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Nearly every spring, usually from March to May, “Asian Dust” originated from the arid areas of Central China has been transported into Eastern China, Korea, Japan, and even to the Pacific Ocean. The elevated levels of particulate matter (PM) during dust storms affect the human health, the visibility, and industrial activities. While the dust particles travel long-distance, they can react with diverse chemical species and/or provide a reaction site for chemicals in the atmosphere. Therefore, Asian Dust can possibly carry other chemical species along with the original soil components. For example, it would react with the atmospheric pollutant gases such as SO_x and NO_x, thus the transport of the reacted dust particles to the East Asian region could result in the deposition of sulfates and nitrates, in addition to mineral dust. The modified Asian Dust particles can affect the global climate, since the direct radiative forcing properties of the dust particles can be changed. Also their effectiveness to serve as cloud condensation nuclei can be altered. Hence, increasing attention has been devoted to the study of physicochemical characteristic changes of Asian Dust particles during long-range transport. In this section, the variation of the physicochemical properties of Asian dust aerosol samples collected during four dust storm events in Korea is discussed.

Four sets of Asian Dust aerosol samples were collected in ChunCheon, Korea on March 7 and April 7 in 2000 and March 22 and May 17 in 2001 (These four samples were notated as “March_00”, “April_00”, “March_01”, and “May_01” samples, respectively). ChunCheon with the population of 200,000 is a small city with a wide suburban area. Back-trajectories for the four Asian Dust events are shown in Fig. 1 (The back-trajectory analyses were performed by using the HYSPLIT model and meteorological data available on the NOAA/ARL website). The circularly marked regions in the figure are the source regions of the Asian Dust storm. Samplings were done in the middle of the dust storms by using a seven-stage May cascade impactor. Some 300 particles for each stage sample were analyzed. The particles on the seventh stage were not analyzed because the sizes of particles were too small to be analyzed by SEM/EDX.

Fig. 1. 
Air-mass backward trajectories for Asian Dust samples collected on (A) March 7, 2000, (B) April 7, 2000, (C) March 22, 2001, and (D) May 17, 2001. Reprinted from permission from ref. Ro et al., 2005. Copyright 2005 American Chemical Society.

Figs. 2-5 show the size dependent relative abundances of chemical species observed in the Asian Dust samples (the aerodynamic cut-offs of stages 1-6 in May impactor are 16, 8, 4, 2, 1, and 0.5, respectively). The relative abundances of each chemical species in a stage sample were obtained by dividing the number of the chemical species by the number of the analyzed particles. The major chemical species found in the Asian Dust particle samples were aluminosilicate, silicon dioxide, calcium-containing, marine-originated, and carbonaceous (specified either as organic or carbon-rich) particles. Overall, the soil-derived particles such as aluminosilicate, silicon dioxide, and calcium-containing particles were the most common in the Asian Dust samples.

Fig. 2. 
Relative abundances of each chemical species in the “March_00” Asian Dust sample (Cut-off diameters of stages 1-6 are 16, 8, 4, 2, 1 and 0.5 μm, respectively). Reprinted from permission from ref. Ro et al., 2005. Copyright 2005 American Chemical Society.

Fig. 3. 
Relative abundances of each chemical species in the “April_00” Asian Dust sample (Cut-off diameters of stages 1-6 are 16, 8, 4, 2, 1 and 0.5 μm, respectively). Reprinted from permission from ref. Ro et al., 2005. Copyright 2005 American Chemical Society.

Fig. 4. 
Relative abundances of each chemical species in the “March_01” Asian Dust sample (Cut-off diameters of stages 1-6 are 16, 8, 4, 2, 1 and 0.5 μm, respectively). Reprinted from permission from ref. Ro et al., 2005. Copyright 2005 American Chemical Society.

Fig. 5. 
Relative abundances of each chemical species in the “May_01” Asian Dust sample (Cut-off diameters of stages 1-6 are 16, 8, 4, 2, 1 and 0.5 μm, respectively). Reprinted from permission from ref. Ro et al., 2005. Copyright 2005 American Chemical Society.

