THE FORMATION AND GROWTH OF MARINE AEROSOLS AND THE DEVELOPMENT OF NEW TECHNIQUES FOR THEIR IN-SITU ANALYSIS

Transcription

1 QUEENSLAND UNIVERSITY OF TECHNOLOGY SCHOOL OF PHYSICAL AND CHEMICAL SCIENCES THE FORMATION AND GROWTH OF MARINE AEROSOLS AND THE DEVELOPMENT OF NEW TECHNIQUES FOR THEIR IN-SITU ANALYSIS A THESIS SUBMITTED BY GRAHAM RICHARD JOHNSON TO THE SCHOOL OF PHYSICAL AND CHEMICAL SCIENCES, QUEENSLAND UNIVERSITY OF TECHNOLOGY, IN PARTIAL FULFILMENT OF THE REQUIREMENTS OF THE DEGREE OF DOCTOR OF PHILOSOPHY. July

2 ABSTRACT Marine aerosols have attracted increasing attention over the past 15 years because of their potential significance for global climate modelling. The size distribution of these aerosols extends from super-micrometer sea salt mode particles down through 150 nm accumulation mode particles, 40 nm Aitken mode particles and nucleation mode particles which extend from 25 nm right down to clusters of a few molecules. The process by which the submicrometer modes form and grow and their composition have remained topics of debate throughout this time in large part because of the difficulties associated with determining their composition and relating it to proposed models of the formation process. The work compared the modality of marine aerosol influencing the South-east- Queensland region with that of other environmental aerosols in the region. The aerosol was found to be consistent with marine aerosols observed elsewhere with concentrations below 1000 cm -3 and frequently exhibiting the distinct bimodal structure associated with cloud processing, consisting of an Aitken mode at approximately 40 nm, an accumulation mode in the range nm and a coarse mode attributed to sea salt between 600 and 1200 nm. This work included the development of two new techniques for aerosol research. The first technique measures aerosol density using a combination of aerosol size distribution and gravimetric mass concentration measurements. This technique was used to measure the density of a number of submicrometer aerosols including laboratory generated NaCl aerosol and ambient aerosol. The densities for the laboratory generated aerosols were found to be similar to those for the bulk materials used to produce them. The technique, extended to super-micrometer particle size range may find application in ambient aerosol research where it could be used to discriminate between periods when the aerosol is dominated by NaCl and periods when the density is more representative of crustal material or sulfates. The technique may also prove useful in laboratory or industrial settings for investigating particle density or in case where the composition is known, morphology and porosity. -2-

3 The second technique developed, integrates the existing physicochemical techniques of volatilisation and hygroscopic growth analysis to investigate particle composition in terms of both the volatilisation temperatures of the chemical constituents and their contribution to particle hygroscopic behaviour. The resulting volatilisation and humidification tandem differential mobility analyser or VH-TDMA, has proven to be a valuable research tool which is being used in ongoing research. Findings of investigations relating the composition of the submicrometer marine aerosol modes to candidate models for their formation are presented. Sea salt was not found in the numerically dominant particle type in coastal nucleation mode or marine Aitken and accumulation modes examined on the Southeast Queensland coast during periods where back trajectories indicated marine origin. The work suggests that all three submicrometer modes contain the same four volatile chemical species and an insoluble non-volatile residue. The volatility and hygroscopic behaviours of the particles are consistent with a composition consisting of a core composed of sulfuric acid, ammonium sulfate and an iodine oxide coated with a volatile organic compound. The volume fraction of the sulfuric acid like species in the particles shows a strong dependence on particle size. -3-

4 KEYWORDS Aerosol size distribution, modality, environmental aerosols, marine aerosols, aerosol density, ambient aerosol, VH-TDMA, particle hygroscopic growth, volatility, iodine oxides, non sea salt sulfate, sea salt aerosols, coastal aerosol, marine biota, algae, photolysis, photochemical, thermal decomposition, volatilisation and humidification tandem differential mobility analyser, ultra fine particles. -4-

5 LIST OF PUBLICATIONS Morawska, L.; Thomas, S.; Jamriska, M.; Johnson, G., The modality of particle size distributions of environmental aerosols, Atmospheric Environment, 1999, 33, Morawska, L.; Johnson, G.; Ristovski, Z. D.; Agranovski, V., Relation between particle mass and number for submicrometer airborne particles, Atmospheric Environment, 1999, 33, Johnson, G. R.; Ristovski, Z.; Morawska, L., Method for measuring the hygroscopic behaviour of lower volatility fractions in an internally mixed aerosol, Journal of Aerosol Science, 2004, 35, Johnson, G.; Ristovski, Z.; Morawska, L., Application of the VH-TDMA technique to coastal ambient aerosols, Geophys Res Lett, 2004, 31, L Johnson, G. R.; Ristovski, Z. D.; D Anna, B.; Morawska, L., The hygroscopic behavior of partially volatilized coastal marine aerosols using the VH-TDMA technique, Journal of Geophysical Research, 2005, In Press.. -5-

