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Mixing properties of submicrometer aerosol particles in the urban atmosphere—with regard to soot particles
Atmospheric Environment 35 (2001) 5765–5772
Mixing properties of submicrometer aerosol particles in the urban atmosphereFwith regard to soot particles Hiroaki Naoe*, Kikuo Okada Meteorological Research Institute, Tsukuba, Ibaraki 305-0052, Japan Received 5 April 2001; received in revised form 3 July 2001; accepted 6 July 2001
Abstract Individual aerosol particles were collected on three days with different meteorological conditions in June 2000 in the urban atmosphere of Tsukuba, Japan. The samples collected with an electrostatic aerosol sampler (EAS) were examined by electron microscopy. The mixing properties of submicrometer aerosol particles of 0.02–0.2 mm radius were studied using the dialysis (extraction) of water-soluble material. Atmospheric aerosol particles were classified into four types with respect to the mixtures of water-soluble and water-insoluble material. The proportions of particles with watersoluble material (hygroscopic particles) ranged from 20% to 80% in the whole radius range and tended to increase with increasing radius. Moreover, by the morphological appearance, soot-containing particles were classified into two types, i.e., externally mixed soot-particles and internally mixed soot-particles. The number fractions of internally mixed sootparticles increased with increasing radius. It is found that the volume fraction of water-soluble material (e) for the internally mixed soot-particles increased with increasing radius. In a ‘‘polluted’’ case, the sample showed a dominant number fraction (75%) of internally mixed soot-particles in the larger radius range of 0.1–0.2 mm. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Hygroscopic particles; Dialysis technique; Electron microscope
1. Introduction Aerosol particles are one of the factors for controlling climate through both direct and indirect radiative forcing (Charlson et al., 1992). Information on the composition and mixing properties of aerosol particles is crucial to evaluate the effect on atmospheric radiation and cloud processes (e.g., Heintzenberg and Covert, 1990). In most of the existing researches the chemical composition of atmospheric aerosol particles has been studied by using the size-segregated bulk samples. These researches have supplied results on particle composition as a function of size. However, the mixing properties of aerosol particles cannot be evaluated by the analysis of the bulk samples alone because atmospheric aerosols *Corresponding author. Fax: +81-298-55-7240. E-mail address: [email protected] (H. Naoe).
consist of many particles with different composition even in a narrow size range. The mixing properties of aerosol particles were studied using the hygroscopic growth as measured by a tandem differential mobility analyzer (TDMA) system (e.g., McMurry and Stolzenburg, 1989; Svenningsson et al., 1992; Zhang et al., 1993; McMurry et al., 1996). They measured the external fraction of hygroscopic and non-hygroscopic particles. Some of these researches also suggested the presence of internally mixed hygroscopic particles. Direct measurements of the mixing properties of individual particles with respect to the hygroscopic properties were carried out by electron microscopy. Okada (1983a, b) measured the mixing properties of individual particles with radius between 0.03 and 0.35 mm in the urban atmospheres of Yokkaichi and Nagoya in Japan using a dialysis method for the extraction of water-soluble material and found that
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 3 6 7 - 3
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H. Naoe, K. Okada / Atmospheric Environment 35 (2001) 5765–5772
more than 80% of the aerosol particles were hygroscopic. The results also showed that 34% of the hygroscopic particles in the Aitken size range (0.03– 0.1 mm radius) and 67% in the large size range (0.1– 0.35 mm radius) were mixed particles. Okada and Hitzenberger (2001) examined individual aerosol particles of 0.1–0.7 mm radius in Vienna and found the increase in the fraction of mixed particles with increase in anthropogenic particle loading. However, information on the mixing properties including those for soot particles is still meager at present. Soot particles are of significant concern in atmospheric radiation because they absorb solar radia-
tion. The mixing properties of soot particles have been studied by hygroscopic growth (Covert and Heintzenberg, 1984) and by optical properties (Covert et al., 1990; Heintzenberg et al., 2001). These studies showed the presence of externally mixed soot-particles. Some researches on nucleation scavenging of particles (Harrison, 1985; Hallberg et al., 1994; Hitzenberger et al., 2000) suggested the presence of internally mixed sootparticles. However, little is known concerning the internal mixture of individual soot particles. The purpose of this paper is to show the mixing properties of individual submicrometer aerosol particles of 0.02– 0.2 mm radius, including those for soot particles,
Table 1 Sampling period and meteorological conditions at 12 JST (Japanese standard time) on each sampling day No.
Date (2000)
Time (JST)
T (1C)
RH (%)
WD
WS (m s–1)
Weather
1 2 3
20 June 21 June 23 June
13:10–17:02 10:35–14:24 09:26–13:17
28.2 25.9 19.3
54 68 88
ENE S NE
3.3 2.2 1.0
Fine Cloudy Rainy
Fig. 1. Number–size distributions of particles for three samples collected in Tsukuba on 20, 21, and 23 June 2000. Note that the distributions for the radius range of 0.02–0.2 mm and for the radius range 0.15–0.5 mm were derived from the samples collected with an EAS and the measurement with an OPC, respectively.
H. Naoe, K. Okada / Atmospheric Environment 35 (2001) 5765–5772
collected in the urban atmosphere of Tsukuba, Japan, under different meteorological conditions.
