9<1 (I}~Reprinted from ENVIRONMENTAL SCIENCE & TECHNOLOGY, Vol. 26, 199r 21,--"

Copyright © 1992by the American Chemical Society and reprinted by permission of the copyright owne .

Characterization of Individual Giant Aerosol Particles above the North Sea

Hans Van Malderen, * Carlos Rojas, and Rene Van Grleken

Department of Chemistry, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Wilrijk-Antwerp, Belgium

• In order to investigate the importance of giant airborneparticles, air sampling was carried out using an aircraftwhich flew at different altitudes over the southern bightof the North Sea. A set of 25 samples was collected witha steel rod on top of the airplane. Some 12500 individualparticles were analyzed using electron probe X-ray mi-croanalysis. Principal factor analysis allowed identificationof four sources of giant aerosol particles: aluminosilicates,combustion processes, industrial processes, and marinesources. Hierarchical and nonhierarchical cluster analysisenabled us to classify the analyzed particles. A cleardistinction was found between flights for which the asso-ciated air masses were marine or continental. The ex-perimentally found size distributions for sea salts, sea saltsenriched with sulfur, and organic, CaS04' and Fe-richparticles were fitted well by the log-normal distribution,and average sizes of these particle types were found to be~3 ~m. Aluminosilicates had a bimodal size distributionwith average size maxima at 4 and 15 ~m.

IntroductionThe North Sea is surrounded by highly populated and

heavily industrialized countries and consequently it issubjected to pollution of industrial, domestic, and agri-cultural origin through different channels. Input of con-taminants, transported by rivers, and direct dumping intothe North Sea have long been known as major sources ofpollution. But is has been shown that atmospheric inputof particulate matter or aerosols by wet and dry depositionprocesses contributes significantly to the North Sea pol-lution (l).

Every aerosol particle type differs in shape, size,density,and chemical composition. Each of these physical pa-rameters has a strong influence on the behavior of theaerosol in the air.

Giant aerosol particles with a radius above 1 ~m, andespecially those larger than 10 ~m, are of extreme im-portance to aerosol fluxes. Their concentration in theatmosphere is low, but is should be emphasized that aparticle of, for example, 10 ~m has 1000 times more massthan a l-~m particle and its deposition velocity is ca. 100times higher (2). Calculations by Dedeurwaerder (3)showed that deposition of giant particles explains 94%(Cd), 96% (Cu), 85% (Pb), and 88% (Zn) of the total drydeposition above the North Sea. Furthermore, if globalaerosol production rates are estimated, discrepancies of0.1-4000 tons/year occur, depending on whether particleswith diameters up to 1 or 1000 ~m are included in thecalculations (4). One should expect giant particles to beof local interest only, since a typical residence time of 1day has been reported by Jaenicke (4); however, thepresence of giant mineral aerosols has been observed inlong-range transport (5). All these facts point to the ex-treme importance of giant aerosol particles for the totaldeposition process.

Due to their relatively low concentrations, it was notuntil the early 19708that serious interest was taken in giantaerosol particles. Noll and Pilat (6) and Whitby et al. (7)pointed to the importance of these particles in the totaldeposition process and cloud and precipitation processes.They stated that, of the atmospheric aerosol mass dis-tribution, approximately 81% of the total mass was ac-

counted for by particles larger than 1 ~m (of which 45%was by particles with diameters between 10 and 50 ~m).Meszaros (8) studied the relation between the giant particlesize distribution and the relative humidity. A major partof the published articles focused on giant sea spray par-ticles (9-11). But sampling difficulties and measurementerrors are still the limiting factor in the study of theseparticles. This probably explains why the total amountof published articles on this topic is very limited until now.This article will approach the collection and analysis ofthe giant particles from a new and different point of view.

Experimental Section1. Sampling Strategy. In order to measure at different

altitudes above the North Sea, a twin engine aircraft(Geosens B.V., Rotterdam, The Netherlands) equippedwith different gas and particulate matter sampling devices,was used. To collect giant aerosol particles, a specialsampling device, consisting of a circular impaction surface(diameter 1 cm) covered with a particle-free sticky tapeand supported by a vertical bar, was exposed outside theairplane perpendicular to the streamlines of the air aroundthe plane and directed upwind. Only the front side of therod was covered with a sticky tape, and therefore, particleswere only collected on this side of the impaction device.Although very simple, this appeared to be an efficient wayof collecting giant aerosols. More details on samplingstrategy and flight plan can also be found elsewhere (12).