Aluminosilicates and Silicon Dioxide

There exists so many different types of aluminosilicate particles as they contain a small amount (less than 5% in atomic concentration) of Na, Mg, S, Cl, K, Ca, and Fe. All these particles were simply grouped as “AlSi (aluminosilicates)”. The particles classified as “AlSi” could be one of typical aluminosilicate minerals such as feldspar or clay mineral. It was observed that many aluminosilicate particles contain carbonaceous species which probably comes from humic substances in soil (Ro et al., 2000). In this case, they were grouped as “AlSi/C”. Some aluminosilicate particles existed as a mixture with CaCO₃, MgCO₃, Fe₂O₃, CaSO₄, Na₂SO₄, or MgSO₄. Since particles internally mixed with aluminosilicates and CaCO₃ were significant in number, they were marked as “AlSi/CaCO₃”. Other mixtures were grouped as “AlSi/misc.”. In all samples, aluminosilicate-containing particles were the most abundant in all size ranges except stage 6. For the stage 6 samples (D_p: 0.5-1 μm), carbonaceous particles were the most abundant. In Fig. 6, overall relative abundances of the significantly encountered particle types in the four samples are shown. The overall relative abundance was calculated by dividing the number of the specific type of particles by the total number of analyzed particles for each sample. As shown in Fig. 6, the relative abundances of aluminosilicate-conatining particle in all samples are overwhelmingly large: the “April_00” sample and “March_01” sample contain more than 60% of aluminosilicate particles. These two samples also contain SiO₂-containing particles more than other two samples. In addition, most of the aluminosilicate-containing particles in the “April_00” sample were found as a single species of aluminosilicate without mixing with other chemical species. The compositional difference in aluminosilicate particles might reflect the characteristics of the soils of the source regions. SiO₂-containing particles were the second most abundantly encountered soil-derived ones (see Fig. 6) and their relative abundances were in the range of 5-18% depending on the size and the samples.

Fig. 6. 
Overall relative abundances of the major particle types in Asian Dust samples. Reprinted from permission from ref. Ro et al., 2005. Copyright 2005 American Chemical Society.

Calcium-containing Particles

CaCO₃-containing particles were encountered with the relative abundances of 2-20%, depending on the size and the sample. Since CaCO₃ can react with SO_x and NO_x to produce CaSO₄ and Ca(NO₃)₂, respectively, it is of great interest to investigate the potential chemical modification of the Asian Dust during its long-range transport. The CaCO₃-containing particles were grouped into four types, i.e., “CaCO₃”, “AlSi/CaCO₃”, “CaCO₃/misc.”, and “reacted CaCO₃”. “AlSi/CaCO₃” particle is a mixture of aluminosilicates and CaCO₃. When a particle was found as an internal mixture of CaCO₃ and other chemical species such as MgCO₃, SiO₂, Fe₃O₃, or carbonaceous species, it was classified as a “CaCO₃/misc.” particle. The products of atmospheric reaction between CaCO₃ and SO_x/NO_x, mostly CaSO₄ or Ca(NO₃)₂ or their mixtures, were classified as “reacted CaCO₃” particles.

As shown in Figs. 4 and 5, the overall relative abundances of particles of “CaCO₃-containing” and “reacted CaCO₃” groups are 9.5% and 0.9%, respectively, for the “March_01” sample and 4.7% and 10.7%, respectively, for the “May_01” sample. Here, the “CaCO₃-containing” group is for particles that contain CaCO₃ species without sulfate and/or nitrate. The “May_01” sample seemed to have experienced very dramatic chemical modification of CaCO₃ species because the number of particles for the “reacted CaCO₃” group was much larger than that of the un-reacted CaCO₃ particles. In addition, the reacted CaCO₃ particles were observed significantly even in stages 1-3 where coarse particles were collected. This means that the sample in question had enough reaction time and reactants to consume even for relatively large CaCO₃ particle. Among 13 reacted CaCO₃ particles found in the “March_01” sample, the numbers of particles that contain Ca(NO₃)₂ and CaSO₄ were 6 and 7, respectively. For this sample, CaCO₃ reacted with NO_x and SO_x in a similar extent. However, for the “May_01” sample, among 142 reacted CaCO₃ particles, particles containing Ca(NO₃)₂ outnumbered those containing CaSO₄ (128 vs. 36).

The reacted CaCO₃ particles were hardly found for the “March_00” and “April_00” samples, therefore it was assumed that these samples had not experienced atmospheric reactions during their long-range transport. By analyzing the chemical composition of the CaCO₃-containg particles, we can see the difference in the extent of chemical modification between samples. “May_01” sample did show the extensive chemical modification of CaCO₃ species and the “March_01” showed a significant chemical modification. On the other hand, the “March_00” and “April_00” did not at all. The reason for these differences for the samples is not clear, but it is certain that many factors such as meteorological conditions and the concentrations of gaseous sulfur and nitrogen species in areas that the air-masses passed through play important roles in chemical modification during long-range transport.

