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K in individual haze particles over Kalimantan during the 1997 Indonesian forest fires
Atmospheric Environment 35 (2001) 4237–4243
Very high weight ratios of S/K in individual haze particles over Kalimantan during the 1997 Indonesian forest fires Miwako Ikegamia,*, Kikuo Okadaa, Yuji Zaizena, Yukio Makinoa, Jorgen B. Jensenb, John L. Grasb, Hery Harjantoc a
Meteorological Research Institute, Tsukuba, Ibaraki 305-0052, Japan b CSIRO, PMB 1, Aspendale, Vic., 3195, Australia c Meteorological and Geophysical Agency, Jakarta, Indonesia
Received 8 December 2000; received in revised form 11 April 2001; accepted 27 April 2001
Abstract The elemental composition of individual aerosol particles of 0.15–3 mm radius, collected over Kalimantan during the 1997 Indonesian forest fire event, was analyzed using a transmission electron microscope equipped with an energydispersive X-ray analyzer (EDX). Although 60–90% of the particles collected at altitudes of 1–5 km contained K, they exhibited high weight ratios of S/K with median values of 9–18 independent of particle size. These were much larger than those (median values of 2–4) obtained from the forest fires in northern Australia. The high weight ratios over Kalimantan are considered to be due to the heterogeneous growth of particles through the oxidation of SO2. In addition to SO2 from the combustion of forest biomass, SO2 originating from the combustion of peat below the ground is believed to have been important in producing the high S/K ratios. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Aerosol particles; Biomass burning; Indonesia; EDX analysis; Elemental composition; Peat combustion
1. Introduction Biomass burning is an important source of gaseous and particulate matter in the troposphere. Particles originating from biomass burning contain sulfate, nitrate and ammonium ions along with organic material, elemental carbon and metallic elements (e.g., Andreae et al., 1988; Gaudichet et al., 1995). These particles affect the transmission of incoming solar radiation and outgoing infrared radiation through the atmosphere. Moreover, these particles will act as cloud condensation nuclei and change the size distribution of droplets in clouds (Kaufman and Fraser, 1997). Hence, particles originating from biomass burning impact the global climate through both direct radiative forcing and changing the radiative characteristics of clouds.
The haze that originated from the 1997 Indonesian forest fires was a significant environmental concern. In order to establish the features of particles, which produced the extensive haze layer, individual aerosol particles were collected on 23 and 25 October 1997 on board an aircraft at several altitudes between 1 and 5 km over Banjarmasin in southern Kalimantan. The aim of this paper is to report the results of the elemental composition of the particles obtained using a transmission electron microscope equipped with an energydispersive X-ray (EDX) analyzer, with special emphasis on the S/K weight ratio. The weight ratios of S/K in the Kalimantan haze particles are also compared with those obtained in smoke particles in northern Australia.
2. Methods *Corresponding author. Fax: +81-298-55-9971. E-mail address: [email protected] (M. Ikegami).
Observations were carried out on 23 and 25 October 1997 on board a research aircraft (Fokker F27) flying at
1352-2310/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 2 4 7 - 3
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altitudes between 1 and 5 km during the PACE-5 (Pacific Atmospheric Chemistry Experiment) campaign over an area about 100–200 km west of Banjarmasin in Kalimantan. Sample air was introduced to the instruments using the CSIRO isokinetic inlet, which is a two-stage decelerator with a boundary layer bleed. Collections of individual aerosol particles were made using an automatic two-stage impactor (one with a jet diameter of 1.0 mm and another of 0.5 mm) made by the Meteorological Research Institute, Japan. In this research, the results described are those obtained from the samples collected with the 1 mm impactor. For this impactor, the air flow-rates were 3.7, 3.4 and 3.0 l min1 at altitudes of 1.3, 2.6 and 4.5 km, respectively. The collection efficiency of the impactor was calculated on the basis of the experimental results of Ranz and Wong (1952) with the assumption that the particles are spheres with a density of 2 g cm3. This gives a collection efficiency of 50% at 0.22 mm radius and 100% at 0.34 mm radius at an altitude of 1.3 km, and an efficiency of 50% at 0.18 mm and 100% at 0.30 mm radius at 4.5 km altitude. The collecting surface used is a nitrocellulose (collodion) film covered with carbon. The collected particles were shadowed with Pt/Pd alloy at an angle of arctan 0.5 ( The to the collecting surface, to a thickness of 7 A. equivalent dry spherical particle radius was calculated by using the shadow length and apparent diameter, as indicated in Okada (1983). The particles were examined with a transmission electron microscope (Hitachi, H-600 and H-6010) to obtain the size and morphology. They were also examined using an EDX analyzer (Kevex, Sigma II) through a Kevex UTW (ultra-thin window) detector to obtain the elemental composition. The electron beam was irradiated at the center of a particle at an accelerating voltage of 50 kV. The diameter of the electron beam was approximately 0.07 mm during the examination. The counting time was 75 s. The weight percentage of elements whose atomic numbers are greater than 11 was calculated using the thin film method (Cliff and Lorimer, 1975).