In each flight, samples were taken at different altitudesmore or less equally spaced between the inversion layerand the sea level in order to obtain a vertical distributionof the giant aerosol. For every flight a set of six sampleswas taken for each altitude. The last track, only 10-30 mabove sea level, was intended to assess particle resuspen-sion by sea spray. All flights were carried out duringrelative cloudless and dry periods. Information about theorigin of the collected air masses was obtained by studyingthe backward trajectories, which were provided by theRoyal Dutch Meteorological Institute (KNMI). Two tra-jectories, shown in Figure 1a and b, are typical for tra-jectories of air masses with a marine and continental origin,respectively. The different pressure levels (1000,925, 850,700 bPa) correspond to the respective altitudes above sealevel (140, 770, 1450, 3015 m) (SPT).

2. Analysis. In 5 different flights, a set of 25 sampleswas collected and the analysis of 12500 particles (500 ineach sample) was performed by electron probe X-raymicroanalysis (EPXMA) on a 733 Superprobe (JEOL,Tokyo, Japan) equipped with a TN-2oo0 system (TracorNorthern). For every analysis, an accelerating voltage of20 kV and a beam current of ca. 1 nA was used. TheEPXMA technique allows one to obtain morphologicaldata such as average diameter and shape factor while thechemical composition can be derived from an energy-dis-persive X-ray spectrum. The single-particle analysisprogram 733B (13) allowed us to perform automated in-dividual particle analysis. Localization of a particle isobtained by successive horizontal scanning with the elec-tron beam. During this process, contour pixels of a particleare stored into the memory. When all contour grid pointshave been stored, area, perimeter, and diameter are cal-culated and an X-ray spectrum is accumulated. All this

5 18 15 11

Figure 1. Backward trajectory of a flight with (a, top) a strong marineinfluence or (b, bottom), a strong continental influence: (1) 140 m, (2)770 m, (3) 1450 m, (4) 3015 m.

information is stored on a disk.3. Multivariate Methods. To allow interpretation of

the obtained information, hierarchical and nonhierarchicalcluster analyses were used to classify particles with similar

chemical composition into "particle groups".Hierarchical cluster analysis was used to classify the 500

particles, measured in each of the 25 samples, into groupsbased on their chemical composition. It was then possibleto calculate the percent abundance of each group in eachsample. Here, the clustering was performed on the nor-malized X-ray elemental intensities of each analyzedparticle. We used the software package DPP (14). Ward'smethod (error sum of squares) was chosen because it yieldsa maximal internal homogeneity (15). The condition forthe joining of two groups, using Ward's method, is that thedecrease in homogeneity is kept as small as possible. Thus,whenever a pair of clusters is joined, the homogeneity iscalculated by determining the sum of square distances ofeach object to the centroid of the cluster. This calculationis done for each pair of clusters and those clusters will bejoined which will lead to the smallest decrease in homo-geneity.

Since the hierarchical clustering cannot be used for si-multaneous classification of all measured particles in allsamples due to the enormous amount of computer timecalculating the distances between 12500 particles in eachcycle of a hierarchical clustering would take, we used thenearest centroid sorting technique. Nearest centroidsorting is a group of nonhierarchical cluster methods, whichclassifies objects (particles) in clusters according to theirdistance from the centroids of a fixed number of previouslychosen training vectors, which are obtained by a hier-archical clustering. The distances between all objects andthe training vectors are calculated. All objects that wereincorrectly classified in the initial clustering are relocatedwith the nearest centroid method. The centroids of thenew clusters are calculated. This procedure is repeateduntil convergence appears. The result of such a clusteringis that objects in a cluster are very similar to each other,while the clusters themselves are well separated. Moretheoretical aspects on nonhierarchical clustering can befound in publications of Anderberg (16) and Bernard (17).

