Characterisation of Amazon Basin aerosols at the individual particle level by X-ray microanalytical techniques

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Atmospheric Environment 41 (2007) 9217–9230 www.elsevier.com/locate/atmosenv

Characterisation of Amazon Basin aerosols at the individual particle level by X-ray microanalytical techniques Anna Worobieca,, Imre Szalo´kib, Janos Osa´nc, Willy Maenhautd, Elz˙ bieta Anna Stefaniaka, Rene Van Griekena a

Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium Institute of Experimental Physics, University of Debrecen, Bem te´r 18/a, H-4026 Debrecen, Hungary c KFKI Atomic Energy Research Institute, P.O. Box 49, H-1525 Budapest, Hungary d Department of Analytical Chemistry, Ghent University, Proeftuinstraat 86, B-9000 Ghent, Belgium

b

Received 17 November 2006; received in revised form 23 July 2007; accepted 27 July 2007

Abstract As a part of the LBA/CLAIRE-98 project (LBA, Large-Scale Biosphere–Atmosphere Experiment in Amazonia; CLAIRE, Cooperative LBA Regional Experiment), an extensive aerosol characterisation study was performed. The field work for the study took place in the Amazon Basin from 23 March to 15 April 1998. The collected aerosol samples were analysed by conventional and thin-window electron probe X-ray microanalysis (EPMA) combined with principal component analysis (PCA). Intensive transport of soil dust particles from the Sahara was observed at the end of March and beginning of April. The absolute number concentration of fine soil dust particles (0.30 mmoparticle diametero2 mm) reached a value of 3.5 million m3. Changes in the total number of particles and reactions of the Saharan dust, i.e., agglomeration with sea salt above the Atlantic Ocean and with local particulate matter, were observed. Particle number concentrations were higher in the fine size fraction, with soil dust dominating in the first part of the campaign. A significant contribution of natural biogenic particles was observed in the last 2 weeks of the campaign, but only in the coarse size fraction. The organic part of the aerosol particles and their agglomeration with other types of particles was studied. The chemical transformation of airborne marine particles in the abundant presence of gaseous pollutants originating from biomass burning and biogenic emissions was observed as well, e.g. N-containing species such as sodium nitrate particles classified as aged sea salt. Carbon was present in almost all particle types. r 2007 Elsevier Ltd. All rights reserved. Keywords: Individual aerosol particles; EPMA; Amazon Basin; Saharan dust; Cluster analysis; Principal component analysis

1. Introduction The tropical regions play a central role in the chemical composition of the Earth’s atmosphere. Corresponding author. Tel.: +32 3 820 23 45; fax: +32 3 820 34 76. E-mail address: [email protected] (A. Worobiec).

1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.07.056

The vast areas of highly active terrestrial ecosystems in the humid tropics are important sources of trace gases and particles that affect oxidation processes and the radiation balance of the atmosphere. Therefore, these regions strongly influence the atmospheric oxidation capacity and the Earth’s climate system. The tropical part of South America, with the Amazon Basin in its heart, contains the

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world’s largest area of humid low-latitude ecosystems (Andreae and Crutzen, 1997; Nobre et al., 2001). Previous studies show that the emission of biogenic particles by the Amazon Basin tropical rain forest accounts for most of the airborne aerosol concentration during the wet season; these biogenic particles are very important in the global aerosol budget (Artaxo et al., 1998). Other investigations show that the aerosol chemistry in Amazonia during the wet season is strongly influenced by long-range transport of soil dust, marine aerosol, and possibly biomass combustion products advected into the central Amazon Basin by large-scale tropospheric circulation (Talbot et al., 1990). Atmospheric inputs of mineral elements into tropical rainforests may constitute an important input of plant nutrients, especially for soils of low inherent fertility. When attempting to deduce external nutrient inputs into a forest, a further complication may be that tropical forests themselves produce aerosols. An important issue is the input into the Amazon Basin of P, often a limiting nutrient. There is negligible transport of P from the tropical oceans as these typically have very low P concentrations in their surface waters. One possibility is the intrusion of mineral dust from desert regions. The importance of dust as a nutrient source is likely for West African rain forests, but the significance of occasional long-term transport of Saharan dust into Amazonia still has to be addressed (Swap et al., 1996). The objective of this paper is to describe the chemical composition and morphological characteristics of atmospheric aerosols during the wet season in the Amazon Basin when the influence from different aerosol origins can be observed. This was achieved at the single particle level using two different electron probe X-ray microanalysis (EPMA) techniques that give detailed information about the chemical composition of aerosol particles and about their size distribution. Additionally, automation provides the possibility to analyse a huge number of single particles for statistically more representative results. However, the identification of low-Z elements in particles by conventional EPMA (with a conventional Be window in front of the Si(Li) X-ray detector) is not possible because of the strong absorption of the characteristic X-rays of the low-Z elements by the Be window. Therefore, the single-particle analysis using thin-window EPMA (TW-EPMA), where the Si(Li) detector is protected by an ultra-thin polymer film, was applied

