Single-Particle Characterization of Atmospheric Aerosols Collected at Gosan, Korea, during the Asian Pacific Regional Aerosol Characterization Experiment Field Campaign Using Low-Z (Atomic Number) Particle Electron Probe X-ray Microanalysis

ABSTRACT

A quantitative energy-dispersive electron probe X-ray microanalysis (ED-EPMA), namely low-Z (atomic number) particle EPMA, was used to characterize the chemical compositions of the individual aerosol particles collected at the Gosan supersite, Jeju Island, Korea, as a part of the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia). On 4–10 April 2001 just before a severe dust storm arrived, seven sets of aerosol samples were obtained by a seven-stage May cascade impactor with a flow rate of 20 L/min. Overall 11,200 particles on stages 1–6 with cutoff diameters of 16, 8, 4, 2, 1, and 0.5 μm, respectively, were examined and classified based on their secondary electron images and X-ray spectra. In general, sea salt particles were the most frequently encountered, followed by mineral dust, organic carbon (OC)-like, (NH₄)₂SO₄/NH₄HSO₄-containing, elemental carbon (EC)-like, Fe-rich, and K-rich particles. Sea salt and mineral dust particles had a higher relative abundance on stages 1–5, whereas OC-like, (NH₄)₂SO₄/NH₄HSO₄-containing, Fe-rich, and K-rich particles were relatively abundant on stage 6. The analysis on relative number abundances of various particle types combined with 72-hr backward air mass trajectories indicated that a lot of reacted sea salt and reacted mineral dust (with airborne NO_x and SO₂ or their acidic products) and OC-like particles were carried by the air masses passing over the Yellow Sea (for sample “10 April”) and many NH₄HSO₄/(NH₄)₂SO₄-containing particles were carried by the air masses passing over the Sea of Japan and Korea Strait (for samples “4–9 April”). It was concluded that the atmosphere over Jeju Island was influenced by anthropogenic SO₂ and NO_x, organic compounds, and secondary aerosols when Asian dust was absent.

IMPLICATIONS

A quantitative energy-dispersive electron probe X-ray microanalysis, called low-Z particle EPMA, was used to characterize aerosol particles collected at Jeju Island, Korea, on 4–10 April of 2001 during the intensive observation period of the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia). The particle size distribution and chemical compositions of individual atmospheric particles over Jeju Island were obtained, and many environmentally important atmospheric particles containing sulfate, nitrate, sea salt, alminosilicate, and carbonaceous species in the size range of 0.5–16 μm were characterized. It was found that anthropogenic SO₂ and NO_x made great impacts on compositions of aerosol particles. The results helped better understand how air pollutants interacted with aerosol particles in the Asia-Pacific region.

INTRODUCTION

Because of its unique location with no nearby and local industrial sources, the Gosan site on Jeju Island of Korea is considered as an ideal place to monitor the regional background atmosphere in the East Asian region and to study the impact of continental outflow events on aerosol composition.1 Aerosol characteristics observed at Gosan have shown that Jeju Island is frequently affected by Asian dust storms originated from Mongolia and northwestern China. For instance, severe dust storms that blanketed much of eastern Asia occurring in April 2001 were observed at the Gosan supersite on 11–13 April and 25–26 April during the intensive observation period (IOP) of the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) field campaign.2

Up to now, many studies on the chemical and physical properties and climatic effects of mineral dust and other types of aerosol particles (of both anthropogenic and natural origins) during Asian dust episodes at Gosan were conducted.3 However, few papers have been published on single-particle characterization of the springtime aerosols in the eve of severe dust storms. In the present study, a quantitative energy-dispersive electron probe X-ray microanalysis (ED-EPMA) method, namely low-Z (atomic number) particle EPMA,4 was used to determine individual aerosol particles collected at Gosan on 4–10 April of 2001 just before an Asian dust storm came. Low-Z particle EPMA has shown powerful advantages in characterization of environmental and geological single particles because it can give the size, morphology, and quantitative chemical compositions of individual particles without a complicated sample pretreatment process, and many environmentally important atmospheric particles, for example, sulfates, nitrates, ammonium, and carbonaceous particles, can be elucidated based on its measurement result.5–7

The objective of the present study is to characterize size-segregated springtime aerosol particles collected at Gosan using the low-Z particle EPMA technique, and to investigate the effects of air pollutants (such as NO_x and SO₂) on the components of the atmospheric particles over Jeju Island, a relatively clean site in Korea.