2. Methods Aerosol particles were collected on three days with different meteorological conditions in June 2000 at the Meteorological Research Institute in Tsukuba which is located 60 km north of Tokyo. Sampling air was kept in relative humidity less than 55%. Individual aerosol particles of 0.02–0.2 mm radius were collected for 4 h on a carbon-coated nitrocellulose (collodion) film with an electrostatic aerosol sampler (EAS) (TSI, Model 3100) with an air flow rate of 5.0 l min–1. Particles collected on the carbon film were coated with a Pt/Pd alloy at a shadowing angle of 26.61 (arctan 0.5) and were examined with an electron microscope (Hitachi, H-600 and H-6010) in order to assess the shape and volume of the individual particles. Since the collection surface is regarded as a semi-permeable membrane, a dialysis technique (e.g., Mossop, 1963; Okada, 1983a, b; Okada and Hitzenberger, 2001) was applied to the samples. The dialysis (extraction) of water-soluble material was done by floating the electronmicroscopic grid on distilled water at a temperature of 401C for 3 h, with the collection surface upward. The water-insoluble residues after the dialysis were again coated with a Pt/Pd alloy at a shadowing angle of 26.61, rotated by 901 with respect to the previous shadow. The ( for each shadowing. The thickness of a Pt/Pd alloy is 7 A mixture of individual particles with respect to watersolubility was obtained by studying the images of the same particles taken before and after the water dialysis.
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The negative films taken at 2000 and 4000 magnifications were processed with a resolution of 1000 dpi in a computer through a scanner. The apparent diameter d was obtained by evaluating the two-dimensional surface area of particle S in the processed image as d ¼ ð4S=pÞ1=2 : Shadow lengths were measured manually. The volume of each particle was calculated on the basis of the measurements of apparent diameter d and shadow length l on the collection surface. If the particle reveals the ratio l=do0:62; its volume was calculated on the assumption that it exists as a spherical cap. If the ratio is more than 0.62, its volume was evaluated by assuming a disk with the apparent diameter and shadow length. From the electron micrographs obtained before and after the dialysis, the volume fraction of water-soluble material in soot-containing particles was estimated by using the method shown by Okada (1983a, b) and Okada and Hitzenberger (2001). Note that the volume of soot was estimated as S1/2 by using the surface area S and shadow length l: X-ray analysis was also applied to some soot particles using an energy-dispersive X-ray (EDX) analyzer through a Kevex UTV detector equipped with the electron microscope. An accelerating voltage of 50 kV was used for the examination. The number concentration of aerosol particles of 0.02–0.2 mm radius collected with the EAS was determined by using the collection efficiency shown by Yamada (1983). Number concentrations of aerosols with radius between 0.15 and 0.5 mm were measured with an optical particle counter (OPC) (Sigmatec Co. Ltd., Model TD100). The meteorological data used in the present study were those obtained at the Tateno Aerological Observatory, which is about 0.5 km north of the Meteorological Research Institute.
Fig. 2. Electron micrographs of individual particles before (a) and after the dialysis (extraction) with water (b). Four types of particles were noted as follows: Type 1, water-insoluble particle; Type 2, mixed particle; Type 3, hygroscopic particle without water-insoluble inclusions; and Type 4, indeterminate particle. Each micrograph is shown in the scale (1 mm 1 mm).
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3. Results 3.1. Number-size distribution of aerosol particles Individual aerosol particles were collected on three days with different meteorological conditions, as shown in Table 1. Sample 1 was obtained on a fine day (20 June) with a dry condition. On 21 June sampling was conducted in the cloudy atmosphere with relative humidity of B70%. The third sample was collected on a rainy day with a calm condition. Fig. 1 shows number-size distributions of aerosol particles for three samples. It is seen that the aerosol concentration on 21 June (cloudy day) is 2–3 times larger than that for other samples. The size distribution on a rainy day (23 June) exhibits low concentration. It is to be noted that the sample collected on 21 June would be regarded as a ‘‘polluted’’ case. The decrease in concentration of aerosols with increasing radius was found clearly in the range of 0.1–0.4 mm radius.
insoluble inclusions (Type 3) increase with increasing radius, but in the radius range of 0.1–0.2 mm they have relatively small proportions (20–30%). The frequencies of hygroscopic particles (Type 2+Type 3) range from 20% to 80% in the whole radius range and tend to increase with increasing radius. The abundance of particles composed of the semi-transparent material
3.2. Classification of particles by hygroscopic properties Fig. 2 shows an example of the electron micrographs of particles before and after the dialysis with water. All particles are classified into four types, as indicated in Okada and Hitzenberger (2001). Type 1 is characterized by no change in morphological appearance before and after the dialysis. The particle composed of electron-opaque material is termed waterinsoluble. Type 2 is called here mixed particle, which is composed of hygroscopic material with water-insoluble material. Type 3 is termed hygroscopic particle without water-insoluble inclusions. Type 4 is termed indeterminate particle, mainly composed of semi-transparent material and shows no change in morphology by the dialysis. This type of particle may be composed of organic material, and the hygroscopic properties would be influenced by the irradiation of electron beam (Okada et al., 2001). 3.3. Abundance of types of particles Fig. 3 shows number proportions of the four types of particles in the four radius ranges between 0.02 and 0.2 mm for each sample. This figure shows that the frequencies of water-insoluble particles (Type 1) have a size dependence that showed the decrease with increasing radius. The mixed particles (Type 2), on the other hand, indicate that the number proportions increase with increasing radius. Especially, in the radius range of 0.1–0.2 mm for the samples 1 (20 June) and 2 (21 June) the number proportions of mixed particles are dominant (B50%). In the radius ranges of 0.02–0.1 mm the number proportions of hygroscopic particles without water-
Fig. 3. Number proportions of the four types of particles in the four radius ranges between 0.02 and 0.2 mm obtained from each sample. Total number of particles examined for each radius range is shown on the top of graph-bar. Samples were collected on 20 (a), 21 (b), and 23 June 2000 (c).