as a complementary analysis. A reverse Monte Carlo quantification procedure thereafter has been shown to provide elemental concentrations within 15% relative accuracy, also for low-Z elements. The knowledge of the low-Z element concentrations leads to more detailed information on the speciation of the particles, which is very important during the tracing and characterising of the environmental pollutants. The TW-EPMA is capable to distinguish different types of S-, N-, and C-rich particles, i.e., organic sulphur and (NH4)2SO4, organic nitrogen and NH4NO3 or NaNO3, and in this way more detailed information about these elements can be obtained. Additionally, the use of the cold-stage method in EPMA (whereby the sample is continuously cooled by liquid nitrogen) allows the measurement of beam sensitive specimens such as ammonium-rich particles. It should be stressed that the determination of low-Z elements at the individual particle level by TW-EPMA was performed for Amazonian atmospheric particulate matter for the first time. Such analytical data have never been published before. This type of analysis provides not only more sophisticated information about the elemental composition at the singe particle level, including the characterisation of C–N–O rich particles, but it also gives information about the compositional complexity of the particles, especially in the case of agglomerates or particles, which have reacted with gaseous compounds. 2. Sampling The ground-based field work for the LBA/ CLAIRE-98 (LBA, Large-Scale Biosphere–Atmosphere Experiment in Amazonia; CLAIRE, Cooperative LBA Regional Experiment) campaign was carried out at the town of Balbina (11590 S, 591120 W; 470 m a.s.l.) in the Amazon Basin, Brazil (see Fig. 1). The aerosol samples for the current study were collected during the wet season from 23 March to 15 April 1998. The samples were taken at 2 m above the ground level. Gent PM10 stacked filter unit (SFU) samplers and cascade impactors were used. The SFU sampler collects particles on 47-mm diameter Nuclepore polycarbonate filters connected to a low-volume vacuum pump (flow rate about 17 L min1). Since during the EPMA step, the localisation of the particles on the filter is based on the backscattered electron image, it is crucial to use an adequate substrate giving high enough contrast for the proper identification (Worobiec et al., 2005).

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Fig. 1. The sampling location (http://geography.about.com/library/cia/blcbrazil.htm; http://lba.cptec.inpe.br/mirror/lbaeco/newareamaps/ Manaus.gif).

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Therefore, coarse particles (2.0–10 mm aerodynamic diameter (AD)) were collected on a 8.0 mm pore-size Nuclepore filter, while for the collection of fine particles (o2.0 mm AD) a Nuclepore filter with 0.4 mm pore size was used. The collection time per SFU sample was approximately 12 h, in order to study differences in the aerosol composition between day and night. The SFU samples were used for conventional EPMA. The cascade impactor samples were taken for low-Z element (TW-EPMA) analysis. They were taken with a PIXE International cascade impactor (PIXE International Corporation, Tallahassee, FL, USA) with only three impaction stages, i.e., stages 1, 2, and 4. The aerodynamic size ranges for these stages were 0.25–0.5, 0.5–2.0, and 2.0–8.0 mm, respectively. Samples were taken at the flow rate of approximately 1 L min1. Aluminium foil was used as impaction surface. The cascade impactor samples were collected only during the daytime. The sampling time for stages 1 and 2 varied from 1 to 10 min. At the beginning of the campaign, during the Saharan dust event, the sampling time was much shorter than that at the end. For stage 4, the sampling time was between 10 and 30 min. The

cascade impactor samples were always taken at times that also SFU samples were collected. Because of the specific geometry of the cascade impactor samples, only relative abundances of the distinguished particle types were calculated. 3. Meteorological conditions Shifts in the position of the Inter-Tropical Convergence Zone (ITCZ) and the northern hemisphere subtropical anticyclone during the sampling campaign resulted in the injection of pulses of air from the northern hemisphere, which may have introduced dust particles from the Sahara region (Talbot et al., 1990). In March 1998, the ITCZ was located at approximately 11N. Brazil, including Balbina, was located in the southern hemisphere (see also Fig. 2 with the air mass backward trajectories for the crucial days during the sampling campaign). As discussed in detail by Formenti et al. (2001), in March 1998 the air masses originated mostly from the Sahara desert and the Atlantic Ocean. The major route of mobilisation of windblown dust from the Sahara crosses the North Atlantic Ocean westward towards the American 28.03.1998