Mter the clustering, a standardless ZAF correction wascarried out in order to convert the relative peak intensitiesinto elemental weight compositions. Performing ZAFcorrections on each of the measured particles separatelywould, in principle, be better, but would require anenormous amount of computer calculating time. There-fore, the ZAF correction is performed after the clustering.

To identify the different sources for giant aerosols,principal factor analysis (PF A) with orthogonal Varimaxrotation was performed on the data set. PF A has alreadybeen used in many other atmospheric studies, e.g., byHenry and Hidy (18, 19), Rojas et a1. (20), Schaug et al.(21), and Wolff et al. (22). The objective of PFA is to takep variables and find linear combinations of these to pro-duce uncorrelated new variables (factors), which are or-dered in decreasing order of importance so that the firstone explains the largest amount of variance. It is a com-mon practice to perform an orthogonal rotation of theoriginal factors in order to facilitate their interpretation.In this case, the Varimax rotation was chosen. This typeof orthogonal rotation consists of maximizing the varianceof the squares of the factor loadings. Here the data matrixconsisted of 13 variables (Na, Mg, AI, Si, P, S, CI, K, Ca,Fe, Cu, Zn, and altitude). An element was considered tobe detected if the X-ray intensity of the element was foundabove detection limit in one of the 500 analyzed particlesfor each sample. Occasionally other elements such as Pb,Ni, Cr, Br, Ti and V were detected in a particle. However,when detected, these elements were found in particles thatalways contributed less than 0.5% of the total aerosol

Table I. Varimax Rotated Factor Loading Matrix for the 25 North Sea Giant Aerosol Samples·

standardvariable factor 1 factor 2 factor 3 factor 4 communality deviation

Na --1).42 0.69 0.36 0.814 0.09Mg 0.75 0.627 0.13Al 0.95 0.928 0.06Si 0.96 --1).18 0.956 0.05P --1).40 --1).85 0.913 0.06S 0.86 0.878 0.08CI --1).31 0.73 0.46 0.883 0.07K --1).28 0.84 0.822 0.09Ca 0.59 --1).60 0.773 0.10Fe 0.90 --1).26 0.887 0.07Cu --1).30 0.83 0.25 0.867 0.08Zn 0.85 0.813 0.09height 0.38 --1).35 0.36 --1).52 0.672 0.11

eigenvalue 4.74 3.54 1.39 1.16

% variance explained 36.3 27.1 10.6 8.9

source ID aluminosilicates marine industrial combustion process

• Absolute values for factor loadings smaller than 3 times their standard deviation were deleted for simplicity.

abundance. Therefore, they were excluded from the datamatrix. One of the most important problems of PF A isthe decision of the dimensionality of the model. The de-cision on how many factors should be retained is oftenrather subjective. But it is usually a "rule of thumb" toretain as many factors as there are eigenvalues greater thanunity. More detailed information on the strategy forperforming PF A on EPXMA data can be found elsewhere(23).

Collection UncertaintiesCollection of giant aerosols is not a straightforward

matter. In aerosol sampling by f1ltration, suction velocitiesare usually too small to collect giant aerosols. Aerosolsampling by impaction as a function of particle size nor-mally requires isokinetic conditions for representativesampling.

In order to calculate the collection efficiency E of theimpaction rod on top of the aircraft, we assumed the samephysics for the impactor as for the scavenging of particlesby rain droplets; the impaction rod was considered as adroplet (diameter 1cm) falling through the air with a speedequal to the speed of the airplane (75 m/s) during col-lection. For the calculations, the equations given by Slinn(24) were used. These equations treat the impactionsurface as a sphere, but during the calculations this wascorrected to the actual shape and dimensions of the im-pactor rod. Further theoretical background can be foundelsewhere (25).

The results show (Figure 2) that an impaction rod, ataircraft speed, can be used to collect particles with a di-ameter above 0.8 Jlm. This seems to compare well withthe obtained EPXMA results, where the smallest detectedparticles had diameters around 0.6 Jlm. The slight increasein collection efficiency for small particles is the result ofcollection by Brownian diffusion. Because Browniandiffusivity of particles increases as the particle size de-creases, this effect is more distinct for small particles.