3. Results During the period from 23 to 25 October 1997, the observation areas were covered with a haze layer up to approximately 4 km altitude, although the height of the boundary layer was approximately 2–3 km. Many cumulus clouds penetrated through the haze layer and relative humidity exceeded 60% below 5 km altitude. The air around the convective clouds was influenced by smoke up to the maximum flight levels of 4.5–4.7 km. The observations were carried out over the areas where the smoke was so dense that visible flames could not be
seen from the aircraft. During the observation, samples were collected in the haze layer and the free troposphere. Fig. 1a shows an electron micrograph of individual particles collected at 1.3 km altitude during the period of 10:10–10:30 GMT on 23 October. The particles are characterized by the presence of an electron-opaque material surrounded by a semi-transparent material. Okada et al. (2001) examined the mixture state of individual aerosol particles collected in this campaign using dialyses (extraction) with water or benzene in conjunction with electron microscopy. Their results showed that the semi-transparent material was an organic material and most of the electron-opaque material in the central part of the particle was sulfate. The results obtained by the EDX analyses of electronopaque material are shown in this paper. Fig. 1b shows an EDX spectrum obtained from the central part of particle A: The spectrum clearly shows the dominant peak of S; along with a small peak of K: Note that most of the C; N; and O peaks originate from the collecting substrate. The presence of K suggests that the particle was originally formed by biomass burning (see for example, Andreae, 1983). We have analyzed other samples collected in the atmosphere over Kalimantan, up to 5 km altitude, and most of the particles in the samples used for the EDX analysis (Table 1) exhibited a morphological appearance similar to the particles shown in Fig. 1a. None of the particles collected below the 3 km altitude produced satellite droplet rings on the collecting surface, which is characteristic of sulfuric acid (Frank and Lodge, 1967). Electron diffraction (Okada et al., 2001) showed the presence of ammonium sulfate in the haze particles. Hence, S in these particles would most probably be in the form of ammonium sulfate. Table 1 summarizes the results of the particle classification and median S/K weight ratios for samples collected over Kalimantan on 23 and 25 October 1997. Samples D4 and G3 were collected just above the haze layer. On the basis of the quantitative EDX analyses, all particles in the radius range of 0.15–3 mm were classified into three types, ‘‘minerals’’, ‘‘seasalt’’ and ‘‘sulfurrich’’. It should be noted that more than 80% of the analyzed particles had submicrometer radius. For the classification, the weight proportion defined as P(X) was used PðXÞ ¼ X=ðNa þ Mg þ Al þ Si þ S þ Cl þ K þ CaÞ: Particles with PðAl þ SiÞX0:1 are considered to be mineral-containing particles and they are termed minerals. Those with PðNaÞX0:1 are classified as seasalt. Other particles with a value of PðS þ KÞX0:8 are classified as sulfur-rich. In the sulfur-rich particles, only 3 particles had PðS þ KÞo0:9 in the three samples collected on 23 October. Moreover, the sulfur-rich particles were divided into those with and without K. The S=K value was obtained from sulfur-rich particles
M. Ikegami et al. / Atmospheric Environment 35 (2001) 4237–4243
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Fig. 1. Electron micrograph of individual particles collected at 1.3 km altitude during the period from 10:10 to 10:30 GMT on 23 October (a), and EDX spectrum obtained from the central part of the particle A (b). The enlarged image of the particle A was also indicated in (b). The peak of Cu* is due to the copper electron microscopic grid. Also, peaks of Pt* and Pd* are from the shadowing material.
with K: For sulfur-rich particles, the elemental composition obtained by the EDX analysis was quite simple; these particles mainly contained S and K: It should be noted that Cl was not detected in most of the sulfur-rich particles. Although mineral and sea-salt particles were found, sulfur-rich particles containing K constituted a large fraction (approximately 80%) of all the particles
collected below 2.6 km altitude. On the other hand, for samples collected at altitudes higher than 4.4 km these comprised a smaller fraction, (57% (D4) and71% (G3)). Fig. 2a shows the weight ratios of S/K in individual particles as a function of particle radius for samples F3 (1.3 km altitude), E2 (2.6 km) and D4 (4.4 km), which were collected on 23 October. The particles used for Fig. 2 are those which contained K (sulfur-rich particles
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Table 1 Abundance of particles and median values of the S/K weight ratio in individual particles of 0.15–2.1 mm radius collected over Kalimantan on 23 and 25 October 1997. The collection was made over areas enclosed by 2.8–3.31S, 112.3–114.01E on 23 October and 3.1–3.31S, 112.2–112.71E on 25 October Sample ID
Altitude (km)
Sampling period (GMT)
Abundance of particlesa (%) Mineral
Seasalt
Median S/K weight ratio Sulfur rich ðK > 0Þ
ðK ¼ 0Þ
23 October D4 E2 F3
4.4 2.6 1.3
08:30–08:45 09:02–09:05 10:10–10:30
13 1 7
0 2 1
57 89 89
30 8 8
9.2 12.5 9.6
25 October G3 H4 G4
4.7 2.6 1.3
02:40–02:44 04:45–04:48 03:25–03:28
0 4 2
0 2 2
71 78 74
29 16 22
15.1 8.9 18.2
a
Number of particles examined is 100 for D4, E2, F3 and G3 samples and 50 for H4 and G4 samples.