Marine Particles

It was well demonstrated that “genuine” and “reacted” sea-salts can be clearly distinguished by the low-Z particle EPMA (Ro et al., 2001b). Since the sea water contains Mg, the Mg X-ray peak was detected when the genuine sea-salt particles were investigated. Therefore, the genuine sea-salt particles were classified as (Na, Mg)Cl. The reacted sea-salts were mostly sodium nitrate and/or sulfate. The particles of sea origin can be verified by the existence of residual Cl in the X-ray spectrum. For the “March_00” sample, either genuine or reacted sea-salts were not significantly observed. It might be due to the fact that the air-mass for the sample did not pass over the Yellow Sea (see Fig. 1(A)). However, for other samples that had passed over the Yellow Sea, a significant number of sea-salt particles were observed. The overall relative abundances of marine-originated particles for the “April_00”, “March_01”, and “May_01” samples were 6.3%, 0.9%, and 4.7 %, respectively. For the “April_00” sample, the reacted sea-salts outnumbered the genuine sea-salts (4.8% vs. 1.5%). Regarding the CaCO₃-containing particles, it is interesting to notice that the reacted CaCO₃ particles were not significantly observed for this sample. The “March_01” sample entrained the least amount of sea-salts particles among the three samples, and only the reacted sea-salts were observed. Again, the reacted CaCO₃ particles were outnumbered by the un-reacted CaCO₃ particles for this sample. This implies that sea-salts had experienced their chemical modification more rapidly and extensively than CaCO₃ particles. Most of the reacted sea-salt particles were encountered in stages 4-6 (where the fine particles were collected), and it might be due to the larger surface-to-volume ratio for the smaller particles. The “May_01” sample also contains only the reacted sea-salts. Since most of reacted sea-salt and reacted CaCO₃ particle were observed as the products of atmospheric reaction, particles in this sample seemed to experience the most extensive chemical modification among the four Asian dust samples.

Among 50 reacted sea-salt particles in the “April_00” sample, the numbers of particles that contain nitrate and sulfate are 13 and 38, respectively. For the “March_01” sample, among 14 reacted sea-salt particles, the numbers of particles that contain nitrate and sulfate are 5 and 11, respectively. For these samples, sea-salt particles reacted with SO_x to a greater extent than with NO_x. However, for the “May_01” sample, among 64 reacted sea-salt particles, particles containing nitrate outnumber those containing sulfate (55 vs. 18). This tendency is similar to that for the reacted CaCO₃ particles observed in the “March_01” and “May_01” samples, where nitrate-and sulfate-containing reacted CaCO₃ particles were observed in a similar extent for the “March_01” sample, and yet nitrate-containing particles outnumber sulfate-containing particles for the “May_01” sample. The “May_01” sample must have experienced a situation in which nitrate was produced more easily than sulfate.

Potassium-containing Particles

The “May_01” sample has another unique feature in terms of the presence of reacted potassium-containing particles. The reacted potassium-containing particles included sulfate, nitrate, carbonate, and mixtures of sulfate, nitrate, and/or carbonate such as K₂SO₄, KNO₃, K(NO₃, SO₄), K(CO₃, SO₄), K(CO₃, NO₃), K(CO₃, NO₃, SO₄). Among 37 potassium-containing particles, chemical species of 15 particles was K₂SO₄, that was the most abundant among the potassium-containing particle types. These K₂SO₄ particles are not natural mineral particles because 12 particles among 15 particles classified as K₂SO₄ appeared to contain a small amount of nitrate in them when their X-ray spectral data were examined in detail. Even though no single K₂CO₃ particle was encountered, it is certain that K₂CO₃ particles reacted in the atmosphere to produce reacted potassium-containing particles. The reason is that the K₂CO₃ species was often found as mixtures of K₂SO₄ and/or KNO₃, implying that atmospheric reaction for those particles was not completed. The reacted potassium-containing particles were mostly encountered in stages 5 and 6 (D_p: 0.5-2 μm). Among the 37 potassium-containing particles, sulfate-containing particles outnumbered nitrate-containing ones (34 vs. 13). As stated before, for the reacted CaCO₃ and sea-salt particles in this sample, nitrate-containing particles outnumber sulfate-containing ones, which is opposite to that for reacted potassium-containing particles. It is not possible to explain the discrepancy between these observations, but it is certainly related to the reactivity of each chemical species. There has not been any report regarding chemical modification of K₂CO₃ during long-range transport of Asian Dust. In this work, it is clearly shown that K₂CO₃ species, in addition to CaCO₃ species and sea-salts, is also important in the study of chemical modification of Asian Dust during transport.