The upper size range of the collected particles is limitedby bounce-off and reentrainment. When a particle strikesthe surface, it can bounce off back into the gas stream orcause a previously collected particle to be knocked off thesurface. In both cases, for single-stage impactors, thismeans that the concentration of particles collected is toosmall. These problems can be minimized by coating thesurface of the impactor to absorb the kinetic energy of theparticle. EPXMA results showed maximum diameters

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~ 0.01 !!'!!'!!!!,;,;,;,;,;,;,;'!'!',;'!!,;!,;!',;'!,;'!'!'!' .'!';';';!!!!'!"!';';!';'!';'!'!';!';!!'!"!';!'8 :::::::::::::::::::::::::::::::::::::::::::::::::::::':::::::::::::::::::::::::::::::::::::::::::::::::

1Diameter van)

Figure 2. Calculated collection efficiency for giant North Sea particlescollected with a direct impaction rod.

around 25 Jlm. Less than 1% of the detected particles werelarger than 25 Jlm.

Results and Discussion1. Principal Factor Analysis. PF A, performed on the

correlation matrix of the data set, which gives the patternof relationships between the factors, resulted in four ei-genvalues greater than 1 (4.74,3.54,1.39, 1.16), while thefifth and sixth eigenvalues were respectively 0.71 and 0.56.Therefore, further calculations were performed with thefour-factor solution.

Results of the Varimax rotated factor analysis are shownin Table I. Only factor loadings greater than 3 times theirstandard deviation are shown, because only these areconsidered as statistically significant (26). The four factorstogether explain 83% of the total variance. Communalitiesare high for all variables, except for Mg, Ca, and "altitude".Factor 1has high loadings for AI, Si, Ca, and Fe. Alu-minosilicates, both windblown soil dust and fly ash, whichhave similar chemical compositions with respect to manyelements, probably correspond to this factor. Factor 1ispositively correlated with altitude, which could be ex-plained by the presence of a long-range-transported min-eral aerosol at high altitudes. Factor 2 most likely rep-resents marine impact. Furthermore, this marine sourceis slightly anticorrelated with altitude. If one considersthe mechanism by which matrix and trace elements areinjected from the sea into the atmosphere (27), high con-

Table II. Results of the Hierarchical Clustering

group abund, av elementsno. % diam, !Lm detected identification

(a) Flights with Little Continental Influence1 31 3.5 Na, CI, S S enriched seaspray2 18 3.3 Na, CI seaspray3 16 2.9 Ca,S gypsum4 10 2.0 Fe Fe-rich5 8 3.9 S, CI sulfates6 6 3.1 Ca, S, CI CI enriched gypsum7 5 3.2 AI, Si, CI aluminosilicates8 5 3.1 none organic9 1 1.9 Na Na-rich

(b) Flights with a Strong Continental Influence1 36 3.1 Ca (Si, P) Ca-rich2 17 2.1 none organic3 12 3.3 Ca,S gypsum4 11 4.3 AI,Si aluminosilicates5 4 2.7 K, CI (Zn) K,ZnCI6 4 3.7 CI, Na seaspray7 3 3.6 Fe Fe-rich8 2 2.6 Si quartz

centrations of Na, Mg, S, and CI are to be expected atlower altitudes, as observed. Factor 3 has high loadingsfor Cu and Zn, suggesting the existence of an industrialsource for Cu and Zn. Generally, Zn is emitted evenly inEurope, whereas emissions of Cu are attributed to Centraland Eastern Europe (28). Heavy metal concentrations inthe atmosphere were reported to reach maximums witheast-southeast winds (29) and should therefore be mainlyfound in flights with a continental air mass history. Thiswas recently confIrmed by Injuk et al. (1). CI and K, whichcharacterize factor 4, are probably the result of combustionprocesses. The burning of organic material leads toemission of K, but also the burning of municipal waste inincinerators leads to potassium and zinc chlorides. Insummary, PFA on 13 variables distinguished 4 classes ofgiant aerosol sources: aluminosilicates (Al, Si, Ca, Fe),marine aerosols (Na, Mg, S, cn, and aerosols from met-allurgical processes (for Zn, Cu) and combustion processes(for K, CI).