6 TABLE OF CONTENTS ABSTRACT...2 KEYWORDS...4 LIST OF PUBLICATIONS...5 STATEMENT OF ORIGINAL AUTHORSHIP...7 ACKNOWLEDGEMENTS...8 CHAPTER 1. INTRODUCTION DESCRIPTION OF SCIENTIFIC PROBLEM INVESTIGATED OVERALL AIMS OF THE STUDY SPECIFIC OBJECTIVES OF THE STUDY ACCOUNT OF SCIENTIFIC PROGRESS LINKING THE SCIENTIFIC PAPERS...13 CHAPTER 2. LITERATURE REVIEW INTRODUCTION THE SIGNIFICANCE OF MARINE AEROSOLS FEATURES OF THE MARINE ENVIRONMENT MARINE AEROSOL FORMATION, TRANSFORMATION AND TRANSPORT THE CHARACTERISATION OF MARINE AEROSOLS SUMMARY OF RESEARCH NEEDS RESEARCH NEEDS TO BE ADDRESSED IN THE RESEARCH REFERENCES...53 CHAPTER 3. THE MODALITY OF PARTICLE SIZE DISTRIBUTIONS OF ENVIRONMENTAL AEROSOLS...63 CHAPTER 4. RELATIONSHIP BETWEEN PARTICLE MASS AND NUMBER FOR SUBMICROMETER AIRBORNE PARTICLES...87 CHAPTER 5. METHOD FOR MEASURING THE HYGROSCOPIC BEHAVIOUR OF LOWER VOLATILITY FRACTIONS IN AN INTERNALLY MIXED AEROSOL CHAPTER 6. APPLICATION OF THE VH-TDMA TECHNIQUE TO COASTAL AMBIENT AEROSOLS CHAPTER 7. THE HYGROSCOPIC BEHAVIOUR OF PARTIALLY VOLATILISED COASTAL MARINE AEROSOLS USING THE VH-TDMA CHAPTER 8. GENERAL DISCUSSION INTRODUCTION PRINCIPAL SIGNIFICANCE OF FINDINGS SCIENTIFIC CHALLENGES FUTURE DIRECTIONS

7 STATEMENT OF ORIGINAL AUTHORSHIP The work contained in this thesis has not been previously submitted for a degree or diploma at any other higher education institution. To the best of my knowledge and belief, the thesis contains no material previously published or written by another person except where due reference is made. Signed: Date: -7-

8 ACKNOWLEDGEMENTS My sincere thanks go to the following people and organisations for their support throughout this research: Prof Lidia Morawska, who has been my supervisor, colleague and friend. Lidia has given me the opportunity of employment in this very diverse and interesting field of research and additionally, the freedom support and guidance needed to pursue this PhD program. Dr Zoran Ristovski, also my supervisor, colleague and friend. Zoran has taken an active interest in this work and I have valued very highly his contributions, advice, and friendship throughout my time here. My friends and colleagues from ILAQH for their friendship and assistance over the years. It has been fascinating, and a great pleasure to work with this very diverse group of people. The science workshop staff; Bob Organ, David Pit, and Jim Drysdale. If you can sketch it, they can build it. George Racz, our resident electronics technician for repairs and design ideas, willingly provided at short notice. Stuart Costin, our resident computer technician for all the tweaks, installations and sleuthing. Finally to my wife Narelle, whose love, understanding and enthusiasm made this research possible. -8-

9 CHAPTER 1. INTRODUCTION 1.1 DESCRIPTION OF SCIENTIFIC PROBLEM INVESTIGATED The environmental and health significance of anthropogenic aerosols within a restricted urban area cannot be understood without knowledge of the natural and anthropogenic aerosol sources which surround that area. Each source represents only one part of a complex system involving gases and particles, both anthropogenic and natural. This system is itself a facet of a still more complex atmospheric system in which aerosols influence and respond to weather phenomena and ultimately the global climate as well. The significance of an aerosol source is not determined purely by its intensity but may lie rather in the shear geographical size of the source region. If this region is large enough even a weak particle source can have important influences which warrant their detailed study. The fate of the gaseous and primary particulate emissions from the most intense discrete source depends on their interaction with the diluting medium. In the case of coastal environments the dominant background air mass frequently originates in the marine environment. At a global level the marine environment is the most extensive, covering 71% of the Earths surface. Aerosols from the marine environment tend to have very low concentrations with relatively consistent and well defined size distributions. The question of the production mechanisms responsible for generating these aerosols has been the subject of ongoing and intensive research for at least the past 15 years or so and as yet no firm conclusion has been reached. The widely held view until recently had been that the submicrometer marine aerosol Aitken mode was derived from the growth of nucleating sulfuric acid or sulfate particles in the marine boundary layer. However recent studies have shown that concentrations of precursor gases in this environment are insufficient to explain the growth of such particles to detectable sizes. Progress in understanding submicrometer ambient aerosol processes has been limited in large part by a lack of time resolved, quantitative and particle size specific information about particle composition. Such -9-

10 knowledge is necessary if one is to relate the formation and growth of marine aerosols to any proposed model for their formation. Recently coastal nucleation processes have attracted increasing interest because of the possibility that they illustrate an as yet unknown mechanism by which submicrometer particles can form in the marine environment. A recent study (O'Dowd, Hameri et al. 2002) suggests that these nucleation bursts, which had previously been thought to be an isolated phenomenon may in fact be widespread with global climatic significance. As yet, little is known with any degree of certainty about the extent of the phenomenon, the processes involved or the composition of the resulting particles. Again a major obstacle to progress has been the inability to determine the particle composition due to their small size and the sporadic nature of their occurrence. 1.2 OVERALL AIMS OF THE STUDY Many questions remain to be answered with respect to marine aerosols, encompassing a diverse range of specialisations. Taking into account the needs identified in the literature review, the tools and facilities available, as well as geographical and time constraints, the following aims were identified as being both worthwhile and achievable: 1 To develop techniques which can provide insights into the composition of coastal marine aerosol particles and to apply these techniques to the task to comparing the compositions of marine background and coastal nucleation mode aerosols with candidate chemical species associated with aerosol formation mechanisms proposed in the literature. 2 To identify conditions giving rise to nucleation burst events in the marine environment. 3 To extend existing knowledge of the physical and physicochemical properties of ambient and laboratory generated aerosols. -10-

