Single Particle Analysis of Aerosols

Single Particle Analysis of Aerosols, Observed in the Marine Boundary Layer during the Monterey Area Ship Tracks Experiment (MAST), with Respect to Cloud Droplet Formation

L. A. DE BOCK 1,P.E.JOOS1, K. J. NOONE 2, R. A. POCKALNY 3 and R. E. VAN GRIEKEN 1 1Department of Chemistry, University of Antwerp (UIA), B-2610 Antwerp, Belgium 2Department of Meteorology, Stockholm University, S-106 91 Stockholm, Sweden 3Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, U.S.A.

(Received: 10 September 1998; accepted: 12 April 2000) Abstract. The chemical composition of individual particles >0.2 µm sampled during the MAST- experiment were analysed by SEM-EDX, in combination with multivariate techniques. The objective of this experiment was to identify the mechanisms responsible for the modification of marine stratocumulus clouds by emissions from ships and in a wider sense to provide information on the global processes involved in atmospheric modification of cloud albedo. Aerosols were examined under different MBL pollution levels (clean, intermediately polluted and moderately polluted) in five different reservoirs: background below-cloud and above-cloud aerosol; background cloud droplet residual particles; below-cloud ship plume aerosol and ship track cloud droplet residual particles. In this study a relation was provided between the aerosol emitted from the ship’s stack to an effect in cloud. Additionally, a large fraction of the ambient aerosol was found to be composed of organic material or other compounds, consisting of low Z-elements, associated with chlorine. Their number fraction was largest in clean marine boundary layers, and decreased with increasing pollution levels. The fraction of ‘transformed sea salt’ (Na, Cl, S), on the other hand, increased with the pollution level in the MBL. Only 20% of the particles fell within the detectable range of the analysis.

Key words: aerosols, cloud formation, microanalysis, ship tracks, climate change.

1. Introduction

Chemical and physical processes acting in clouds are very complex, since clouds are multiphase systems in which gaseous compounds, interstitial aerosol particles and liquid droplets coexist. All of the processes occurring in clouds are mutually connected and proceed simultaneously. Among the different existing cloud types, marine stratocumulus clouds seem to play a crucial role in the control of the radiative budget of the earth’s atmosphere. By reflecting incoming solar radiation they are able to reduce the earth’s surface temperature. Anthropogenic activity may 300 L. A. DE BOCK ET AL. influence cloud reflectivity (albedo), perturbing the radiative balance of the earth (Kim and Cess, 1993). The objective of the Monterey Area Ship Tracks Experiment (MAST), which took place in June 1994 off the coast of central California, U.S.A., was to acquire a better understanding of the mechanisms responsible for the modification of marine stratocumulus clouds by emissions from ships (Durkee et al., 1997). Ever since the launching of the first TIROS satellites, the anomalous cloud lines (or ship tracks) in satellite images have been suspected to be induced by additional aerosol particles produced by ships (Conover, 1966; Twomey, 1968). Moreover, recent evidence for this correlation was provided by King et al. (1993), Radke et al. (1989), Platnick and Twomey (1994), Ferek et al. (1997) and Hindman and Bodowski (1996). The overall objective of the MAST project was to provide information on the processes involved in anthropogenic modification of cloud albedo. For a long time the characterisation of aerosols and cloud droplets in the cloud multiphase systems was approached, like several other environmental topics, by bulk analysis techniques. The single particle analysis approach (SPA) is presently recognised as a powerful tool to reveal detailed information, inaccessible by bulk techniques, concerning the particle origin, formation, reactivity, transformation reactions and their environmental impact (De Bock et al., 1996). In this study, scanning electron microscopy in combination with energy dispersive X-ray analysis (SEM-EDX, one of the most commonly used micro-analytical techniques in SPA) was applied to determine the composition and size of individual cloud droplet residual particles and ship track and ship plume aerosol particles, with diameters down to 0.2 µm. Droplet residual particles are those particles remaining after cloud droplets have been sampled and evaporated. Compositional differences found between particles collected in these different reservoirs will help to link the aerosol produced by ships to that present in clouds, and to identify the processes by which ship tracks are formed and become recognisable on satellite images. SEM-EDX was considered to be a particularly ap- propriate technique since the instrument is strongly computerised and automated, offering the possibility to analyse several hundreds of particles in a few hours time. In combination with multivariate techniques, the analysed particles can be assigned to specific sources and classified in different types from which the abundances can reveal the source strength.

2. Experimental

2.1. SAMPLING As part of the MAST sampling campaign, 12 research flights with the University of Washington’s C131 aircraft were flown off the coast of central California, U.S.A. In addition to its normal payload, the aircraft was equipped with a number of additional instruments to investigate the nature of the particulate and gaseous emissions from ships and the chemistry and microphysics of clouds. Particle measurements included a continuous-flow cloud condensation nuclei (CCN) spectrometer (Hud- SINGLE PARTICLE ANALYSIS OF AEROSOLS 301 son, 1989), to investigate CCN properties; a radial differential mobility analyser (Zhang et al., 1995) to determine aerosol size distributions, and a counterflow virtual impactor (CVI; Noone et al., 1988; Orgen et al., 1985) to collect residual aerosol particles. This paper will discuss the chemical analysis of particles sampled with the CVI. The chemical composition of droplet residual particles will reflect the composition of the particles on which the cloud droplets actually formed, plus any non-volatile material produced in the droplets during their lifetime in the cloud. By comparing the composition of cloud droplet residual particles with the composition of particles in (a) the background below-cloud aerosol, (b) the ship plume aerosol, and (c) the aerosol above the subsidence inversion, we can determine the contribution of the aerosol in each of these reservoirs to the number of cloud droplets formed in the clouds we sampled. Perturbations in cloud albedo are dependent on the number of additional cloud droplets formed; SPA gives us a particularly powerful tool to determine the source of these additional droplets, since it is a number-based analysis technique. The major disadvantage of the technique in this regard is that the smallest particle which can be analysed is 0.2 µm in size. Öström et al. (1997) stated in a study of residual particle distributions observed in clouds forming in boundary layers of varying degrees of pollution, that typically, only 20% of the number of residual particles found in the cloud droplets are larger than 0.2 µm. While the composition of the particles controlling cloud droplet number can not be definitively determined with SPA in this case, the relative contribution of various different particle classes to the droplet population can nonetheless be determined. It is known that for the small particles, i.e., smaller than 0.2 µm, sulfate is very important. However, these particles are, as said before, not measurable with SPA. Another unavoidable limitation in this work is that highly volatile particles might disappear in the vacuum of the SEM-instrument.

