cloud formation

Sanna Ekström

©Sanna Ekström, Stockholm 2010

ISBN 978-91-7447-175-5, pp. 1-43

Printed in Sweden by US-AB, Stockholm 2010

Distributor: Department of Applied Environmental Science (ITM)

”Tralala lilla molntuss,

kom hit skall du få en

puss” -Bob hund

List of papers

This thesis is based on the following papers. They are referred by their Roman numerals.

I. Ekström, S., Nozière, B., and Hansson, H.-C. 2009. The cloud condensation nuclei (CCN) properties of 2- methyltetrols and C3–C6 polyols from osmolality and sur- face tension measurements. Atmospheric Chemistry and Physics, 9, 973–980.

II. Ekström, S., Nozière, B., Hultberg, M., Alsberg, T., Mag- nér, J., Nilsson, E. D., Artaxo, P. 2010. A possible role of ground-based microorganisms on cloud formation in the atmosphere. Biogeosciences, 7 387-394.

III. Nozière, B., Ekström, S., Alsberg, T., and Holmström, S.

2010. Radical-initiated formation of organosulfates and sur- factants in atmospheric aerosols. Geophysical Research Let- ters, 37, L05806, doi:10.1029/2009GL041683.

IV. Ekström, S., Wittbom, C., Svenningsson, B., Nozière, B.

2010. Biosurfactants as CCN: comparison between on-line and off-line measurements. Manuscript.

Papers I, II and III have been reproduced by permission from European

Geosciences Union (I, II) and American Geophysical Union (III), re-

spectively.

Contents

1. Introduction ... 11

1.1 Aerosols, clouds and climate ... 11

1.2 Köhler theory ... 14

1.3 Surface active organic compounds ... 15

1.4 The effect of life on earth on cloud formation ... 16

1.5 Main objectives ... 17

2. Measuring the cloud forming potential ... 18

2.1 Off-line technique ... 18

2.2 On-line techniques ... 19

3. Sampling and analysis ... 21

3.1 Aerosol sampling sites ... 21

3.2 Instrumentation ... 22

3.3 Chemical characterization of surfactant molecules ... 23

4. Results ... 25

4.1 Paper I - “The Cloud Condensation Nuclei (CCN) properties of 2- methyltetrols and C3-C6 polyols from osmolality and surface tension measurements” ... 26

4.2 Paper II - “A possible role of ground-based microorganisms on cloud formation in the atmosphere” ... 27

4.3 Paper III - “Radical-initiated formation of organosulfates and surfactants in atmospheric aerosols” ... 29

4.4 Paper IV - “Biosurfactants as CCN: comparison between on-line and off- line measurements” ... 30

5. Key conclusions ... 31

6. Outlook ... 32

Appendix – contribution to the papers ... 34

Acknowledgements ... 35

Bibliography ... 36

1. Introduction

This thesis concerns the ability of organic compounds present in aerosol particles to form clouds, and specifically the contribution of compounds emitted by living organisms. First, an introduction to the topic.

1.1 Aerosols, clouds and climate

The climate is very likely to be changing as a consequence of hu- man activities. The greenhouse gases are warming the troposphere by absorbing incoming short wave radiation from the sun. Aerosols, small particles suspended in air, can on the other hand interact with incoming solar radiation either by absorbing or scattering the light.

This so-called direct aerosol effect can thus lead to both cooling and warming, which is illustrated together with the cloud albedo effect in Figure 1. All cloud droplets are formed by the condensation of water vapor on aerosol particles, referred to as cloud condensation nuclei (CCN). The cloud albedo effect, or the indirect aerosol effect, is the influence that aerosols have on the climate by producing cloud drop- lets that can scatter and absorb radiation, which is illustrated in Figure 2. The direct and indirect aerosol effects result in a net cooling on the climate and they are the largest unknown factors in the atmospheric radiation budget (IPCC, 2007).

The atmosphere can contain from a few particles per cubic centi- meter in very clean environments and up to a million in polluted areas.

There can also be large differences in size distribution and the chemi-

cal composition between various atmospheric environments (Heint-

zenberg, 1989). The smallest particles are in the nanometer size range

(10

m) and the largest several hundred micrometers in diameter. A

particle is classified as “fine” if it has a diameter smaller than 2.5 µm,

and “coarse” if the diameter is larger than 2.5 µm. The size class indi-

cates the physical, chemical and health related properties of the par-

ticle. A former common generalization was that natural or aged par-

ticles are larger than newly formed or anthropogenic particles. There

are now evidences showing that this is not always true. For instance,

natural aerosol particles containing polyols, which are mainly emitted from fungi, exist in both fine and coarse aerosol (Graham et al., 2003;

Ion et al., 2005).

Figure 1: Components of radiative forcing (RF) for emissions of aerosols and aero- sol precursors. Values represent RF in 2005 due to emissions and changes since 1750. (IPCC 2007).