11 1.3 SPECIFIC OBJECTIVES OF THE STUDY The specific objectives of the study can be summarised as follows: 1 To determine the physical characterisation of the marine aerosol in the south-east Queensland region, identifying the main size distribution modes and their associated particle concentrations. The characterisation is necessary in order to understand the degree of similarity between the physical characteristics of local marine aerosols and marine aerosols reported elsewhere. The physical processes governing particle growth are common to all aerosols so an understanding of the dominant particle size distribution modes and concentrations gives significant insights into the aerosols history. The resulting knowledge is also an important first step in determining requirements in terms of particle size and concentration to be addressed in the development of any prospective compositional characterisation technique. This objective is addressed within three papers. The physical description of the marine background aerosol in terms of the modality of its size distribution is described and compared with other natural and anthropogenic environmental aerosols in CHAPTER 3 (Morawska, Thomas et al. 1999). Details of the particle concentration and modality for both the background marine and coastal nucleation mode aerosols are further described in CHAPTER 6 (Johnson, Ristovski et al. 2004) and CHAPTER 7 (Johnson, Ristovski et al. 2005). 2 To develop a tool capable of comparing the composition of the nucleation, Aitken and accumulation modes of the marine aerosol to compositions expected for aerosols produced by the most prominent mechanisms for their formation found in the literature including gas to particle phase conversion mechanisms involving sulfates, volatile organic compounds and iodine oxides and primary particle production through bubble burst aerosolisation and soot formation in combustion processes. Two techniques identified as having good potential for development are density determination using a combination of size distribution and mass concentration measurements and TDMA techniques combining volatilisation and hygroscopic growth. -11-

12 2.1 The first technique developed, derives estimates of average particle density from particle size distribution and mass concentration measurements taken across a common size range. This technique is examined in CHAPTER 4 (Morawska, Johnson et al. 1999). 2.2 The second technique developed involves the simultaneous real time thermovolumetric and hygroscopic analysis of high resolution size fractionated ambient aerosols. The resulting instrumental system is called the volatilisation and humidification tandem differential mobility analyser (VH-TDMA) and it is described and demonstrated with laboratory generated aerosols in CHAPTER 5 (Johnson, Ristovski et al. 2004). 3 To apply the developed tool to the task of assessing the ability of the proposed particle formation mechanisms to explain the composition of the various modes of marine aerosol as observed from the south east Queensland coast during conditions where the air parcel is representative of the marine environment. This objective is addressed in two chapters. 3.1 The first is CHAPTER 6 (Johnson, Ristovski et al. 2004). This paper contains the first published demonstration of the new VH-TDMA technique for ambient aerosols. 3.2 The second is CHAPTER 7 (Johnson, Ristovski et al. 2004). This paper gives the detailed physicochemical speciation of the marine background aerosols Aitken and Accumulation modes and of coastal marine nucleation mode aerosol. -12-

13 1.4 ACCOUNT OF SCIENTIFIC PROGRESS LINKING THE SCIENTIFIC PAPERS Measurement of the local marine aerosol size characteristics has been carried out (Morawska, Thomas et al. 1999). The results were found to be consistent with similar measurements from a number of locations around the world. These measurements also confirmed the frequently distinct bimodal appearance of the marine boundary layer submicrometer aerosol size distribution. Several somewhat different mechanisms have been described in the literature in attempts to explain such results. Although these mechanisms can under the right conditions produce the observed physical characteristics, different particle compositions are predicted. For example accumulation mode particles may achieve their position in the aerosol size distribution by growing in size through sulfate accumulation during the cloud processing of smaller particles which were themselves formed through sulfate nucleation or alternatively through the bubble burst aerosolization of seawater. The resulting particles may have the same size distribution but will differ in composition. This highlights the need for techniques, which can distinguish between particles of different composition particularly with respect to small constituent fractions which may reflect the origins of the seeding particle. Of the various physicochemical characterisation techniques investigated, the density and VH-TDMA techniques proved to be the most suitable for ambient aerosol characterisation. The density characterisation technique does however suffer from a number of inherent limitations which make it unsuitable for the size resolved characterisations required. In particular the technique lacks the sensitivity needed for investigations involving the very low mass concentrations found in submicrometer marine aerosols. A further technique based on the continuous mass distribution monitor (Ristovski, Morawska et al. 1998) having the potential to measure aerosol density size distribution on a continuous basis through simultaneously measurement of the aerosol mass and volume size distribution was also found to lack the necessary sensitivity for ambient aerosol investigations. -13-

14 The VH-TDMA technique proved to be more than capable of the high temporal and size resolutions required for the investigations providing meaningful results even at the lowest aerosol concentrations encountered in the marine environment. Subject to careful interpretation of the results, the system was found to be capable of discriminating between the various chemical species in marine aerosols. In the first published ambient aerosol analyses obtained using a VH-TDMA system (Johnson, Ristovski et al. 2004), the instrument provided compositional data for ambient aerosols in urban and marine environments. Several physicochemically distinct species were resolved and their contribution to the particle volume was determined. Comparison of the characteristic behaviour of these components with laboratory generated candidate aerosols based on the various available models for marine aerosol formation was used to arrive at a quantitative compositional description of the particles. The last of the papers to be presented in this thesis discusses the application of the technique to the examination of the marine background aerosol and coastal nucleation mode aerosol. The experiment was performed at a time when the aerosols modelled back trajectory and characteristics clearly demonstrate that the air sample was of marine origin. The VH-TDMA of the nucleation Aitken and accumulation modes allowed the elimination of several candidate aerosol formation models which would otherwise be considered strong contenders for the formation mechanism while providing significant support for others. -14-