Fig. 2. Weight ratios of S=K in individual particles as a function of particle radius for samples F3 (1.3 km altitude; open circle), E2 (2.6 km, closed circle) and D4 (4.4 km; cross) which were collected over Kalimantan on 23 October 1997 (a), and for samples G4 (1.3 km; open circle), H4 (2.6 km; closed circle) and G3 (4.7 km; cross) collected on 25 October (b). The particles used for Fig. 2 are those that contained K (particles with K>0 in Table 1).
with K > 0 in Table 1). The ratios range widely from 0.6 to 68. Fig. 2b shows the S=K ratios as a function of particle radius for samples G4 (1.3 km), H4 (2.6 km) and G3 (4.7 km) collected on 25 October. The ratios in Fig. 2b are from 0.3 to approximately 100, which are larger than those on 23 October. Both figures show that the S/K ratios have no clear size and height dependence. The median values of the S=K weight ratios were 9.2– 12.5 on 23 October and 8.9–18.2 on 25 October (Table 1). Note that the S=K ratio for K2SO4 is 0.41, which is much smaller than the measured ratios. During the PACE-5 aircraft observation, we also carried out an observation of forest fires over Arnhem Land (near Katherine) in northern Australia in October 1997. Table 2 summarizes the results of particle classification and median S=K weight ratios in two samples collected on 16 October. The procedures for the particle classification and the S=K ratio evaluation are the same as those used for Table 1. In the sulfur-rich particles, only one particle had PðS þ KÞo0:9 in the sample G1. The number of sulfuric acid containing particles in the sulfur-rich particles is also shown in parenthesis in Table 2. Sample G1 was collected at 1.5– 2.8 km altitude over the leeward area of a fire front. Sample G2 was collected in the free troposphere at 5.1 km altitude over an area, which was leeward of another fire front. It contained sea-salt particles, (approximately 30%), and also sulfuric acid particles. The abundance of sulfur-rich particles with K and seasalt particles in the sample G2 shows a vertical transport of these surface-derived particles by convective clouds. Moreover, sulfuric acid particles which showed satellite droplet rings on the collecting surface were found in the sample G2. As shown in Table 2, the sulfur-rich particles with K in both the samples comprised 64–52% of analyzed particles, with median S=K weight ratios of 4.0 (G1) and 2.6 (G2).
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Table 2 Abundance of particles and median values of the S/K weight ratio in individual particles of 0.21–1.2 mm radius collected over Arnhem Land (near Katherine) in northern Australia on 16 October 1997. The collection was made over areas enclosed by 14.6–14.81S, 132.0– 133.21E on 16 October. Sample G1 was collected at 1.5–2.8 km altitude leeward of a fire front. Sample G2 was collected at 5.1 km altitude over the area, which was leeward of another fire front Sample ID
Altitude (km)
Sampling period (GMT)
Abundance of particlesa (%) Mineral
Seasalt
Median S/K weight ratio Sulfur-rich ðK > 0Þ
ðK ¼ 0Þ 16 (4)b 21
G2
5.1
06:00–06:15
1
31
52
G1
1.5–2.8
05:03–05:13
11
4
64
a b
2.6 4.0
Number of particles examined is 100 for both samples. Abundance of sulfuric acid particles.
4. Discussion Submicrometer KCl and K2SO4 particles are produced by biomass burning (Akagawa, 1993; Gaudichet et al., 1995; Christensen et al., 1998). The weight ratio of S/K in particles originating from biomass burning has been measured by several researchers. Gras et al. (1992) carried out analyses of smoke particles from controlled burning of dried eucalyptus timber and leaf litter and found S=K weight ratios of less than 0.1. Gaudichet et al. (1995) collected particles close to savanna fires on the Ivory Coast and found low S=K ratios (0.052). They also found that the ratio in a smoldering condition was relatively high, ðS=K ¼ 0:2Þ: Christensen et al. (1998) analyzed the composition of particles originating from the combustion of straw and found that the average S/K ratio in submicrometer particles was 0.12. The composition of submicrometer aerosol particles in the haze associated with biomass burning has also been reported. S=K weight ratios measured by bulk analyses were 0.4– 2.3 in haze layers over the Amazon Basin (Andreae et al., 1988; Artaxo et al., 1994, 1988; Talbot et al., 1988) and the African rain forest (Bingemer et al., 1992). The S=K ratios in the present work are derived from individual micro-analysis while those given in the literature are obtained by bulk chemical analysis. However, the median values of S=K we have found over northern Australia are close to the values for haze particles reported in previous researches. Although the types of plants and combustion conditions may influence the composition of particles emitted to the atmosphere (Echalar et al., 1995; Gaudichet et al., 1995), the presence of particles with very high S=K ratios over Kalimantan implies the accumulation of S in particles in the atmosphere after their emission. Moreover, the absence of Cl suggests Cl loss from KCl in particles by chemical reactions probably with H2SO4 and/or HNO3. S in the particles was not in the form of sulfuric acid. From previous research (e.g., Andreae
et al., 1988), ammonia would be generated from biomass burning and would neutralize sulfuric acid. This is consistent with the finding of Okada et al. (2001) that the central electron-opaque crystals in individual haze particles over Kalimantan were identified as ammonium sulfate. One of the remarkable characteristics of forest fires in Indonesia was the occurrence of ground fires in peat swamp forests, which are located in the lowlands of Sumatra, Borneo and west New Guinea (e.g., Malingreau et al., 1985; Goto, 1998; Levine, 1999). Levine (1999) estimated the emission amount of particles by the combustion of peat in the 1997 Indonesian biomass burning and it constituted >90% of the total mass emission from the burning. Although not shown here, some results by electron microscopy showed that the particles of >0.1 mm radius generated from the combustion of the peat were considered to be composed of organic material with very small fractions of sulfur. The average S=K ratios for particles that originated just after the combustion of the 10 peat samples in Kalimantan were 1.80 while the ratios from the combustion of the 10 timber samples were 0.60 (CSIRO, 1999). The particles from peat combustion appear to be a factor 3 enriched in sulfur as compared to the particles from timber combustion. However, the S=K value (1.80) for peatgenerated particles is far below the S=K ratios obtained over Kalimantan in the present research. Biomass burning is a significant source of SO2 (Hegg et al., 1987; Andreae et al., 1988; Bates et al., 1992). In our observation, SO2 concentrations in the Kalimantan haze layer were 0.5–3 ppbv, which is larger than those (o0:5 ppbv) in the plume from biomass burning over northern Australia. In order to assess SO2 emission from this source, peat was burnt in the laboratory by heating dried peat, which was sampled in April 1998 below the ground near Banjarmasin. The peat was burnt in a chamber and the evolved SO2 concentration was measured with a SO2 analyzer (Thermo Electron Co.