The chemical composition varies amongst an aerosol population

and it is likely to consist of a large amount of compounds of which

hundreds can be organic compounds (Hahn, 1980; Simoneit and Ma-

zurek, 1982; Graedel, 1986; Rogge et al., 1993a-c). Organic particles

have been shown to play an important role in cloud formation (Nova-

kov and Penner, 1993; Matsumoto et al., 1997). The contribution of

organic particles to CCN can be up to 80% of the total number of

CCN in marine regions (Rivera-Carpio et al., 1996) and 20% at a con-

tinental semi-rural site (Chang et al., 2007). The aerosol can be exter-

nally, internally mixed, or both, which means chemicals can exist in-

dividually or together in each single particle. One particle may have a

liquid organic coating on top of an insoluble core, another might be a

completely mixed liquid of several compounds, and the third could be

a particle consisting of pure salt (Gieray et al., 1997; Ghermandi et al.,

2005). It is now well established that organic compounds almost al-

ways exist as internal mixtures with inorganic compounds in atmos-

pheric particles (Middlebrook et al., 1998; Murphy et al., 2006; Zhang

et al., 2007, Froyd et al., 2009). The presence of the organic material might alter the properties that influence cloud forming efficiency (Saxena et al., 1996; Cruz and Pandis, 2000; Dick et al., 2000), and the cloud forming properties of inorganic compounds are better un- derstood than that of pure organic compounds and mixtures. To sum- marize, the properties of atmospheric aerosols vary widely between each individual particles and this complicates the attempt to estimate their behaviour in the climate system.

Figure 2: The global annual mean radiation balance reprinted from Kiehl and Tren- berth (1997).

An important reason to obtain knowledge about the size spectrum of the atmospheric particles is that different physical processes depend on size. Particles with a diameter in the order of magnitude of 100 nm are commonly involved in cloud processes as CCN. The largest par- ticles will activate very easily no matter what compound(s) they con- sist of whilst the smaller will depend more on chemical composition (Dusek et al., 2006).

The particle numbers in a given aerosol mass can vary tremend-

ously if it consists of mainly fine or coarse particles. The mass, or vo-

lume, is mostly dominated by particles from approximately 0.1 µm to

50 µm in diameter and the number of particles is mostly contributed

by around 100 nm or smaller particles.

Large scale climate models calculate the radiative forcing from aerosols by inserting assumed primary emissions of particles, estima- tions of secondary particle formation and using parameterizations to estimate cloud droplet numbers. A parameterization can be derived from observations or physical relationships. Therefore, although this thesis focuses on molecular-scale properties, the results could affect the prediction of cloud formation at a global scale.

The main uncertainties in the existing descriptions of cloud forma- tion are the aerosol emissions, the aerosol population and composi- tion, and the effect of composition on cloud droplet formation. This results in different predictions in optical properties of the clouds.

There is a need to improve the approximation of the cloud albedo ef- fect, and the parameterizations plugged in the models. These physical and chemical parameters that are required to estimate the cloud drop- let numbers from an existing aerosol population in climate models are expressed using Köhler theory.

1.2 Köhler theory

Clouds would not exist if there were no particles for the water va- por to condense upon. Not all particles are equally efficient as conden- sation nuclei due to differences in both size and chemistry. Köhler theory (Köhler, 1936) expresses the required amount of water vapor in the atmosphere necessary for a particle to activate and grow into a cloud droplet. The solute effect and the surface tension effect are in- fluenced by the chemical composition of the particles.

Forces between the dissolved molecules in the droplet will affect its properties. Dissolved organic compounds influence the solute ef- fect, or Raoult effect but salts have a particularly strong effect by dis- sociating into ions which interact with each other.

The surface tension effect, or Kelvin effect, describes the vapor pressure over a curved surface, which is always larger than that of a flat surface. Droplets with a diameter larger than approximately 100 nm have a negligible surface tension effect, which is a significant force during the initial cloud droplet growth. Organic compounds gen- erally have a larger influence on the Kelvin effect than salts.

By combining the Raoult effect and the Kelvin effect we obtain the so-called Köhler equation. The Köhler equation describes cloud droplet activation and can be expressed as:

4 1

exp   

 



 



P w

sol w

w

RT D

a M

SS 

 , (1)

where SS is the supersaturation of water vapor, a

is the water activi- ty, D

is the droplet diameter, M

is the molecular weight of water,



is the droplet solution surface tension, R the gas constant, T the temperature, and 

is the density of water.

1.3 Surface active organic compounds

Köhler theory implies that the ability to reduce the surface tension of water is one of the important properties that an aerosol particle can have to enhance its cloud forming properties. Yet, it is only in the last decade or so that the surface tension lowering compounds have been acknowledged as important contributors to cloud formation (Facchini et al. 1999). In their pioneering work, Facchini et al. evidenced a cor- relation between the concentration of organic acids and the surface tension of fog droplet water. As this effect was ignored in cloud mod- els it was likely to result in an error in the indirect radiative effect of aerosols on climate.

Since this study, several other types of organic compounds in aerosol have been found to have surface active properties, such as

“HUmic-LIke Substances” or “HULIS” (Taraniuk et al., 2007). How- ever, their surface tension effect has been reported as small during cloud droplet activation (Wex et al., 2008). Moreover, the surface ten- sion of aerosol water extracts has never been reported below 50 mN/m (McFiggans et al 2005 and references therein).

There are many other molecules present in the natural environment that have better surfactant properties than the organic compound stu- died in these earlier works. One group of natural compounds that are known to be exceptionally strong surfactants is biosurfactants which are produced by microorganisms (Desai et al., 1997; Gautam and Tyagi 2006). Some of the biosurfactants can lower the surface tension of water as low as 28 mN/m and at concentration of µM (micromole per liter), many orders of magnitude lower than that of organic acids and HULIS.