15 CHAPTER 2. LITERATURE REVIEW 2.1 INTRODUCTION The term marine aerosol primarily refers to aerosols for which the important features, including number, area and mass distribution as well as the particle composition and particle structure are assumed to be predominantly a result of the air parcels history in the marine environment. This classification may be extended to include coastal marine aerosols where the influence of the surf-line, aerosol formation from river outfall affected marine areas and the diurnal recirculation of air between land and sea areas may have a significant influence. Ambient aerosols including marine aerosols and coastal marine aerosols are further distinguished as being either remote or anthropogenically influenced (polluted). In practise the extent to which an air parcel is attributed to the marine environment is decided on the basis of its back trajectory (its historical path) and certain characteristics which are symptomatic of continental or anthropogenic influence in aerosols with long histories over remote marine areas. For example, an aerosol with a back trajectory showing in excess of 72 hours over the remote marine environment and having a submicrometer number concentration of the order of 1000 cm -3 or less would be described as a marine aerosol. Much larger concentrations would suggest unknown processes or persisting anthropogenic influences being preserved by unusual meteorological conditions. 2.2 THE SIGNIFICANCE OF MARINE AEROSOLS An examination of the literature concerning marine aerosols reveals that their significance ranges from local public health to global climate research. While the major focus for public health policy concerning ambient aerosols lies in the control of anthropogenic contributions, the significance of those contributions must be considered in the context of the existing natural background aerosols which determine the minimum achievable aerosol exposure. In coastal cities, the marine environment is -15-

16 a major natural source of background aerosols. In less polluted suburban areas of Brisbane for example, sea salt contributes 10% of the aerosol mass for particles with aerodynamic diameters less than 2.5 μm (PM2.5) and about 25% (the largest single source) of the aerosol mass for particles with aerodynamic diameter less than 10 μm (PM10) (Chan, Simpson et al. 1999). Background aerosol particles are also known to act as sites for the formation of new aerosol material (Kleeman and Cass 1999; Kiss, Varga-Puchovny et al. 1998; Zhuang, Chan et al. 1999). Table 2-1 shows estimates of the contribution of various aerosol sources to the finer (diameter<2.5 μm) and coarser (diameter>2.5 μm) fractions of aerosol in a suburban area of Brisbane from the work of Chan et al (Chan, Simpson et al. 1999). These figures show that after anthropogenic primary particle emissions the main source of aerosol mass is secondary aerosol formation, followed by marine aerosols (including sea salt) and crustal matter. Table 2-1 Comparison of contribution of sources to fine and coarse aerosols (Chan, Simpson et al. 1999) If urban primary aerosol emission levels fall strongly as a result of emission control policies, background aerosols will become the main site of secondary aerosol formation. Background aerosols would then strongly influence the resulting size distribution of semi-volatile materials including toxic organic species (Kleeman and Cass 1999). Understanding the health and visibility effects of shifting this material to -16-

17 a different particle size range will require consideration when forming future policies regarding gas and particle emission controls. In addition to sea salt, marine aerosols are believed to include a variety of other substances including sulfates (formed in the air from dimethyl sulfide gas released by phytoplankton and other marine organisms), organic materials and micro-organisms. The size distribution, composition, hygroscopicity and particle shape and structure of the pre-existing aerosol are all critical factors in determining the particle size distribution after gas to particle conversion processes have occurred. It is therefore necessary to establish these properties for marine aerosols. Marine aerosol generation mechanisms also concentrate and redisperse pollutants deposited in the oceans. Sea foam, formed from natural and man-made surfactants when entrained air bubbles rise to the ocean surface, concentrates a range of materials including persistent organic substances, heavy metals and bacteria from sewage outfalls from very large volumes of seawater into a thin surface film (Oppo, Bellandi et al. 1999; MacIntyre 1972; Blanchard 1963; Blanchard 1975). This material is then aerosolised by bursting bubbles and can be carried by the sea breeze to populated areas. For example, Mycobacterium intercellulare is a pathogenic bacterium known to survive well in seawater and to be enriched by factors of several hundred in the aerosols formed from bubble bursting. This bacterium is known to cause pulmonary disease, and correlations have been observed between immune reaction to this bacterium and proximity to coastal areas where the bacterium exists in the seawater and can be isolated from ambient aerosols (Blanchard 1983; Gruft, Falkinham III et al. 1981; Gruft, Katz et al. 1975). Atmospheric aerosols also have climatic influences through their ability to scatter and absorb radiation and to affect the lifetime and optical properties of cloud (Charlson et al., 1987). Aerosols influence cloud droplet size and therefore play a significant role in precipitation and cloud albedo (Twomey 1974; Porch, Borys et al. 1999; Khain, Pokrovsky et al. 1999). Cloud albedo and that of the aerosol itself are major considerations in climate modelling and are thus highly relevant in the global warming debate (Twomey 1974; Charlson, Lovelock et al. 1987; Russell, Livingston et al. 1999; Seinfeld and Pandis 1998; Coakley, Cess et al. 1983). Since marine areas -17-