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Ltd., Model 43A). The emission factor of SO2 was evaluated to be X1.5 102 mg g1. Rice hulls were also burnt and the emission factor was evaluated as a reference. It was found that the emission factor of SO2 for peat combustion is nearly one order of magnitude larger than that measured for the combustion of rice hulls. Large amounts of SO2 from the combustion of peat would be oxidized in the particles and then the growth of particles would occur through the heterogeneous oxidation of SO2 accompanying the intake of ammonia from the burning (e.g., Andreae et al., 1988). We conclude that SO2 released by the combustion of peat below the ground contributed to the formation of particles with high S=K ratios. Moreover, the absence of height dependence of the S=K weight ratios in the haze particles was probably due to the active vertical mixing of air over Kalimantan.
5. Conclusions Collections of individual aerosol particles were made on 23 and 25 October 1997 on board an aircraft at several altitudes between 1 and 5 km as part of the PACE-5 campaign over Kalimantan during the 1997 Indonesian forest fires. Observations were also made over forest fires in northern Australia. The elemental composition of particles of 0.15–3 mm radius was examined using a transmission electron microscope equipped with an EDX analyzer. Although 60–90% of the particles collected at altitudes of 1.3, 2.6 and 4.4 km over Kalimantan contained K; they exhibited high weight ratios of S/K, with median values between 9 and 18. These were much larger than those (median values of 2–4) obtained from the forest fires in northern Australia. We conclude that the high S=K weight ratios in Kalimantan haze particles were due to the growth of particles, probably through the heterogeneous oxidation of SO2 that originated from the combustion of peat below the ground.
Acknowledgements The authors are grateful to Dr. Y. Tsutsumi and Mr. Y. Sawa of the Meteorological Research Institute (MRI), the crews of AFTS, the staff of CSIRO and Showa Avi. Co. for their help during the observations. They also wish to express their gratitude to Mr. S. Diharto of MGA, Dr. S. Indroyono and Ms. T. Sribmawati of BPPT for their kind advice and support. The authors also wish to express their gratitude to Dr. K. Ohkushi, University of Tsukuba, for his help for the EDX analysis. This study was carried out as a
sub-project of the Japanese Study of the Behavior of the Greenhouse Gases and Aerosols supported by the Science and Technology Agency of Japan.
References Akagawa, H., 1993. Elemental composition of atmospheric soot particles and their capability as cloud condensation nuclei. Master thesis, Tsukuba University, 79p (in Japanese). Andreae, M.O., 1983. Soot carbon and excess fine potassium: long-range transport of combustion-derived aerosols. Science 220, 1148–1151. Andreae, M.O., Browell, E.V., Garstang, M., Gregory, G.L., Harriss, R.C., Hill, G.F., Jacob, D.J., Pereira, M.C., Sachse, G.W., Setzer, A.W., Silva Dias, P.L., Talbot, R.W., Torres, A.L., Wofsy, S.C., 1988. Biomass-burning emissions and associated haze layers over Amazonia. Journal of Geophysical Research 93, 1509–1527. Artaxo, P., Gerab, F., Yamsoe, M.A., Marins, J.V., 1994. Fine mode aerosol composition at three long-term atmospheric monitoring sites in the Amazon Basin. Journal of Geophysical Research 99, 22857–22868. Artaxo, P., Storms, H., Bruynseels, F., Grieken, R.V., Maenhaut, W., 1988. Composition and sources of aerosols from the Amazon Basin. Journal of Geophysical Research 93, 1605–1615. Bates, T.S., Lamb, B.K., Guenther, A., Dignon, J., Stoiber, R.E., 1992. Sulfur emissions to the atmosphere from natural sources. Journal of Atmospheric Chemistry 14, 315–337. Bingemer, H.G., Andreae, M.O., Andreae, T.W., Artaxo, P., Helas, G., Jacob, D.J., Mihalopoulos, N., Nguyen, B.C., 1992. Sulfur gases and aerosols in the above the equatorial African rain forest. Journal of Geophysical Research 97, 6207–6217. Christensen, K.A., Stenholm, M., Livbjerg, H., 1998. The formation of submicron aerosol particles, HCl and SO2 in straw-fired boiler. Journal of Aerosol Science 29, 421–444. Cliff, G., Lorimer, G.W., 1975. The quantitative analysis of thin specimens. Journal of Microscopy 103, 203–207. CSIRO, 1999. Air pollution from Indonesian biomass fires: extended report to AusAID. CSIRO Atmospheric Research, Australia. Echalar, F., Gaudichet, A., Cachier, H., Artaxo, P., 1995. Aerosol emissions by tropical forest and savanna biomass burning: characteristic trace elements and fluxes. Geophysical Research Letters 22, 3039–3042. Frank, E.R., Lodge Jr., J.P., 1967. Morphological identification of airborne particles with the electron microscopy. Journal de Microscopie 6, 449–456. Gaudichet, A., Echalar, F., Chatenet, B., Quisefit, J.P., Malingre, G., Cachier, H., Buat-Menard, P., Artaxo, P., Maehaut, W., 1995. Trace elements in tropical African savanna biomass burning aerosols. Journal of Atmospheric Chemistry 22, 19–39. Goto, Y., 1998. Peat fires in Jambi, Indonesia. Abstracts of International Study Conference on the 1997 Indonesian Forest Fire Event, Tokyo, 20 March 1998. Gras, J.L., Gillett, R.W., Bentley, S.T., Ayers, G.P., Firestone, T., 1992. CSIRO-EPA Melbourne aerosol study