2.2. INSTRUMENTATION – ANALYSIS A total of 71 aerosol samples were analysed using a JEOL JSM 6300 SEM (JEOL, Tokyo, Japan). The instrument is equipped with a PGT (Princeton Gamma Tech, Princeton, NJ, U.S.A.) energy dispersive X-ray detector. The PA6300 program was applied for the automated analysis of 500 individual particles per sample at an acceleration voltage of 20 kV and a beam current of 1 nA. This number of particles is statistically (Poisson distribution) sufficient enough to get a reasonable view about the presence of a certain class of compounds. The average total particle number concentration during exactly the time of the sampling for single particle analysis is not known, but the total particle concentration results during the MAST experiment are discussed in Noone et al. (2000). For every sample, 500 particles are analysed; a number concentration of detected particles cannot be given. The X-ray accumulation time was fixed at 20 s. When the PA6300 program is run, 302 L. A. DE BOCK ET AL. the area of the sample to be analysed is subdivided in different fields and the particle analysis starts with the collection of a backscattered electron image of the field. Based on this image, the area, perimeter and diameter of the particles in the field are calculated and compared with a selection criterion. The X-ray spectrum is accumulated by rastering the electron beam on 40 points within the particle’s contour. Spectrum evaluation is performed by a tophat filter and storage on optical disk of images and spectra allows a re-evaluation of the acquired data. Clear drawbacks of SEM-EDX are the poor detection limits (about 0.1%) and the limitation to detect only elements with Z > 10, due to the presence of a Be- window in front of the Si-Li detector in a conventional SEM instrument. The X-rays of elements, like C, O and N are too low in energy to penetrate this window. Therefore, in SEM-EDX a particle group is classified as organic material, if no characteristic X-rays are collected or, in other words, when the sum of characteristic X-ray peak intensities in this group is very low.

2.3. MULTIVARIATE TECHNIQUES

Reduction of the data set was obtained by performing, on each of the aerosol samples, the cluster analysis (CA) program in the Integrated Data Analysis System (IDAS), a Windows based software package for both CA (Treiger et al., 1995) and Factor analysis (De Bock et al., 1998), producing a classiﬁcation into groups of particles with chemically similar composition. The hierarchical cluster analysis (HCA), used in this technique, starts with n particles or clusters from which the most similar ones are joined successively into new clusters. In this way a tree- like structure, called dendrogram, is obtained. Different hierarchical strategies are possible to join two similar clusters; Ward’s error sum method is the one we applied because it creates a maximum internal homogeneity into the separated groups (Bernard and Van Grieken, 1992). To facilitate the choice of the correct number of clusters, stopping rules can be used in IDAS which indicate at what level the dendrogram should be cut. From the three stopping rules, available in IDAS, the Consistent Akaike Information Criterion (CAIC) (Bozdogan, 1987) revealed the best results. The Davies-Bouldin Index and the Total Error Sum of Squares Cri- terion can not always provide useful information due to their smaller application ﬁeld compared to CAIC and due to the sometimes-complicated character of the experimental data set. Figures 1(a–d) illustrate the application of HCA, in IDAS, on the SEM-EDX data obtained for an ambient cloud droplet residual aerosol sample taken on 27 June in moderately polluted boundary layer clouds. SINGLE PARTICLE ANALYSIS OF AEROSOLS 303

Figure 1a. Dendrogram showing the initial results of HCA for the ambient cloud droplet residual aerosol sample taken on 27 June.

Figure 1b. The Consistent Akaike Information Criterion indicates, by its ﬁrst sharp decline, that the dendrogram should be cut at three clusters. Decline at 3 clusters, indicating a solution with 3 clusters. 304 L. A. DE BOCK ET AL.

Figure 1c. Average composition of the 3 clusters of the ambient cloud droplet residual aerosol sample taken on 27 June.

Figure 1d. Pie chart of the cluster populations of the ambient cloud droplet residual aerosol sample taken on 27 June.

3. Results and Discussion

3.1. DATA STRATIFICATION The data were stratified in two different ways. The primary characterisation is in terms of the ‘reservoir’ in which the samples were obtained. Five different reservoirs will be discussed: (1) background below-cloud aerosol; (2) background cloud droplet residual particles; (3) above-cloud aerosol; (4) below-cloud ship plume SINGLE PARTICLE ANALYSIS OF AEROSOLS 305 aerosol; and (5) ship track cloud droplet residual particles. Within each of these reservoirs (with the exception of the above-cloud cases) the data were stratified into three different levels of pollution in the boundary layer; clean, intermediately polluted and moderately polluted. The purpose of the dual stratification is to exam- ine differences in particle chemistry between both the different reservoirs and the different pollution levels.

3.2. BACKGROUND BELOW-CLOUD The composition of the marine boundary layer (MBL) aerosol, regarding cloud droplet formation, has been the subject of a number of scientific investigations. In 1968 Radke already reported that Na-containing aerosol particles >0.2 µm (dry diameter) in the mid-latitude marine air accounted for only 1% or less of the total CCN concentration and the remaining 99% was surmised to be dominated by excess sulfate particles. A combination of bulk and single particle analysis applied by Andreae et al. (1986) revealed that most cloud droplets in marine settings form on CCN composed of sulfate and only a small fraction of the droplets have sea salt particles as their CCN. The fact that sea salt contributes little or no mass to the fine-mode aerosol in the MBL was also confirmed by Clarke et al. (1987). According to Covert (1988) the chemical form of the fine mode sulfate aerosol resembles ammonium bisulfate, based on the average ammonium to sulfate molar ratio. However, the aerosol composition in general remains strongly dependent on both the origin of the air masses present in the MBL as well as on the prevailing wind speed. A correlation between NaCl particles and wind speed, even down to particle sizes of 0.05 µm, was derived by O’Dowd (1993), which indicated that sea salt can become the primary source of CCN in stratiform clouds in cases of clean air associated with high wind speeds. Classification of the 13 ambient below-cloud aerosol samples, collected during 8 research flights and investigated by single particle SEM-EDX analysis, was done using the criteria described in Öström et al. (1997). The MBL was considered to be clean (C), intermediately polluted (IP) and moderately polluted (MP) at condensation nuclei (CN) concentrations below 300 cm−3, between 300–900 cm−3 and in the range of 900–1600 cm−3, respectively. HCA results of the data are presented in Table I. Although the three samples taken under ‘clean’ conditions appear to be quite different in composition (Figure 2(a)), we tend to believe that the chemical composition of a ‘clean marine boundary layer’ is probably best represented by the aerosol sample collected on 29 June. Sampling on that day was characterised by an anticyclone, positioned over the Central Pacific and inducing a northerly airflow with a surface wind speed of 6–9 m/s and cloud-level wind speeds of 11–14 m/s. The total boundary layer particle concentration was around 50–200 cm−3 (Öström et al., 1997). Some 77% of the analysed particles in this sample appeared to be pure sea salt, consistent with O’Dowd et al. (1993). According to their calculations, 306 L. A. DE BOCK ET AL. m) µ m) (mean diameter µ (0.9) 1(Si,S,K) m) (mean diameter µ HCA results of the ambient below cloud aerosol particle samples m) (mean diameter Table I. µ NaCl elements S NaCl Transf. C.P. No Cl-rich Cl-rich S-rich K, Cl Si-rich S, Ca, Cl Ambientbelow cloudaerosol Marine % material abundance (mean diameter 6/8Intermediate (0.6)6/12 Organic materialClean (0.6) % abundance 446/15 (0.6)Moderate 176/15 (0.4)Moderate 37 11 (0.4)6/15 (0.6)Moderate 30 57 (0.4) (0.7)6/15 (0.4) (0.4)Moderate 37 44 21 (0.4) (0.4)6/21 (0.5)Moderate 27 54 6 (0.4) Si-rich 26 (0.5) particles 36/21 (0.4) (0.4)Moderate 28 50 25 % abundance (0.3) (0.4)6/27 (0.5)Moderate 9 74 Mixed 20 particles (0.3)6/28 5 (0.5) (0.5)Intermediate (0.5) 50 9 % 8 abundance 27 (0.4) (0.5) (0.6)6/29 (0.4) 38 (0.6) 49 (0.5) 7 (0.3)6/29 (0.5) (0.7) 8Clean 49 (0.3) 77 106/30 18 14 Intermediate 22 8 56 (0.3) 23 (0.4) (0.4) 23 (1.0) (0.5) 49 (0.4) (0.4) (0.6) 3 (0.3) 19 9 (0.3) 17 5 3 37 (0.4) (0.4) (0.9) (0.3) 11 27 2 4 (0.8) (0.4) 1 (S, Si, Al) 2 Clean (0.3) (0.3) SINGLE PARTICLE ANALYSIS OF AEROSOLS 307