MATERIALS AND METHODS

Sampling

A seven-stage May cascade impactor at a flow rate of 20 L/min was used to collect seven sets of aerosol samples in the period from 4 to 10 April of 2001 at Gosan (126.17˚E, 33.28˚N), which is located on the western edge of Jeju Island, Korea, and has an altitude of ∼60 m above sea level (Figure 1). The sampling duration varied from several minutes to hours for different stages (stages 1–6) to obtain a good loading of particles without agglomeration. The collected samples were put in plastic carriers, sealed, and stored in desiccators before EPMA measurement.

Figure 1. Schematic map of the sampling site.

Measurement and Analysis

Overall 11,200 particles on stages 1–6 with nominal cutoff diameters of 16, 8, 4, 2, 1, and 0.5 μm, respectively, were investigated by using a JEOL JSM-6390 scanning electron microscope (SEM) equipped with an Oxford Link SATW ultrathin window energy-dispersive X-ray (EDX) detector (Japan Electron Optics Laboratory, Co., Ltd.) (100 particles per stage for the samples of stage 1 and 300 particles per stage for the samples of stages 2–6). The resolution of the detector was 133 eV for Mn Kα X-ray. The net X-ray intensities of the chemical elements were fitted by the nonlinear, least squares method using the Analysis of X-ray speetra by iterative least-squares fitting (AXIL) program.8 A Monte Carlo simulation with successive approximation was employed for quantification.9 The “expert system” program that can rapidly and reliably perform chemical speciation was used to determine the formula concentrations and group distributions.10 The basic classification rule for the measured particles was given elsewhere.^5,6 In general, particle samples collected on substrates may suffer from some artifacts, which included evaporation of volatile and semivolatile species during the sampling, storage, and measurement. In the vacuum of electron microscope and under the electron beam of EPMA, water, NH₄NO₃, NH₄Cl, and certain volatile and semivolatile organic materials, if present, might be lost. Despite this, the compositions of the particles we measured should reflect the majority of their original compositions before collection. And the lost compositions had no influence on accuracy of the analytical approach of low-Z EPMA. 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 KNO₃.11

The speciation of carbonaceous particles was carried out based on the morphologies and the C and O content of particles (in general a particle containing more than 90% of C and O in atomic concentration was regarded as an organic or carbon-rich particle).¹¹ Since the presence of hydrogen failed to be detected in EPMA, elemental carbon (EC) and organic carbon (OC) were defined in a somewhat arbitrary way. To avoid confusion, we used OC-like and EC-like rather than OC and EC in the following description. The EC-like particle, for example, soot aggregate, tar ball, and char or coal dust, was easily identified by its shape and by the ratio of carbon to oxygen content (the atomic concentration of C was usually 3 times larger than that of O in its X-ray spectra), whereas the OC-like particle had an irregular shape and the atomic concentration of C was not so much larger than that of O (often, N signal could be detected in it).

Air Mass Transport History

To investigate the air mass transport pathways and the potential aerosol source regions, 3-day (72-hr) backward air mass trajectories were obtained using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model from the National Oceanic and Atmospheric Administration (NOAA) Air Resources Laboratory's Web server (http://www.arl.noaa.gov/ready/hysplit4.html). Backward trajectories at the altitude of 60 m were calculated at the end of sampling.