24.03.1998

01.04.1998

10.04.1998

Fig. 2. Ten-days air mass backward trajectories calculated by the Hybrid Single-Particle Lagrangian (HYSPLIT) Model for selected days of the sampling campaign (ref: http://www.arl.noaa.gov/ready/hysplit4.html); arrival height 500 a.s.l.

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continent (Prospero, 1999). March 1998 was also characterised by anomalously dry weather conditions in the Amazon Basin, likely modulated by the effects of El Nin˜o (Peters, 1998). This resulted in enhanced biomass burning over Brazil, Surinam and the neighbouring countries. Large fires were observed in the Roraima State (northwestern Brazil), at the border with Venezuela, which also was affected by the transport of these smoke plumes (Formenti et al., 2001). During sampling, the average temperature was approximately 30 1C. The relative humidity (RH) was 50–60% over day and about 100% during the night. 4. Measurements methods A JEOL JSM 6300 scanning electron microscope (JEOL, Tokyo, Japan), equipped with backscattered electron (BSE) and secondary electron (SE) detectors and an energy dispersive X-ray detection system, was used for the conventional EPMA. A Si(Li) X-ray detector (FWHM ¼ 150 eV at E ¼ 5.9 keV) coupled to a PGT (Princeton Gamma Tech; Princeton, NJ, USA) system was employed for acquiring the X-ray spectra (acquisition time was 20 s). For analysis, typical electron energy of 20 keV and a beam current of 1 nA were used. The average diameter of the particles was determined from the backscattered electron image (BEI). The intensities of the characteristic peaks in the spectra were determined by the top-hat filter method (Van Espen and Janssens, 1993). A home-written program, PRODPP, was used for peak identification. In total, 41 fine and 40 coarse filter samples were analysed by conventional EPMA. Approximately 400 particles were examined this way for each sample, resulting in some 32,000 single particle analyses. Considering the uniform particle distribution on the Nuclepore polycarbonate filter, the absolute number of particles per filter for each size fraction was calculated in the following way. During the automated analysis, a selected area of the filter was analysed and divided into several fields. Their number depended on the selected size of filter area and the magnification, which defines the size of a single field (e.g., at a magnification of 1000x, the field was 120 mm wide and 88 mm long). The software controlled the movements of the sample stage from one field to another inside the defined area, until the fixed number of spectra from particles (in this case 400) was collected. The number of analysed fields, scanned to find the fixed

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number of particles, was recorded for the further calculations. Assuming a uniform distribution of the particles on the filter and knowing the total sampled area of the filter (i.e., the area of the aerosol deposit), the total number of the particles on the filter was estimated (Worobiec et al., 2005). Using this value and the sampled air volume, the number of particles in m3 per size fraction was then calculated. It should, however, be stressed that the number of particles can only be seen as an approximated value. The low-Z measurements were carried out for the cascade impactor samples. They were done on a JEOL-733 electron probe micro-analyser equipped with an OXFORD energy-dispersive X-ray detector with atmospheric thin window (FWHM ¼ 150 eV, E ¼ 5.9 keV). Since the aerosol particles were collected on Al-foil, working in the automated analysis mode, based on the BEI, was not possible, because the difference between the average atomic number of the substrate and the particles was too low. Therefore, the TW-EPMA was done manually in the secondary electron image (SEI) mode. Since the Al concentration in the samples could not be calculated, the classification of the aluminosilicates was based on the combination of the elements Si and Fe. When the concentrations of these elements were approximately 20% for Si, 3–10% for Fe, while very low concentrations of C were observed, the particles were classified as aluminosilicates. To achieve a low background in the X-ray spectra and a high sensitivity for light element analysis, a 10 kV accelerating voltage and 0.5 nA beam current were applied. In order to limit the beam-damage for beam-sensitive particles, each measurement was carried out using a cold sample-holder at the temperature of liquid nitrogen, i.e., at 193 1C (Szalo´ki et al., 2001; Worobiec et al., 2003). The size of each individual particle was measured from a high-magnification SEI. The samples were analysed manually and spectra were obtained for approximately 100 particles per sample, resulting in some 4500 single particle analyses. The characteristic X-ray spectra from the TWEPMA were evaluated by non-linear least-squares fitting, using the AXIL code (Vekemans et al., 1994). Semi-quantitative calculation of the particle composition including light elements such as C, N and O was performed by an approximation method (EPPROC) (Osa´n et al., 2000; Szalo´ki et al., 2000, 2004). The algorithm is based on the combination of Monte Carlo simulations and successive approximation as