2. Hierarchical Clustering. Only 6 out of 25 samplescorrespond to a wind sector which is representative ofmarine air masses, whereas the remaining 19 samplescorrespond to other wind directions, which have as majorsources Belgium, The Netherlands, Germany, Poland,Czechoslovakia,and Denmark. Depending on the sample,three to seven different aerosol groups were distinguished.Overall results of the hierarchical clustering for flights with,respectively, little and strong continental influences arelisted in Table ITaand b. In these tables, average diameter,abundance (in percent), and the identifIcation of eachparticle type are shown.

(a) Marine Air Masses. The flights with little con-tinental influence are characterized by southwest winddirections. Then air masses come directly from the At-lantic Ocean through the English Channel, and anthro-pogenic influence is consequently low.

In these marine flights, mainly untransformed andtransformed sea salts were found, accounting for up to 49%of the total aerosol abundance. Sea salt and transformedsea salt fInd their existence mostly in the mechanism ofwhite cap formation during high-speed winds and bubblebursting. The sea-spray particles can then react with othercompounds in the atmosphere. Eriksson (30) stated thatthe Clloss in sea salt is the result of pH-lowering reactionsbetween NaCI and acid atmospheric components such asH2S04, and this was later confirmed by Hitchcock et al.

(31). As a result, degassing of HCI takes place and CI- isreplaced by SOi- (32). The abundance of the transformedsea salt enriched with S was positively correlated withaltitude. This seems in agreement with the fact that theseaged aerosols had a residence time long enough to allowchemical reactions with other compounds.

Ca-rich (and especially CaS04) particles account for upto 22% of the total aerosol abundance. Ca-rich particles,which mayor may not be associated with sulfur, werefound in all flights. CaS04 particles were very commonin marine as well as in continental flights. CaC03 couldbe transformed into CaS04 in the atmosphere by H2S04(33). Some of the caS04 particles contain CI-. This mightpoint out that caS04 particles are partly of marine origin:interaction between marine CaC03, mainly coming fromskeletons of pelagic organisms, and H~04 in the air couldbe a possible source. Fractionated crystallization of gyp-sum (CaS04·2H20) out of sea spray and splitting off of thegypsum crystals could be another source (34). Othermarine elements (such as Na, Mg, and K) were not ob-served in this type of particle.

In the lower tracks (under 300 m), a considerable partof the detected particles was of anthropogenic origin. Theoccurrence of these anthropogenic particles explains thealuminosilicates (5%) and organic (5%) and Fe-rich (10%)particles found in the hierarchical clustering. Particleresuspension could be a possible source for the anthro-pogenic particles, but more likely direct emission from landcould explain the occurrence of these particle types. Inthe predominantly marine flights, air masses at lower levelscame over the northwest of France. In the region ofRoubaix and Le Havre (France) the iron industry is veryintense, which could explain the high abundances of Fein the lower tracks. Also the power plants in northwestFrance can partly explain the high abundance of CaS04'

(b) Continental Air Masses. Particle abundancesduring periods in which the air masses came from thecontinent were mostly dominated by Ca-rich, organic, andcaS04 particles and aluminosilicates. Ca-rich particles anddifferent combinations mainly containing Ca (e.g., Ca-P,Ca-Si-S, and Ca-Fe) yield 36% of the aerosol abundance.Abundances of Ca-rich and CaS04 particles were alwayshigher for flights with a continental air mass history thanfor marine flights. Ca-bearing particles can be related tohigh-temperature combustion processes as well: largequantities of CaC03 are used in thermal power plants, asa desulfurization agent, to enhance the oxidation of S02and neutralize the S03; this leads to sulfates. Furthermore,some of the caS04 particles were enriched with Si. Thisparticle type is associated with coal combustion (35).

Particles with no major X-ray intensities were classifIedas organic (17%); they seem anticorrelated with altitudeand could therefore be related to biogenic material suchas pollen, spores, and bacteria. Organic particles are alsoemitted during combustion of fossil fuels.

Aluminosilicates are clearly less abundant in typicalmarine flights than in flights with continental influence,where they account for 11% of the particle abundance.The aluminosilicates can be divided into two groups:windblown soil dust and fly ash. Even though theseparticles have roughly the same chemical composition, theyusually differ in morphology, as will be discussed later. Aconsiderable amount of the aluminosilicates contained S,which can be related to the fact that a substantial part ofthese particles originates from coal combustion (fly ash).On the other hand, this might be a result of a sulfur layerformed on the soil dust particles, as reported by Win-chester et al. (36,37).

