Environ. Sci. Technol. 2005, 39, 1409-1419
Single-Particle Characterization of Four “Asian Dust” Samples Collected in Korea, Using Low-Z Particle Electron Probe X-ray Microanalysis C H U L - U N R O , * ,† H E E J I N H W A N G , † HYEKYEONG KIM,† YOUNGSIN CHUN,‡ AND R E N EÄ V A N G R I E K E N § Department of Chemistry, Inha University, Incheon, 402-751, Korea, Applied Meteorology Research Laboratory, Meteorological Research Institute, Seoul 156-720, Korea, and Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610, Antwerp, Belgium
A single-particle analytical technique, named low-Z particle electron probe X-ray microanalysis (low-Z particle EPMA), employing an ultrathin window X-ray detector and enabling the quantitative determination of even low-Z elements such as C, N, and O, is applied to characterize “Asian Dust” samples, collected in ChunCheon, Korea, during four Asian Dust storm events on March 7, 2000, April 7, 2000, March 22, 2001, and May 17, 2001. In this study, it is demonstrated that single-particle analysis using the low-Z particle EPMA provides detailed information on various types of chemical species in the samples. The most abundantly encountered particles, both in coarse and fine fractions, are aluminosilicates. The relative abundances of those particles on the basis of their size are different between the four Asian Dust samples. The sample collected on March 7, 2000 did not experience any chemical modification during its transport because the sample does not contain particles of chemical species that result from atmospheric reactions. The sample collected on April 7, 2000 contains both genuine and reacted seasalt particles. The genuine sea-salts are in the form of a mixture of NaCl and MgCl2 entrained during their passage over the Yellow Sea. The reacted sea-salts particles are encountered very much in fine fraction. The sample collected on March 22, 2001 shows somewhat significant chemical modification both for CaCO3 and sea-salts particles. For this sample, a significant number of reacted CaCO3 and seasalt particles, such as those containing nitrate and/or sulfate, are encountered, implying that CaCO3 and sea-salts particles have reacted with sulfur or nitrogen oxide species during their long-range transport. The sample collected on May 17, 2001 experienced the most extensive chemical modification during its transport. In addition to the observation of the extensively reacted CaCO3 and sea-salt particles, reacted K2CO3 particles are also extensively * Corresponding author phone: +82 32 860 7676; fax: +82 32 867 5604; e-mail:
[email protected]. † Inha University. ‡ Meteorological Research Institute. § University of Antwerp. 10.1021/es049772b CCC: $30.25 Published on Web 01/27/2005
2005 American Chemical Society
encountered in this sample, which implies that K2CO3 species should be regarded as an additional important chemical species in the study of the chemical modification of Asian Dust particles during long-range transport.
Introduction Nearly every spring, usually from March to May, “Asian Dust” originating mostly in Central China’s arid areas is transported into Eastern China, the industrialized regions of China, and over the Yellow Sea to Korea, Japan, and even the Pacific Ocean. During the Asian Dust’s long-distance travels, it can react with diverse chemical species and/or provide a reaction site for chemicals in the air. Therefore, Asian Dust can possibly carry other chemical species along with its original soil components. For example, it would react with chemical species such as SOx and NOx so that the transport of modified Asian Dust to the East Asian region could result in the deposition of sulfate and nitrate, in addition to mineral dust, in this area (1-3). It was also calculated that dust particles can contain more sulfate in aerosol form during high dust periods by up to a factor of 2, with almost all the sulfate being present on the surface of the dust particles (4). It is well-known that sulfate aerosols in the accumulation mode, both directly and indirectly, reflect solar radiation back to space and thus they play an important role in climate change (5). If the reaction of SOx with mineral dust occurs in the air during long-range transport, the radiative forcing due to the sulfate component will be reduced; the size of Asian Dust is large enough to have much smaller radiative forcing than the accumulation range sulfate aerosols, and the sulfate on the surface of Asian Dust does not change the radiative properties of the dust particles very much (4). In addition, the chemistry of the Earth’s atmosphere is influenced by reducing photolysis rates of important atmospheric gas-phase species, through the heterogeneous chemical reaction of mineral dust particles (6, 