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a numerical procedure. This quantification procedure utilises a modified version (Ro et al., 1999, 2003) of a Monte Carlo code called CASINO (Hovington et al., 1997), which was originally designed to model interactions between a low-energy electron beam and atomic-bound electrons. The iteration procedure is based on a reverse Monte Carlo method: in each iteration step, the simulation program calculates the characteristic intensities and a new set of concentration values for the particle elements is determined. Using EP-PROC, the concentrations of the detected elements in standard particles down to 0.3 mm can be calculated with good agreement between the expected and calculated concentrations, within 3–8% relative (Szalo´ki et al., 2000). The very large and poorly organised data matrix from the analysis of many particles per sample (100–400) by both conventional and TW-EPMA has to be subjected to multivariate analysis to reduce the amount of data and to extract the relevant (pertinent) information. The multivariate analysis procedure used was developed by Bernard et al. (1986) and considers compositional data only. Essentially, it is based on a sequence of hierarchical and non-hierarchical cluster techniques (HCA and NHCA). For comparison of the different aerosol samples of one type (coarse SFU samples, fine SFU samples, stage 1, 2 or 4 of the cascade impactor), all particles were classified by non-hierarchical clustering analysis based on the Forgy algorithm (Massart and Kaufmann, 1983), using DPP software (Van Espen, 1984). The initial centroids were selected by a sequence of hierarchical cluster analyses performed with the IDAS program (Bondarenko et al., 1996). As the initial data, the normalised X-ray intensities conventional EPMA were used, while the calculated elemental weight concentrations (wt%) were used for results obtained by TW-EPMA. A non-hierarchical cluster analysis was performed on the fine and coarse size fractions independently. To investigate the correlations between the different samples, i.e., to see which samples had a similar chemical composition, the results from the NHCA were used as input for a principal component analysis (PCA). The PCA calculations were carried out using the SIMCA-P software package (Umetri AB and Ericsson Erisoft AB, Sweden), based on the covariance matrix of the abundances of the different particle classes. During the calculations no data scaling, only a data centralisation was applied.

5. Results and discussion 5.1. Conventional EPMA 5.1.1. Coarse particle fraction In the coarse particles data set, 14 different particle types were identified: aluminosilicates, P–S–Cl–K-rich, Si-rich, low-Z-rich, K–P-rich, NaCl, aluminosilicates agglomerated with fresh sea salt, K-rich, K–S-rich, CaSO4, P-rich, Na–P–S–Clrich, Fe-rich and Ti-rich. The different particle types and the possible relationships between their compositions were achieved by PCA, performed on the cluster analysis results. The loading plot of the PCA for the coarse size fraction is shown in Fig. 3. The distribution of the particle types over a two-dimensional plot shows that in the area of positive loadings for component 1, particle types of a composition typical for the soil dust can be found (Al–Si–Fe–Ti-rich species). Since the soil dust re-suspension is very small in the Amazonian forest (high humidity) and considering the backward trajectories (see Fig. 2), the composition of the particle types and their time of appearance, one can conclude that these particles are advected from the Sahara desert. For the same reason (backward trajectories, composition and appearance time), the particles located in the area of positive loadings of component 2 can be associated with the Atlantic Ocean. The area of positive loadings for both components 1 and 2 contains the mixed transported particles. Most of the particle types with negative loadings for both components contain P and K, which in this case can be considered as the fingerprint of the biogenic particles (P–S–Cl–K-rich, K–P-rich, K-rich, P-rich and low-Z-rich), and these particles are undoubtedly related to the Amazon forest. During the manual analysis by scanning electron microscopy (SEM), various kinds of biogenic particles were recognised, such as parts of plants, pollen, spores, algae, and insect fragments, especially in the samples collected during April, after the invasion of the Saharan dust into the Amazon Basin. Especially, the simultaneous presence of K, P, S was considered as ‘‘biogenic fingerprint’’, since these elements are essential in the fluids circulating within the superior plants. The PCA indicates also a relationship between these particle types and the K–S-rich type, which would usually be associated with a marine origin. However, its position on the loading plot indicates