Table III. Average Composition, Size, and Shape for the Groups Found by the Nearest Centroid Sorting Performed on theEPXMA-Analayzed North Sea Giant Particles

group group diam,no. abund, 0/0 !Lm Na Mg Al Si P

1 22 3.2 0.6 0.5 0.6 2.7 0.62 21 4.2 14 1.0 0.1 0.8 03 20 3.0 0.9 1.5 0.7 2.2 0.14 17 2.7 1.6 4.0 2.7 4.2 3.15 14 3.8 0.1 0.5 8.1 28 0.16 5 2.6 0 0.3 0.3 1.7 0.17 1 2.2 69 0 0 0 0

relative abundance per element, wt 0/0S CI K Ca Cr Fe Ni Cu Zn 0

0.9 0 0 0 310.3 0 0 1.1 02.4 0.3 0.1 0.1 465.7 0.6 1.1 11 324.7 0 0 0.1 47

60 0.6 0 0.2 310 0 0.3 0 27

1.3 1.1 0.1 61 03.7 75 1.6 2.3 0

21 2.0 0.7 22 0.16.7 11 10 5.0 0.41.6 1.2 2.0 6.1 00.1 1.4 0.2 1.5 1.82.1 0.8 0.4 0 0

80

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60

In flights with continental air mass history, potassiumand zinc chlorides and phosphates were found. Theyprobably result from burning organic material and mu-nicipal waste (38, 39). Results point to a low content ofheavy metals in the giant particle size range. Since heavymetals are commonly emitted in high-temperature com-bustion processes, they are mostly drained into the at-mosphere as gases, which condense from the vapor phaseto predominantly sub micrometer particles at suddencooling. The occurrence of submicrometer aerosols con-taining heavy metals above the North Sea has indeed beenreported in EPXMA results (40).

The sea-salt particles (4%) were only found in the lowesttracks (20 m). The iron-rich and quartz particles were onlyfound in small amounts (3-4%).

In summary, in flights with air masses with marine in-fluence, the samples were mainly characterized by sea salts,sea salts enriched with sulfur, and CaS04 particles. Sam-ples from flights correlated with air masses of a continentalorigin were high in aluminosilicates and Ca-rich, organic,and CaS04 particles.

3. Nonhierarchical Clustering. To get an overallview on the particle types collected in all samples together,a nonhierarchical clustering was performed. In order toclassify the 500 measured particles into 7 particle groupsin each of the 25 collected samples, a hierarchical clustering(Ward's method) was performed on each of the 25 samplesseparately. The average elemental abundances of 175 (7X 25) particle groups (7 in each sample), obtained fromthe clustering of the 25 samples, were clustered again,which provided us again with 7 groups of average elementalabundances now for all 25 samples together. These werefurther used as centroids for the nonhierarchical clustering.Results of the ZAF -corrected nonhierarchical clusteringare given in Table III. The oxygen in Table III has beencalculated by assuming all elements were present in theform of oxides.

From Table III it appears that 22% of the detectedparticles consist of Ca-rich (61 % Ca) particles. Nearly asmuch (21 %) of the North Sea giant aerosols are sea salts,while 20% of the particles are found to be caS04 particles.From Table III it can be seen that the CI/Na ratio in group2 is 5.4, which is greater than the seawater value. This isa result of the clustering. Since not all particles, whichare brought together in the so-called "sea-salt" cluster,necessarily contain the same amount of Na and CI, theCI/Na weight ratio after the clustering may significantlydiffer from the actual CI/Na seawater ratio. Group 4 canbe identified as potassium, zinc chlorides. The abundanceof this particle type (18%) is very high compared to theresults obtained from the hierarchical clustering, it is likelythat they have been mixed up in the nonhierarchicalclustering with particles without major X-ray intensities,previously identified as organic particles. Group 5 ischaracterized by high contents of AI, Si, Ca, and Fe (re-spectively 8%,28%,6%, and 5%) and therefore it can be

9 14 19Diameter (JIm)

Figure 3. The log-normal fitted size distribution of the aluminosilicates.

ascribed to aluminosilicates (14%). Group 6 containsFe-rich (60% Fe) particles (5%), while the low-abundancegroup (1%) is identified by a high content of Na (69% Na).The size of the particles varies between 2.2 /.Lmfor theNa-rich particles and 4.2 mm for the sea salts.