18 account for most of the globes cloud cover, marine aerosols are highly significant in determining planetary albedo. Marine aerosols also link ozone depletion to global warming as increased UV-B radiation levels due to ozone depletion are expected to increase marine sulfate aerosol formation with a resultant cooling effect on the troposphere (Kulmala 1999; Pirjola 1999). Kulmala et al (Kulmala, Vehkamaki et al. 2004) has presented a global review of ultrafine atmospheric particle observations. Few observations of coastal or marine aerosol size distribution have been carried out in the southern hemisphere and most of these were conducted in the southern ocean or on the Antarctic coastline. Observations of in-situ measurements at different points on the globe, of background aerosol characteristics such as particle size, composition and concentration are needed to allow the extraction of useful information about global aerosol behaviour from remote sensing satellites data (Tanre, Remer et al. 1999). 2.3 FEATURES OF THE MARINE ENVIRONMENT The region of the atmosphere that is of most interest in the study of ambient aerosols is the troposphere, which is the layer closest to the Earths surface containing 80% of the atmospheres mass and most of the atmospheric moisture and latent heat. The troposphere is characterised by a negative temperature gradient, and extends up to a boundary region known as the tropopause at an altitude of 15 km over the equator and 10km over the poles. The troposphere is well mixed by convection processes, thunderstorms and mid-latitude weather systems. Above the tropopause lies the relatively unmixed stratosphere. The temperature gradient in the lower portion of the stratosphere is zero, but it becomes positive at higher altitudes due to the absorption of short wavelength radiation by ozone. Over the oceans, the troposphere contains a thermal inversion layer, which when cloud forms coincides with a layer of stratus and stratocumulus cloud known as the marine stratus deck. This inversion layer acts as a partition between the underlying air, known as the marine boundary layer (MBL) and the overlying, main portion of the troposphere, referred to as the free troposphere (FT). The marine boundary layer is -18-

19 a more mixed region of higher humidity extending up to about 1300m (Raes, Van Dingenen et al. 1997). Various authors examining the interaction of cloud with marine aerosols, have made use of estimates of average cloud occurrence in the marine boundary layer (Pandis, Russell et al. 1994; Hoppel and Frick, 1990). Hoppel and Frick for example, assume that a parcel of air in the marine boundary layer passes through nonprecipitating clouds on average before encountering a precipitating cloud which can remove particles from the atmosphere. Mixing of aerosol from the free troposphere into the marine boundary layer may be responsible for some apparent inconsistencies in observations of marine boundary layer aerosols. For example the Aitken peak frequently observed in the marine boundary layer is assumed to result from the growth through condensation of newly formed particles, however the assumed formation process is only rarely observed. The free troposphere has been promoted as a possible source of these particles, with their appearance in the marine boundary layer being attributed to entrainment from the free troposphere (Gregory, et al., 1996; Raes, 1995; Berresheim et al., 1990). Modelling of the effects of mixing between the free troposphere and marine boundary layer requires knowledge of entrainment velocities of air in the free troposphere moving into the marine boundary layer. Estimates have put this global average downward velocity at around 0.4 cm s -1 (Clarke et al., 1996; Huebert, et al., 1996; Kritz, 1983). Inversion layer formation and atmospheric stability can strongly influence ambient aerosol concentrations in the marine boundary layer by partitioning the atmosphere and suppressing interaction between the marine boundary layer and free tropospheric air masses. For example the entrainment of free troposphere air into the marine boundary layer is expected to be more pronounced under cloudy as opposed to clear conditions (Boers and Betts, 1988). Weather systems such as fronts, highs and lows have the capacity to displace preexisting air masses. This can make it difficult to distinguish between aerosol changes caused by air mass displacement and the cleansing effects of precipitation scavenging. -19-

20 According to Remer et al, the day to day variation of coastal aerosol size distribution is due to meteorological variability rather than from source strength variation. The aerosol is particularly sensitive to variations in the residence time for the air mass and the duration of stagnant conditions (Remer, Kaufman et al. 1999). Mixing processes in the ocean itself may also play a significant role in marine aerosol formation by helping to bring surface-active material into contact with air. This material then becomes concentrated on the ocean surface where it can participate in aerosol formation. Alternating left and right helical vortices known as Langmuir circulations are generated in the ocean surface water by the action of the wind. The axes of these circulations lie parallel to the direction of the wind and the depth of the circulation can be in excess to 20 m. The vortices bring surface active materials to the surface and compress them into visible parallel windrows or slicks where adjacent circulations converge (Blanchard 1983). Particle Size Distribution Most literature concerning marine aerosols under conditions of minimum anthropogenic influence, reports number concentrations of between 200 and 1000 cm - 3 (Russell, Huebert et al. 1996; Fitzgerald 1991; Hoppel, Fitzgerald et al. 1990; Hoppel and Frick 1990; Gras and Ayers 1983; Andreae, Andreae et al. 1994). The sub-micrometer portion of the size distribution of such aerosol is found to be consistently bimodal (having two peaks) in most reports, with one peak in the range 0.02 to 0.1 μm and another in the range 0.1 to 0.5 μm (Morawska, Thomas et al. 1999). The modal diameters for the two peaks in these distributions are usually reported to be around 0.06 and 0.2 μm (Fitzgerald, 1991). This marine boundary layer aerosol size distribution is remarkably robust in the face of ever changing conditions (Katoshevski, Nenes and Seinfeld 1999). An ultra-fine mode (diameter<0.02 μm) has also been observed in the (Wiedensohler et al., 1996; Covert et al., 1996), and near Tasmania (Fitzgerald, 1991; Jaenicke, 1979) although similar measurements in the same location by other researchers (Gras and Ayers, 1983) do not show such a peak. -20-