H. Naoe, K. Okada / Atmospheric Environment 35 (2001) 5765–5772
(Type 4) is very low especially in the radius ranges below 0.1 mm. Only the sample 3 (on 23 June) has a relatively high proportion of the particles in the radius range of 0.1–0.2 mm.
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an example of the electron micrographs of these soot particles, examined before and after the dialysis with water.
3.4. Mixing properties of soot particles In this subsection we have focused on the mixing state of soot particles. Since particle composed of chain aggregations of electron-opaque spherules is considered to be soot (e.g., Ogren and Charlson, 1983) or metallic oxide (e.g., Green and Lane, 1964), we apply the EDX analysis to an aggregated particle. Fig. 4 shows an electron micrograph of particles collected on 21 June, together with X-ray spectra. The X-ray spectrum of aggregated particle (marked by ‘‘A’’) shows the dominant peaks of C and S, and the X-ray spectrum of the film (marked by ‘‘B’’) shows the dominant peaks of C and O. These results show that the particle ‘‘A’’ has no metallic compounds. The peak height ratio C/O of particle ‘‘A’’ is much larger than that of the film. This means that the particle ‘‘A’’ contains carbon. Hence, soot particles can be determined by the morphological appearance. The particles containing soot are classified into two types; i.e., Type 5 is called externally mixed sootparticle that does not contain water-soluble material, and Type 6 is internally mixed soot-particle that is covered with water-soluble material. Fig. 5 shows
Fig. 5. Electron micrographs of individual soot-containing particles before (a) and after the dialysis with water (b). Two types of the particles were noted as follows. Type 5 is called externally mixed soot-particle; Type 6 internally mixed soot particle.
Fig. 4. X-ray spectra of particle ‘‘A’’ (a) and the film ‘‘B’’(b), and the electron micrograph of individual particles (c) collected with an impactor of 0.5-mm diameter jet during the period of 13: 21–13: 24 JST on 21 June 2000. The Pt* peak is due to the Pt/Pd alloy for shadowing. The Cu* peak is due to the copper electron microscopic grid. The examination was made by the point analysis.
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Fig. 6 shows number proportions for the two types of soot-particles in the four radius ranges between 0.02 and 0.2 mm for each sample. This figure shows that the externally mixed soot-particles (Type 5) have proportions that show the decrease with increasing radius. Namely, the number proportions of internally mixed soot-particles (Type 6) increase with increasing radius. Especially, the results obtained on 21 June (‘‘polluted’’ case) exhibit a dominant number proportion (75%) of
Fig. 6. Number proportion of the two types of soot particles (externally mixed soot particles and internally mixed soot particles) for the samples collected on 20 (a), 21 (b), and 23 June 2000 (c). Total number of particles examined for each radius is shown on the top of graph bar.
internally mixed soot-particles in the larger radius range of 0.1–0.2 mm. The results collected on a rainy day (23 June) have showed the relatively low fractions of internally mixed soot-particles in the radius ranges below 0.1 mm. A summary of analyses for all particles examined in this study is given in Table 2. From this table it is found that generally the numbers of the externally mixed sootparticles (Type 5) are dominant in total water-insoluble particles (Type 1) and that the internally mixed sootparticles (Type 6) have a low ratio in total mixed particles (Type 2). However, the number ratios of the internally mixed soot-particles to total mixed particles are characterized by relatively high proportions (0.3–0.6) in the results obtained on 21 June (‘‘polluted case’’). Therefore, it is interesting to focus on the characteristics of the volume fraction of water-soluble material for this ‘‘polluted case’’. The volume fraction of water-soluble material in individual particles is important for assessing the nucleation scavenging properties of atmospheric aerosol and its influence on the radiative properties. Fig. 7 shows the volume fraction of water-soluble material (e) in internally mixed soot-particles collected on 21 June (‘‘polluted’’ case). Note that the waterinsoluble particle (Type 1) has an e of 0 and the hygroscopic particle (Type 3) does an e of 1. The results from this figure that showed the increase in value of e with increasing radius are qualitatively consistent with the results of McMurry et al. (1996), who found that ‘‘less’’ hygroscopic particles (i.e., smaller e) included chain agglomerates, flakes, irregular shapes, and contained mostly carbon, while ‘‘more’’ hygroscopic particles (i.e., larger e) appear to be liquid spheres and contained sulfur, and sometimes carbon or cations. However, their observation was insufficient to explain the amount of the carbon and hygroscopic compounds as a function of particle size, and thus our methodology has an advantage to provide more qualitative and comprehensive insights into the size dependent trends. It is considered that the water-insoluble soot-particles would act as the nucleus for heterogeneous processes such as catalytic chemical reactions (e.g., Novakov et al., 1974), and then internally mixed soot-particles can be formed. Extremely high proportions of externally mixed soot-particles in the smaller radius range would be attributable to the particles formed just after the combustion process. Large abundance of internally mixed soot-particles in larger particles would be due to the occurrence of heterogeneous processes on soot particles during their aging. The internally mixed soot particles can form cloud droplets, and they are scavenged from the atmosphere. This may lead to the low abundance of internally mixed soot-particles on a rainy day.