RESULTS AND DISCUSSION

Particle Types

Based on the X-ray spectral data and “expert system” classification, the particle types were identified. They were (1) sea salt group, including (a) genuine sea salt, (b) reacted (or aged) sea salt with sulfur and nitrogen oxides or their acidic products in the air, and (c) the mixture of genuine or reacted sea salt with mineral dust particles; (2) mineral dust group, including (a) genuine mineral dust such as aluminosilicate, quartz (SiO₂), and calcite (CaCO₃)/dolomite (CaMg(CO₃)₂), and (b) reacted (or aged) mineral dust particles such as sulfate- and/or nitrate-containing aluminosilicate and CaCO₃/CaMg(CO₃)₂; (3) (NH₄)₂SO₄/NH₄HSO₄-containing particles; (4) carbonaceous particles, including EC-like and OC-like particles; (5) Fe-rich particles; and (6) K-rich particles. The characteristics of their morphologies and chemical compositions have been described in detail elsewhere.5–7

Particle Size Distribution

Particle size distribution plays an important in our understanding the physical and chemical properties of particles and their reactivity with other matters. Herein, the size distribution of particles at different stages of the May cascade impactor was investigated on the basis of the information provided by EPMA. Our study focused on the particulate matter with aerodynamic diameters (ADs) of 0.5–16 μm on stages 1–6, since smaller particles (on stage 7, AD <0.5 μm) were not fit to be detected by EPMA. In the measured particles, 96.9% of them were in the size range of 0.5–16 μm (Table 1). Probably, some size misclassification might have occurred owing to particle bounce-off during sampling.12 For stages 1 and 2, most of the measured particles had the equivalent diameter of 4–16 μm (87.6% for stage 1 and 92.2% for stage 2, respectively); for stage 3, 83.8% of the measured particles were in the size range of 2–8 μm; for stage 4, 84.6% of the measured particles were in the range of 1–2 μm; for stage 5, 83.4% of the measured particles were in the size range of 0.5–2 μm; and for stage 6, 72.2% of the measured particles were in the size range of 0.25–1.0 μm.

Table 1. Size distribution of the analyzed particles collected at Gosan on 4–10 April 2001

Percent of Measured Aerosol Particles in Different Diameter Range (%)
Size	<0.249 μm	0.25–0.49 μm	0.5–0.99 μm	1–1.99 μm	2–3.99 μm	4–7.99 μm	8–15.99 μm	16–31.99 μm	>32 μm	Total
Stage 1						42.0	45.6	10.8	1.6	100
Stage 2					7.4	63.6	28.6	0.3		100
Stage 3				15.3	47.6	36.2	0.9			100
Stage 4			7.0	27.8	56.9	8.3				100
Stage 5		5.3	28.4	55.0	11.3					100
Stage 6	6.1	31.0	41.2	21.7						100

Three-Day (72-Hr) Backward Air Mass Trajectories

Based on the 72-hr backward air mass trajectories at receptor height of 60 m above the sea level at the sampling site, three categories of air mass transport routes were classified (Figure 2a–g). The first category is for samples collected in the afternoon on 4, 5, and 6 April of 2001 (Coordinated Universal Time [UTC]), in which the air masses originated from northeastern China or Korea, traveled over the Sea of Japan and Korea Straits, and then arrived at the sampling site from the east direction (Figure 2a–c). The second category is for samples collected in the afternoon on 7, 8, and 9 April of 2001 (UTC), in which the air masses originating from near the coast of Japan passed over the Sea of Japan and Korea Strait, and arrived at the sampling site from the east or northeast direction (Figure 2d–f). The third category is for the sample collected in the afternoon on 10 April of 2001 (UTC), in which the air masses originated from and traveled over the Yellow Sea, and arrived at the sampling site from the northwest direction (Figure 2g). In early April of 2001, a severe dust storm occurred.13 The dust clouds originated from the arid deserts of Mongolia (the Gobi Desert) and passed over China and the Yellow Sea. On 11 April, the dust storm front reached Gosan with high wind speed (Figure 2h).

Figure 2. Three-day (72-hr) backward air mass trajectories at sampling height (60 m above the sea level) from 4 to 10 April 2001. The backward air mass trajectories at the sampling site were obtained using the Hybrid Lagrangian Integrated Trajectory (HYSPLIT) model from the NOAA Air Resources Laboratory's web server (http://www.arl.noaa.gov/ready/hysplit4.html). On 11 April 2001, a severe dust storm front arrived at Gosan, so the air mass back trajectory was different from those on 4–10 April. (a) 04 April 2001; (b) 05 April 2001; (c) 06 April 2001; (d) 07 April 2001; (e) 08 April 2001; (f) 09 April 2001; (g) 10 April 2001; (h) 11 April 2001.