Figure 2. (a)–(c): Particle types encountered in the MBL under different pollution levels. 308 L. A. DE BOCK ET AL. the concentration of NaCl between 0.1–0.15 µm radius increases from 1.4 cm−3 to 15 cm−3 at wind speeds of 5 m/s and 15 m/s, respectively, and the ratio of the concentrations of NaCl to non sea salt (Nss) sulfate aerosol particles grows from 0.7 to 3. Moreover, in the O’Dowd et al. (1993) results, Nss sulfate particles seemed to become only wind speed dependent for larger particle sizes and showed a heterogeneous chemical composition. Sievering et al. (1991) ascribed this het- erogeneity to the rapid transferring of SO2 to the sea salt aerosol by condensation on to its surface, where it becomes subsequently oxidised to sulfate. The latter process, although more significant in the larger particle size ranges, could explain the 22% abundance of NaCl particles containing low concentrations of S and described in Table I as ‘Transformed NaCl’ on 29 June. The other sample classified as ‘clean’ reveals besides sea salt and transformed sea salt particles also a number of particles (9%) identified as marine crystallisation products (C.P. in Table I) which are produced by the crystallisation (crystallisation occurs in the laboratory during storage and due to the low pressure in the microscope) of successive salts (CaCO3, dolomite, CaSO4,NaCl...)upon evaporation of sea water drops (Bochert, 1965). The remaining particles, characterised in this sample (11%), seemed to be composed of organic or other low-Z material, exclusively associated with a characteristic Cl-peak in the X-ray spectrum. However, we have no further evidence that these particles are in fact truly organic or whether they are elemental carbon or other low Z-elements. These low Z-elements can not be measured with the micro- analytical technique used here, since the low energy X-rays of these elements are totally absorbed by the beryllium window of the detector. However, recent pub- lications on the abundance of organohalogen compounds in nature (Asplund and Grimval, 1991; Gribble, 1992; Gribble, 1994, Murphy et al., 1998) illustrate that organic compounds, and more specifically organohalogen compounds, like methyl chloride, can be present in reasonable amounts. Moreover, these studies emphasise clearly that the concentration of these compounds in the atmosphere exceeds the amount that can be explained by anthropogenic emissions. Marine life can be classified, according to Gribble (1992), as the largest source of these naturally occurring organohalogen components. If this is the case, the abundance of these ‘chlorine-rich organic particles’ increased even up to 74% in the case of the ‘clean’ sample collected on 12 June. During that time, the operation area was influenced by a low pressure system with surface wind speeds of 13–18 m/s from the north. Under these meteorological conditions however, we would expect a predominance of pure sea salt particles, which makes the occurrence of this presumed Cl-rich organic particle type even more interesting. As such, one can state that these particles can be in fact ammonium chloride, which is not measurable with SEM-EDX as well. However, since the number of individual organohalogen compounds in marine organisms was estimated to be at least 1500 and that only a small number of marine organisms have been investigated in terms of their chemical content, including the knowledge that different species seem to produce organochloride components, one can expect a positive correlation with organochlorides present in marine air. SINGLE PARTICLE ANALYSIS OF AEROSOLS 309

Since more than 80 plants are already known to produce halometabolites (Engvild, 1986) and laboratory investigations by Asplund and Grimval (1991) demonstrated clearly the production of organohalogens by spruce trees, plants can probably also be considered as contributors to the organohalogen budget of the atmosphere. Estimates on the production of CH3Cl by a macroalgae (giant kelp) revealed a flux of 2,000 tons/year (Manley and Dastoor, 1987), with a total of 4.9 million tons/year by the ocean in general (Isidorov, 1990). Other estimates are lower, namely 0.3 million tons/year. Organic material can, on the other hand also be present as surface-active organic molecules constituting a film on the particle’s surface (Gill and Graedel, 1983). Evidence for the transport of micro-components ‘via marine aerosols’ was also supplied by Cinni et al. (1996), who investigated sea salt and organic micro-components in Antarctic snow. Another possibility is that Cl-radicals react with organic compounds, e.g., polycyclic aromatic hydrocarbons (PAHs). Russel et al. (1997) found during the MAST experiment high concentrations of benzo(e)pyrene and benzo(k)fluoranthene in the background boundary layer, present in the marine atmosphere, giving organohalogen compounds. According to Varotsos (1994) these chlorine atoms are formed at sunrise from ClNO2(g), one of the reaction products of N2O5(g) with NaCl(s) in sea salt particles. Such reactions between gases and solid interfaces appear to be important in polluted marine areas as well as in remote ones like the Arctic. By all means, this problem has to be evaluated further. Therefore, if we talk about organic compounds further in this article, we have to take in mind that this is only a possibility and that also other low Z-elements are possible candidates for assignment. One possibility is the reaction products between HCl and ammonia or amines, which also will fell in this category, when analyzed with conventional SEM. Although the number of samples characterising an ‘intermediately polluted’ MBL is again limited (Figure 2(b)), the differences between their compositions is not so significant as in the case of the ‘clean’ MBL. The samples collected on 8 June and 30 June appear quite similar in chemical particle composition which is consistent with the results obtained for the aerosol number concentrations and prevailing wind speed conditions (Öström et al., 1997). The fact that a slightly higher level of pollution is associated with a decrease in pure sea salt particle abundance in comparison with the ‘clean’ sample collected on 29 June (77% NaCl) is clearly seen. The abundance of transformed sea salt particles remains more or less constant, except for the sample taken on 28 June where it increased up to 49%. The MBL on 28 June was indeed characterised by higher aerosol number concentrations, compared to 8 and 30 June, and was therefore more strongly influenced by polluted air masses containing either aged sea salt particles or elevated levels of SO2 which could easily condense and be oxidized on sea salt particles, as discussed above. The majority of below-cloud aerosol samples were taken under ‘moderately polluted’ conditions. The results presented in Table I and graphically illustrated in Figure 2(c) indicate a strong similarity between the samples with regard to the marine material. Therefore the moderately polluted marine boundary layer aerosol 310 L. A. DE BOCK ET AL. larger than 0.2 µm can be characterised by marine material containing on the average: 40% pure sea salt, 30% transformed sea salt and 10% marine crystallisation products.