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H. Naoe, K. Okada / Atmospheric Environment 35 (2001) 5765–5772 Table 2 Number concentrations of total particles (dN) and numbers of each type that were found for each samplea Date (2000)
Radius (mm)
20 June
0.02–0.03 0.03–0.05 0.05–0.1 0.1–0.2
21 June
23 June
dN (cm–3)
Type 1
Type 2
Type 3
1900 2600 2100 570
111 91 37 7
4 33 56 23
56 107 76 12
0.02–0.03 0.03–0.05 0.05–0.1 0.1–0.2
5400 9400 5900 2800
29 68 14 9
7 27 29 36
0.02–0.03 0.03–0.05 0.05–0.1 0.1–0.2
1700 2000 2400 1500
79 88 66 42
3 10 24 23
Type 4
Total of Types 1–4
Type 5
Type 6
Total of Types 5–6
0 2 4 1
171 233 173 43
24 33 23 6
0 3 10 2
24 36 33 8
23 92 54 23
2 2 2 7
61 189 99 75
15 27 10 8
2 9 12 23
17 36 22 31
18 36 69 38
0 0 8 30
100 134 167 133
38 59 47 6
0 1 5 4
38 60 52 10
a
Note: Type 1, water-insoluble particle; Type 2, mixed particle; Type 3, hygroscopic particle without water-insoluble inclusions; Type 4, indeterminate particle; Type 5, externally mixed soot-particle; and Type 6, internally mixed soot-particle. The numbers of Type 1 and 2 particles include those of Type 5 and 6 particles, respectively.
Fig. 7. The volume fraction of water-soluble material (e) in internally mixed soot-particles collected on 21 June 2000. Note that the uncertainty of e is estimated to be 0.3 at 0.05 mm radius and to be 0.1 at 0.14 mm radius.
4. Summary Individual aerosol particles were collected on three days with different meteorological conditions in June 2000 in the urban atmosphere of Tsukuba in Japan. The samples collected with an electrostatic aerosol sampler (EAS) were examined by electron microscopy. The mixing properties of submicrometer aerosol particles with radius between 0.02 and 0.2 mm were studied by using the dialysis (extraction) of water-soluble material. Aerosol particles are classified into four types, waterinsoluble particle, mixed particle, hygroscopic particles without water-insoluble material and indeterminate particle (which might be organic material). The number proportions of water-insoluble particles tended to decrease with increasing radius. On the other hand, the frequencies of mixed particles showed the opposite size
dependence. The proportions of particles with watersoluble material (hygroscopic particles) ranged from 20% to 80% in the whole radius range and tended to increase with increasing radius. With respect to the soot-containing particles, two types of the particles were classified; i.e., externally mixed soot-particle and internally mixed soot-particle. The frequencies of internally mixed soot-particles also have the similar size dependence to mixed particles. Particles collected in a ‘‘polluted’’ case showed that the volume fraction of water-soluble material increased with increasing the radius of soot-containing particle in the radius range of 0.03–0.2 mm. This suggests that waterinsoluble soot-particles would act as the nucleus for heterogeneous processes such as catalytic chemical reactions, and then internally mixed soot-particles can be formed during their aging. The results collected on a rainy day have showed relatively low fractions of the internally mixed soot-particles probably due to the efficient nucleation scavenging of internally mixed soot-particles in the atmosphere.
References Charlson, R.J., Schwartz, S.E., Hales, J.M., Cess, R.D., Coakley, J.A., Hansen, J.E., Hofman, D.J., 1992. Climate forcing by anthropogenic aerosols. Science 255, 423–430. Covert, D.S., Heintzenberg, J., 1984. Measurement of the degree of internal/external mixing of hygroscopic compounds and soot in atmospheric aerosols. Science of the Total Environment 36, 347–352.
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Covert, D.S., Heintzenberg, J., Hansson, H.-C., 1990. Electrooptical detection of external mixtures in aerosols. Aerosol Science and Technology 12, 446–456. Green, H.L., Lane, W.R., 1964. Particulate Clouds: Dusts, Smoke and Mists. E & F N Spon, London, p. 471. Hallberg, A., Ogren, J.A., Noone, K.J., Okada, K., Heintzenberg, J., Svenningsson, I.B., 1994. The influence of aerosol particle composition on cloud droplet formation. Journal of Atmospheric Chemistry 19, 153–171. Harrison, L., 1985. The segregation of aerosols by cloudnucleating activity. Part II: observation of an urban aerosol. Journal of Climate and Applied Meteorology 24, 312–321. Heintzenberg, J., Covert, D.S., 1990. On the distribution of physical and chemical particle properties in the atmospheric aerosol. Journal of Atmospheric Chemistry 10, 383–397. Heintzenberg, J., Okada, K., Luo, B.P., 2001. The distribution of optical properties among atmospheric submicronmeter particles of given electrical mobilities. Journal of Geophysical Research, in press. Hitzenberger, R., Berner, A., Kromp, R., Kasper-Giebl, A., Limbeck, A., Tscherwenka, W., Puxbaum, H., 2000. Black carbon and other species at a high-elevation European site (Mount Sonnblick, 3106 m, Austria): concentrations and scavenging efficiencies. Journal of Geophysical Research 105, 24637–24645. McMurry, P.H., Litchy, M., Huang, P.-F., Cai, X., Turpin, B.J., Dick, W.D., Hanson, A., 1996. Elemental composition and morphology of individual particles separated by size and hygroscopicity with the TDMA. Atmospheric Environment 30, 101–108. McMurry, P.H., Stolzenburg, M.R., 1989. On the sensitivity of particle size to relative humidity for Los Angeles aerosols. Atmospheric Environment 23, 497–507.