Relative Abundances of Various Particle Types

Jeju Island is regarded as an ideal location to monitor the atmospheric chemistry of East Asia, since it is surrounded by China, the Korean peninsula, and the Japanese islands to the west, north, and east, respectively (about 100 km south of the Korea mainland, 250 km west of Kyushu, Japan, 500 km east-northeast of Shanghai, China, and 1000 km north-northeast of Taipei), and is a famous resort with no large industrial sources.14 It is one of the cleanest areas in Korea with very low emissions of primary air pollutants and the atmosphere over it may be affected by long-range transport of air pollutants from outside of the island.¹⁴ In the days before air parcels carrying Asian dust arrived at Gosan site (1–10 April 2001), the daily mass concentrations of particulate matters of aerodynamic diameters ≤10 (PM₁₀) and ≤2.5 (PM_2.5) μm were approximately 55 and 25 μg/m³, respectively.15 When the dust storm advected off the Chinese mainland and arrived at the Jeju Island, the daily mass concentrations of PM₁₀ and PM_2.5 over Gosan were sharply increased to around 140 and 71 μg/m³, respectively (on 13 April 2001).2,15

In the single-particle analysis, the characteristics of aerosol components were reflected by relative abundances of particle types. They were obtained by dividing the number of a specific type of particles by the total number of particles analyzed for a sample (Figure 3 and Table 2). By comparisons of relative abundances of major particle types at different size levels on different sampling days, we expected to find the major components of aerosols in the atmosphere of Jeju Island and to understand how the air mass histories affected them.

Figure 3. Relative abundances of various particle types in different size ranges for the aerosol samples collected at Gosan, Korea, on 4–10 April 2001.

Table 2. Relative abundances of different types of particles in different size ranges (on stages 1–6) for the samples collected at Gosan on 4–10 April 2001

		Relative Abundance on Average in Different Size Range (%)
No.	Particle Type	Stage 1	Stage 2	Stage 3	Stage 4	Stage 5	Stage 6
1	Genuine sea salt	3.65 ± 1.04	5.57 ± 1.23	4.56 ± 2.65	6.65 ± 2.21	4.08 ± 1.28	3.08 ± 2.79
2	Reacted sea salt (+ mixture)	36.49 ± 5.90	53.92 ± 6.59	53.74 ± 4.60	66.59 ± 5.77	66.26 ± 4.74	20.93 ± 3.57
	(1) containing nitrates	25.39 ± 4.54	33.93 ± 5.90	36.85 ± 3.95	48.42 ± 4.71	29.49 ± 4.31	2.15 ± 1.41
	(2) containing SO4 ^2–/CH3SO3 ^–	3.05 ± 0.78	6.38 ± 1.45	6.07 ± 2.04	7.94 ± 2.16	14.85 ± 1.90	11.19 ± 2.88
	(3) containing both	9.05 ± 1.80	13.61 ± 1.53	10.82 ± 1.75	10.23 ± 1.60	21.92 ± 4.16	7.59 ± 1.09
3	Genuine mineral dust	44.27 ± 6.77	30.01 ± 4.98	30.41 ± 4.19	11.63 ± 1.63	12.92 ± 3.22	11.04 ± 3.03
4	Reacted mineral dust	2.20 ± 0.41	8.84 ± 2.94	7.24 ± 1.52	7.04 ± 1.74	6.05 ± 1.43	11.21 ± 4.29
	(1) containing nitrates	1.47 ± 0.20	1.14 ± 0.22	3.14 ± 1.19	2.85 ± 0.85	1.58 ± 0.55	3.34 ± 1.94
	(2) containing sulfates	0.44 ± 0.30	5.53 ± 2.59	1.63 ± 0.25	1.77 ± 0.49	2.91 ± 0.81	5.83 ± 3.02
	(3) containing both	0.29 ± 0.19	2.17 ± 0.71	2.47 ± 0.69	2.42 ± 0.74	1.56 ± 0.29	2.04 ± 0.96
5	(NH4)2SO4/NH4HSO4-containing	0	0	0.15 ± 0.07	0.26 ± 0.15	5.26 ± 2.01	38.42 ± 8.08
6	Elemental carbon (EC)-like	5.96 ± 2.51	0.54 ± 0.42	0.68 ± 0.25	0.59 ± 0.26	0.59 ± 0.14	0.79 ± 0.38
7	Organic carbon (OC)-like	6.98 ± 1.89	0.96 ± 0.13	2.33 ± 0.65	6.49 ± 4.86	3.34 ± 0.72	11.99 ± 4.24
8	Fe-rich	0.45 ± 0.21	0.16 ± 0.11	0.88 ± 0.30	0.56 ± 0.15	0.40 ± 0.29	1.82 ± 0.71
9	K-rich	0	0	0	0.19 ± 0.14	1.10 ± 0.30	0.71 ± 0.37
Note : All data are represented as mean ± SE.