4. Particle Size and Distribution. A description ofthe size distribution of several of the most abundantparticle types was achieved by means of a log-normaldistribution. The experimentally found size distributionsof sea salts, sea salts enriched with sulfur, and CaS04,Fe-rich, and organic particles turned out to be fitted wellby the log-normal distribution, and they also had averagesizes between 1 and 3 /.Lm. Aluminosilicates, on the otherhand, presented a bimodal distribution; the total distri-bution could be split up into two separate distributionswith average diameters centered at 4 and 15 /.Lm. Thissuggests the presence of two completely different sourcesof aluminosilicates. Closer observation of the particle sizeand shape revealed that the smaller sizes were round inshape, while the fraction of the particles with diametersaround 15 /.Lmwere often irregularly shaped. This suggeststhat the smaller, spherical fraction originates from high-temperature combustion processes, which lead to fly ash.Especially when the sampled air masses came from EasternEurope, these particles were responsible for the major partof the aluminosilicates (>60%). The irregularly shapedlarger aluminosilicates, on the other hand, are windblownsoil dust particles. The log-normal fitted size distributionof the aluminosilicates is shown in Figure 3.

In Figure 4, a number distribution for all 12500 particlesas a function of their size is shown. Attention must bedrawn here to the fact the size spectrum is cut off at thelower end by the impaction rod. Therefore, Figure 4 only

Q) 15Ol

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0.2-0707·1,010-2.02.0-3.030--4040-5.050-7.07.0-10.010-15 1S-25 25-50 SO·100

Diameter (/lm)

Figure 4. The number distribution (in percent) of all analyzed particlesversus their diameter.

shows the size distribution of the detected particles andnot the real aerosol size distribution as it can be found inthe atmosphere. It can be seen that only 10% of thedetected particles were smaller than 0.7 j.Lm, which con-firms the calculated cutoff point of 0.8 j.Lm. Althoughparticles with diameters up to 100 j.Lmwere found, only2% had diameters larger than 15 j.Lm. The relative lowabundance of these very large particles might be a resultof the splintering of big particles during the impact, es-pecially of liquid drops (41).

Summary and ConclusionIn order to investigate the importance of giant airborne

particles, air sampling was carried out over the southernbight of the North Sea. Analysis of the collected sampleswas done by EPXMA.

Four sources of giant aerosol particles were found byPF A: aluminosilicates, combustion processes, and an in-dustrial and a marine source. Hierarchical and nonhier-archical cluster analysis enabled us to find a clear dis-tinction between flights for which the associated air masseswere marine or continental.

The most abundant particle types were fitted with alog-normal size distribution. The experimentally founddistributions for sea salts, sea salts enriched with sulfur,and organic, CaS04' and Fe-rich particles were fitted wellby the log-normal distribution, while aluminosilicates hada bimodal size distribution with average size maximumsat 4 and 15 j.Lm.

To confirm the importance of giant aerosol particles,further research on giant particle size distributions andcollection efficiencies is needed. In the near future, moreexperiments with the impaction rod should lead to a me-thod which enables us to calculate the upper size limitsof the collected giant particles. The design and use ofimproved and new giant aerosol samplers should allow usto obtain a more precise view on the giant aerosol sizedistribution and composition.

Registry No. Na, 7440-23-5; Mg, 7439-95-4; AI, 7429-90-5; Si,7440-21-3; P, 7723-14-0; S, 7704-34-9; Cl, 7782-50-5; K, 7440-09-7;Ca, 7440-70-2; Fe, 7439-89-6; Cu, 7440-50-8; Zn, 7440-66-6; Cr,7440-47-3; O2, 7782-44-7.

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Received for review May 6,1991. Revised manuscript receivedSeptember 18,1991. Accepted November 7, 1991. We aregratefulto Rijkswaterstaat, Dienst Getijdewateren (The Netherlands)for their financial support under grants NOMIVE*2 No. DGW-920 and No. DGW-217.