7). Furthermore, as mineral dust reacts in the air, the physicochemical properties of the particles change and thus their effectiveness to serve as cloud condensation nuclei also changes (8, 9). Hence, increasing attention has been devoted to the study of physicochemical characteristic changes of Asian Dust particles during longrange transport. Many studies have been carried out on the chemical modification of Asian Dust particles during long-range transport. The chemical analysis of Asian Dust has been performed either by bulk analysis or single particle analysis. Even though results of the bulk analysis could not provide direct evidence for the chemical modification of Asian Dust, research conducted in Korea reported that Asian Dust collected in Korea contained aerosols reacted with NOx and SOx, especially after it had passed through an industrialized region of China (10, 11). Asian Dusts collected in Japan and Hong Kong were also reported to contain aerosols reacted with NOx, SOx, and sea-salts (12-14). However, it was reported that the NO3- and SO42- contents of aerosols collected in the spring at three Chinese cities (Minqin, Qingdao, and Qianliyan) are rather lower than those collected in the summer, suggesting that the concentration of aerosols reacted with NOx and SOx did not increase during Asian Dust events in China (15). Direct evidence of the chemical modification of Asian Dust can be provided by the application of single particle analysis because this can provide detailed knowledge about the chemical composition and the morphology of individual VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Samplings during Asian Dust Storm Events Asian Dust Storm Event
Sampling
sample
start
end
duration
start
end
duration
March 7, 2000 April 7, 2000 March 22, 2001 May 17, 2001
Mar. 7, 11:20 Apr. 7, 8:20 Mar. 22, 3:30 May 16, 15:00
Mar. 7, 16:50 Apr. 8, 18:30 Mar. 22, 20:30 May 19, 11:20
5 h 30 min 34 h 10 min 17 hr 68 h 20 min
Mar. 7, 15:00 Apr. 7, 11:00 Mar. 22, 15:00 May 17, 14:00
Mar. 7, 16:45 Apr. 7, 12:10 Mar. 22, 17:00 May 17, 18:00
1 h 45 min 1 h 10 min 2 hr 4 hr
particles on a micrometer scale. Using single particle analysis, Asian Dust samples collected in China were reported to contain less sulfate and nitrate than non-Asian Dust samples (16, 17). In addition, it was reported that more than 90% of Asian Dust particles collected in Qingdao, China during three Asian Dust events in the spring of 2001 were not disturbed by sulfate, nitrate, and/or sea-salts (18). However, there were reports saying that Asian Dust samples collected in Japan had experienced chemical modification during their transport (19-21). Those single particle analyses were performed with conventional energy-dispersive electron probe X-ray microanalysis (ED-EPMA), where the energy-dispersive X-ray detector has a thick Be window so that low-Z elements such as C, N, and O cannot be determined. An EPMA technique called “low-Z particle EPMA” has been developed which uses an X-ray detector equipped with an ultrathin window and thus allows EPMA to determine concentrations of low-Z elements as well in individual particles of micrometer size. In our previous study, it was found that excitation interactions between electrons and the matrix atoms and the geometric and matrix effects of electron-induced X-ray signals for low-Z elements in individual atmospheric microparticles can be described by Monte Carlo simulation (22). By the application of a quantification method, which employs Monte Carlo simulation combined with successive approximations, it was also shown that quantitative specification of the chemical compositions can be done (23-25). Furthermore, the chemical species, in addition to elemental composition, in individual particles can be determined by the application of the low-Z particle EPMA (26-30). The capability of the quantitative determination of low-Z elements in individual environmental particles improves the applicability of single particle analysis; many environmentally important atmospheric particles (e.g., sulfates, nitrates, ammonium salt, and carbonaceous particles) contain low-Z elements. Therefore, this low-Z particle EPMA is expected to provide more conclusive and detailed analysis on the chemical composition of Asian Dust particles than single particle analysis performed by conventional EPMA. In this study, the microanalyses for characterizing the chemical composition of four Asian Dust samples collected in ChunCheon, Korea, on March 7, 2000, April 7, 2000, March 22, 2001, and May 17, 2001 were carried out using the low-Z particle EPMA technique. It is demonstrated that this single particle analysis provides detailed information on the chemical modification that the samples experienced during long-range transport.