3.3. BACKGROUND CLOUD RESIDUALS The characterisation of the chemical composition of the residual or cloud droplet nuclei (CDN) in the ambient cloud is of great importance to identify the sources of cloud forming aerosol. Additionally the fraction of aerosol scavenged into cloud droplets in the ambient cloud will provide information on the susceptibility of the ambient cloud to ship track formation. Classification of 23 ambient cloud residual samples, analysed by SEM-EDX, was done according to Öström et al. (1997) and based on the nearest measured out-of-cloud condensation nuclei (CN) concentration. Table II(a–b) presents the HCA results of these data. It must be noted here that, as with the below-cloud aerosol, only 20% by number of the cloud droplet residual particles were sufficiently large to have been analysed by the SPA technique. If in the text is referred to the fractions of droplets forming on particles of various different compositions, the text implicitly refers to those droplets forming on particles larger than 0.2 µm diameter, which represented at most 20% of the total number of droplets in the clouds. Taking a closer look at the ‘clean’ ambient cloud residual data, graphically represented in Figure 3(a), we notice that cloud droplets were mainly formed on pure sea salt particles and on Cl-rich particles in combination with low Z-elements (organic material, ammonium, ...).Bothparticle types were detected in almost all of the samples, although their abundances varied, depending on the sample. Average values of 26 17% for sea salt and 38 22% for supposed organic cloud droplet residuals were found. Considering the marine versus the low-Z component, an ‘anti-correlation’ seems to exist, or in other words a decrease in marine material abundance appears to be associated with an increase in the abundance of low-Z material and vice versa. The presence of organic particles or other low-Z particles as CDN would imply that these particles are hydrophilic or water-soluble. Little information is known however on the molecular composition of the water-soluble fraction, although a critical review by Saxena and Hildemann (1996) hypothesizes that this fraction constitutes a substantial part of the total organic mass and in- cludes C2 to C7 multifunctional compounds like diacids, polyols, aminoacids and other oxygenated multifunctional groups. Aerosol particles composed of slightly soluble organic compounds or provided with a slightly soluble organic surface coating, may delay or suppress cloud formation (Bigg, 1986; Shulmann, 1996) and are therefore less probable to be detected in cloud residual aerosol samples. The fact that water solubility is clearly related to the ability of particles to serve as CCN was confirmed by Russel et al. (1997). The activation index for a PAH compound, determined by the ratio of the frequency with which this compound was detected in-cloud over out-of-cloud during the MAST sampling campaign, SINGLE PARTICLE ANALYSIS OF AEROSOLS 311 m) µ (0.4) m) (mean diameter µ m) (mean diameter µ HCA results of the ambient cloud residual samples Table IIa. m) (mean diameter µ NaCl elements S Cl NaCl Transf. C.P. No Cl-rich Cl-rich S-rich K, Cl Si-rich Fe-rich S, Ca, Cl Ambientcloudresiduals Marine material (mean % diameter abundance6/8Intermediate (0.5)6/11 Organic materialModerate (0.5) 326/11 (0.4) (0.4) %Moderate abundance 32 18 (0.5)6/12 (0.5)Clean 53 20 11 (0.4) (0.5)6/12Clean 70 22 12 (0.3) (0.5)6/12 (0.4) Clean 39 10 0.5)6/21Moderate 20 (0.6) Si-rich 37 particles6/21 (0.6)Moderate 57 13 (0.5) (0.4)6/21 (0.4) %Moderate abundance 24 54 Mixed particles (0.6) (0.4)6/21 (0.4)Moderate 33 42 13 (0.4) (0.3)6/27 (0.4) (0.3) 41Moderate % 50 abundance 37 12 (0.6) 2 (0.3) (0.4) (0.5) (0.4) 36 (0.4) 18 43 (0.7) 6 (0.4) (0.3) (0.5) 1 66 19 (0.9) 10 7 0.3) 11 1 3 (0.6) (0.4) 18 (0.5) (0.4) 10 (0.4) (0.5) 11 11 35 25 (0.8) 9 (0.8) (0.7) 1 (0.3) 1 1 2 (0.4) 1 312 L. A. DE BOCK ET AL. m) µ (0.4) (0.3) 3 2 m) (mean diameter µ (0.4) (0.9) 4 4 m) (mean diameter µ HCA results of the ambient cloud residual samples Table IIb. m) (mean diameter µ NaCl elements S Cl NaCl Transf. C.P. No Cl-rich Cl-rich S-rich K, Cl Si-rich Fe-rich S, Ca, Cl Ambientcloudresiduals Marine material (mean % diameter abundance6/27Moderate6/28 Organic (0.4) material 42 (0.4) % abundance 33 65 31 (0.4) Si-rich particles 25 % abundance Mixed particles % abundance Intermediate (0.4)6/28Intermediate (0.4) (0.4)6/28Intermediate (0.4) 56 (0.4)6/28Intermediate 40 (0.5) 41 (0.4)6/29 (0.4)Clean 27 (0.6) 246/29Clean 9 (0.7) 38 9 (0.3)6/29Clean (0.3) 20 (0.5)6/29 (0.4) 4 (0.3)Clean 49 16 (0.3) 10 (0.4)6/30 (0.4) (0.3)Intermediate 31 14.2 11 19 (0.3) (0.5) (0.4)6/30 (0.4)Intermediate (0.4) 9 (0.4) 58 41 (0.4)6/30 (0.3) (0.5) Intermediate 30 0.4) 47 (0.3) 23 25 9 (0.4) 1 35 (0.3) (0.4) 44 11 (0.4) (0.4) 67 30 66 18 (0.3) 14 (0.3) 14 (0.4) (0.4) 2 21 (1.2) (0.3) 6 (0.3) 1 2 SINGLE PARTICLE ANALYSIS OF AEROSOLS 313