M. Ikegami et al. / Atmospheric Environment 35 (2001) 4237–4243 (final report). CSIRO Atmospheric Research, Melbourne, 194p. Hegg, D.A., Radke, L.F., Hobbs, P.V., Brock, C.A., Riggan, P.J., 1987. Nitrogen and sulfur emissions from the burning of forest products near large urban areas. Journal of Geophysical Research 92, 14701–14709. Kaufman, Y.J., Fraser, R.S., 1997. The effect of smoke particles on clouds and climate forcing. Science 277, 1636–1639. Levine, J.S., 1999. The 1997 fires in Kalimantan and Sumatra, Indonesia: gaseous and particulate emissions. Geophysical Research Letters 26, 815–818. Malingreau, J.P., Stephens, G., Fellows, L., 1985. Remote sensing of forest fires: Kalimantan and North Borneo in 1982–83. Ambio 14, 314–321.
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Okada, K., 1983. Nature of individual hygroscopic particles in the urban atmosphere. Journal of the Meteorological Society of Japan 61, 727–736. Okada, K., Ikegami, M., Zaizen, Y., Makino, Y., Jensen, J.B., Gras, J.L., 2001. The mixture state of individual aerosol particles in the 1997 Indonesian haze episode. Journal of Aerosol Science (in press). Ranz, W.E., Wong, J.B., 1952. Impaction of dust and smoke particles on surface and body collectors. Industrial Engineering Chemistry 44, 1371–1381. Talbot, R.W., Andreae, M.O., Andreae, T.W., Harriss, R.C., 1988. Regional aerosol chemistry of the Amazon Basin during the dry season. Journal of Geophysical Research 93, 1499–1508.
Very high weight ratios of S/K in individual haze particles over Kalimantan during the 1997 Indonesian forest fires Miwako Ikegamia,*, Kikuo Okadaa, Yuji Zaizena, Yukio Makinoa, Jorgen B. Jensenb, John L. Grasb, Hery Harjantoc a
Meteorological Research Institute, Tsukuba, Ibaraki 305-0052, Japan b CSIRO, PMB 1, Aspendale, Vic., 3195, Australia c Meteorological and Geophysical Agency, Jakarta, Indonesia
Received 8 December 2000; received in revised form 11 April 2001; accepted 27 April 2001
Abstract The elemental composition of individual aerosol particles of 0.15–3 mm radius, collected over Kalimantan during the 1997 Indonesian forest fire event, was analyzed using a transmission electron microscope equipped with an energydispersive X-ray analyzer (EDX). Although 60–90% of the particles collected at altitudes of 1–5 km contained K, they exhibited high weight ratios of S/K with median values of 9–18 independent of particle size. These were much larger than those (median values of 2–4) obtained from the forest fires in northern Australia. The high weight ratios over Kalimantan are considered to be due to the heterogeneous growth of particles through the oxidation of SO2. In addition to SO2 from the combustion of forest biomass, SO2 originating from the combustion of peat below the ground is believed to have been important in producing the high S/K ratios. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Aerosol particles; Biomass burning; Indonesia; EDX analysis; Elemental composition; Peat combustion
1. Introduction Biomass burning is an important source of gaseous and particulate matter in the troposphere. Particles originating from biomass burning contain sulfate, nitrate and ammonium ions along with organic material, elemental carbon and metallic elements (e.g., Andreae et al., 1988; Gaudichet et al., 1995). These particles affect the transmission of incoming solar radiation and outgoing infrared radiation through the atmosphere. Moreover, these particles will act as cloud condensation nuclei and change the size distribution of droplets in clouds (Kaufman and Fraser, 1997). Hence, particles originating from biomass burning impact the global climate through both direct radiative forcing and changing the radiative characteristics of clouds.
The haze that originated from the 1997 Indonesian forest fires was a significant environmental concern. In order to establish the features of particles, which produced the extensive haze layer, individual aerosol particles were collected on 23 and 25 October 1997 on board an aircraft at several altitudes between 1 and 5 km over Banjarmasin in southern Kalimantan. The aim of this paper is to report the results of the elemental composition of the particles obtained using a transmission electron microscope equipped with an energydispersive X-ray (EDX) analyzer, with special emphasis on the S/K weight ratio. The weight ratios of S/K in the Kalimantan haze particles are also compared with those obtained in smoke particles in northern Australia.