This work focuses on surfactants that are amphiphilic by consist-

ing of one hydrophilic part and one or two hydrophobic tails. Because

of the structure, these compounds re-arrange themselves on the sur-

face of the liquid and can also form micelles, which are spherical ag-

gregates in the bulk liquid. The molecules at the water surface stick

out their hydrophobic tails and the molecules in micelles will have the

hydrophilic part facing out towards the surrounding water.

1.4 The effect of life on earth on cloud formation

Organic surfactants of biological origin that are present in aerosols would contribute to the climate system by affecting the formation of clouds. The natural cycles of which these compounds are part of would therefore contribute to the climate as feedbacks.

The possible contribution of the biosphere to cloud formation has been studied for decades. One classic example is the CLAW hypothe- sis (Charlson 1987, 2001), which suggests that the cloudiness is partly controlled by oceanic phytoplankton. The phytoplankton emit dime- thylsulfide (DMS) as they digest nutrients. In the atmosphere the DMS is converted into sulfate salts (Shaw 1983), which are excellent CCN (Charlson et al., 1987). The sulfate salts can be involved in for- mation of clouds that dampen the phytoplankton activity by blocking out sunlight. This theory could be supported by the study of phytop- lankton blooms and the observed increased cloudiness by Meskhidze and Nenes (2006) but is yet to be proven unambiguously.

A potential influence from airborne microorganisms on cloud for- mation has also been studied, which lead to the conclusion that several bacteria are competent ice nucleating agents (Schnell and Vali, 1972;

Schnell, 1976; Bauer et al., 2003; Pratt et al., 2009). Their possible contribution to liquid clouds has also been investigated (Moehler et al., 2007) but has not been considered to be nearly as an important contributor to CCN as inorganic salts. An important aspect of the question whether bacteria can act as CCN themselves is the size of bacteria, which is very large. Large particles are generally too few to contribute to CCN numbers in warm clouds.

A study at a semirural and a remote continental location showed

that 5-50% of the number of particles suspended in air are primary

biological aerosol particles injected directly into the atmosphere (e.g.,

cellular particles plant fragments, pollen, spores, bacteria, algae, fungi,

viruses, protein “crystals” and more) (Jaenicke, 2005). About 74% of

the total aerosols mass and up to 40% of the fine aerosol over the

Amazon rainforest have been estimated to consist of biological par-

ticles (Graham et al., 2003). But still, no compounds emitted by bacte-

ria and fungi had been proven to be an important contributor to the

CCN numbers.

1.5 Main objectives

The main focus of this thesis was to investigate the ability of bio- genic organic compounds in aerosol particles to form clouds.

The scientific objectives are:

 Investigate whether highly water-soluble organic com- pounds have a high cloud forming potential as suggested in the literature (Silva Santos et al., 2006; Meskhidze and Nenes 2006), and if this could be explained by their solute effect or surface tension effect.

 Study the cloud forming ability of biogenic compounds with a very strong surface reducing effect on water (biosur- factants).

 Show evidences for the presence of biosurfactants in aero- sol samples. This includes finding a suitable extraction technique for these compounds in aerosol samples and de- veloping a suitable analytical technique.

 Quantify the surface tension effect of these biosurfactants on real aerosols.

 Investigate if chemical reactions of organic precursors in

aerosol particles can produce surfactants. As an example

study sunlight-initiated formation of organosulfates and

their effect on the surface tension.

2. Measuring the cloud forming potential

There are two main approaches of measuring the CCN potential of aerosol particles: on-line and off-line. The principles of these ap- proaches are explained below but their common objective is to deter- mine the critical supersaturation (SS

). The SS

is defined as the min- imum amount of water vapor that is required for a dry particle of a certain size and composition to grow into a cloud droplet. This infor- mation is important when predicting cloud formation from aerosol emissions.

2.1 Off-line technique

Kiss and Hansson (2004) developed a new technique which esti- mates the critical supersaturation theoretically by using the Köhler equation and measurements of physical parameters. This technique utilizes water solutions of one or several compounds in a range of concentrations. The osmolality and surface tension are measured for each solution and the water activity is calculated using the osmolality.

Thus the Köhler equation can be used to plot the Köhler curve for an assumed dry particle radius.

The advantages of the osmolality/surface tension approach are that it eliminates problems such as unknown molecular weights, solubility, interactions in the growing droplet, and assumed van’t Hoff factors (determined by the colligative nature of the solutes) of aerosol consti- tuents. The disadvantages are that the method depends on the Köhler equation and that the measured physical properties of large droplets may not be the same for much smaller cloud droplets, as suggested by surface partitioning theory (Bianco and Marmur, 1992).

The overall measurement uncertainty of this method has been es-

timated to 4-7% for the critical supersaturation (Ekström et al.,

2009). Strongly surfactant compounds have a larger relative error due

to their rapid decrease of surface tension as a function of concentration

and the low minimum surface tension. The uncertainty of the water

activity is less than 2 % of errors for up to 1.5 mol kg of solute (Kiss and Hansson, 2004).