Carbonaceous Particles

In all samples, carbonaceous particles such as organic and carbon-rich were significantly encountered in the stages 5 and 6 (D_p: 0.5-2 μm). It is well known that anthropogenic carbonaceous and ammonium sulfate particles are major chemical species in urban atmospheric aerosols (Ro et al., 2002), thus it can be assumed that the carbonaceous particles might be from local anthropogenic sources. However, in these samples, ammonium sulfate particles were rarely observed, which implies that these carbonaceous particles could be a part of the Asian Dust particles entrained during their transport.

For typical urban aerosols, a significant portion of aerosols is anthropogenic in origin. The gaseous SO_x and NO_x emitted from industry and mobile combustion engines cause the production of secondary aerosols such as sulfate and nitrate. Chemical composition of urban aerosols can be variable depending on the surrounding environment, such as population and the geography. Compositions of atmospheric aerosols in three cities in Korea, namely Seoul, CheongJu, and ChunCheon, were investigated. Seoul, the capital of Korea, is a megacity with high population (population: 10.3 million, area: 605 km²), whereas CheongJu is relatively small (population: 0.57 million, area: 153 km²) and has industrial complexes in the city. ChunCheon is the least densely populated (population: 0.25 million, area: 1,116 km²) among the three, and is free of industrial complexes. The three cities have different urban characteristics in terms of their population, size, and industrial circumstances. The results of single particle analysis for the three cities having different environment were expected to provide the detailed information on their atmospheric characteristics.

All the samplings were done during the winter of 1999. In Seoul, the sampling was done on December 11, 1999, on the roof of St. Mary’s Hospital building (30 m above ground level), which is located at the Youido district in central Seoul. In ChungJu, the sample was collected on the roof of a campus building of ChungBuk National University (3 m above ground level) on November 13, 1999. The campus is located in downtown CheongJu. Industrial complexes inside the city of CheongJu are just 2 km north from the sampling site. In ChunCheon, the sampling was performed on the roof of a campus building of Hallym University (12 m above ground level) on November 27, 1999.

Particles were sampled on Ag foil (99.95%, 0.025 mm, Goodfellow) using a seven-stage May cascade impactor. For each sample, some 300 particles for each stage in #1-6 were analyzed. The particles on the seventh stage were not analyzed because the sizes of particles were too small to be analyzed by SEM/EDX. Since the samples were stored for a couple of months before the analysis, it might be possible that some of semi-volatile particles such as ammonium nitrate could have partially been lost during storage. The measurements were carried out on a JEOL 733 electron probe micro-analyzer equipped with an Oxford Link SATW ultra-thin window EDX detector.

The relative abundances of the chemical species found in atmospheric aerosols collected in Seoul, ChungJu, ChunCheon are shown in Figs. 7-9. The major chemical species encountered in urban atmosphere were: carbonaceous particles (specified either as organic, carbon-rich, or biogenic), aluminosilicate, silicon dioxide, calcium carbonate, iron oxide, marine-originated (such as sodium chloride and sodium nitrate), and ammonium sulfate particles. It is well known that these types of particles are common in urban atmospheres (Polissar et al., 2001; Zhang and Friedlander 2000; Hughes et al., 1999; Chow et al., 1992; Van Borm and Adams, 1989). The size dependences of the population of the chemical species are clearly shown in Figs. 7-9. In general, soil derived particles, e.g. aluminosilicates, silicon dioxide, calcium carbonate, and iron oxide, and also sea salts were mainly observed in the coarse fraction (i.e., on impactor stages 1-3, where their cut-offs are 16, 8, and 4 μm, respectively), while carbonaceous and ammonium sulfate particles that may have anthropogenic origin were the major species in the fine fraction (in stages 4-6, where their cut-offs are 2, 1, and 0.5 μm, respectively).

Fig. 7. 
Relative abundances of each chemical species in the Seoul sample (Cutoff diameters of stages 1-6 are 16, 8, 4, 2, 1, and 0.5 μm). Reprinted from permission from ref. Ro et al., 2002. Copyright 2002 American Chemical Society.

Fig. 8. 
Relative abundances of each chemical species in the ChungJu sample (Cutoff diameters of stages 1-6 are 16, 8, 4, 2, 1, and 0.5 μm). Reprinted from permission from ref. Ro et al., 2002. Copyright 2002 American Chemical Society.