2. Methods *Corresponding author. Fax: +81-298-55-9971. E-mail address: [email protected] (M. Ikegami).
Observations were carried out on 23 and 25 October 1997 on board a research aircraft (Fokker F27) flying at
1352-2310/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 2 4 7 - 3
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M. Ikegami et al. / Atmospheric Environment 35 (2001) 4237–4243
altitudes between 1 and 5 km during the PACE-5 (Pacific Atmospheric Chemistry Experiment) campaign over an area about 100–200 km west of Banjarmasin in Kalimantan. Sample air was introduced to the instruments using the CSIRO isokinetic inlet, which is a two-stage decelerator with a boundary layer bleed. Collections of individual aerosol particles were made using an automatic two-stage impactor (one with a jet diameter of 1.0 mm and another of 0.5 mm) made by the Meteorological Research Institute, Japan. In this research, the results described are those obtained from the samples collected with the 1 mm impactor. For this impactor, the air flow-rates were 3.7, 3.4 and 3.0 l min1 at altitudes of 1.3, 2.6 and 4.5 km, respectively. The collection efficiency of the impactor was calculated on the basis of the experimental results of Ranz and Wong (1952) with the assumption that the particles are spheres with a density of 2 g cm3. This gives a collection efficiency of 50% at 0.22 mm radius and 100% at 0.34 mm radius at an altitude of 1.3 km, and an efficiency of 50% at 0.18 mm and 100% at 0.30 mm radius at 4.5 km altitude. The collecting surface used is a nitrocellulose (collodion) film covered with carbon. The collected particles were shadowed with Pt/Pd alloy at an angle of arctan 0.5 ( The to the collecting surface, to a thickness of 7 A. equivalent dry spherical particle radius was calculated by using the shadow length and apparent diameter, as indicated in Okada (1983). The particles were examined with a transmission electron microscope (Hitachi, H-600 and H-6010) to obtain the size and morphology. They were also examined using an EDX analyzer (Kevex, Sigma II) through a Kevex UTW (ultra-thin window) detector to obtain the elemental composition. The electron beam was irradiated at the center of a particle at an accelerating voltage of 50 kV. The diameter of the electron beam was approximately 0.07 mm during the examination. The counting time was 75 s. The weight percentage of elements whose atomic numbers are greater than 11 was calculated using the thin film method (Cliff and Lorimer, 1975).
3. Results During the period from 23 to 25 October 1997, the observation areas were covered with a haze layer up to approximately 4 km altitude, although the height of the boundary layer was approximately 2–3 km. Many cumulus clouds penetrated through the haze layer and relative humidity exceeded 60% below 5 km altitude. The air around the convective clouds was influenced by smoke up to the maximum flight levels of 4.5–4.7 km. The observations were carried out over the areas where the smoke was so dense that visible flames could not be
seen from the aircraft. During the observation, samples were collected in the haze layer and the free troposphere. Fig. 1a shows an electron micrograph of individual particles collected at 1.3 km altitude during the period of 10:10–10:30 GMT on 23 October. The particles are characterized by the presence of an electron-opaque material surrounded by a semi-transparent material. Okada et al. (2001) examined the mixture state of individual aerosol particles collected in this campaign using dialyses (extraction) with water or benzene in conjunction with electron microscopy. Their results showed that the semi-transparent material was an organic material and most of the electron-opaque material in the central part of the particle was sulfate. The results obtained by the EDX analyses of electronopaque material are shown in this paper. Fig. 1b shows an EDX spectrum obtained from the central part of particle A: The spectrum clearly shows the dominant peak of S; along with a small peak of K: Note that most of the C; N; and O peaks originate from the collecting substrate. The presence of K suggests that the particle was originally formed by biomass burning (see for example, Andreae, 1983). We have analyzed other samples collected in the atmosphere over Kalimantan, up to 5 km altitude, and most of the particles in the samples used for the EDX analysis (Table 1) exhibited a morphological appearance similar to the particles shown in Fig. 1a. None of the particles collected below the 3 km altitude produced satellite droplet rings on the collecting surface, which is characteristic of sulfuric acid (Frank and Lodge, 1967). Electron diffraction (Okada et al., 2001) showed the presence of ammonium sulfate in the haze particles. Hence, S in these particles would most probably be in the form of ammonium sulfate. Table 1 summarizes the results of the particle classification and median S/K weight ratios for samples collected over Kalimantan on 23 and 25 October 1997. Samples D4 and G3 were collected just above the haze layer. On the basis of the quantitative EDX analyses, all particles in the radius range of 0.15–3 mm were classified into three types, ‘‘minerals’’, ‘‘seasalt’’ and ‘‘sulfurrich’’. It should be noted that more than 80% of the analyzed particles had submicrometer radius. For the classification, the weight proportion defined as P(X) was used PðXÞ ¼ X=ðNa þ Mg þ Al þ Si þ S þ Cl þ K þ CaÞ: Particles with PðAl þ SiÞX0:1 are considered to be mineral-containing particles and they are termed minerals. Those with PðNaÞX0:1 are classified as seasalt. Other particles with a value of PðS þ KÞX0:8 are classified as sulfur-rich. In the sulfur-rich particles, only 3 particles had PðS þ KÞo0:9 in the three samples collected on 23 October. Moreover, the sulfur-rich particles were divided into those with and without K. The S=K value was obtained from sulfur-rich particles
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Fig. 1. Electron micrograph of individual particles collected at 1.3 km altitude during the period from 10:10 to 10:30 GMT on 23 October (a), and EDX spectrum obtained from the central part of the particle A (b). The enlarged image of the particle A was also indicated in (b). The peak of Cu* is due to the copper electron microscopic grid. Also, peaks of Pt* and Pd* are from the shadowing material.