Mossop, S.C., 1963. Stratospheric particles at 20 km. Nature 199, 325–326. Novakov, T., Chang, S.G., Harker, A.B., 1974. Sulfates as pollution particulates: catalytic formation on carbon (soot) particles. Science 186, 259–261. Ogren, J.A., Charlson, R.J., 1983. Elemental carbon in the atmosphere: cycle and lifetime. Tellus 35B, 241–254. Okada, K., 1983a. Volume fraction of water-soluble material in individual aerosol particles. Journal of Aerosol Science 14, 301–302. Okada, K., 1983b. Nature of individual hygroscopic particles in the urban atmosphere. Journal of the Meteorological Society of Japan 61, 727–736. Okada, K., Hitzenberger, R.M., 2001. Mixing properties of individual submicrometer aerosol particles in Vienna. Atmospheric Environment, in press. Okada, K., Ikegami, M., Zaizen, Y., Makino, Y., Jensen, J.B., Gres, J.L., 2001. The mixture state of individual aerosol particles in the 1997 Indonesian haze episode. Journal of Aerosol Science, in press. Svenningsson, I.B., Hansson, H.-C., Wiedensohler, A., Ogren, J.A., Noone, K.J., Hallberg, A., 1992. Hygroscopic growth of aerosol particles in the Po Valley. Tellus 44B, 556–569. Yamada, Y., 1983. A new method for the determination of collection efficiency of an aerosol sampler by electron microscopy. Atmospheric Environment 17, 369–372. Zhang, X.O., McMurry, P.H., Hering, S.V., Casuccio, G.S., 1993. Mixing characteristics and water content of submicron aerosols measured in Los Angeles and at the grand canyon. Atmospheric Environment 27A, 1593–1607.
Mixing properties of submicrometer aerosol particles in the urban atmosphereFwith regard to soot particles Hiroaki Naoe*, Kikuo Okada Meteorological Research Institute, Tsukuba, Ibaraki 305-0052, Japan Received 5 April 2001; received in revised form 3 July 2001; accepted 6 July 2001
Abstract Individual aerosol particles were collected on three days with different meteorological conditions in June 2000 in the urban atmosphere of Tsukuba, Japan. The samples collected with an electrostatic aerosol sampler (EAS) were examined by electron microscopy. The mixing properties of submicrometer aerosol particles of 0.02–0.2 mm radius were studied using the dialysis (extraction) of water-soluble material. Atmospheric aerosol particles were classified into four types with respect to the mixtures of water-soluble and water-insoluble material. The proportions of particles with watersoluble material (hygroscopic particles) ranged from 20% to 80% in the whole radius range and tended to increase with increasing radius. Moreover, by the morphological appearance, soot-containing particles were classified into two types, i.e., externally mixed soot-particles and internally mixed soot-particles. The number fractions of internally mixed sootparticles increased with increasing radius. It is found that the volume fraction of water-soluble material (e) for the internally mixed soot-particles increased with increasing radius. In a ‘‘polluted’’ case, the sample showed a dominant number fraction (75%) of internally mixed soot-particles in the larger radius range of 0.1–0.2 mm. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Hygroscopic particles; Dialysis technique; Electron microscope
1. Introduction Aerosol particles are one of the factors for controlling climate through both direct and indirect radiative forcing (Charlson et al., 1992). Information on the composition and mixing properties of aerosol particles is crucial to evaluate the effect on atmospheric radiation and cloud processes (e.g., Heintzenberg and Covert, 1990). In most of the existing researches the chemical composition of atmospheric aerosol particles has been studied by using the size-segregated bulk samples. These researches have supplied results on particle composition as a function of size. However, the mixing properties of aerosol particles cannot be evaluated by the analysis of the bulk samples alone because atmospheric aerosols *Corresponding author. Fax: +81-298-55-7240. E-mail address: [email protected] (H. Naoe).
consist of many particles with different composition even in a narrow size range. The mixing properties of aerosol particles were studied using the hygroscopic growth as measured by a tandem differential mobility analyzer (TDMA) system (e.g., McMurry and Stolzenburg, 1989; Svenningsson et al., 1992; Zhang et al., 1993; McMurry et al., 1996). They measured the external fraction of hygroscopic and non-hygroscopic particles. Some of these researches also suggested the presence of internally mixed hygroscopic particles. Direct measurements of the mixing properties of individual particles with respect to the hygroscopic properties were carried out by electron microscopy. Okada (1983a, b) measured the mixing properties of individual particles with radius between 0.03 and 0.35 mm in the urban atmospheres of Yokkaichi and Nagoya in Japan using a dialysis method for the extraction of water-soluble material and found that
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 3 6 7 - 3
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H. Naoe, K. Okada / Atmospheric Environment 35 (2001) 5765–5772
more than 80% of the aerosol particles were hygroscopic. The results also showed that 34% of the hygroscopic particles in the Aitken size range (0.03– 0.1 mm radius) and 67% in the large size range (0.1– 0.35 mm radius) were mixed particles. Okada and Hitzenberger (2001) examined individual aerosol particles of 0.1–0.7 mm radius in Vienna and found the increase in the fraction of mixed particles with increase in anthropogenic particle loading. However, information on the mixing properties including those for soot particles is still meager at present. Soot particles are of significant concern in atmospheric radiation because they absorb solar radia-
tion. The mixing properties of soot particles have been studied by hygroscopic growth (Covert and Heintzenberg, 1984) and by optical properties (Covert et al., 1990; Heintzenberg et al., 2001). These studies showed the presence of externally mixed soot-particles. Some researches on nucleation scavenging of particles (Harrison, 1985; Hallberg et al., 1994; Hitzenberger et al., 2000) suggested the presence of internally mixed sootparticles. However, little is known concerning the internal mixture of individual soot particles. The purpose of this paper is to show the mixing properties of individual submicrometer aerosol particles of 0.02– 0.2 mm radius, including those for soot particles,
Table 1 Sampling period and meteorological conditions at 12 JST (Japanese standard time) on each sampling day No.