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-0.5 -0.4

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0.0

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Fig. 3. Principal component loadings for the first two components, as derived from PCA on the conventional EPMA data for the coarse SFU samples (using the relative X-ray intensities); eigenvalue 1: 7.64; eigenvalue 2: 2.96, and explained variance: 76%.

clearly that these particles are rather associated with a biological origin. The sea-salt and CaSO4 particles form the marine type group. Both have positive loading values for the second component. The group of mixed particles (aluminosilicates+sea-salt) has positive loadings for both components. Its position suggests that both the marine source and the Sahara desert have contributed to their formation. This type is situated between the sea-salt and aluminosilicates particles, but it is clearly separated from the other groups, which implies a very different chemical character. Once the correlation of the different particle types was established, the evolution of their contribution in the air was plotted as a graph of five types, making the interpretation easier. Fig. 4 shows the absolute abundances of the distinguished particle types for the coarse size fraction. The sum of the abundances gives the total coarse particle number concentration in the air for each SFU sample. The samples from the beginning of the campaign (13–28 March) were characterised by a very high contribution of soil dust (sum of aluminosilicates, Si-rich, Fe-rich and Ti-rich particles). The abundances of the marine type aerosol particles (sum of NaCl and CaSO4) follow a similar trend. This similarity is especially clear in the case of CaSO4 particles. For the pure sea-salt particles not only a high abundance can be observed during the Saharan dust event, but later some peaks with higher sea-salt share can also be noticed. In the beginning of the campaign, the marine type and the mixed soil dust/sea-salt

particles follow a similar trend, which strongly suggests that part of the soil dust carried by the air masses from the Sahara has agglomerated with sea salt during its transport over the ocean as this transport occurred within the trade winds at low altitude (below 800 hPa). The latter explains also the high correlation between the pure soil dust and the mixed particles. The mixed soil dust/sea-salt particles were only found in the coarse size fraction. During the manual analysis, agglomerates of soil dust particles from the Sahara and sea-salt were observed. Mostly the soil dust particles were covered by a layer of sea salt or crystals of halite were seen on the particles. In this way, the particle diameter became larger, which explains why pure halite particles were mostly present in the coarse size fraction. At Balbina, the transport of mineral dust from Africa within the NE trade winds took place without interruptions between 24 and 28 March. A north–south shift in the source region (from Morocco to the Sahel) was observed depending on the pressure level considered. After the change in meteorological conditions, a drastic decrease of both soil dust and mixed soil dust/sea-salt particles was noticed. In Fig. 4, a clear transition to the ‘‘clean’’ period can be seen when almost no soil dust particles were observed (30 March–11 April). The particle loading was much lower and the relative contribution of the biogenic and organic (low-Z) particles clearly increased. Athough Fig. 4 indicates that soil dust and sea-salt particles were still present in the clean period, they were much lower in abundance. The soil dust particles during this

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Mixed aluminosilicates/sea salt particles 5.E+05

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0.E+00

sample

Fig. 4. Absolute abundances of the coarse particle types—results obtained by conventional EPMA on samples collected at Balbina, Amazon Basin, during the LBA/CLAIRE-98 field campaign.

period are likely attributable to local or regional soil dust re-suspension; their low abundance indicates, though, that the relative contribution from local/ regional dust was negligible during the Saharan dust episode. The absolute abundances of the coarse biogenic particles (sum of types: P–S–Cl–K-rich, K–P-rich, K-rich, K–S-rich, P-rich) were much lower than those of the soil dust particles. However, the relative abundances of various particle types varied with meteorological conditions. During the Saharan dust event, the biogenic particles were also observed but since the concentrations of the soil dust and marine particles were very high, the relative contribution of the biogenic particles to the total atmospheric particle population was almost negligible. During the clean period, a spectacular increase in the relative abundances of these particles could be observed, while at the same time, the total coarse particle concentration was much lower (see Fig. 4). Moreover, the concentration of biogenic and low-Z particles was very high in the samples collected during the night, while in the samples collected during the day it decreased to 10% of the night-time value. Most likely, this was related to the night-time inversions, causing less vertical dilution.