with K: For sulfur-rich particles, the elemental composition obtained by the EDX analysis was quite simple; these particles mainly contained S and K: It should be noted that Cl was not detected in most of the sulfur-rich particles. Although mineral and sea-salt particles were found, sulfur-rich particles containing K constituted a large fraction (approximately 80%) of all the particles
collected below 2.6 km altitude. On the other hand, for samples collected at altitudes higher than 4.4 km these comprised a smaller fraction, (57% (D4) and71% (G3)). Fig. 2a shows the weight ratios of S/K in individual particles as a function of particle radius for samples F3 (1.3 km altitude), E2 (2.6 km) and D4 (4.4 km), which were collected on 23 October. The particles used for Fig. 2 are those which contained K (sulfur-rich particles
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Table 1 Abundance of particles and median values of the S/K weight ratio in individual particles of 0.15–2.1 mm radius collected over Kalimantan on 23 and 25 October 1997. The collection was made over areas enclosed by 2.8–3.31S, 112.3–114.01E on 23 October and 3.1–3.31S, 112.2–112.71E on 25 October Sample ID
Altitude (km)
Sampling period (GMT)
Abundance of particlesa (%) Mineral
Seasalt
Median S/K weight ratio Sulfur rich ðK > 0Þ
ðK ¼ 0Þ
23 October D4 E2 F3
4.4 2.6 1.3
08:30–08:45 09:02–09:05 10:10–10:30
13 1 7
0 2 1
57 89 89
30 8 8
9.2 12.5 9.6
25 October G3 H4 G4
4.7 2.6 1.3
02:40–02:44 04:45–04:48 03:25–03:28
0 4 2
0 2 2
71 78 74
29 16 22
15.1 8.9 18.2
a
Number of particles examined is 100 for D4, E2, F3 and G3 samples and 50 for H4 and G4 samples.
Fig. 2. Weight ratios of S=K in individual particles as a function of particle radius for samples F3 (1.3 km altitude; open circle), E2 (2.6 km, closed circle) and D4 (4.4 km; cross) which were collected over Kalimantan on 23 October 1997 (a), and for samples G4 (1.3 km; open circle), H4 (2.6 km; closed circle) and G3 (4.7 km; cross) collected on 25 October (b). The particles used for Fig. 2 are those that contained K (particles with K>0 in Table 1).
with K > 0 in Table 1). The ratios range widely from 0.6 to 68. Fig. 2b shows the S=K ratios as a function of particle radius for samples G4 (1.3 km), H4 (2.6 km) and G3 (4.7 km) collected on 25 October. The ratios in Fig. 2b are from 0.3 to approximately 100, which are larger than those on 23 October. Both figures show that the S/K ratios have no clear size and height dependence. The median values of the S=K weight ratios were 9.2– 12.5 on 23 October and 8.9–18.2 on 25 October (Table 1). Note that the S=K ratio for K2SO4 is 0.41, which is much smaller than the measured ratios. During the PACE-5 aircraft observation, we also carried out an observation of forest fires over Arnhem Land (near Katherine) in northern Australia in October 1997. Table 2 summarizes the results of particle classification and median S=K weight ratios in two samples collected on 16 October. The procedures for the particle classification and the S=K ratio evaluation are the same as those used for Table 1. In the sulfur-rich particles, only one particle had PðS þ KÞo0:9 in the sample G1. The number of sulfuric acid containing particles in the sulfur-rich particles is also shown in parenthesis in Table 2. Sample G1 was collected at 1.5– 2.8 km altitude over the leeward area of a fire front. Sample G2 was collected in the free troposphere at 5.1 km altitude over an area, which was leeward of another fire front. It contained sea-salt particles, (approximately 30%), and also sulfuric acid particles. The abundance of sulfur-rich particles with K and seasalt particles in the sample G2 shows a vertical transport of these surface-derived particles by convective clouds. Moreover, sulfuric acid particles which showed satellite droplet rings on the collecting surface were found in the sample G2. As shown in Table 2, the sulfur-rich particles with K in both the samples comprised 64–52% of analyzed particles, with median S=K weight ratios of 4.0 (G1) and 2.6 (G2).
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Table 2 Abundance of particles and median values of the S/K weight ratio in individual particles of 0.21–1.2 mm radius collected over Arnhem Land (near Katherine) in northern Australia on 16 October 1997. The collection was made over areas enclosed by 14.6–14.81S, 132.0– 133.21E on 16 October. Sample G1 was collected at 1.5–2.8 km altitude leeward of a fire front. Sample G2 was collected at 5.1 km altitude over the area, which was leeward of another fire front Sample ID
Altitude (km)
Sampling period (GMT)
Abundance of particlesa (%) Mineral
Seasalt
Median S/K weight ratio Sulfur-rich ðK > 0Þ
ðK ¼ 0Þ 16 (4)b 21
G2
5.1
06:00–06:15
1
31
52
G1
1.5–2.8
05:03–05:13
11
4
64
a b
2.6 4.0
Number of particles examined is 100 for both samples. Abundance of sulfuric acid particles.