Figure 3. (a)–(c): Particle types encountered in the ambient cloud under different pollution levels. 314 L. A. DE BOCK ET AL. appeared to be higher for higher water soluble compounds and lower for less water soluble compounds. On the other hand, individual cloud droplets can also contain an organic surface film as a result of the scavenging of organic vapours in-cloud. This increases however the lifetime of the cloud droplets by inhibition of water vapour evaporation and reduces the efficiency of compound scavenging from the surrounding air (Gill et al., 1983). Whether this coating will remain on the residual particles, after evaporation of the cloud droplets in the CVI depends on the volatility of the organic film material. Whether or not it is here the case, the fact that organic aerosols can indeed dominate the CCN concentration and in this way also the CDN concentration, was confirmed by Rivera-Carpio et al. (1996). Comparing the composition of the ambient cloud residual particles collected in clean and intermediately polluted clouds (Figure 3(a) versus Figure 3(b)), we observe an increase in sea salt and transformed sea salt abundance for the IP clouds, with average values changing from 2617% to 4614% and 1012% to 2810% for NaCl and transformed NaCl, respectively. Although their abundances varied, both particle types became present in each of the IP samples. On the other hand, the contribution of Cl-rich low-Z particles to the total particle abundance in IP clouds seems to decrease and they could now only be detected in 50% of the IP samples. Crystallisation products, which occur more frequently in IP clouds, develop into the third component of the MP cloud droplet nuclei (Figure 3(c)). In addition, the abundance of transformed sea salt particles increases even more under MP conditions and this mainly at the expense of the sea salt particles. This could again indicate that the MP clouds were subjected to air masses containing either elevated levels of aged sea salt particles or higher concentrations of SO2 which condensed on pure sea salt particles. Finally, the diversity in particle types declines from 10 to 7 types, when going from clean to MP clouds. The SPA results from the ambient cloud residual samples of particles larger than 0.2 µm diameter indicate that, under clean conditions, cloud droplets that grow on these particles are mainly formed on pure sea salt and Cl-rich organic and/or low-Z containing particles and their abundances seem to be anti-correlated. The higher abundance of transformed sea salt detected in CDN under IP and MP conditions matches with a similar tendency found in the MBL.

3.4. AMBIENT ABOVE-CLOUD AEROSOL

Ambient above-cloud aerosol particles were sampled during the MAST experiment to determine their impact, as a result of a possible scavenging into cloud droplets, on the chemical composition of the ambient cloud residual particles. The HCA results of ten ambient above-cloud aerosol samples, analysed by single particle SEM-EDX analysis, are shown in Table III and visually represented in Figure 4. The samples were only classiﬁed according to the day of collection. Since our main concern lies in identifying the chemical composition of those particles rather than SINGLE PARTICLE ANALYSIS OF AEROSOLS 315 m) µ (0.5) 7 m) (mean diameter µ m) (mean diameter µ (0.4) (0.4) (0.4) (0.4) (0.4) (0.4) (0.3) (0.3) (0.8) elements HCA results of the ambient above cloud aerosol samples Table III. (0.5) (0.4) (0.4) (0.5) (0.3) (0.3) m) (mean diameter µ (0.5) (0.5) (0.5) (0.4)(0.5) (0.3) (0.6) (1.6) (0.4) (0.3) (0.7) (0.3) (0.8) (0.3) (0.4) (0.6) (0.4) (0.4) 6/27 14 10 66 10 6/276/28 116/286/28 17 9 17 24 3 33 13 81 9 80 50 41 11 2 6/29 78 21 1 6/30 22 78 6/21 2 26 73 Ambientabove Marine materialcloudaerosol % abundance (mean diameter NaCl6/12 Na, Cl, K Ca, K. Cl K, Cl, S6/21 No 1 Organic material % Cl-rich abundance K, Cl S-rich 13 K-rich Si, Al-rich Aluminosilicate particles 24 9 Anthropogenic Fe-rich % abundance Fe, Cr, Ni 45 91 % abundance 316 L. A. DE BOCK ET AL.

Figure 4. Particle types encountered above cloud. the particle’s origin, a further classification revealing the history of the air masses above-cloud did not seem strictly necessary. In general, the ambient above-cloud aerosol samples appeared to be mainly composed of K, Cl-rich organic particles as well as pure organic material and/or other exclusively low-Z containing compounds (Figure 4). Marine particles are less abundant and if present, they seem to occur associated with K. Whether these particles really did affect the chemical composition of the CDN can only be evaluated by comparing, for each sampling period, the chemical composition of the three reservoirs: the MBL, the ambient cloud and above the ambient cloud. Taking a closer look, we surmise that the high abundance of K, Cl-rich organic material in the above-cloud area (91%) on 12 June, could possibly be responsible for some of the K, Cl-rich abundance found in the CDN. However 14% of this particle type was also present in the MBL. On the other hand, the Cl-rich containing low-Z material detected in the CDN is more likely to have been supplied by the MBL. Although the composition of the two above-cloud samples taken on 21 June is not completely similar, part of its pure organic material or other low-Z containing compounds could have been incorporated into the cloud droplets. The presumed Cl-rich organic material in the CDN could originate from both reservoirs, while the marine material probably found its origin in the MBL. K, Cl-rich organic particles or particles, containing only low-Z elements from the above-cloud sample could neither be detected in the CDN nor in the MBL. This was also the case for the remaining seven above-cloud samples, which could be classified in two groups. The first group, containing the above-cloud samples taken on 27 and 28 June, could have been responsible for the abundance of pure and Cl-rich organic material or other low-Z compounds in the CDN, whereas the MBL mainly supplied particles identified as marine material. The second group, composed of the above-cloud samples collected on 29 and 30 June, seemed to have no influence on the ambient SINGLE PARTICLE ANALYSIS OF AEROSOLS 317 cloud residual composition, except perhaps for some marine material which was associated with K. The MBL aerosol clearly determined the CDN composition on these days. Based on our SPA data we find no evidence for a substantial contribution of above-cloud particles to the composition of CDN. However it seems that if K, Cl- rich organic material and/or other exclusively low-Z containing compounds are present in the CDN, it will find its origin in the above cloud area rather than in the MBL, while the Cl-rich and pure organic material could have been supplied by both reservoirs. Marine material, as would be expected, was provided by the MBL.