Fig. 9. 
Relative abundances of each chemical species in the ChunCheon sample (Cutoff diameters of stages 1-6 are 16, 8, 4, 2, 1, and 0.5 μm). Reprinted from permission from ref. Ro et al., 2002. Copyright 2002 American Chemical Society.

Carbonaceous Particles

They were found both in coarse (stages 1-3) and fine fractions (stages 4-6). In terms of the particle size, most of carbonaceous particles in coarse fraction might be naturally emitted and those in fine fraction might be anthropogenic. Observation of the organic particles mostly in the fine fraction implies their anthropogenic origin. Biogenic organic particles such as pollen were found as coarse particles in stages 1-3. In the Seoul and ChunCheon samples, the carbonaceous particles were ca. 60% in stage 6, i.e., the majority of the finest particles were carbonaceous particles. Whereas, in the CheongJu sample, ammonium sulfate was the main component in the fine fraction (ca. 70% in stage 5 and ca. 80% in stage 6), and the relative abundance of the carbonaceous particles was overwhelmed by sulfate particles. Detailed discussion on the sulfate particles will be given below.

Aluminosilicate and Silicon Dioxide Particles

They have been observed abundantly in urban atmosphere and mostly regarded as soil-derived, being resuspended by mechanical forces such as running automobiles, strong winds, and construction activities. The aluminosilicate particles were abundantly encountered in coarse fraction (in stages 1-3) of all samples (see Figs. 7-9). However, in the Seoul sample, they were significantly found even in stages 5 and 6. Similarly, silicon dioxide particles were significantly present in all stages for the Seoul sample, whereas they were mostly encountered in stages 1-3 for the CheongJu and ChunCheon samples. Zhang and Friedlander (2000) reported that PM_2.5 and PM_2.0 urban particles collected in various cities of China also had high silica contents (ca. 18%) next to carbonaceous and ammonium sulfate and the concentrations of the fine silica component in Beijing and Lanzhou, China, were 26-36 times higher than those in Los Angeles, USA. They claimed that the fine silica probably came from coal combustion, crustal sources, and rice straw burning. Silicon dioxide and aluminosilicate particles in the Seoul sample seemed to have two sources, namely soil-derived and anthropogenic. In other study (Ro et al., 2001a), soil-derived silicon dioxide particles in the coarse fraction usually were observed to contain small amount of aluminum (possibly from aluminosilicate) and/or carbon (from humic substance in soil) elements. However, silicon dioxide particles in stage 6 in Seoul sample might be fly ashes since they were “pure”. A possible local source of anthropogenic silicon oxide particles is an incinerator (location: Mok-dong, Seoul; daily capacity of 550 ton; average running rate of 60%) located 3 km west from the sampling site.

Calcium-containing Particles

They were observed as carbonate, sulfate, and nitrate. As shown in Figs. 7-9, calcium carbonate particles were significantly encountered in the coarse fraction of all the samples. However, there were some differences in their size distributions among the samples. In the Seoul sample, the relative abundance of the CaCO₃ particles was relatively constant in all stages. In the ChunCheon sample, CaCO₃ particles were significantly observed in stages 1-3 and not in the fine fraction (stages 4-6). This finding implies that most of the calcium carbonate particles in the ChunCheon sample were soil-derived. In the CheongJu sample, the overall relative abundance of the Ca-containing particles was smaller than that of others and, in addition, fine calcium carbonate particles were not encountered. The two types of calcium-containing particles, namely sulfate and nitrate, were significantly observed only in the CheongJu sample. Calcium nitrate particles were significantly encountered in stages 2-4, whereas, sulfate in stages 1-5. The formation of Ca(NO₃)₂ by the reaction between CaCO₃ and HNO₃ was proposed (Dentener et al., 1996) and directly observed (Carmichael et al., 1997). A recent study claimed that soil-derived aerosol particles were more suitable for the internal mixture of sulfate and nitrate than the other kinds of particles, and this claim was based on analyzing individual particles with the combination of microchemical test and EPMA techniques (Zhang et al., 2000). Other study reported the observation of the internal mixtures of CaCO₃ and Ca(NO₃)₂ (Ro et al., 2001a). It seemed that the reaction had not completely occurred for these particles. In addition, since some of the Ca(NO₃)₂ particles also contained small amount of aluminosilicates, Ca(NO₃)₂ seemed to be most probably the product of a reaction between anthropogenic nitric acid and soil-derived CaCO₃ species. In the ChungJu sample, about 30% of Ca-containing particles were found as internal mixture of CaCO₃ and Ca(NO₃)₂ species. In addition, observation of some calcium sulfate particles including calcium nitrate and/or carbonate species implies that some of the calcium sulfate particles were also formed by an atmospheric reaction.