2.2 On-line techniques

For online techniques, the ability to act as CCN is usually speci- fied as the potential cloud droplet concentration at a specified water vapor supersaturation, or as a spectrum of potential droplet concentra- tions which depends on the supersaturation. The typical approach is to generate particles of uniform size and composition and expose them to the same supersaturation, varying either the size or the supersaturation gradually. The point where half of the dry particles are activated into cloud droplets is considered to be the critical supersaturation or the critical dry particle diameter, depending on which factor that is varied.

Several different CCN chambers have been developed since the end of the 1950’s each having advantages and drawbacks. Some of the drawbacks are that the CCN instruments only operate above certain supersaturations, are time consuming, have unstable supersaturation, or have a relatively poor accuracy of supersaturation defined by the instrument. Some examples of instrumentation for on-line measure- ments are the chemical diffusion chamber, the static thermal diffusion chamber, the isothermal haze chamber, the diffusion tube, the CCN

“remover”, and the continuous-flow chamber.

The static diffusion cloud chamber (SDCC) initially designed by

Twomey (1963), is comprised of two parallel metal plates having wet-

ted surfaces held at different temperatures. This temperature gradient

results in a supersaturation, which is assumed to be at maximum in the

center between the plates. Other variants of the chambers are the Con-

tinuous Flow Parallel Plate Diffusion Chamber (CFDC) (Sinnarwalla

and Alofs, 1973) with a centered, continuous, particle flow, the Fukuta

continous flow spectrometer (FCNS) (Fukuta and Saxena, 1979) with

the capability of measuring at different supersaturations by particles

following different streamlines, or the Hudson continuous flow spec-

trometer (HCNS) (Hudson, 1989), which operates through increasing

supersaturations along the streamline and estimate the CCN spectrum

from the size of the outlet droplets. This allows quicker measurements

within a wider range of supersaturations (0.01-1%) than the previously

described thermal diffusion instruments, which requires supersatura-

tions above 0.1%.

These on-line instruments have been criticized as having an uncer- tainty caused by kinetic properties of the potential CCN. They subject the particles to a supersaturation during a certain time, which may not correspond to the actual time of exposure in a real cloud. A cloud par- cel model has estimated that the peak supersaturation is around 10 seconds long in a representative cloud (cf. Rogers and Yau, 1989).

Different particles also have different kinetic properties and equili-

brium may not be achieved in the cloud chamber, which can result in

errors in the estimated critical supersaturation (Chuang et al., 1997). A

fairly new continuous-flow instrument (Stratmann et al., 2004) has

been developed in order to mimic realistic cloud conditions, which

would be the atmospherically most relevant to use.

3. Sampling and analysis

3.1 Aerosol sampling sites

In paper II we collected ambient aerosol samples from two conti- nuous measuring stations, SMEAR II, Finland and Aspvreten, Swe- den. Aerosol was also collected during a campaign in the Cuieiras Reserve in Brazil and by using a bubble tank to produce aerosol from water sampled in the Stockholm archipelago in Sweden. The sampling of aerosols in this thesis was performed at four locations, also marked in Figure 3:

Aspvreten, Sweden - a forested coastal area located at 58°47´N 17°23´E, from April to May 2008.

Stockholm Archipelago, Sweden – marine area located at 59°47´5N 19°30´5E, July 2007.

Hyytiälä, Finland – temperate forest located at 61°85´N 24°28´E, July 2008.

Cuieiras reserve, Brazil – tropical forest located at 2°37´N 60°12´W, May 2008.

The filters used for collecting aerosol were baked in an oven dur- ing 12 hours at 500°C for evaporation of impurities before sampling.

All sampled filters were extracted in pure water (Milli-Q) in order to retrieve the water-soluble material only. Some of the extracts were extracted further with a technique that was developed to enrich the specific amphiphilic compounds. Various extraction materials were examined; a bag filled with polystyrene-divinylbenzene, polyethylene, and finally silicone, which was determined to be best suited.

The aerosol samples were extracted in two steps. The first extrac-

tion was with respect to the water soluble fraction by letting the filters

soak in pure water. The water extract was further extracted with thin

silicone material, which absorbs the non-water-soluble part. The com-

pounds taken up by the silicone were then extracted from the silicone

with methanol. The methanol extract should, thus, contain the amphi-

philic compounds from the aerosol. Silicone material has been used

earlier for absorption of organic compounds in water (Valor et al.,

2001). The recovery depends on the polarity of the absorbent of which

less hydrophilic compounds absorb more efficiently than more hydro- philic compounds. The extraction efficiency is also dependent on the thickness of the silicone and the extraction time.

Figure 3: Measurement sites in this work.

3.2 Instrumentation

The equipment that was used in paper I, II and IV to measure the

properties relating to cloud formation was an FTÅ 100 tensiometer

and a K-7000 vapor pressure osmometer. The tensiometer is used to

determine the surface tension of a compound in water solution and the

osmometer measures the temperature difference which depends on the vapor pressure difference between a solution droplet and a pure water droplet. The tensiometer was also used in paper III.

In Paper II a bubble tank was used to produce and collect marine aerosol in a controlled environment. Surface water rich in blue green algae from the Stockholm archipelago was sampled and brought to the laboratory. The water was circulated in a steel tank and particles were produced as the water hit the surface and generated bursting bubbles.