21 Table 2-2 summarises some findings on submicrometer marine aerosol mode positions reported in the literature. Table 2-2 Some reported diameters for marine aerosol modes in the submicrometer size range. Location Ultrafine Aitken Accum- Number Reference Mode Diameter (μm) Mode Diameter (μm) ulation Mode Diameter (μm) Conc (cm -3 ) Canary Raes, et al Islands North Russell et al., 1996 Atlantic 1000 Atlantic Hoppel et al., 1990 Tasmania Gras and Ayers, 1983 Arctic <20 nm * <100 Wiedensohler et al., 1996; Covert et al., 1996 * focus here was below the size range of most studies The extent to which the differences in such findings are due to variations in sampling location, meteorological conditions, or different errors inherent in the various techniques used is uncertain. Investigations of the vertical distribution of sea salt aerosol show that the concentration changes by less than a factor of three from a height of 15 m to 100 m (Fitzgerald, 1991; Blanchard et al., 1984; Warneck, 1998). Humidity influences particle size, particularly for strongly hygroscopic particles, and airborne measurements in the remote marine boundary layer have shown strong correlations between fluctuations in particle volume, effective diameter, and light scattering coefficients with changes in relative humidity (Baumgardner and Clarke, 1998). -21-

22 Particle Compositional Size Distribution Marine boundary layer aerosols are often categorised as either sea-salt derived or nonsea-salt derived, depending on similarities between their composition and that of sea salt. The non-sea-salt aerosol is widely thought to be composed of low volatility gas phase reaction products such as sulfuric acid and ammonium sulfate (Hoppel and Frick, 1990; Hoppel et al., 1990; Charlson et al., 1987; Gras and Ayers, 1983). Some authors have however found a sea-salt core surrounded by sulfate while others have found a predominance of organic mater. O Dowd et al (O Dowd et al., 2002c) have found evidence that newly formed coastal marine aerosol particles may be composed largely of condensable iodine species and in particular iodine oxides and that the production of these particles appears likely to be widespread along Atlantic European coastlines. The coarse mode particles (diameter>0.5 μm) are composed of sea salt (Gras and Ayers, 1983; Blanchard, and Woodcock, 1957) as well as natural surfactants (Oppo, et al. 1999; Blanchard, 1975) and small quantities of Si and other elements which may be of continental origin (O'Dowd et al., 2004; Hoppel et al., 1990). A comparison of compositions given by various authors is summarised in Table 2-3. In order to be able to understand the global effects of atmospheric aerosols more information is needed on diurnal and spatial variations in concentration and composition as well as a better understanding of the formation and growth mechanisms involved (Kulmala 1999). -22-

23 Table 2-3 Marine aerosol composition according to various authors Location Composition Reference Ultrafine Mode d<0.02 μm Aitken Mode 0.03 d μm Accumulation Mode 0.1 d 0.4 μm Coarse Mode d>0.5 μm Canary Islands not sea salt not sea salt Raes, et al North Atlantic H 2 SO 4, MSA, (NH 4 ) 2 SO 4 H 2 SO 4, MSA, (NH 4 ) 2 SO 4 sea salt Hoppel et al., 1990 North Atlantic Southern Ocean Macquarie Island Not Sea salt, Sea salt: mostly resolved sulfate, organic matter 5-73%, sulfate: 10-20%, organic matter: 15-60% sea salt MSA, ammonium bisulfate O'Dowd et al., 2004 O'Dowd et al 1997 mainly H 2 SO 4 or MSA, sea salt sea salt Kreidenweis et al organic matter, sea salt sea salt Oppo, et al Tasmania (NH 4 ) 2 SO 4, H 2 SO 4 sea salt Gras and Ayers, 1983 Tropical ammonium bisulfate, MSA Andreae et south Atlantic al., 1995 Southern Ocean sea salt sea salt Clarke

24 2.4 MARINE AEROSOL FORMATION, TRANSFORMATION AND TRANSPORT. Formation Mechanisms Fitzgerald claims that the available evidence shows that the marine boundary layer background aerosol is probably of marine origin rather than being transported aged continental aerosol. This implies that there is an in situ source of small particles, which allows the recovery of the normal background level after losses during precipitation (Fitzgerald, 1991). As discussed previously, proposed sources of the marine boundary layer aerosols include the entrainment of aerosol from the free troposphere, sea spray evaporation, and the homogenous nucleation of vapour molecules. The resulting aerosols may then be modified both physically and chemically by a variety of mechanisms. The evaporation of the water from the airborne water droplets created by the action of wind on the ocean might be expected to leave an aerosol composed of the salts and other materials originally contained in the seawater. Raes et al, however, have found that wind speed based estimates of sea spray particle production give less that 10% of the number concentrations observed for marine aerosols (Raes, et al. 1997). The number concentration is dominated by sulfate aerosol under almost all conditions, accounting for 89% with no wind and 69% even for wind speeds of 17 m s -1 (Katoshevski, Nenes and Seinfeld 1999). Mass concentration however is dominated by sea salt particles contributing 62% with no wind and 98% at 17 m.s -1. Reasonable correlation was observed by Bates et al, between the total number of dried coarse (number mean diameter 0.54 ± 0.07 μm at 10% relative humidity) mode particles (almost all were sea salt) and local wind speed averaged over six hours (Bates et al., 1998). Berg et al, did not observe correlation between the number of 0.15 μm sea salt particles with local wind speed (Berg et al., 1998b). O Dowd and Smith, however, observed strong correlation down to 0.05 μm (O'Dowd and Smith, 1993). -24-