Iron Oxide Particles

They constituted ca. 2.0% of total analyzed particles. Iron oxides could be originated from soil or anthropogenic emissions (e.g. as fly-ash). Iron oxides were observed in stages 2-5; for the Seoul sample, the particles were more abundantly encountered in stages 4 and 5 (more in the fine fraction), whereas in the CheongJu and ChunCheon samples, the particles were more abundantly observed in stages 2 and 3 (more in the coarse fraction). It implies that the iron oxide particles in the Seoul sample might be originated from anthropogenic emission. The observation of anthropogenic fine SiO₂ particles in the Seoul sample supports the assertion. Studies carried out on urban particles in Europe reported quite abundant iron oxide particles in the air (Van Borm et al., 1990). In the study on aerosols in Antwerp, Belgium, using the low-Z EPMA, the iron oxide particles were believed to have two sources, namely soil and anthropogenic, although the anthropogenic source was not determined (Ro et al., 2000).

Marine Particles

They were observed as “genuine” or “reacted” ones. Since sea-water contains significant amount of Mg (the mole ratio of Mg to Na in sea-water is 0.12 to 1 (Weast et al., 1984)), the Mg peak can be seen in their X-ray spectrum when marine-originated particles are analyzed using EPMA. All the NaCl particles in the sample were found to be genuine sea-salts and most of NaNO₃ particles also contain Mg in them. Furthermore, the observation of the existence of residual Cl in NaNO₃ particles might be the direct evidence of the heterogeneous atmospheric reaction between the sea-salt particle and gaseous NO_x in the air. Gard et al., (1998) reported that the atmospheric reaction between sea-salts and HNO₃ to produce NaNO₃ occurred in a relatively short time (on a time scale of about a half day). Observation of overwhelmingly abundant NaNO₃ particles over genuine sea-salts in the CheongJu and ChunCheon samples implies that the air-masses had spent a sufficient time in the air. However, lack of Na₂SO₄ particles implies that sulfate formation from the sea-salts does not occur as easily as that of nitrate. The similar observation that the Na₂SO₄-containing particles were much less abundant than the NaCl-and NaNO₃-containing particles was seen for the aerosol samples collected in Cheju Island, Korea (Ro et al., 2001b).

Ammonium Sulfate Particles

They were observed in the fine fraction (stages 4-6) for all the three samples (see Figs. 7-9). It is well known that ammonium sulfate particles in the air are anthropogenic and they are the major fine particles which can be found in urban atmosphere. The ammonium sulfate particles were most abundantly encountered in the CheongJu sample, and significantly in the ChunCheon sample, but relatively less in the Seoul sample. In addition, in the CheongJu sample, some ammonium sulfate particles were found even in the coarse fraction (i.e., on stage 3). As discussed previously on the Ca-containing particles in the ChungJu sample, most of Ca-containing particles were observed as nitrate and/or sulfate without carbonate particles. It seemed that there was an anthropogenic emission during the sampling period in ChungJu (ChungJu has an industrial complex in the city.).

Many people within the worldwide metropolitan areas commute using underground subway transportation and spend considerable time in the underground subway environment on a daily basis. The atmospheric environments of underground subway stations have relatively unique characteristics since there is a peculiar source of aerosol by the movement of train and the ventilation is restricted. Therefore, there has been increasing attention to air quality in the underground subway system because of its possible adverse influence on public health (Karlsson et al., 2006; Karlsson et al., 2005; Seaton et al., 2005; Chillrud et al., 2004). Due to the limited ventilation, in general, the PM concentrations of the underground subway system are higher than those of outdoor atmosphere. It has been reported that the PM₁₀ concentrations in various subway stations around the world were a few times higher than those of ambient outdoor atmosphere (Branis, 2006; Aarnio et al., 2005; Johansson and Johansson, 2003; Furuya et al., 2001). Since PM emission sources in the underground subway stations are very different from those in outdoor urban environments, the chemical compositions of subway particles are unique. Subway particles are dominated by Fe and Si, and contain Mn, Cr, Cu, Ca, and K as minor components (Salma et al., 2007; Birenzvige et al., 2003; Sitzmann et al., 1999). In particular, concentrations of Fe, Mn, and Cr in PM_2.5 samples collected in underground subway stations of New York City were found to be more than 100 times higher than those in aboveground ambient samples (Chillrud et al., 2004). Studies have shown that the subway particles were more genotoxic and more likely to induce oxidative stress in cultured human lung cells than the particles from welding fume and the wood combustion (Karlsson et al., 2006; Karlsson et al., 2005; Seaton et al., 2005).