5.1.2. Fine particle fraction The following particle types were identified in the fine samples: aluminosilicates, Fe-rich aluminosilicates, Si-rich (quartz), S-rich, low-Z-rich, P–S-rich, S-rich aluminosilicates, Fe-rich, P-rich, S–Cl–Krich, CaSO4, Ti-rich, Cl-rich and K-rich. In comparison to the coarse size fraction, the soil dust particles are more diversified here. Fig. 5 shows the loading plot of the first two components from the PCA. The majority of the distinguished particle types are located quite close to the point (0, 0), which indicates their small impact on the overall fine aerosol composition. To this group belong gypsum particles, biogenic types (Cl-rich, S–Cl–K-rich, K-rich and P-rich) and soil dust, such as Ti-rich, Fe-rich and Si-rich aluminosilicates. Taking into account the information provided by the backward trajectories (Fig. 2) and the position of the particle groups in the PCA plot (Fig. 5), one can conclude that the positive loadings of the first component represent the African dust as the source, while the negative loadings indicate the Amazon Basin (local/regional) origin. It also suggests that only two sources should be considered for the fine size fraction.

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0.7 0.5 K-rich

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S-rich aluminosilicates Aluminosilicates

-0.4

-0.2

0

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component 1

Fig. 5. Principal component loadings for the first two components, as derived from PCA on the conventional EPMA data for the fine SFU samples (using relative X-ray intensities); eigenvalue 1: 8.80; eigenvalue 2: 2.37, and explained variance: 80%.

Together with the Fe-rich aluminosilicates, the pure aluminosilicates and the Si-rich particles are located in the area of the positive loadings for the first component, suggesting a similar origin. Here, again the Sahara desert is the most probable source of these particle types. The S-rich particle type, the low-Z elements particles and the P-S-rich particles are negatively loaded on the first component. The biogenic (P–S-rich) particles are clearly separated from the S-rich and low-Z element particles, probably because of different origins. Whereas the P–S-rich particles may be associated with biogenic emissions, the low-Z element and the S-rich particles may originate from biomass burning. The location of the S-rich particles together with low-Z element particles suggests that we rather deal with organic sulphur and not sulphates from marine origin. Neither pure sea-salt particles (halite) nor mixed particles were distinguished in the fine size fraction. The evolution of the different particle contributions in the fine size fraction is shown in Fig. 6. As can be seen by comparing Figs. 6 and 4, during the Saharan dust event the number concentration of fine particles was substantially larger than that of the coarse particles. After 28 March 1998, the concentration of fine particles decreased drastically and reached a level lower than that for the coarse size fraction (ca. 200,000 particles m3). In the case of the coarse size fraction (2.0–10 mm AD), the particle number concentration during the clean period was not higher than 300,000 particles m3, probably because of the large contribution of the biogenic aerosols, which are not abundant in the

fine size fraction (0.30 mmodiametero2 mm); the abundance of the fine biogenic particles was approximately constant over the whole clean period (see Fig. 6). Since the time trends for the biogenic and low-Z elements particles are rather similar, it is possible that the fine biogenic particles originated from biomass burning rather than from biogenic emission by the rain forest. The aluminosilicates are the most abundant particle type in the fine size fraction. The sum of all types of aluminosilicates accounted for 450% of the total fine aerosol population, while, in the coarse fraction, only 33% of the particles were classified as aluminosilicates. Generally, the soil dust particles (aluminosilicates, Si-rich, Ti-rich and Fe-rich) were relatively more abundant in the fine size fraction than in the coarse one (average values: 70% versus 44%). But in the Saharan dust event the contribution of the soil dust particles reached up to 98%, while the contribution of the other particle types was almost negligible (Fig. 6). This is not so surprising, as the long-range transported Saharan dust aerosol has relatively more fine particles than the sea-salt or biogenic particles. The time trend of the fine S-rich particles exhibits two maxima. One of them occurred on 29 March, when the backward trajectories show a transport of air masses from Europe over the Atlantic Ocean (see Fig. 2). The other one is on 7 April, which may have been a period of enhanced biogenic emissions. During this second maximum, there were also higher abundances of S–Cl–K-rich particles, which may also be from biogenic origin. In general, the

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23.03.98D 23.03.98N 24.03.98D 24.03.98N 25.03.98D 25.03.98N 26.03.98D 26.03.98N 27.03.98D 27.03.98N 28.03.98D 28.03.98N 29.03.98D 29.03.98N 30.03.98D 30.03.98N 31.03.98D 31.03.98N 01.04.98D 01.04.98N 02.04.98D 02.04.98N 03.04.98D 03.04.98N 04.04.98D 04.04.98N 05.04.98D 05.04.98N 06.04.98D 06.04.98N 07.04.98D 07.04.98N 08.04.98D 08.04.98N 09.04.98D 10.04.98D 10.04.98N 11.04.98D 11.04.98N 12.04.98D

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sample

Fig. 6. Absolute abundances of the fine particle types—results obtained by conventional EPMA on samples collected at Balbina, Amazon Basin, during the LBA/CLAIRE-98 field campaign.