25 It appears likely that a wind velocity correlated mechanism such as sea spray evaporation, generates a small number of large particles, which usually dominate the mass distribution. The spray evaporation mechanism probably creates predominantly sea salt particles with the inclusion of some surfactant organic matter. The tail of the particle number distribution peak produced by this mechanism may extend to submicrometer particles as small as 0.05 μm. If this is true the smaller particles are frequently masked by the formation of other particulate matter (eg sulfate) formation processes either onto the existing particles or as new particles. The mechanisms producing these other particles appear to be unrelated to wind conditions. Breaking Wave Aerosol Generation and Sea Spray Evaporation Aerosols attributed to sea spray evaporation include sea salt and natural surfactant particles. The coarse mode particles (diameter > 0.5 μm) produced by this mechanisms are composed primarily of sea-salt, but also include significant amounts of biologically derived material. Sea spray droplets small enough to remain aloft and evaporate are formed through breaking wave aerosolisation. Breaking wave aerosolisation results from the evaporation of droplets ejected from the sea surface when air entrained by a breaking wave reaches the water surface in the form of bubbles. The formation of droplets by bursting bubbles has been studied by Blanchard and Cipriano (Blanchard 1983; Cipriano et al., 1982; Blanchard, 1975; Cipriano and Blanchard, 1981). As the bubbles burst, three main types of droplets are formed: Jet Droplets: Jet drops are only formed for bubbles of less than about 6mm diameter. The diameter, ejection height and the number of jet drops formed per bubble is related to bubble diameter. For bubbles less than 2 mm in diameter, jet drop diameters are approximately one-tenth the bubble diameter. The ejection height for these droplets is about 100 times the bubble diameter (Blanchard 1983). The number of droplets increases to a maximum of about seven per bubble as bubble diameter falls. -25-

26 Film Droplets: Film drops are formed from bubbles larger than 0.3 mm in diameter. Bubbles with diameters of 6mm produce about 1000 film droplets and numbers increase rapidly with bubble size. The film droplet size distribution has a peak at 5 μm and for larger bubbles, has a long tail extending to beyond 30 μm (Blanchard 1983). Shearing droplets: Strong winds (15-20 m/s) shear water directly from the wave crest forming shearing drops, which are very large and have a very short lifespan. Because of their large size the droplets fall back to the ocean surface before they evaporate and don t usually participate in aerosol formation. A breaking wave mixes entrained air to a depth of several meters. The bursting of the resulting small bubbles as they reach the surface favours jet droplet formation (Blanchard 1983). Whitecaps, which are the main source of bubbles in the ocean, are formed when the wind speed exceeds 3.5 m/s. The prolonged collapse of white caps in front of a wave creates large bubbles, which preferentially form film drops. Two to five (~ 50 μm diameter) jet droplets and a large number of submicrometer film droplets for each bubble burst are ejected to an altitude of less than 2 cm. Wania citing Waldichuk states that at any time whitecaps cover 3-4% of the ocean surface resulting in an estimated 0.1 jet droplets.cm 2.s -1 and 0.07 surface film droplets.cm -2.s -1 (Wania et al., 1998). Other bubble sources include oxygen supersaturation through photosynthesis, and methane bubbles from anaerobic decomposition and rainfall. Temperature and pressure changes due to mixing processes can also result in supersaturation and bubble production. Moderate rain (or snow) fall on the sea surface produces similar numbers of bubbles per unit area to that of whitecaps (Wania et al., 1998; Blanchard and Woodcock, 1957; Green and Houk, 1979). The Role of Surfactant Organic Matter in Sea Spray Aerosolisation: The rotational motion of a wave mixes surface water and entrained air to several meters below the surface and during high winds, turbulent mixing and Langmuir -26-

27 circulations can carry the resultant bubbles to depths as great as 20m (Blanchard 1983). The extent of this mixing allows organic material dispersed in very large volumes of seawater to come into contact an air/water interface. Several authors have referred to a significant organic matter content in the submicrometer fraction of remote marine aerosol. The volume of organic matter in sea salt aerosol has been found to be between 20 and 50% of volume of sea salt. This level of enrichment corresponds to an enrichment factor for organic material in total marine aerosol (ie cumulative across all sizes) of between 7000 and times that found in bulk seawater (Oppo, et al. 1999). These organic constituents may even cause the formation of submicrometer marine sulfate aerosol. Leck et al, suggest that liquid organic particles produced by bubble bursting are chemically transformed into a substance capable of promoting sulfate nucleation, leading to the predominance of sulfate usually found in the submicrometer marine aerosol fraction (Leck et al., 1999). Bubble formation is known to concentrate organic matter through its adsorption onto air water interfaces, and the presence of surface-active matter enhances this adsorption process in rough seas (MacIntyre, 1972; Blanchard, 1963; Blanchard, 1975). In the laboratory, non-foaming bubble enrichment can extract more than 95% of surfactants from water for concentrations below 10-2 ppm into a volume 1/200 of the original sample volume if the process is not degraded by the presence of colloidal and particulate matter (Loglio, 1981). The breaking wave appears to act as a surfactant pumping system. Air entrained in the wave produces bubbles several metres below the surface. As these bubbles rise surfactant organic matter diffusing to the bubble surface becomes attached and is swept down to the bottom surface by the drag forces associated with the relative motion of the water (Blanchard, 1975). The amount of organic matter concentrated around the lower bubble surface increases with bubble speed and with time. The jet drops preferentially created when these bubbles reach the surface and collapse eject a portion of this surfactant material into -27-