3.5. SHIP PLUMES

The different stages involved in plume development and the basic aerosol phe- nomena occurring in plumes, generated by point sources, were recently discussed by Kerminen and Wexler (1995). In the MAST experiment, ship plume particles, gases, heat and water vapour, were sampled to investigate whether they were responsible for new cloud droplet formation and the radiative features of ship tracks. Sampling in ship plumes during the experiment proved to be difficult due to the relatively short amount of time spent in the plume, and the difficulties in avoiding plume edges. The composition as well as the size of the sub-micrometer aerosol particles produced by the ships stack seemed to depend clearly on the type of fuel burned by the ships (Hobbs et al., 1997 and Noone et al., 2000). An obvious example is provided by the nuclear-powered ships, which emit primarily heat and relatively few combustion-derived particles. In general, plume particles were mainly identified as organic, possibly associated with sulfuric acid produced by gas-to-particle conversion of SO2 and contained only 10% of water soluble material (Hobbs et al., 1997). The presence of organic material in plume particles was confirmed by the measurements of Russel et al. (1997). They detected several PAH compounds to be present in the plume particles from which four, namely pyrene, benzo(a)fluorene, benzo(b)fluorene and 9,10-anthracenedione exceeded the ambient concentrations. The HCA results of the single particle analysis data, obtained by automated SEM-EDX analysis of eight ship plume samples, are shown in Table IV. Each sample is described by the name of the ship, which emitted the plume sample, the type of engine, the day of collection and the degree of pollution of the background MBL. The composition of the background pollution should be taken into account to clarify which particles are actually produced by the ships stack. The only plume sample collected in a clean MBL was produced by the stack of a commercial ship, the Star Livorno (Table IV, Figure 5(a)). This ship was equipped with a diesel engine and ran on low grade fuel (Hobbs et al., 1997). Based on the composition of the ambient below-cloud aerosol particles on the same day, 29 June in Table I, we have to conclude that the sample is primarily characterised by the 318 L. A. DE BOCK ET AL. m) µ (0.5) 8 31 69 40 40 21 (0.4) (0.4) (0.4) (0.4) (0.6) m) (mean diameter µ HCA results of the ship plume aerosol samples Table IV. m) (mean diameter µ NaCl elements S 61 32 7 61 22 17 69 2069 8 2910 3 2 1 87 2 55 25 13 % abundance(mean diameter % abundance % abundance NaCl Transf. C.P. No Cl-rich Cl-rich S-rich K, Cl K, Cl, P K, Cl, S Si-rich Diesel 6/29 Clean NYK Sunrise Diesel 6/30 Intermediate U.S.S. Copeland Gas turbine6/30 Intermediate (0.4) (0.4) (0.3) Diesel6/27 Moderate U.S.S. Mt. Vernon Steam turbine6/27 Moderate (0.5)Star Livorno (0.5) (0.4) (0.6) (0.4) (0.5) Ship plumes Marine materialU.S.S. Abe Lincoln Nuclear6/15 Moderate U.S.S. Abe Lincoln Organic materialNuclear6/15 Moderate (0.5)U.S.S. Kansas (0.7) City Steam turbine6/21 Moderate (0.5) (0.4)Tai (0.5) he (0.4) (0.7) (0.4) (0.5) (0.4) Si-rich particles (0.3) SINGLE PARTICLE ANALYSIS OF AEROSOLS 319

Figure 5. (a)–(c): Particle types encountered in the ship plume under different pollution levels. 320 L. A. DE BOCK ET AL. composition of the ships burned fuel. The analysed plume particles consisted of organic material, associated with K and Cl and K, Cl and P. With respect to the plume sample taken under IP conditions (Table IV, Fig- ure 5(b)) on 30 June from the NYK Sunrise, with comparable ship characteristics, similar particle types could be identified. The presence of S probably may be due to the gas-to-particle conversion of SO2, as mentioned earlier. A large increase of S, present as sulfate, compared to the ambient conditions, was confirmed by the IC analysis of the Teflon filter (Hobbs et al., 1997). The 20.6% abundance of Si- rich particles can be attributed to the combustion of diesel fuel, since silicon is used in the distillation cracking process of diesel fuel. The second plume sample of 30 June was collected from the stack of one of the United States navy ships, the U.S.S. Copeland, a gas-turbine powered ship. In contrast with commercial ships, U.S. Navy gas- and steam-turbine as well as diesel ships are not powered by the cheap low grade fuel but by a cleaner distillate fuel, leading to less stack emissions. Noone et al. (2000) show that the majority of the particles emitted from a ship similar to the U.S.S. Copeland were too small to act as CCN, and also too small to have been analysed with the SPA technique used here. The composition of five aerosol plume samples collected from the stack of four different ships under MP conditions is graphically represented in Figure 5(c). The results will again be discussed according to the type of ship engine and combustion fuel. The two aerosol samples taken from the U.S. Navy nuclear powered aircraft carrier the U.S.S. Abraham Lincoln, seemed to be composed of the same particle types as detected in the ambient air samples of the same day (Table I). This is not surprising since nuclear powered ships are expected to emit mainly heat, as discussed above, although the U.S.S. Abraham Lincoln is effectively a floating city and does therefore represent a significant particle source. Nevertheless, the abundance of pure sea salt particles appears to be significantly higher in the plume (69%), compared to its abundance in the MBL (48%). Initially we thought that the increase could be attributed to a possible uplift of sea water from the ships water wake. However Frick and Hoppel (1997) found no convincing evidence of an observable aerosol particle increase over the water wake of several ships. Sea salt particles produced by the bubble breaking in the water wake, can only become important if their number is comparable to the integrated sea salt source strength along the upwind fetch. Therefore, we assume that this increase can be linked to the NaCl aerosol particle dispersion from the sea water, which is used for the cooling of the nuclear engine and emitted along with the steam air wake by the ship stack. The apparently small decrease in transformed sea salt abundance from 31% in the MBL to 25% in the plume as well as the reduction of the marine crystallisation product abundance to sometimes half of its value, is probably due to the strong dominance of pure sea salt particles in the plume. The fact that the relative X-ray intensity of S in the majority of these transformed sea salt particles is rather low, indicates that the coagulation between NaCl and sulfate occurred only partially, or that the capturing of this particle type took place in the early stage of the transformation reaction. SINGLE PARTICLE ANALYSIS OF AEROSOLS 321