Sea Salt Particles

The genuine sea salt particles, which did not experience chemical reactions after being freshly emitted into the air by the so-called bubble bursting or sea spray process, were identified by the presence of Na and Cl peaks in their X-ray spectra, often with minor C, O, and Mg signals.^5–7 The C and O signals might result from organic matter that adhered to sea salt as coatings. Also, the O signal probably came from NaOH shell (an alkaline hygroscopic coating around the NaCl), which was generated at the air-solution interface by the reaction of NaCl with OH^– (OH^– was produced from photolysis of O₃ on water vapor).16 When the genuine sea salt particles reacted with nitrogen and sulfur oxides species in the atmosphere, the reacted (or aged) particles were formed, usually resulting in chlorine loss. Based on their secondary electron image (SEI) and X-ray spectral data, three types of reacted sea salt particles were classified. The first type was for those containing nitrates such as Na(Cl, NO₃) and (Na, Mg)(Cl, NO₃), which were generated when sea salt reacted with NO_x or HNO₃, mostly from anthropogenic emission. The second type was for those containing sulfates (SO₄ ^2–) or methanesulfonate (CH₃SO₃ ^–) such as (Na, Mg)SO₄ and (Na, Mg)CH₃SO₃, which were generated from the reactions of sea salt with anthropogenic SO₂/H₂SO₄ and/or methylsulfonic acid (MSA) from oxidized dimethylsulfide (DMS).17,18 DMS, the most abundant volatile sulfur compound emitted into the atmosphere from the ocean (usually produced by oceanic phytoplanktons), can be oxidized in the atmosphere to form SO₂, MSA, non-sea-salt sulfate (nss-SO₄ ^2–), etc. Hence, SO₄ ^2– and CH₃SO₃ ^– were likely to be present simultaneously in the sulfur-containing reacted sea salt particles. It was regarded that nss-SO₄ ^2– were generated when the atomic concentration ratio of [S] to [Na] was >0.06 because the concentration ratio of sea salt sulfate (ss-SO₄ ^2–) to sodium in bulk seawater was 0.06.17 However, since the contribution of biologically produced DMS was not significant for nss-SO₄ ^2– over the North Yellow Sea and the East China Sea (the ratio of the calculated biogenic nss-SO₄ ^2– to the measured total nss-SO₄ ^2– ranged from 1.2% to 11.5% [average 5.8%]),18 the dominant component for atmospheric SO₄ ^2– budget in this region was attributed to anthropogenic sources. So, herein, sulfate aerosols generated from oxidized DMS were neglected. The third type was for those containing both NO₃ ^– and SO₄ ^2–/CH₃SO₃ ^–. In addition, some reacted sea salt particles internally mixed with mineral dust species (i.e., CaCO₃, Ca(NO₃)₂, and CaSO₄) were classified into the group of “reacted sea salt + mixture”.