S-rich particles can be from different sources, such as the marine environment, biomass burning (organic sulphur), biogenic emissions, and anthropogenic SO2. A significant fraction of the biogenic aerosol in forested areas consists of secondary aerosol formed by gas-to-particle conversion of organic, nitrogen-, and sulphur-containing gases (Jaenicke and Mathias-Maser, 1992). As already mentioned, March 1998 was characterised by anomalously dry weather conditions. This may explain the high concentrations of low-Z element and S-rich particles, which in this case may probably be classified as organic. 5.2. Thin-window EPMA Fig. 7(A)–(C) shows the relative abundances of the distinguished particle types for the impactor stages 1, 2, and 4, respectively. Particles with an AD of 0.25–0.5 and 0.5–2.0 mm (impactor stages 1 and 2) were classified as fine particles; those with an AD of 2.0–8.0 mm (stage 4) as coarse particles. Unfortunately, the evolution of the absolute abundance of an individual particle type could not be derived, since only relative abundances could be calculated.

When comparing the results for the 0.25–0.5 mm size fraction of TW-EPMA with the fine size fraction results of the conventional EPMA, it appears that TW-EPMA resolved two types of aluminosilicate particles with and without organic content, and that the two types together were the most abundant group of particles. As already indicated when discussing the results from conventional EPMA, the aluminosilicate particles can be related to the air masses transported from the Sahara, but it seems that once the particles reached the Amazon Basin, they react or agglomerate with the organic matter which originates from the Amazon Basin itself. This is important information about the specification of the soil dust particles and their composition, which could not be obtained by conventional EPMA. The same observation was made for the Si-rich particles, which in some cases also seemed to be agglomerated with organic material. Such particles were especially present in the finest size fraction. The organic particles were most abundant in the fine size fraction. This suggests that the organic matter originated to a large extent from biomass burning, of which several events in the neighbourhood were observed

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Soot 100%

Oxides

90%

Organic Biogenic

80% 70% 60%

Soil dust

50% 40% 30% 20%

Soil dust + organic 10% 0% 25.03.98 27.03.98 28.03.98 29.03.98 31.03.98 01.04.98 02.04.98 03.04.98 04.04.98 09.04.98 10.04.98 11.04.98 12.04.98

Organic 100% 90% 80%

Biogenic Aged sea salt + soil dust

70% 60%

Aged sea salt + organic

50% 40% 30% 20% 10%

Soil dust + organic 0% 25.03.98 26.03.98 27.03.98 29.03.98 30.03.98 01.04.98 02.04.98 06.04.98 07.04.98 08.04.98 10.04.98 11.04.98 12.04.98

Sea salt + organic Fig. 7. Relative abundances of distinguished particle types from TW-EPMA on cascade impactor samples collected at Balbina, Amazon Basin, during the LBA/CLAIRE-98 field campaign for fraction. (A) Stage 1 (0.25–0.5 mm AD), (B) stage 2 (0.5–2.0 mm AD), (C): stage 4 (2.0–4.0 mm AD).

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100% 90%

Biogenic 80%

Aged sea salt 70%

+ organic

60% 50%

Sea salt 40% 30%

Organic 20% 10%

Soil dust 12.04.98

11.04.98

10.04.98

09.04.98

08.04.98

07.04.98

06.04.98

05.04.98

04.04.98

03.04.98

02.04.98

01.04.98

31.03.98

30.03.98

29.03.98

28.03.98

27.03.98

26.03.98

25.03.98

0%

Fig. 7. (Continued)

during the sampling campaign. It is well known that biomass burning aerosol consists predominantly of fine particles. Biogenic particles occurred as a separate group and were particularly abundant in the coarse size fraction. Generally, the particle types distinguished in the 0.25–0.5 mm size fraction could be classified into two basic aerosol groups: (1) Soil dust particles: pure ones (aluminosilicates and oxides) and soil dust agglomerated with organic matter. Saharan dust is the most probable source of these particles. (2) Organic matter and biogenic particles: regarding the biogenic particles (containing elements as P, K, and S) a local source can be considered. However, taking into account both the observations from the visualisation of the particles, the results from conventional EPMA and backward trajectories (Fig. 2), biogenic emissions are rather unlikely for these particles in the finest size fraction. Particles from biogenic emissions are more expected in the coarse size fraction. Therefore, for the small biogenic-related particles, biomass burning is suspected as the most possible source.