The method has been shown to produce aerosol chemically and physi- cally similar to atmospheric marine aerosol (Mårtensson et al., 2003;

Facchini et al., 2008). A Leckel sampler SEQ47/50, with either PM2.5 or PM10 inlets was used to collect aerosol samples.

For paper II and III tandem mass spectrometry (MSMS) and a Qu- adrupole Time-Of-Flight (Waters, Manchester, United Kingdom) mass spectrometer with an electrospray ion source was used to charac- terize compounds chemically.

In Paper IV we used an atomizer, a Tandem Differential Mobility Analyser (TDMA) coupled with a Cloud Condensation Nucleus Counter (CCNC) equipped with a single growth column which pro- duces a supersaturation through a vertical thermal gradient. An optical particle counter was used in the CCNC for measurements of cloud condensation nuclei.

3.3 Chemical characterization of surfactant molecules

When examining compounds with respect to their cloud forming properties, one must often use standard compounds, but their link to the atmospheric particles is also necessary. The largest problem with atmospheric aerosol samples is that even sampling during several days results in a miniscule amount of material collected on each filter dur- ing clean conditions. So-called Aerosol Mass Spectrometer (AMS) instruments can measure individual particles on-line and determine what each particle consist of, but the AMS is limited to measuring ions and major classes. The organic content is at the most divided into crude sections, such as dividing between oxygenated organic aerosol (OOA) and hydrocarbon-like organic aerosol (HOA).

We chose to use a very powerful method for determining which

organic species a solution contains, hybrid mass spectrometry

(MSMS) with a Quadrupole Time-of-Flight Premier (Waters, Man-

chester, UK) mass spectrometer with an electrospray ion source. The

solutions from extracted filters were either injected directly using a syringe pump or separated on an Acquity BEH C8 column mounted in the UPLC system. In paper III the organic reaction products were se- parated from salt on a 2.1 times 20 mm Sequant Hilic column fitted to the Acquity UPLC (Ultra Performance Liquid Chromatography) sys- tem (Waters, Milford, USA).

The systems drawback is the necessity for off-line measurements of bulk aerosol extracts but by using Electrospray Ionization High Resolution Hybrid Mass Spectrometry it is possible detect complex organic molecules and fragment them down into smaller pieces for determination of their structures. However, the success of the identifi- cation is highly dependent on the availability of reference compounds.

We could obtain some commercially available surfactants but this project relied to a large extent on the contact with a microbiologist who provided valuable expertise and biosurfactant samples.

Numerous standard surfactants were analysed chemically, such as

rhamnolipids, galactolipids (mono- and di-galactosyl diacylglyceride),

lipopeptides, (surfactin and an unknown cyclic lipopeptide extracted

from bacteria), sodium dodecyl sulfate, and a fatty acid (linoleic acid)

to achieve some knowledge about typical patterns and behaviour dur-

ing different instrumental conditions. This knowledge is valuable

when measuring aerosol extracts and reaction solutions looking for

surfactant molecules. The extraction technique to be used prior to the

chemical analysis is also essential. Salt must be removed, the surfac-

tants must be concentrated, and the extraction method must add as

little amount of contaminants as possible.

4. Results

This work investigated the ability of organic compounds in aerosol particles to form clouds, especially the biogenic contribution. The aim was to find natural organic compounds that are potentially good cloud condensation nuclei. We wanted to target compounds from living or- ganisms that have the essential chemical and physical properties for cloud formation, determine how efficient they are as CCN, find them in real aerosol and quantify their effect.

Initially we measured the CCN properties of pure standard com- pounds that are believed to be an oxidation product from isoprene and the sugar compounds polyols emitted by fungi. Isoprene and its prod- ucts have received a lot of attention in previous years due to their quantities and size, which is small enough to be an interesting poten- tial CCN (Kourtchev et al., 2005; Böge et al., 2006). The question still remained whether the oxidation products 2-methylthreitol and 2- methylerythritol had good cloud forming properties. Polyols are high- ly water-soluble compounds which are emitted in large quantities, both in coarse and fine aerosol. (e.g. Graham et al., 2003). This was the first step of investigating compounds with origin from living or- ganisms on earth’s surface and also of using and validating a recently developed technique for determining CCN properties based on mea- surements of the osmolality and the surface tension. (Kiss and Hans- son, 2004).

The following work focused on the CCN properties of a group of

surfactants from bacteria that are exceptionally surface active. No one

had investigated the cloud forming potential of standard biosurfactants

before this work. This potential role of biosurfactants on cloud forma-

tion introduced another possible contribution from living organisms to

climate. We also extracted amphiphilic fractions of aerosol samples

taken from various locations with mainly natural sources in an attempt

to confirm their occurrence in atmospheric aerosol, either directly by

chemical characterization or indirectly by examining the surface activ-

ity of the extracts.

The third paper has a slightly different direction but is also related to cloud formation. We investigated the possible role of sulfates in the chemical transformations of organic compounds. Organosulfates are common in atmospheric aerosols (Maria et al., 2003; Romero and Oehme, 2005; Reemtsma et al., 2006; Gao et al., 2006; Iinuma et al., 2007; Surratt et al., 2007, 2008; Gómez-González et al., 2008; Lukács et al., 2009) and rainwater (Altieri et al., 2009) and might be formed by such transformations. Organosulfates have been measured in sub- micron (<1 µm particle diameter) particles (Maria et al., 2003), which makes them eligible as cloud condensation nuclei. My work consisted mainly in the study of how the surface tension, and thus the cloud forming efficiency, varies with reaction time when organosulfates are formed in the studied chemical reactions.