The relative abundances of genuine and reacted (aged) sea salt particles were calculated and illustrated in Figure 3 and Table 2. They could provide information on whether and how the aerosols at Gosan were influenced by NO_x and/or SO₂. The fact that the reacted sea salt (+ mixture) particles were abundant on every stage and greatly outnumbered the genuine ones (20.9–66.6% of relative abundance on average vs. 3.1–6.7%) revealed that the sea salt aerosol particles at Gosan were easily affected by anthropogenic NO_x and SO₂ produced in China and/or Korea. It was consistent with the reports that marine-originated particles such as NaNO₃ ^–- and Na₂SO₄ ^–-containing particles were popular over Jeju Island19 and the aerosol at Jeju Island was enriched in sulfate, nitrate, and ammonium, but deficient in chlorine (relative to seawater).20 The reacted sea salt particles had nearly 2-fold higher abundances on stages 2–5 than on stages 1 and 6 (Table 2), indicating that they were neither too large nor too small in size, being frequently encountered in the size range of 1–16 μm. In all the reacted sea salt particles, those containing NO₃ ^– significantly outnumbered the SO₄ ^2–/CH₃SO₃ ^– and both-containing ones on stages 1–5, whereas those containing SO₄ ^2–/CH₃SO₃ ^– peaked on stage 6 (Figure 4a and Table 2). It had a strong implication that nitrates tended to form in supermicrometer particles and sulfates tended to be produced in submicrometer ones, which agreed with the observations made by Sullivan et al.21 We also observed that many reacted sea salt particles (with more than 1.0 µm in equivalent diameter) contained nitrates in the samples collected at Tokchok Island (one of the cleanest areas in Korea with few local emissions of air pollutants)⁵ and on the marine boundary layer of the Yellow Sea,⁶ indicative of great influences made by NO_x/HNO₃ on aerosol components over the Yellow Sea.

Figure 4. Relative abundances of reacted (aged) sea salt and mineral dust particles containing nitrates, sulfates, and both. (a) Reacted sea salt particles; (b) reacted mineral dust particles.

After comparing the average ratios of the relative abundance of the genuine to the reacted sea salt particles among the samples from three categories of air mass sources (13.7%, 10.8%, 6.1% on average for the first, second, and third categories, respectively) based on the 72-hr back trajectories (see Figure 2), we speculated that there were more air pollutants for reaction with sea salt particles over the Yellow Sea than over the Sea of Japan and Korea Strait.

Mineral Dust Particles

Mineral dust particles appear irregular and bright on their SEIs.^5,6 The typical genuine mineral dust particles include aluminosilicate (with strong X-ray peaks of Al, Si, and O), SiO₂, and CaCO₃/CaMg(CO₃)₂. The reacted (or aged) ones mainly include “reacted CaCO₃/CaMg(CO₃)₂” and “aluminosilicate + (N, S)”, where the (N, S) notation represents compounds containing either nitrates, sulfates, or both. They are produced when aluminosilicate (especially Ca²⁺-containing) and CaCO₃/CaMg(CO₃)₂ particles react with airborne sulfur and nitrogen oxides in the presence of moisture or with “secondary acids” such as H₂SO₄, HNO₃, and HCl, and can be formed from the adsorption of NH₄NO₃ or (NH₄)₂SO₄/NH₄HSO₄ on particle surface.21 In the present study, the genuine mineral dust particles had higher abundances than the reacted ones on stages 1–5 (about 11–44% vs. 2–7% on average) (Figure 3 and Table 2). In particular, on stage 1, the abundance of the genuine particles was nearly 20 times higher than that of the reacted ones. It suggested that mineral dust particles larger than 1.0 µm in diameter did not easily react with SO₂ and NO_x as the sea salt particles did. On stage 6, the genuine and reacted particles had similar distribution in abundance (both around 11% on average), implying that the smaller mineral dust (less than 1.0 µm in diameter) were prone to reacting with air pollutants. Because of larger surface to volume ratio, the particles smaller in size might have an increased reactivity with SO₂ and NO_x adsorbed on dust surface compared to the bigger ones. Alternatively, these smaller reacted mineral dust (or soil-derived) particles likely came from remote sources, so they had more reacting chances with SO₂ and/or NO_x in the process of their long-range transport.

By calculating the abundance ratio of the reacted to genuine mineral dust particles on stage 6 according to the air mass histories, we found that the ratio for the third category of air mass back trajectories was markedly higher than that for the first and second categories (5.6 vs. 0.9 and 0.4), indicating that the air masses passing over the Yellow Sea carried more reacted mineral dust particles than those passing over the Sea of Japan and Korea Strait. Possibly, it was attributed to the higher mass concentrations of airborne SO₂ and NO_x over the Yellow Sea due to its being adjacent to the mainland of China. Although the mineral dust particles carried by the air masses passing over the Sea of Japan and Korea Strait (Figure 2e and f) might float for longer time than those found in the trajectories coming directly from the mainland, it didn't mean that these “older” particles must become the reacted ones. Many factors such as meteorological condition, mass concentrations of ambient SO₂ and NO_x, and saturation effects of uptake of H₂SO₄ and/or HNO₃ on dust surface influenced the formation of reacted (aged) mineral dust.