Experimental Section Samples. Aerosol samples were collected on the roof of a campus building of Hallym University (12 m above ground level), which is located in the downtown district of ChunCheon, Korea. ChunCheon (37°89′ N, 127°73′ E) is a relatively small city (population 0.25 million, area 1116 km2), has a mostly rural character outside the central district, and is also free of industrial complexes. Samplings were done on March 7 and April 7 in 2000 and March 22 and May 17 in 2001 when Asian Dust storm events occurred (see Table 1). For each of 1410
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the dust storm events, the samplings were started after the dust storm events had progressed at least for a few hours and finished before the dust storm events ended. According to land-based observation data, the Asian Dust sampled on March 7, 2000 in ChunCheon, Korea, originated from the area located in the Provinces of Northeastern China (40-45° N, 120° E) (“source region” in Figure 1A designates where the Asian Dust storm originated). The Asian Dust storm event started at 0300 UTC on March 6, 2000, and had drifted toward the Korean peninsula (see Figure 1A for its air-mass backward trajectory. The back-trajectory analyses shown in Figure 1A-D were performed by using the HYSPLIT model and meteorological data available on the NOAA/ARL website). The origin of the sample collected on April 7, 2000 was the Gobi Desert of China (40-50° N, 100-120° E). The Asian Dust storm event started at 0600 UTC on April 5, 2000 and lasted until 0900 UTC on April 6, 2000, and it passed over the Bo Sea and Yellow Sea on its way to the Korean peninsula (see Figure 1B). The two Asian Dust samples collected on March 22 and May 17 in 2001 originated from the areas of 38-45° N, 105-126° E and 38-45° N, 114-116° E, respectively. The first Asian Dust event started at 0000 UTC on March 21, 2001 and lasted until 2100 UTC of the same day. The second one started at 0900 UTC on May 15, 2001. This air-mass moved more slowly compared to the other airmasses and passed over the Yellow Sea before arriving in Korea. Particles were sampled on Ag foil using a seven-stage May cascade impactor (31). The May impactor has, at a 20 L/min sampling flow, aerodynamic cutoffs of 16, 8, 4, 2, 1, 0.5, and 0.25 µm for stages 1-7, respectively. The seventh stage was not analyzed because the very-small-sized particles collected on the stage were not suitable for EPMA measurements. The sampling duration varied from about 10 min (for stage 6) to several hours (for stage 1) in order to obtain a good loading of particles at the impaction slots. Since the number concentration of smaller particles in the air is larger than that of bigger particles, it is necessary to collect smaller particles in shorter sampling duration, to avoid collection of agglomerated particles. The sampling durations were somewhat different between the Asian Dust storm events, reflecting the different intensities of the Asian Dust storm events (see Table 1). Since uncoated collecting substrates were used, it is possible that some size misclassification could have occurred owing to particle bounce-off. The collected samples were put in plastic carriers, sealed, and stored in a desiccator. Some 200-300 particles for each stage sample were analyzed totaling some 5700 particles (1400, 1100, 1600, and 1600 particles for samples collected on March 7, 2000, April 7, 2000, March 22, 2001, and May 17, 2001, respectively). EPMA Measurements. The measurements were carried out on a JEOL 733 electron probe microanalyzer equipped with an Oxford Link SATW ultrathin window EDX detector. The resolution of the detector is 133 eV for Mn KR X-rays. The spectra were recorded by a CANBERRA S100 multichannel analyzer under the control of homemade software. To achieve optimal experimental conditions, such as a low background level in the spectra and high sensitivity for low-Z
FIGURE 1. Air-mass backward trajectories for samples collected on (A) March 7, 2000, (B) April 7, 2000, (C) March 22, 2001, and (D) May 17, 2001. element analysis, a 10 kV accelerating voltage was chosen. The beam current was 1.0 nA for all the measurements. To obtain statistically enough counts in the X-ray spectra while limiting beam damage effects on sensitive particles, a typical measuring time of 10 s was used. The cold stage of the electron microprobe allowed the analysis of particulate samples at liquid nitrogen temperature (approximately -193 °C), which minimized contamination and also reduced beam damage to the samples. A more detailed discussion on the measurement conditions is given elsewhere (22). Computer-controlled X-ray data acquisition for individual particles was carried out automatically in point analysis mode, i.e., the electron beam was focused on the center of each particle, and X-rays were acquired while the beam remained fixed on this single spot. The localization of the particles was based on inverse backscattered electron contrast. Morphological parameters,
such as diameter and shape factor, were calculated by an image processing routine. These estimated geometrical data were set as input parameters for the quantification procedure. Data Analysis. The net X-ray intensities for the elements were obtained by nonlinear least-squares fitting of the collected spectra using the AXIL program (32). The elemental concentrations of individual particles were determined from their X-ray intensities by the application of a Monte Carlo calculation combined with reverse successive approximations (23-25). The quantification procedure provided results accurate within 12% relative deviations between the calculated and nominal elemental concentrations when the method was applied to various types of standard particles such as NaCl, Al2O3, CaSO4‚2H2O, Fe2O3, CaCO3, and KNO3, except for C and K where the characteristic X-rays overlap with those from the Ag substrate (27). More details VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Typical secondary electron image showing well-separated individual particles. on the quantification procedure can be found elsewhere (23, 24, 27). The determination of chemical species in individual particles was done in a way that fully utilized the information contained in their X-ray data. The chemical composition of any one particle is never exactly the same as that of others. It is also rather rare to see particles composed of only one pure chemical species. Also, particles with two or more chemical species have different compositions. The low-Z particle EPMA can provide quantitative information on the chemical composition, and particles can be classified on the basis of their chemical species. The analytical procedure for determining chemical species, and the way to perform classification, are described in more detail elsewhere (26, 28). Here, we will briefly summarize how particles were classified. First, particles were regarded to be composed of just one chemical species when the chemical species constituted at least 90% in atomic fraction. Second, efforts were made to specify chemical species, even for particles internally mixed with two or more chemical species. Since many different types of internally mixed particles were identified, mixture particles were grouped on the basis of all the chemical species with >10% in formula fraction. Third, it is known that ED-EPMA has high detection limits of 0.11.0% in weight mainly due to its high Bremsstrahlung background level. Since the low-Z particle EPMA is used for the analysis of a microscopic volume (pg range in mass for a single particle of micrometer size), the elements at trace levels could not be reliably investigated; thus, we do not include elements with less than 1.0% of atomic concentration in the procedure of chemical speciation. This classification procedure takes a substantial amount of time if done manually, since we analyze several thousands of particle data for each sample. Thus, an expert system that can determine chemical species from the elemental concentration data has been developed and applied to these Asian Dust sample data. By applying the expert system, the time necessary for chemical speciation becomes significantly shorter, and detailed information on particle data can be saved and extracted when more information is needed for further analysis. A detailed description of the expert system, which tries to mimic the logic used by experts to determine 1412
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the chemical species of individual particles using the low-Z particle EPMA data, is given elsewhere (28).