Herein, subway particles collected in Hyehwa station, one of the underground subway stations in Seoul, Korea were investigated. The Seoul subway system contained 8 lines with a total of 327 stations in 2005. According to statistics provided from the Seoul metro transportation center (http://www.seoulmetro.co.kr), approximately 6.7 million commuters use the Seoul subway system on a daily basis. The Hyehwa station is one of the busiest subway stations with 58,000 commuters per day. In 2005, the average PM₁₀ in Seoul was 58.0 μg/m³ and that in Hyehwa station was 121.5 μg/m³. By the combined use of the morphological and chemical information obtained from the low-Z particle EPMA, it was observed that subway particles were mostly iron-containing particles which were originated from the wear processes at rail-wheel-brake interfaces (Kang et al., 2008).

Sampling was done at the platform of the Hyehwa station using a 7-stage May cascade impactor. Subway particle samples were collected twice every season during 2004-2005: Dec. 16 and 17, 2004; May 3 and 5, July 4 and 6, and Nov. 23 and 25, 2005. The measurements were carried out using a Hitachi S-3500N environmental scanning electron microscope equipped with an Oxford Link SATW ultrathin window EDX detector. Due to the SEM resolution, particles on stage 1-5 were analyzed. About 300 particles for each stage were analyzed (1,300 particles for each sample).

Based on their chemical compositions, 4 major particle types were significantly encountered: Fe-containing; soil-derived; carbonaceous; and secondary nitrate and/or sulfate particles. In Table 1, overall relative abundances of those particle types for the 8 samples are shown. Since the particles collected on stages 1-5 were analyzed, the results shown in Table 1 is for the aerosols with D_p>1 μm. Fe-containing particles were overwhelmingly abundant (more than 60%) in the subway station environment. Other studies for the subway aerosols in London, New York, Helsinki, Tokyo, and Budapest have also reported the very high concentrations (above 42%) of iron (Salma et al., 2007; Aarnio et al., 2005; Chillrud et al., 2004; Furuya et al., 2001; Sitzmann et al., 1999).

Fe-containing Subway Particles

They are shown as a secondary electron image in Fig. 10. In this study, Fe-containing particles were classified into three different types: iron metal (denoted as “Fe”), partially oxidized (denoted as “Fe/FeOx”), and fully oxidized (denoted as “FeOx”). A major advantage of EPMA technique is that the morphology of the particle can be obtained with the chemical composition. The combined information makes it possible to deduce the source of the particle. Most of Fe-containing particles were observed as oxidized form and some nano-sized iron oxide particles formed an agglomerated lump on the surface of Fe-containing particles (see particles (a)-(c) in Fig. 11B). Similar way to the formation of arc welding particles (Zimmer and Biswas, 2001), the nano-sized iron particles were probably formed by the condensation of gaseous iron species generated from the sparking between the third rail and the electricity guide of subway trains. Iron metal particles also can be produced by the mechanical friction process at the rail-wheel-brake interface. It is probable that the surface of the primary iron particles must be active enough so that oxygen in the air can easily react on the metallic surface, resulting in the formation of iron oxides. Since the surface area is larger for smaller particles, oxidized iron particles were more frequently encountered in fine fraction (on stages 4 and 5). In addition, the existence of gaseous iron species in the subway environment can be explained by the observation of minor iron species contained in almost all the subway particles, even for soil particles that came from outdoor.

Fig. 10. 
A typical secondary electron image for subway particles collected at Hyehwa subway station in Seoul, Korea. Reprinted from permission from ref. Kang et al., 2008. Copyright 2008 American Chemical Society.

Fig. 11. 
Magnified Fe-containing particles shown in Fig. 10. Reprinted from permission from ref. Kang et al., 2008. Copyright 2008 American Chemical Society.

Iron metal particles were clearly distinguishable from oxidized iron particles in the secondary electron images. Iron metal particles were observed mostly in the sliced form, implying that they were scraps peeled off from train wheels or rails (see Fig. 11A). In the secondary electron image, the iron metal particles look darker compared to iron oxides particles. The difference in brightness mainly results from the difference in the secondary electron yields between conducting metal and insulating oxide particles. The insulating oxide particles can develop a negative voltage by the accumulation of primary electrons, resulting in a bright contrast due to a larger emission of secondary electrons by repulsion (Goldstein et al., 2003).