Similar to the reacted sea salt aerosols, the reacted mineral dust particles were also classified into three types. The first type was for nitrate-containing species, which mostly include Ca(NO₃)₂, the mixture of Ca(NO₃)₂ and CaCO₃, and “AlSi + (N)”. The (N) notation represents compounds containing nitrates. The second type was for sulfate-containing species, which mostly include CaSO₄, the mixture of CaSO₄ and CaCO₃, and “AlSi + (S)”. The (S) notation represents compounds containing sulfates. The third type was for those containing both nitrates and sulfates, which include Ca(NO₃, SO₄) and “AlSi + (N, S)”. As shown in Figure 4b, for particles on stages 2, 5, and 6, the sulfate-containing species outnumber nitrate- and nitrate + sulfate-containing ones (especially for samples “4 April”, “7 April”, and “10 April”), whereas for particles on stages 1, 3, and 4, the nitrate-containing species outnumber sulfate- and nitrate + sulfate-containing ones. Hence, it was difficult to judge which type of reacted mineral dust particles was predominant. However, during 30 April to 1 May of 2006 when an Asian dust storm originating from Mongolia passed over the Yellow Sea, it was the nitrate-containing soil-derived particles that were significantly increased, suggesting that Asian dust aerosols in springtime were an important carrier of gaseous inorganic nitrogen species.⁶

Carbonaceous Particles

Carbonaceous aerosol particles are generally differentiated into two groups: elemental carbon (EC) and organic carbon (OC), which have large but not absolutely distinct difference in their chemical properties.³ Herein, EC-like (also termed “carbon-rich particles”) and OC-like particles were identified using the ED-EPMA. EC-like particles mainly result from complete or incomplete combustion of liquid or solid fuels.22 In this work, they were in lower abundances on stages 2–6 (less than 0.8% on average) compared to on stage 1 (6.0% on average) in all samples (Table 2). In the sample “6 April”, their relative abundance on stage 1 was even up to 20.4%. It suggested that the EC-like particles were very big (at least more than 8 µm in diameter). Probably, they were char or coal dust particles. But it was not clear where they came from.

OC-like particles were dominant in the carbonaceous aerosols based on the observation that the abundance of OC-like particles outweighed that of EC-like particles on every stage (Figure 3). Especially on stage 6, it was about 15 times higher than the abundance of EC-like particles (11.99% vs. 0.79%) (Table 2), implying that the OC-like particles on stage 6 were abundant. In the three categories of air mass back trajectories, the air masses passed over the Yellow Sea (the third category) carried more OC-like particles than those passed over the Sea of Japan and Korea Strait, as the abundance for the sample “10 April” was up to 35.1%, around 4 times higher than that for the samples “4–9 April”. These OC-like particles were believed to mainly result from anthropogenic emissions and atmospheric transport and/or transformation of anthropogenic organic species from outside of Jeju Island.14 However, a part of them might be from biogenic emissions,19 since the Jeju Island are covered by plenty of forest and closed by seawater.