The forth paper focuses on the biosurfactants that were examined in paper II, this time using a more conventional, on-line technique for measuring the CCN properties, a Cloud Condensation Nuclei Counter (CCNC). We also compare the on-line results with results obtained by offline osmolality/surface tension measurements, this time taking sur- face partitioning into account. The partitioning theory stems from the fact that the physical parameters inserted in the Köhler equation are measured for droplets much larger than cloud droplets.

Short descriptions and main findings of the four studies are pre- sented next.

4.1 Paper I - “The Cloud Condensation Nuclei (CCN) properties of 2-methyltetrols and C3-C6 polyols from osmolality and surface tension measurements”

The focus of this work was to study compounds from natural sources which could be efficient CCN. The selected compounds were polyols (glycerol, erythritol, arabitol, and mannitol), which are sugars emitted by fungi and 2-methyltetrols (2-methylerythritol and 2- methylthreitol), compounds that are believed to be formed as an oxi- dation product of isoprene which is emitted by vegetation. These high- ly water-soluble aerosol particles could consist of up to 5% of the total organic aerosol in forested (e.g. Graham et al., 2003; Decesari et al., 2006; Fuzzi et al., 2007), and marine areas (e.g.Simoneit et al., 2004).

They could potentially have a significant solute effect caused by their

high solubility.

The CCN properties of both the pure compounds and also mix- tures with ammonium sulfate or sodium chloride were examined by determining their effect on osmolality and surface tension. Measure- ments of the CCN properties of organic acids (malonic acid, succinic acid, and adipic acid) were also performed as they have been ex- amined numerous times before by the common technique CCNC (cloud condensation nuclei counter) and serves as a validation that the unconventional technique provides reliable results.

Our measurements gave comparable critical supersaturations to the previous results from CCNC measurements. Adipic acid, which is a more potent surface active compound, showed slightly different re- sults between techniques but also gave different results between dif- ferent comparable equipment. The critical supersaturations of pure polyols and 2-methyltetrols were higher than those of the correspond- ing organic acids. The Raoult effect was estimated by osmolality mea- surements, which is influenced both by dissociation and by electrostat- ic interactions between molecules. Salts like sodium chloride disso- ciate completely while polyols and 2-methyltetrols do not. They do not influence the surface tension much either. This means that the CCN properties of the polyols and 2-methyltetrols are mostly deter- mined by the electrostatic interactions. These were shown to give a minor contribution to the lowering of the critical supersaturation.

The results from the measurements with salt mixtures revealed that the addition of salt resulted in lower critical supersaturation in all cas- es except for methylerythritol. This unusual effect might depend on the fact that the methylerythritol in partly soluble but liquid and not solid as the polyols or dicarboxylic acids. The liquid could thus form a film on the droplet and hinder water uptake.

Our results imply that a compound with high water solubility is not necessarily an efficient CCN and that dissociation is the main in- fluence on the Raoult effect. The 2-methyltetrols and polyols are not expected to contribute significantly to the CCN population in the at- mosphere.

4.2 Paper II - “A possible role of ground-based microorganisms on cloud formation in the atmosphere”

In this work we decided to focus on natural compounds that influ-

ence the Kelvin effect, or surface tension effect. Microorganisms are

known to produce very powerful surfactants that can reduce the sur-

face tension of water from 73 mN/m to below 30 mN/m at extremely low concentrations. Initially, pure standard surfactants; rhamnolipids, glycolipids and a lipopeptide, were investigated to see whether they have good cloud-forming properties by using the same osmolality and tensiometry techniques as in Paper I. Surfactants mixed with small amounts of salt were studied because bacteria normally excrete bio- surfactants into the environment along with water-soluble compounds.

The results showed that these strong surfactants had potential to dis- play better CCN properties than pure inorganic salts even.

The second part was to show that biosurfactants actually exist in atmospheric aerosols. Aerosol samples were taken from Aspvreten, Sweden (forested, coastal region), Hyytiälä, Finland (temperate for- est), the Amazon rainforest in Brazil and a marine sample from the Swedish archipelago during algae bloom season. The amphiphilic fraction of the sampled aerosol was extracted using the technique de- scribed further in section “Sampling and analysis”.

The first attempt was to identify known surfactants chemically by using a UPLC (Ultra Performance Liquid Chromatography) coupled with MSMS (hybrid mass spectrometer) However, neither water ex- tracts of aerosol samples nor silicone extracted fractions of the water extracts yielded high enough concentrations for detection. Unfortu- nately, the majority of the work on chemical characterization was not publishable.

A different approach was taken and the typical surface tension re-

duction signature which biosurfactants give was studied. The extrac-

tion method with silicone was used again and this time the methanol

extract was gently dried with nitrogen gas. After drying, the extracted

compounds were dissolved in purified water and the surface tension

was measured in different dilutions. The results were curves which

resemble the surface tension curves from the standard biosurfactants

and the minimum surface tension reached down below 30 mN/m for

all aerosol samples. Such low surface tension values of aerosol ex-

tracts have never been reported before this work. The fog water sam-

ples from the Po Valley in Italy reduced the surface tension by 30 per-

cent (Facchini et al., 1999) while this study resulted in reductions of

around 60 percent. The volumes of the extracts in our study were larg-

er than the volume of the aerosol, which implies a similar surface ten-

sion effect in the initial aerosol. This indicated a possible new link

between living organisms and climate.