Date (2000)
Time (JST)
T (1C)
RH (%)
WD
WS (m s–1)
Weather
1 2 3
20 June 21 June 23 June
13:10–17:02 10:35–14:24 09:26–13:17
28.2 25.9 19.3
54 68 88
ENE S NE
3.3 2.2 1.0
Fine Cloudy Rainy
Fig. 1. Number–size distributions of particles for three samples collected in Tsukuba on 20, 21, and 23 June 2000. Note that the distributions for the radius range of 0.02–0.2 mm and for the radius range 0.15–0.5 mm were derived from the samples collected with an EAS and the measurement with an OPC, respectively.
H. Naoe, K. Okada / Atmospheric Environment 35 (2001) 5765–5772
collected in the urban atmosphere of Tsukuba, Japan, under different meteorological conditions.
2. Methods Aerosol particles were collected on three days with different meteorological conditions in June 2000 at the Meteorological Research Institute in Tsukuba which is located 60 km north of Tokyo. Sampling air was kept in relative humidity less than 55%. Individual aerosol particles of 0.02–0.2 mm radius were collected for 4 h on a carbon-coated nitrocellulose (collodion) film with an electrostatic aerosol sampler (EAS) (TSI, Model 3100) with an air flow rate of 5.0 l min–1. Particles collected on the carbon film were coated with a Pt/Pd alloy at a shadowing angle of 26.61 (arctan 0.5) and were examined with an electron microscope (Hitachi, H-600 and H-6010) in order to assess the shape and volume of the individual particles. Since the collection surface is regarded as a semi-permeable membrane, a dialysis technique (e.g., Mossop, 1963; Okada, 1983a, b; Okada and Hitzenberger, 2001) was applied to the samples. The dialysis (extraction) of water-soluble material was done by floating the electronmicroscopic grid on distilled water at a temperature of 401C for 3 h, with the collection surface upward. The water-insoluble residues after the dialysis were again coated with a Pt/Pd alloy at a shadowing angle of 26.61, rotated by 901 with respect to the previous shadow. The ( for each shadowing. The thickness of a Pt/Pd alloy is 7 A mixture of individual particles with respect to watersolubility was obtained by studying the images of the same particles taken before and after the water dialysis.
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The negative films taken at 2000 and 4000 magnifications were processed with a resolution of 1000 dpi in a computer through a scanner. The apparent diameter d was obtained by evaluating the two-dimensional surface area of particle S in the processed image as d ¼ ð4S=pÞ1=2 : Shadow lengths were measured manually. The volume of each particle was calculated on the basis of the measurements of apparent diameter d and shadow length l on the collection surface. If the particle reveals the ratio l=do0:62; its volume was calculated on the assumption that it exists as a spherical cap. If the ratio is more than 0.62, its volume was evaluated by assuming a disk with the apparent diameter and shadow length. From the electron micrographs obtained before and after the dialysis, the volume fraction of water-soluble material in soot-containing particles was estimated by using the method shown by Okada (1983a, b) and Okada and Hitzenberger (2001). Note that the volume of soot was estimated as S1/2 by using the surface area S and shadow length l: X-ray analysis was also applied to some soot particles using an energy-dispersive X-ray (EDX) analyzer through a Kevex UTV detector equipped with the electron microscope. An accelerating voltage of 50 kV was used for the examination. The number concentration of aerosol particles of 0.02–0.2 mm radius collected with the EAS was determined by using the collection efficiency shown by Yamada (1983). Number concentrations of aerosols with radius between 0.15 and 0.5 mm were measured with an optical particle counter (OPC) (Sigmatec Co. Ltd., Model TD100). The meteorological data used in the present study were those obtained at the Tateno Aerological Observatory, which is about 0.5 km north of the Meteorological Research Institute.
Fig. 2. Electron micrographs of individual particles before (a) and after the dialysis (extraction) with water (b). Four types of particles were noted as follows: Type 1, water-insoluble particle; Type 2, mixed particle; Type 3, hygroscopic particle without water-insoluble inclusions; and Type 4, indeterminate particle. Each micrograph is shown in the scale (1 mm 1 mm).