4. Discussion Submicrometer KCl and K2SO4 particles are produced by biomass burning (Akagawa, 1993; Gaudichet et al., 1995; Christensen et al., 1998). The weight ratio of S/K in particles originating from biomass burning has been measured by several researchers. Gras et al. (1992) carried out analyses of smoke particles from controlled burning of dried eucalyptus timber and leaf litter and found S=K weight ratios of less than 0.1. Gaudichet et al. (1995) collected particles close to savanna fires on the Ivory Coast and found low S=K ratios (0.052). They also found that the ratio in a smoldering condition was relatively high, ðS=K ¼ 0:2Þ: Christensen et al. (1998) analyzed the composition of particles originating from the combustion of straw and found that the average S/K ratio in submicrometer particles was 0.12. The composition of submicrometer aerosol particles in the haze associated with biomass burning has also been reported. S=K weight ratios measured by bulk analyses were 0.4– 2.3 in haze layers over the Amazon Basin (Andreae et al., 1988; Artaxo et al., 1994, 1988; Talbot et al., 1988) and the African rain forest (Bingemer et al., 1992). The S=K ratios in the present work are derived from individual micro-analysis while those given in the literature are obtained by bulk chemical analysis. However, the median values of S=K we have found over northern Australia are close to the values for haze particles reported in previous researches. Although the types of plants and combustion conditions may influence the composition of particles emitted to the atmosphere (Echalar et al., 1995; Gaudichet et al., 1995), the presence of particles with very high S=K ratios over Kalimantan implies the accumulation of S in particles in the atmosphere after their emission. Moreover, the absence of Cl suggests Cl loss from KCl in particles by chemical reactions probably with H2SO4 and/or HNO3. S in the particles was not in the form of sulfuric acid. From previous research (e.g., Andreae
et al., 1988), ammonia would be generated from biomass burning and would neutralize sulfuric acid. This is consistent with the finding of Okada et al. (2001) that the central electron-opaque crystals in individual haze particles over Kalimantan were identified as ammonium sulfate. One of the remarkable characteristics of forest fires in Indonesia was the occurrence of ground fires in peat swamp forests, which are located in the lowlands of Sumatra, Borneo and west New Guinea (e.g., Malingreau et al., 1985; Goto, 1998; Levine, 1999). Levine (1999) estimated the emission amount of particles by the combustion of peat in the 1997 Indonesian biomass burning and it constituted >90% of the total mass emission from the burning. Although not shown here, some results by electron microscopy showed that the particles of >0.1 mm radius generated from the combustion of the peat were considered to be composed of organic material with very small fractions of sulfur. The average S=K ratios for particles that originated just after the combustion of the 10 peat samples in Kalimantan were 1.80 while the ratios from the combustion of the 10 timber samples were 0.60 (CSIRO, 1999). The particles from peat combustion appear to be a factor 3 enriched in sulfur as compared to the particles from timber combustion. However, the S=K value (1.80) for peatgenerated particles is far below the S=K ratios obtained over Kalimantan in the present research. Biomass burning is a significant source of SO2 (Hegg et al., 1987; Andreae et al., 1988; Bates et al., 1992). In our observation, SO2 concentrations in the Kalimantan haze layer were 0.5–3 ppbv, which is larger than those (o0:5 ppbv) in the plume from biomass burning over northern Australia. In order to assess SO2 emission from this source, peat was burnt in the laboratory by heating dried peat, which was sampled in April 1998 below the ground near Banjarmasin. The peat was burnt in a chamber and the evolved SO2 concentration was measured with a SO2 analyzer (Thermo Electron Co.
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Ltd., Model 43A). The emission factor of SO2 was evaluated to be X1.5 102 mg g1. Rice hulls were also burnt and the emission factor was evaluated as a reference. It was found that the emission factor of SO2 for peat combustion is nearly one order of magnitude larger than that measured for the combustion of rice hulls. Large amounts of SO2 from the combustion of peat would be oxidized in the particles and then the growth of particles would occur through the heterogeneous oxidation of SO2 accompanying the intake of ammonia from the burning (e.g., Andreae et al., 1988). We conclude that SO2 released by the combustion of peat below the ground contributed to the formation of particles with high S=K ratios. Moreover, the absence of height dependence of the S=K weight ratios in the haze particles was probably due to the active vertical mixing of air over Kalimantan.
5. Conclusions Collections of individual aerosol particles were made on 23 and 25 October 1997 on board an aircraft at several altitudes between 1 and 5 km as part of the PACE-5 campaign over Kalimantan during the 1997 Indonesian forest fires. Observations were also made over forest fires in northern Australia. The elemental composition of particles of 0.15–3 mm radius was examined using a transmission electron microscope equipped with an EDX analyzer. Although 60–90% of the particles collected at altitudes of 1.3, 2.6 and 4.4 km over Kalimantan contained K; they exhibited high weight ratios of S/K, with median values between 9 and 18. These were much larger than those (median values of 2–4) obtained from the forest fires in northern Australia. We conclude that the high S=K weight ratios in Kalimantan haze particles were due to the growth of particles, probably through the heterogeneous oxidation of SO2 that originated from the combustion of peat below the ground.
Acknowledgements The authors are grateful to Dr. Y. Tsutsumi and Mr. Y. Sawa of the Meteorological Research Institute (MRI), the crews of AFTS, the staff of CSIRO and Showa Avi. Co. for their help during the observations. They also wish to express their gratitude to Mr. S. Diharto of MGA, Dr. S. Indroyono and Ms. T. Sribmawati of BPPT for their kind advice and support. The authors also wish to express their gratitude to Dr. K. Ohkushi, University of Tsukuba, for his help for the EDX analysis. This study was carried out as a
sub-project of the Japanese Study of the Behavior of the Greenhouse Gases and Aerosols supported by the Science and Technology Agency of Japan.
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