4.3 Paper III - “Radical-initiated formation of organosulfates and surfactants in atmospheric aerosols”

Inorganic and organic compounds often exist mixed together in atmospheric particles. One objective of this study was to test if the organosulfates that have been found in aerosol could be explained by reactions of organic compounds with sulfate radicals in the presence of sunlight. We also wanted to investigate whether the same reactions could produce long-chained organosulfates that could have surfactant properties.

The studied unsaturated organic compounds were isoprene, methyl vinyl ketone, methacrolein, or -pinene. The compounds were mixed with water or concentrated ammonium sulfate. Some of the mixtures were exposed to UV light (280-320 nm) and others were kept in the dark. The products obtained in the different cases were identified us- ing Liquid Chromatography with Quadrupole Time-of-Flight hybrid mass spectrometry (LC/ESI-MS and LC/ESI-MSMS) and compared with organosulfates previously identified in other studies. We con- firmed that exposing sulfate/organic mixtures produced organosulfates of which several have been identified in atmospheric samples. The reaction time was much faster (9 hours) than a previously proposed mechanism which requires 4600 days.

My main contribution in this study was showing how the surface

tension of the solution varied with reaction time. The surface tension

of the bulk liquid was determined for a small portion of the reacting

solution each step of the reaction during several hours by using an

FTÅ tensiometer. The surface tension was shown to be lowered by 25-

35% for isoprene, methyl vinyl ketone and methacrolein in persulfate

exposed to UV light. The surface tension of the solutions not exposed

to UV light or in pure water did not change within the experimental

error. We propose that the surface tension reduction is caused by the

production of long-chained oligomer organosulfates. Their organic

part would be quite hydrophobic and the sulfate part hydrophilic. This

amphiphilic structure is typical for surface tension reducing com-

pounds. -pinene reacting with persulfate, on the other hand, did not

produce lower surface tension. This supports the theory since the -

pinene is less likely to form linear chains as oligomers. The main con-

clusion is that mixed sulfate organic aerosol could have higher CCN

properties when exposed to sunlight than when kept in the dark as a

result of these reactions. This could explain previous atmospheric ob-

servations (Baumgardner et al., 2004).

4.4 Paper IV - “Biosurfactants as CCN: comparison between on-line and off-line measurements”

The objective of this study was to examine the CCN properties of biosurfactants using conventional, on-line techniques. We studied the CCN properties of rhamnolipid and surfactin mixed with 20/50/80 wt%

salt using a Tandem Differential Mobility Analyser (TDMA) coupled with a Cloud Condensation Nucleus Counter (CCNC) to determine cloud forming efficiencies. A mixture of 80 wt% sodium dodecyl sulfate and 20 wt% sodium chloride was also measured for comparison as it is a previously measured strong surfactant. We also compare the results with surface tension and osmolality measurements of the same mixtures ratios, this time with bulk to surface partitioning taken into account. The con- clusion is that the results from surface tension and osmolality mea- surements do not correlate with the CCNC results if partitioning is not taken into account. This would be consistent with the suggestions in the previous paper of Varga et al. (2007).

By using a simple partitioning calculation the osmolality/surface tension technique could be used for the rhamnolipid/salt mixtures for weight percentages of 20 and 50 weight percent biosurfactant and also for surfactin in the 20 weight percent biosurfactant solution. The mix- ture with 80 weight percent rhamnolipid and 20 weight percent so- dium chloride resulted in an overestimation of the calculated partition- ing supersaturation compared with the corresponding measured su- perstation. Rhamnolipid is easy to dissolve in water and a possible explanation for this is an overestimation of the surface partitioning.

The results for the mixtures with 50 and 80 weight percent surfactin indicate poor dissolution since the results resemble pure salt, but there are other plausible reasons for the results.

From CCNC measurements we conclude that particles containing

surfactin or rhamnolipid mixed with sodium chloride do not have bet-

ter CCN potential than pure sodium chloride, although it is still good.

5. Key conclusions

 Organic compounds with high water-solubility are not neces- sarily good cloud condensation nuclei material. In particular, 2-methyltetrols and polyols are not likely to be important cloud forming aerosol particles either pure or mixed with salt.

 The surface tensions of aerosol extracts indicate the presence of biosurfactants in atmospheric particles sampled at several locations.

 The surface tension of aerosol water extracts can be as low as 30 mN/m, while previously lowest reported values were around 50 mN/m.

 Particles consisting of biosurfactant and salt have good cloud forming potential but give different results between two tech- niques.

 Surface partitioning could explain the discrepancy between de- termination of cloud condensation nuclei properties by off-line (osmolality and surface tension) and on-line techniques (Cloud Condensation Nuclei Counter).

 Exposing organic compounds in sulfate salts solutions to sun- light can produce compounds with surface active properties.

Thus, mixed sulfate/organic aerosol could have higher CCN properties after exposure to light than those kept in the dark.