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3. Results 3.1. Number-size distribution of aerosol particles Individual aerosol particles were collected on three days with different meteorological conditions, as shown in Table 1. Sample 1 was obtained on a fine day (20 June) with a dry condition. On 21 June sampling was conducted in the cloudy atmosphere with relative humidity of B70%. The third sample was collected on a rainy day with a calm condition. Fig. 1 shows number-size distributions of aerosol particles for three samples. It is seen that the aerosol concentration on 21 June (cloudy day) is 2–3 times larger than that for other samples. The size distribution on a rainy day (23 June) exhibits low concentration. It is to be noted that the sample collected on 21 June would be regarded as a ‘‘polluted’’ case. The decrease in concentration of aerosols with increasing radius was found clearly in the range of 0.1–0.4 mm radius.
insoluble inclusions (Type 3) increase with increasing radius, but in the radius range of 0.1–0.2 mm they have relatively small proportions (20–30%). The frequencies of hygroscopic particles (Type 2+Type 3) range from 20% to 80% in the whole radius range and tend to increase with increasing radius. The abundance of particles composed of the semi-transparent material
3.2. Classification of particles by hygroscopic properties Fig. 2 shows an example of the electron micrographs of particles before and after the dialysis with water. All particles are classified into four types, as indicated in Okada and Hitzenberger (2001). Type 1 is characterized by no change in morphological appearance before and after the dialysis. The particle composed of electron-opaque material is termed waterinsoluble. Type 2 is called here mixed particle, which is composed of hygroscopic material with water-insoluble material. Type 3 is termed hygroscopic particle without water-insoluble inclusions. Type 4 is termed indeterminate particle, mainly composed of semi-transparent material and shows no change in morphology by the dialysis. This type of particle may be composed of organic material, and the hygroscopic properties would be influenced by the irradiation of electron beam (Okada et al., 2001). 3.3. Abundance of types of particles Fig. 3 shows number proportions of the four types of particles in the four radius ranges between 0.02 and 0.2 mm for each sample. This figure shows that the frequencies of water-insoluble particles (Type 1) have a size dependence that showed the decrease with increasing radius. The mixed particles (Type 2), on the other hand, indicate that the number proportions increase with increasing radius. Especially, in the radius range of 0.1–0.2 mm for the samples 1 (20 June) and 2 (21 June) the number proportions of mixed particles are dominant (B50%). In the radius ranges of 0.02–0.1 mm the number proportions of hygroscopic particles without water-
Fig. 3. Number proportions of the four types of particles in the four radius ranges between 0.02 and 0.2 mm obtained from each sample. Total number of particles examined for each radius range is shown on the top of graph-bar. Samples were collected on 20 (a), 21 (b), and 23 June 2000 (c).
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(Type 4) is very low especially in the radius ranges below 0.1 mm. Only the sample 3 (on 23 June) has a relatively high proportion of the particles in the radius range of 0.1–0.2 mm.
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an example of the electron micrographs of these soot particles, examined before and after the dialysis with water.
3.4. Mixing properties of soot particles In this subsection we have focused on the mixing state of soot particles. Since particle composed of chain aggregations of electron-opaque spherules is considered to be soot (e.g., Ogren and Charlson, 1983) or metallic oxide (e.g., Green and Lane, 1964), we apply the EDX analysis to an aggregated particle. Fig. 4 shows an electron micrograph of particles collected on 21 June, together with X-ray spectra. The X-ray spectrum of aggregated particle (marked by ‘‘A’’) shows the dominant peaks of C and S, and the X-ray spectrum of the film (marked by ‘‘B’’) shows the dominant peaks of C and O. These results show that the particle ‘‘A’’ has no metallic compounds. The peak height ratio C/O of particle ‘‘A’’ is much larger than that of the film. This means that the particle ‘‘A’’ contains carbon. Hence, soot particles can be determined by the morphological appearance. The particles containing soot are classified into two types; i.e., Type 5 is called externally mixed sootparticle that does not contain water-soluble material, and Type 6 is internally mixed soot-particle that is covered with water-soluble material. Fig. 5 shows
Fig. 5. Electron micrographs of individual soot-containing particles before (a) and after the dialysis with water (b). Two types of the particles were noted as follows. Type 5 is called externally mixed soot-particle; Type 6 internally mixed soot particle.
Fig. 4. X-ray spectra of particle ‘‘A’’ (a) and the film ‘‘B’’(b), and the electron micrograph of individual particles (c) collected with an impactor of 0.5-mm diameter jet during the period of 13: 21–13: 24 JST on 21 June 2000. The Pt* peak is due to the Pt/Pd alloy for shadowing. The Cu* peak is due to the copper electron microscopic grid. The examination was made by the point analysis.
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Fig. 6 shows number proportions for the two types of soot-particles in the four radius ranges between 0.02 and 0.2 mm for each sample. This figure shows that the externally mixed soot-particles (Type 5) have proportions that show the decrease with increasing radius. Namely, the number proportions of internally mixed soot-particles (Type 6) increase with increasing radius. Especially, the results obtained on 21 June (‘‘polluted’’ case) exhibit a dominant number proportion (75%) of
Fig. 6. Number proportion of the two types of soot particles (externally mixed soot particles and internally mixed soot particles) for the samples collected on 20 (a), 21 (b), and 23 June 2000 (c). Total number of particles examined for each radius is shown on the top of graph bar.