(NH₄)₂SO₄/NH₄HSO₄-Containing Particles

The (NH₄)₂SO₄/NH₄HSO₄-containing particle is denoted as (C, N, O, S)-containing particle because of the peaks of C, O, S, and N in their EDX spectra.⁵ It is a secondary species produced from the reactions of ambient sulfate or sulfuric acid (formed by the oxidation of SO₂ from anthropogenic sources) with ammonia (largely emitted from regions of high agricultural activity and livestock farming), often internally mixed with soot and organic matter.23,24 Here, the (NH₄)₂SO₄/NH₄HSO₄-containing particles were in higher abundance on stages 5 and 6 (5.3% and 38.4% on average, respectively) than on stages 1–4 (0%, 0%, 0.2%, and 0.3% respectively), indicating that they were small in size. Hayami and Carmichael25 reported that over Jeju Island NH₄ ⁺ was most strongly associated with nss-SO₄ ^2–, and peak values of NH₄ ⁺ and SO₄ ^2– were in the fine mode, whereas Na⁺, Ca²⁺, Mg²⁺, NO₃ ^–, and Cl^– peaks were in the coarse mode. On stage 6, there were higher abundances of (NH₄)₂SO₄/NH₄HSO₄-containing particles in the samples “6–9 April” (44.3%, 58.1%, 37.1%, and 71.6%, respectively), revealing that a large number of (NH₄)₂SO₄ or NH₄HSO₄ could be formed over the Sea of Japan and Korea Strait.

Fe-Rich and K-Rich Particles

The Fe-rich particles looked bright on their SEIs and usually contained obvious Fe and O peaks in their X-ray spectra (often with minor C, Si, and Al). They were generally in the form of iron ((oxy)hydr)oxides, being interpreted as goethite, hematite, or magnetite in atmospheric aerosols. Human activities such as mining, steel production, metallurgical industries, the abrasion of brake lining, and erosion of asphalted road might lead to distinctly higher loads of Fe/FeO_x.26 The K-rich particles, in which K₂SO₄ was the most abundant species in the study, were regarded to mostly originate from biomass burning.27 In the present study, Fe-rich and K-rich particles on stages 1–6 for all the samples had lower abundances (most of them were less than 2%), of which the abundances of particles on stages 5 and 6 outweighed those on stages 1–4 (especially for samples “5 April”, “6 April”, and “8 April”).

CONCLUSIONS

This study presented the single-particle observations of chemical compositions of aerosols at different size level (stages 1–6) at Gosan, Jeju Island, Korea, in the eve of a typical dust storm during the ACE-Asia 2001 field campaign. Seven sets of particle samples were collected on 4–10 April of 2001 using a seven-stage May cascade impactor, and 11,200 particles on stages 1–6 with cutoff diameters of 16, 8, 4, 2, 1, and 0.5 μm were analyzed by a quantitative ED-EPMA technique. The measurement on the equivalent diameter of every particle indicated that 96.9% of the analyzed particles were in the size range of 0.5–16 μm. Based on secondary electron images and X-ray spectra of particles, particle types were identified and their relative number abundances were calculated. Results showed that

a.	Particles of sea origin were the most abundant, followed by mineral dust, carbonaceous, NH4HSO4/(NH4)2SO4-containing, Fe-rich, and K-rich particles.
b.	In sea salt particles, the reacted ones with SO2 and NOx (or their acidic products) were predominant, greatly outnumbering the genuine ones. The nitrate-containing reacted sea salt particles were frequently encountered on stages 1–5, whereas the sulfate-containing ones were frequently encountered on stage 6. In mineral dust particles, the genuine ones greatly outnumbered the reacted (aged) ones. A lot of reacted sea salt and reacted mineral dust particles were carried by the air masses passing over the Yellow Sea.
c.	In the carbonaceous aerosols, OC-like particles were dominant compared to EC-like particles. They were frequently encountered on stage 6 and largely present in the air masses passed over the Yellow Sea.
d.	(NH4)2SO4/NH4HSO4-containing particles had higher abundance on stages 5 and 6 (5.3% and 38.4% on average, respectively) than on stages 1–4. The 72-hr air mass back trajectories showed that many of them came from the marine boundary layer of the Sea of Japan and Korea Strait.
e.	Fe-rich and K-rich particles had low abundances. They were rarely encountered on stages 1–4.

In conclusion, the single-particle characterization of chemical compositions of aerosols at Gosan indicated the reacted (aged) sea salt, organic compounds, and secondary aerosols such as (NH₄)₂SO₄ greatly influenced the atmosphere over Jeju Island before a typical dust storm originating from Mongolia arrived.

ACKNOWLEDGMENTS

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0018881) and by the fund provided from the Research Project supported by Shanxi Scholarship Council of China (2010) OS and the Basic Science Research Program of Shanxi Natural Science Foundation (2011011009-2).