 The surface active compounds are proposed to be long-chained

organosulfates with hydrophilic and hydrophobic parts, similar

to other amphiphilic surfactants.

6. Outlook

Aerosols are very diverse and have complex chemical compositions which can change during their atmospheric lifetime. Also, as noticed in paper I, even two small molecules containing the exact same atoms that only differ in their conformation in space can have different cloud forming abilities. The climate system is very intricate and a cloud model containing all possible details will not be a likely solution.

There is a need to understand more about the climate system and learn more about the details for a more accurate, yet simplified, parameteri- zation in the climate models. Aim to focus on getting the big picture, rule out what are less important, find feedbacks and key parts.

In this thesis we concluded that 2-methyltetrols and polyols are not likely to be important CCN and that high water-solubility does not necessarily mean good cloud-forming efficiency. We also concluded that biosurfactants have a good potential to affect cloud formation but we did not manage to determine which specimen of surfactants that existed in our atmospheric samples and we still do not know their concentration. The organosulfates, which could be produced by solar radiation and biogenic organic compounds, were also shown to be interesting contributors to the potential CCN particles. All these com- pounds are products of nature and will probably have a feedback loop if the temperature, wetness or income of radiation changes.

Questions that would be worth looking further into are:

 Are biosurfactants present in aerosol, in which quantities, to which extent do they influence the surface tension of indi- vidual aerosol particles, and which particle sizes are influ- enced?

 Can we develop extraction techniques for different classes

of surfactants without contaminating the sample? Can we

use tracers to detect surfactant compounds?

 How can we best describe the behaviour of strong surfac- tants in aerosol and their effect on cloud formation in mod- els? For instance, how do we take surface partitioning into account? Would several surfactants mixed together have the same effect on the surface tension as individual ones or will they lose their surface tension reducing capability?

 How will the natural cycles and environmental factors such

as sunlight and wetness affect the amount of surfactant ma-

terial in aerosols, and how much would this influence the

radiative effect of clouds on climate?

Appendix – contribution to the papers

Paper I: The experimental method and the investigated compounds were chosen with directions by Barbara Nozière. I performed the mea- surements and most of the analyses. Barbara Nozière and I wrote the paper together.

Paper II: I chose the studied biosurfactants. The commercially non- available compounds were provided by Malin Hultberg, who I in- itiated collaboration with. The extraction method was investigated by me with advice from Jörgen Magnér. I performed the physical mea- surements, extraction and most of the analyses. The chemical charac- terization of standard surfactants and analyses were made by Tomas Alsberg and me. Barbara Nozière wrote most of the paper.

Paper III: The idea that the paper is based on was presented by Bar- bara Nozière, who also determined the reaction time. I performed the measurements of the surface tension reduction during the reaction. I participated in the chemical analyses but the main contribution was by Tomas Alsberg and also Barbara Nozière. Barbara Nozière wrote the paper with minor comments from me.

Paper IV: I had a leading role in the development of this paper with

guidance mainly from Birgitta Svenningsson but also Barbara No-

zière. I performed all off-line measurements. Cerina Wittbom and I

measured biosurfactant mixtures with the on-line technique. The on-

line measurements of SDS and the calibration of the instruments were

performed by Cerina Wittbom. I calculated the surface partitioning

with essential guidance from Birgitta Svenningsson. The paper was

written by me, with comments from all co-authors.

Acknowledgements

First of all I would like to thank my main supervisor, Barbara No- zière, for being engaged in my work, for always taking the time to guide me, and for giving me an inspiring PhD topic. I would also like to thank my other supervisors, Tomas Alsberg and H-C Hansson, for their support. Tomas, I appreciate your humble kindness and your rock n’ roll t-shirts. H-C, you have the rare quality of knowing intui- tively when I need a pep talk and I value the advice to have given me.

A big thank you to all co-authors. Especially Malin Hultberg, for a treasured collaboration, which probably meant a lot more to us than to you. Cerina Wittbom and Birgitta Svenningsson, you are incredibly friendly and I really enjoyed working with you.

Many thanks to all the colleagues at ITM and Luftlab. Ulla Wi- deqvist, your expertise in the lab has been essential for my work.

Hasse Karlsson, for being the sampling master. Anki Andersson, for patiently answering the same stupid questions over and over again, I wish I was as cool as you. Paul Glantz, for giving me the opportunity to work on things outside the PhD work, it meant a lot to me.

Thanks to the fellow PhD students, and innebandy team members from MISU and ITM that I have met during my four years, especially Johan, Lars, Dan, Johannes, Kim, Anders, Matthias and Nelida.

I am also grateful for the support from Peter Lundberg, Gunilla Svensson, Heiner Körnich, and Henning Rodhe during my time at MISU. Peter, I still eat ketchup, all activists do.

I would also like to mention my fellow union warriors at Saco- SULF. It has been interesting to be a small part of your work. It also led me to the highly therapeutic conversations with Peter Emsheimer.

Thanks to all my dear friends which make life after work so good.

Gödigt, så sett!

Special thanks to my family Maud, Bo, Jessica, Malena; I know you are proud of me even though some of the things I write about are complete Gibberish to you.

Last but not least…Per, thanks for being a part of my life and for encouraging me to follow my dreams, by getting out of bed earlier?!



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