internally mixed soot-particles in the larger radius range of 0.1–0.2 mm. The results collected on a rainy day (23 June) have showed the relatively low fractions of internally mixed soot-particles in the radius ranges below 0.1 mm. A summary of analyses for all particles examined in this study is given in Table 2. From this table it is found that generally the numbers of the externally mixed sootparticles (Type 5) are dominant in total water-insoluble particles (Type 1) and that the internally mixed sootparticles (Type 6) have a low ratio in total mixed particles (Type 2). However, the number ratios of the internally mixed soot-particles to total mixed particles are characterized by relatively high proportions (0.3–0.6) in the results obtained on 21 June (‘‘polluted case’’). Therefore, it is interesting to focus on the characteristics of the volume fraction of water-soluble material for this ‘‘polluted case’’. The volume fraction of water-soluble material in individual particles is important for assessing the nucleation scavenging properties of atmospheric aerosol and its influence on the radiative properties. Fig. 7 shows the volume fraction of water-soluble material (e) in internally mixed soot-particles collected on 21 June (‘‘polluted’’ case). Note that the waterinsoluble particle (Type 1) has an e of 0 and the hygroscopic particle (Type 3) does an e of 1. The results from this figure that showed the increase in value of e with increasing radius are qualitatively consistent with the results of McMurry et al. (1996), who found that ‘‘less’’ hygroscopic particles (i.e., smaller e) included chain agglomerates, flakes, irregular shapes, and contained mostly carbon, while ‘‘more’’ hygroscopic particles (i.e., larger e) appear to be liquid spheres and contained sulfur, and sometimes carbon or cations. However, their observation was insufficient to explain the amount of the carbon and hygroscopic compounds as a function of particle size, and thus our methodology has an advantage to provide more qualitative and comprehensive insights into the size dependent trends. It is considered that the water-insoluble soot-particles would act as the nucleus for heterogeneous processes such as catalytic chemical reactions (e.g., Novakov et al., 1974), and then internally mixed soot-particles can be formed. Extremely high proportions of externally mixed soot-particles in the smaller radius range would be attributable to the particles formed just after the combustion process. Large abundance of internally mixed soot-particles in larger particles would be due to the occurrence of heterogeneous processes on soot particles during their aging. The internally mixed soot particles can form cloud droplets, and they are scavenged from the atmosphere. This may lead to the low abundance of internally mixed soot-particles on a rainy day.
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H. Naoe, K. Okada / Atmospheric Environment 35 (2001) 5765–5772 Table 2 Number concentrations of total particles (dN) and numbers of each type that were found for each samplea Date (2000)
Radius (mm)
20 June
0.02–0.03 0.03–0.05 0.05–0.1 0.1–0.2
21 June
23 June
dN (cm–3)
Type 1
Type 2
Type 3
1900 2600 2100 570
111 91 37 7
4 33 56 23
56 107 76 12
0.02–0.03 0.03–0.05 0.05–0.1 0.1–0.2
5400 9400 5900 2800
29 68 14 9
7 27 29 36
0.02–0.03 0.03–0.05 0.05–0.1 0.1–0.2
1700 2000 2400 1500
79 88 66 42
3 10 24 23
Type 4
Total of Types 1–4
Type 5
Type 6
Total of Types 5–6
0 2 4 1
171 233 173 43
24 33 23 6
0 3 10 2
24 36 33 8
23 92 54 23
2 2 2 7
61 189 99 75
15 27 10 8
2 9 12 23
17 36 22 31
18 36 69 38
0 0 8 30
100 134 167 133
38 59 47 6
0 1 5 4
38 60 52 10
a
Note: Type 1, water-insoluble particle; Type 2, mixed particle; Type 3, hygroscopic particle without water-insoluble inclusions; Type 4, indeterminate particle; Type 5, externally mixed soot-particle; and Type 6, internally mixed soot-particle. The numbers of Type 1 and 2 particles include those of Type 5 and 6 particles, respectively.
Fig. 7. The volume fraction of water-soluble material (e) in internally mixed soot-particles collected on 21 June 2000. Note that the uncertainty of e is estimated to be 0.3 at 0.05 mm radius and to be 0.1 at 0.14 mm radius.
4. Summary Individual aerosol particles were collected on three days with different meteorological conditions in June 2000 in the urban atmosphere of Tsukuba in Japan. The samples collected with an electrostatic aerosol sampler (EAS) were examined by electron microscopy. The mixing properties of submicrometer aerosol particles with radius between 0.02 and 0.2 mm were studied by using the dialysis (extraction) of water-soluble material. Aerosol particles are classified into four types, waterinsoluble particle, mixed particle, hygroscopic particles without water-insoluble material and indeterminate particle (which might be organic material). The number proportions of water-insoluble particles tended to decrease with increasing radius. On the other hand, the frequencies of mixed particles showed the opposite size
dependence. The proportions of particles with watersoluble material (hygroscopic particles) ranged from 20% to 80% in the whole radius range and tended to increase with increasing radius. With respect to the soot-containing particles, two types of the particles were classified; i.e., externally mixed soot-particle and internally mixed soot-particle. The frequencies of internally mixed soot-particles also have the similar size dependence to mixed particles. Particles collected in a ‘‘polluted’’ case showed that the volume fraction of water-soluble material increased with increasing the radius of soot-containing particle in the radius range of 0.03–0.2 mm. This suggests that waterinsoluble soot-particles would act as the nucleus for heterogeneous processes such as catalytic chemical reactions, and then internally mixed soot-particles can be formed during their aging. The results collected on a rainy day have showed relatively low fractions of the internally mixed soot-particles probably due to the efficient nucleation scavenging of internally mixed soot-particles in the atmosphere.
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