28 the air while the rest is left on the surface to participate in the formation of the larger foam bubbles at the face of whitecaps. This process can increase the surfactant concentration in the surface and sub-surface water involved in the wave motion by several orders of magnitude over that in calm conditions (Oppo, et al. 1999). Small quantities of surfactants tend to spread until they form a mono-layer (approximately one surfactant molecule thick) of the order of 10 nm thick at air water interfaces, so droplets of all sizes are likely to have surface films of similar thickness. The concentration of surfactant in the water droplet should therefore increase as the droplet size decreases becoming much higher than that in bulk sea-water. The saturation surface film thickness for dry humic substances can be in excess of 10 nm (Hunter and Liss, 1981). Therefore, the surfactant organic matter enrichment ratio, defined as the ratio of the film volume to the overall droplet volume, should remain near zero until the droplet radius approaches 10 nm at which point it should begin to rise rapidly, reaching 100% at a radius of about 10 nm. Such humic substances have been found to make up 40-60% of the marine surfactant organic matter. Another smaller component in surfactant organic matter is fatty acid, which has been shown to form a saturated film thickness of 2 nm (Gabrielli et al., 1989). Droplets of the order of 10nm are therefore expected to be composed almost entirely of surfactant organic matter and those materials, which have an affinity for surfactant organic matter (Oppo, et al. 1999). Oppo et al, have measured surfactant organic matter enrichment in marine aerosols using Na concentration as a measure of the sea water content of aerosol and characteristic fluorescence behaviour as a measure of surfactant organic matter content. Fluorescence spectra show that for an excitation wavelength of 308nm the maximum fluorescence intensity for the fluorescent-surfactant organic matter in seawater is in the range nm which is characteristic of humic (cell decomposition products) substances (Oppo, et al. 1999; Cini, 1993). According to Hunter and Liss, this fluorescent component is the most important fraction of soluble surfactant organic matter (Hunter and Liss, 1981). -28-

29 Size fractionated bulk marine aerosol samples examined by Oppo et al show that the enrichment of fluorescent-surfactant organic matter increases rapidly with decreasing diameter, at much larger diameters ( 0.5 μm) than expected ( 0.01 μm) (Oppo, et al. 1999). They suggested that this result may be due to the presence in the seawater component of the larger droplets, of colloidal micelles (diameter<0.45 μm) composed of humic substances (Note, this is also the diameter region at which sea salt starts to be overwhelmed by sulfate). Other supporting evidence for the Oppo at al model is the similar enrichment of materials (Cu, Cd, Pb, Mn, Fe, Al and many organic compounds) in seawater which have a demonstrated affinity for surfactant organic matter (Piotrowicz, 1979). This affinity is due to a large number of functional groups (Mantoura, 1981) in the structure of humic substances. For example surfactant organic matter also contains fatty acids and lipids which form soaps with metal ions (Hunter and Liss, 1981). The soaps of bivalent ions such as Ca and heavy metals are insoluble, and form solid films. Oppo et al, attribute the differing enrichment for the various heavy metals to the varied binding capacities of different components in fluorescent-surfactant organic matter as reported by Mantoura (Mantoura, 1981). K, Mg, Ca and Na are used as a marine signature in aerosol studies (Oppo, et al. 1999). Many studies report a K and Ca excess however, and attribute it to non-marine origins such as crustal Ca (Hoffman, 1980; Chesselet, 1972). Oppo at al conducted lab measurements comparing aerosols generated from artificial seawater, natural seawater and a mixture of gas-bubble enriched seawater with untreated seawater which showed the same level of surfactant organic matter enrichment as found in seawater during high whitecap coverage. They found that the enrichment for K, Ca (and zero enrichment for Mg) was similar to that found in marine aerosol. These findings suggest that the greater K and Ca concentrations are actually a result of marine surfactant organic matter interaction. Oppo et al, attribute this to the fatty acids in soluble surfactant organic matter interacting differently with various metals according to their valance, and to the fact that the lower molecular weight and greater surface activity of fatty acids leads to their greater enrichment in the smallest marine -29-

30 aerosol particles (Oppo, et al. 1999). They also conclude that the presence of heavy metals in marine aerosols results not from direct crustal contributions but from the recycling of such material after its deposition on the sea surface. There is an unexpected enrichment at 7μm in the Oppo data for the heavy elements, which also appears for fluorescent-surfactant organic matter enrichment. Enrichment of K and Ca at 3 and 7μm is also observed. Oppo et al, suggest the Ca enrichment may be due to bacteria in the larger drops. They suggest that the much greater enrichment of the heavy metals is due to the much different concentration of K, Ca, and Mg in seawater compared to that of the other metals (much lower) and to the type of interactions between the elements and fluorescent-surfactant organic matter (Oppo, et al. 1999). Re-aerosolisation of Persistent Organic Pollutants: Elevated levels of persistent organic pollutants such as poly-chlorinated biphenyls (PCBs) and organo-chlorine pesticides have been found in organisms feeding in remote marine waters, indicating the global presence of these compounds in the marine environment (Wania et al., 1998). In coastal areas, riverine inflows, direct and atmospheric deposition are higher due to the proximity of the largely land based sources of theses pollutants. Wania citing Sodergren states that persistent organic pollutants have been shown to accumulate in the organic surface film formed by amphiphilic organic compounds (those with hydrophobic and hydrophilic parts) (Wania et al., 1998). Seasonal dependency of these effects is likely, due to the fact that most of the sea surface microlayer is of biological origin. Wania, et al citing Schwarzenbach et al, state that persistent organic pollutants have limited affinity for water and thus tend to associate with particulate organic carbon in the ocean by dissolution into a porous organic matrix (rather than through surface adsorption) (Wania et al., 1998). Wania, claims that persistent organic pollutants may also sorb into dissolved organic carbon such as humic substances and fatty acids (Wania et al., 1998). The dissolved organic carbon holds together organic -30-