Results and Discussion The low-Z particle EPMA technique was applied to characterize four Asian Dust samples collected in ChunCheon, Korea, on March 7, 2000, April 7, 2000, March 22, 2001, and May 17, 2001 (the samples will be represented as “March_00”, “April_00”, “March_01”, and “May_01” samples, respectively). Figure 2 shows a secondary electron image obtained from a sample collected on the stage 4 of the May impactor. The particles are well separated from each other and X-ray spectra for individual particles are obtained from the corresponding particles. Overall, some 5700 particles (1400, 1100, 1600, and 1600 for the March_00, April_00, March_01, and May_01 samples, respectively) were analyzed and classified on the basis of the chemical species of the particles. In Figures 3-6, relative abundances of the chemical species at different stages for the four samples are shown. Particles abundantly encountered in the samples are soil-derived such as aluminosilicate, silicon dioxide, and calcium-containing particles; carbonaceous particles, specified either as organic or carbon-rich; and marine-originated particles. It is well-known that Asian dust particles collected during Asian Dust events and at source areas are mainly composed of mineral particles (15, 33, 34). Aluminosilicates and Silicon Dioxide. Aluminosilicate particles can exist in many different forms by containing various minor elements such as Na, Mg, S, Cl, K, Ca, and Fe. All these particles are simply grouped as “AlSi(aluminosilicates)” in Figures 3-6. It was observed that many soilderived aluminosilicate particles contain carbonaceous species which probably come from humic substances in soil (27). In this case, they are grouped as “AlSi/C” in Figures 3-6. Also, many particles exist as a mixture of aluminosilcates and other chemical species, such as CaCO3, MgCO3, Fe2O3, CaSO4, Na2SO4, or MgSO4. Since particles internally mixed with aluminosilicates and CaCO3 are sometimes significant in number, the particles are denoted by “AlSi/CaCO3” in the Figures. The other mixture particles are grouped as “AlSi/misc.”. For all the Asian Dust samples, the aluminosilicate-containing particles are the most abundant in all
FIGURE 3. Relative abundances of each chemical species in the March_00 sample (cutoff diameters of stages 1-6 are 16, 8, 4, 2, 1, and 0.5 µm).
FIGURE 4. Relative abundances of each chemical species in the April_00 sample (cutoff diameters of stages 1-6 are 16, 8, 4, 2, 1, and 0.5 µm). size ranges except stage 6, in which the cutoff diameter of the stage is the smallest and carbonaceous particles are the most abundant. The relative abundances of the aluminosilicate-containing particles are in the range of 3-76%, depending on the size and the specific Asian Dust samples. In Figure 7, overall relative abundances of significantly encountered particle types are shown for the four samples. The overall relative abundance is calculated by dividing the encountered number of the specific type of particles by the total number of analyzed particles for each sample. As shown in Figure 7, the April_00 sample contains the most abundant aluminosilicate species among the four samples, and the March_01 sample is the next, whereas the alumionosilicatecontaining particles in the March_00 and May_01 samples are approximately half of the analyzed particles. In addition, the April_00 sample is significantly different from the other three samples in that the aluminosilicate particles exist more as single species (see Figures 3-6). In Figure 8, the relative frequencies of minor chemical elements encountered in the aluminosilicate-containing particles are shown. C, Na, Mg, S, K, Ca, and Fe elements are frequently observed in the aluminosilicate-containing particles, whereas the frequencies to observe P, Cl, Ti, V, and Mn are small (