The Tai He, classified as a diesel engine commercial ship, burning low grade fuel, revealed in her plume sample mainly particles from the marine boundary layer. The 7.7% Si-rich particle abundance, from which 1% was already encountered in the ambient air, refers again to the distillation cracking process of the diesel fuel, applied for combustion. Organic material associated with K and Cl; K, Cl and P; and K, Cl and S could not be located in this plume sample, despite their dominance in plume samples taken from similar ship types like the Star Livorno and the NYK Sunrise. This may be explained by the fact that sometimes the spatial extent of the plume appeared to be rather small and less predictable in-flight, causing a possible collection of particles in the plume edges, where emissions were more dilute. The two remaining plume samples were both collected from U.S. Navy steam turbine engine ships, the U.S.S. Kansas City and the U.S.S. Mont Vernon, but on different days. Although both used the same distillate fuel, large differences can be observed in their plume compositions. The emissions from the Kansas City were primarily characterised by high abundances of chlorine-rich organic material, largely exceed- ing the abundances detected in the MBL. According to investigations by Cooper et al. (1996) concerning ferry traffic, evidence for the direct emission of Cl-containing organics like polychlorinated biphenyls (PCB) and hexachloro-benzene (HCB) as well as hydrocarbons and PAHs was found for two passenger ferries. Although global PCB emissions in the investigated region appeared to be rather small, up to 1 kg/year, elevated levels of PCBs can be obtained when poorer grade fuels are utilised. However, since we know that the U.S.S. Kansas City mainly runs on distillate fuel, probably only part of the Cl-rich organic material can be attributed to direct emissions similar to the ones found for ferry traffic. The other part of this Cl-rich organic material was presumably supplied by the MBL itself. The plume sample emitted by the U.S.S. Mont Vernon contained three different marine particle types, which were also encountered in the MBL with rather similar abundances. In conclusion, the SPA results of the plume samples indicate that, under clean and IP conditions, diesel engines burning low grade fuel emit organic material rich in K and Cl; K, Cl and P or K, Cl and S (clean/IP) and Si-rich particles (IP). Difficulties in locating the exact position of the plume are probably responsible for the lack of organic material in the plume sample taken, from a similar diesel engine ship, under MP conditions. The plume sample emitted by both a gas turbine (IP) and nuclear powered steam engine ship (MP), seemed to consist mainly of MBL particles. The higher amount of NaCl particles, compared to the MBL, encountered in the latter plume sample can probably be attributed to the cooling of the nuclear engine with sea water. The general composition of plume samples emitted by steam engine ships, under MP conditions, remains inconclusive because both samples showed different particle types. 322 L. A. DE BOCK ET AL.

3.6. SHIP TRACK RESIDUAL PARTICLES

The HCA results obtained for the fifteen ship track samples are reported in Table V(a, b) and visually shown in Figures 6(a–c). Sample description is ana- logous to the ones provided for the samples in Table II(a, b). The five ship track samples taken under clean conditions were all produced by diesel engined ships. All ships ran on low-grade fuel, except the U.S.S. Safeguard which burned distillate fuel. No information on the type of fuel was available for the sample classified as ‘Other ship tracks’. Based on Table V(a, b) and Figure 6(a) we observe that the cloud droplet nuclei (CDN) of the ship tracks on 12 June, occurring under clean conditions, were mainly composed of organic material rich in Cl and in K and Cl. The relative abundance of marine material seems to increase, whenever the abundance of the organic material decreases. A similar trend was already discovered for the ambient cloud residual particles. Taking a closer look now into these data, we can clearly notice a higher abundance of organic material in the track samples that can not be assigned to the ambient cloud. We assume, according to the composition of the plume sample from 29 June collected under similar circumstances, that this difference in organic material abundance, especially for the K, Cl- rich part, can be attributed to the plumes of both ships. However, the ship track sample produced by the ship Star Livorno on 29 June, devi- ates rather strongly from the previously discussed samples. It seemed to be mainly composed of marine material and although the organic material appeared to be present in high abundances in the ambient cloud residual samples, it only accounts for 11% of the total aerosol abundance in the ship track sample. Moreover, K, Cl- rich organic particles and/or other low-Z containing compounds characterising the plume sample could not be spotted in this track sample. Finally we can conclude that the composition of this sample rather refers to the composition of the ambient below-cloud aerosol (second sample of 29 June in Table I). A total of five ship track samples was also collected under intermediately polluted conditions. These tracks were produced by different ship types. Two ships, the Fremo Scorpius and the NYK Sunrise were, equipped with diesel engines burning low grade fuel, the U.S.S. Mt. Vernon and U.S.S. Truxtun, both supplied by the U.S. Navy, were classified as a steam turbine ship burning distillate fuel and a nuclear powered ship, respectively. No information on the type of fuel was available for the sample classified as ‘Mixed plumes’. Although the U.S.S. Mont Vernon and the U.S.S. Truxtun did not produce a real track on any of the satellite images (Hobbs et al., 1997, Noone et al., 2000), the samples taken in the aerosol plume encountered in the cloud downwind of both ships will nevertheless be referred to as track samples, to avoid confusion with the actual plume samples. In general, the CDN present in these ship track samples seemed to consist mainly of pure and transformed sea salt particles (Figure 6(b)). However to identify the real origin of these track residual aerosol particles, the composition of the ambient cloud residuals as well as the plume composition should be taken into account. Considering SINGLE PARTICLE ANALYSIS OF AEROSOLS 323 m) µ m) (mean diameter µ 64 36 19 36 46 HCA results of the ship track residual samples m) (mean diameter µ Table Va. NaCl elements 20 32 4 44 29 1212 528 1 17 52 1 12 78 43 9 37 62 2 47 42 11 % abundance(mean diameter % abundance % abundance NaCl Transf. C.P. No Cl-rich S-rich K-rich K, Cl Cu-rich Si-rich Diesel6/8 Intermediate Skaugran Diesel6/11 Moderate Brazil Vitoria (0.4)Diesel (0.3)6/11 Moderate Safeguard/Moku Pahu (0.4) (0.4) (0.5) (0.4) (0.5) (0.3) (0.5) (0.9) Ship track residuals Marine materialFremo Scorpius Organic materialDiesel/Diesel6/12 Clean Moku Pahu Si-rich particles (0.3) (0.5) (0.4) Diesel6/12 Clean U.S.S. Safeguard Diesel6/12 Clean Other ship tracks Unknown6/12 Clean U.S.S. Kansas (0.4) City Steam turbine (0.4)6/21 Moderate (0.5) (0.4) (0.3) (0.4) (0.4) (0.4) (0.4) (0.5) (0.3) (0.5) (0.3) (0.5) (0.5) 324 L. A. DE BOCK ET AL. m) µ 2 (1.0) m) (mean diameter µ 28 9 58 5 HCA results of the ship track residual samples m) (mean diameter µ Table Vb. NaCl elements 61 34 5 44 34 21 41 3563 11 3346 2 18 11 22 15 3 41 47 12 % abundance(mean diameter % abundance % abundance NaCl Transf. C.P. No Cl-rich S-rich K-rich K, Cl Cu-rich Si-rich Ship track residuals Marine materialTai He Diesel6/27 Moderate U.S.S. Mt. Vernon Organic material (0.4) (0.4) (0.4) Si-rich particles Steam turbine6/27 Moderate U.S.S. Truxtun/Mt. Vernon Nuclear/steam turbine6/28 Intermediate (0.4)U.S.S. (0.4) Truxtun (0.4) (0.4)Diesel6/29 Clean NYK Sunrise (0.6) (0.4) (0.3) (0.3) (0.3) (0.3) (1.1) Nuclear6/28 Intermediate Star Livorno (0.3) (0.3) (0.3) (0.4) Diesel6/30 Intermediate Mixed plumes (0.4) (0.4) (0.4) Unknown6/30 Intermediate (0.4) (0.4) (0.3) (0.5) SINGLE PARTICLE ANALYSIS OF AEROSOLS 325