Pure marine and mixed aerosol particles were not observed in the finest size fraction. Such particles are rather present in the coarse size fraction. Regarding the 0.5–2.0 mm size fraction, an important contribution of marine-related particles was observed in three general forms: (1) pure sea-salt particles, most abundant in the beginning of the sampling campaign; (2) aged sea salt (mixed types of NaNO3/NaCl or pure NaNO3) observed in the period when the contribution of the pure sea-salt particles decreased; and (3) agglomerates of sea salt and soil dust, which were observed especially in the beginning of the sampling campaign. These observations confirmed the conventional EPMA results, but important additional information was obtained regarding the complex composition of the aged sea-salt particles. It is worth mentioning that all marine particles contained carbon. Additional aged sea-salt particle types were identified, such as mixed NaCl/NaNO3 or pure NaNO3, which cannot be seen by conventional EPMA. A significant contribution of marine related

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particles was also observed in the coarse size fraction (2–4 mm); however, here only the aged sea salt seems to be agglomerated with organic matter. The aged sea salt results from reactions with the gaseous pollutants SO2 and NOx, which mainly originated from biomass burning. The agglomerates of aged sea salt with organic matter are likely the result of a two-step process, namely the reaction of fresh sea salt from the Atlantic Ocean with gaseous pollutants, followed by the agglomeration with organic matter. The time schedule of appearance of the two particle types provides evidence for such explanation. The reaction with the gases and the agglomeration of the sea salt with organic matter need time and this may lead to a shift in time of appearance of various particle types (see Fig. 7). This is even observed when considering only relative abundances. 6. Conclusions The experiment was designed to study the natural aerosol of the Amazon Basin forest. Both local and long-range transport sources were identified. Backward trajectories indicated transport of air masses from North Africa and the EPMA results showed how much the Saharan dust contributes to the composition of the particulate matter above the Amazon Basin and under which form it is transported to the basin. Changes in particle number concentrations and reactions of the Saharan dust as well as agglomeration with sea salt above the Atlantic Ocean were observed. High abundances of dust particles were clearly noticed during the period 23–29 March. The maximum was on 25 March for all particles types in both the coarse and fine size fractions. The number concentration of the fine soil dust particles reached a value of 3.5 millions m3. The number concentrations were higher in the fine size fraction, with soil dust dominating in the first part of the campaign. A significant contribution of natural biogenic aerosol particles was observed in the last 2 weeks of the sampling campaign, but only in the coarse size fraction. Elements associated with soil dust (Al, Si, Ti, Mn, and Fe) and biogenic aerosol (K, S, Cl, Ca, P, Zn, and others) dominated the aerosol composition. Conventional EPMA combined with PCA provided high-quality information about the composition and the chemical properties of the Amazonian aerosol particles, as well as about the relationship between different aerosol types.

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The comparison of the results from thin-window and conventional EPMA indicated that the two techniques confirm and complement each other. With conventional EPMA, absolute abundances of the different particle types can be obtained, while TW-EPMA gives more information on the particle composition. The information from both techniques appeared crucial in the interpretation of the results. With TW-EPMA important information was obtained concerning the contribution from organic aerosol particles and the agglomeration of various types of particles. It should be emphasised that carbon was found in almost all particle types. Furthermore, chemical transformation of the marine particles in the air was corroborated. This was particularly true for aged sea-salt particles, especially in the abundant presence of particles originating from biomass burning and biogenic emissions. The sea-salt particles reacted with the gaseous pollutants generated during biomass burning; therefore, e.g., sodium nitrates were formed. This observation was only possible due to the application of TW-EPMA and the cold-stage technique, since the nitrates are beam sensitive. Acknowledgement The support from the FWO (Fund for Scientific Research, Flanders, Belgium) for the post-doctoral researcher Anna Worobiec is appreciated.

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