Figure 6. (a)–(c): Particle types encountered in the ship tracks under different pollution levels. 326 L. A. DE BOCK ET AL. the track sample collected on 8 June in comparison with the ambient cloud residual particles on the same day (Table II), we observe an increase in transformed sea salt at the expense of pure sea salt and a small increase of Cl-rich organic material or NH4Cl. Although no plume sample was available, we suggest that the condensation of SO2, emitted by the low grade fuel burning diesel engine of the Fremo Scorpius, on pure sea salt particles can be responsible for this increase in transformed sea salt particles. No useful information could be achieved from the two samples taken on 28 June due to their large compositional differences and the variable composition of the associated ambient cloud residual particles. Moreover no plume sample was available which could help determine their composition. 30 June was the only IP day where samples were taken in all reservoirs and provides some evidence that ship plume particles can affect ambient stratocumulus clouds and give rise to ship tracks. Taking a closer look at the plume and track sample collected from the ship NYK Sunrise on 30 June, we can indeed discover that from the 21% abundance of Si-rich particles, characterising the plume sample, 2% appeared as CDN in the ship track sample. Since no Si-rich particles could be traced down in any other reservoir, the ship’s plume is the only possible origin of the Si-rich particles present in the track sample. The other particle types encountered in the track sample were also identified in the ambient cloud residual particles but with somewhat different abundances. The absence of the K, Cl- rich organic particles in the CDN of ship track samples, despite its high abundance in the plume, can only be explained by assuming that these particles remain unactivated in the interstitial aerosol of the track. The remaining five ship track residual samples were collected under moderately polluted conditions. The three diesel engined ships the Skaugran,theBrazil Vitoria and the Tai He all burned low grade fuel and produced real ship tracks, while the U.S.S. Kansas City and the U.S.S. Mont Vernon, both steam turbine ships burning distillate fuel, did not, despite large increases in their total aerosol concentrations (Noone et al., 1998). The reason why we can find no tracks on the satellite images for both ships is explained by Noone et al. (2000) and Hoppel et al. (1997) as being the result of the size of the particles emitted from the ships stack. The smaller particles emitted by both U.S. Navy ships (<0.03 µm) seemed to require a higher supersaturation to act as CCN compared to the larger particles (0.01–0.1 µm) produced by the other ships. The latter two samples were therefore again taken in the aerosol plume encountered in the cloud downwind of both ships, as discussed above. In general, the composition of the CDN from the ship tracks under MP conditions (Figure 6(c)) seemed to consist of two major particle types, sea salt and transformed sea salt, and one minor particle type, identified as Cl- rich organic material. Since no associated ship plume samples were available for the track samples taken on 11 June, no additional information could be extracted from both samples. We could only observe that their composition clearly resembles the one found for the ambient cloud residual particles, but with somewhat different abundances. Comparing the ship plume and ship track sample from the U.S.S. Kan- SINGLE PARTICLE ANALYSIS OF AEROSOLS 327 sas City, we found that from the 87% Cl-rich organic particle abundance detected in the plume, probably originating from the stack and the MBL, 43% could also be encountered in the track. The marine material found in the track sample was probably supplied by the ambient cloud. In view of the fact that the composition of the plume sample of the U.S.S. Mont Vernon clearly matches with the MBL composition, it is not surprising that the ship track particle types are similar to the ones characterised in the MBL and the ambient cloud. Finally the ship track produced by the Tai He seemed to consist of the same particle types as found in the ambient cloud residual samples, more specifically as in the second sample taken on 27 June in Table II(b). The increase in transformed sea salt can probably again be attributed to the condensation of SO2 on sea salt particles, as discussed previously. The Si-rich particles, which accounted for up to 8% of the total particle abundance in the plume sample, could not be detected in this track sample, as it was the case for the NYK Sunrise track samples. In summary, the SPA data of the ship track residual samples were not so easy to interpret, since often the corresponding ship plume sample was not available. Nevertheless a first indication, under clean conditions, that ship plume particles play an active role in the formation of ship tracks, was probably provided by the excess of K, Cl- rich organic material in the track (12 June) that could not have been supplied by the ambient cloud residual particles. Indisputable evidence for this hypothesis was however obtained from the samples taken on 30 June under IP conditions. Some of the Si-rich particles, discovered only in the plume sample from the ship Kansas City, did appear as CDN in the ship track sample. An increase in the abundance of transformed sea salt particles in the track samples, compared to the ambient cloud, was also observed in two cases and is probably the result of the condensation of SO2 from the plume in the track.

4. Conclusions We have analysed the chemical composition of individual aerosol particles (>0.2 µm), sampled during the Monterey Area Ship Tracks (MAST) experiment. We have stratiﬁed the data into periods of differing pollution levels in the boundary layer (clean, intermediately and moderately polluted) and have examined aerosol in several different reservoirs; below-cloud ambient aerosol, ambient cloud droplet residual particles, ship plumes below-cloud, ship track cloud droplet residuals and above-cloud aerosol. In terms of the objectives of the MAST experiment, we found some evidence that combustion-derived particles emitted from a ship stack can result in cloud droplet formation in a ship track. A particle class containing Si (used in the distillation of diesel fuel) was found in the below-cloud plume and ship track residual particles sampled from a diesel-powered ship, and in none of the other samples taken on the day of sampling. These results provide a direct link between aerosols emitted from the ship’s stack to an effect in cloud. 328 L. A. DE BOCK ET AL.

We found that a surprisingly large fraction of the ambient aerosol (both below- cloud and cloud droplet residual particles) were possibly composed of organic material or organic material associated with chlorine. Another possibility is that this fraction constitutes of other low-Z and chlorine containing compounds, like NH4Cl. The number fraction of these particles was largest in clean marine boundary layers, and decreased with increasing pollution levels. These particles were effective as CCN, accounting on one of the sampling days (12 June) for up to 80% of the number of cloud droplet residual particles >0.2 µm diameter. The fraction of ‘transformed sea salt’ (Na, Cl, S) increased with pollution level in the MBL, reﬂecting the increased anthropogenic impact in the more polluted boundary layers. However, we can not overemphasize that this has to be investigated further, since no conclusive evidence can be generated from the analyis method used (SEM-EDX) that the particles are truly organic.

Acknowledgements The MAST experiment was funded primarily by the Ofﬁce of Naval Research. We are indebted to Dr. Ron Ferek, and to the crew of the University of Washington C-131A aircraft for their excellent work during the experiment. L. A. De Bock acknowledges ﬁnancial support by the Instituut voor de be- vordering van het Wetenschappelijk-Technologisch Onderzoek in de Industrie (IWT).

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