Many Fe-containing particles classified either as iron metal, partially, or fully oxidized iron oxides usually contained small amount of C, N, Mg, Al, Si, S, and/or Ca. When the atomic concentrations of those elements were larger than 10%, the elements were regarded as major chemical species. For example, the notation of “FeOx/C/Si/Ca” for particles #25 and #34 in Fig. 10 indicates that the particles are iron oxides internally mixed with C, Si, and Ca species. Using single particle analysis, Sitzmann et al. (1999) discussed in depth the sources of subway particles containing C, Si, and/or Ca together with Fe: (a) particles containing both Fe and Si were generated from the friction between brake block composed of iron and glass fibers and train wheels, (b) particles containing Fe and Ca together were also generated in the same way because brake blocks contain CaCO₃, and (c) C element was from cast iron used for wheels and rails. In this study, these elements were also very frequently encountered in Fe-containing subway particles. For example, among 29 Fe-containing particles in Fig. 10, 22 particles contained Si and 12 particles contained Ca. According to Korean Standard Database (http://www.standard.go.kr), cast irons used for rails and wheels contain C (0.5-0.7%), Si (0.1-0.35%), Mn (0.6-0.95%), P (<0.045%), and S (<0.05%). Since the contents of carbonaceous species in Fe-containing subway particles are much higher than those of cast iron, the extra carbon observed in subway particles must come from the heterogeneous oxidation reaction of volatile organic carbons (VOCs) in indoor subway environment on the active surface of Fe-containing particles when the particles are generated during the wear process. The reaction of VOCs with Fe-containing particles explains why C element is observed in almost all Fe-containing particles.

Among the four seasonal samples, the summertime samples showed the most abundant Fe-containing particles. Other studies have reported similar observation that the concentrations of Fe-containing particles were higher in the summer than in the winter (Adams et al., 2001a, b). Since whole areas of the Hyehwa subway station were air-conditioned during summertime, it is likely due to less air-exchange between indoor and outdoor areas. In general, it showed that the morning samples contained more abundant iron-containing particles than the afternoon samples. Three samples (Dec. 17, July 4, and Nov. 25) were collected in the morning and others in the afternoon. A reason for the observation of more abundant Fe-containing particles in the morning samples can be that the accumulated Fe-containing particles overnight drift in the air due to the movement of train. In addition, the frequency of train runs in the morning was greater (12 times/hour) than that in the afternoon (10 times/hour). Similar observations were reported in other studies; the PM in subway was proportional to the train passing frequency and reached a maximum during rush hour (Birenzvige et al., 2003).

Carbonaceous Particles

They were significantly encountered with their relative abundances ranging in 5.4-13.2% (see Table 1). Since hydrogen cannot be detected in the low-Z particle EPMA, it has a limitation on the speciation of organic particles. Thus, particles were simply classified as carbonaceous particles when the sum of the atomic % of the carbon, nitrogen, and oxygen in the particle was larger than 90%. It was suggested that carbonaceous species in subway particles could originate from the carbon contained in cast iron (Sitzmann et al., 1999). In that case, it is expected that carbonaceous species should be combined with Fe-containing particles. Indeed, many Fe-containing particles also contained carbon and those were classified as Fe-containing particles. However, the particles classified as carbonaceous particles were mostly composed of carbon, nitrogen, and oxygen without iron. It is likely that these anthropogenic carbonaceous particles came from the urban outdoor atmosphere.

Soil-originated Particles

Soil-originated particles such as aluminosilicates, SiO₂, and CaCO₃ were the second most abundantly encountered in subway platform samples, with relative abundances ranging in 6.0-14.0% (Table 1). Some of soil-derived particles were found as internal mixtures of carbonaceous species (denoted as Aluminosilicates/C and SiO₂/C). Soil-derived particles are one of the most abundant aerosol types in ambient urban atmosphere, especially in the coarse fractions, whereby it is expected that those particles came from an outdoor environment. However, it was suggested that some of the Si-, Al-, and S-rich particles in subway aerosol could come from the wear of paints at subway tunnels, or from construction activities in the tunnels (Sitzmann et al., 1999). If the particles were generated in the indoor subway system, the particles may contain iron species which is ubiquitous in subway indoor environments. Indeed, some of aluminosilicates and CaCO₃ particles were found as internal mixture of iron oxide. In this study, they were classified as Fe-containing particles. Considerably less amounts of soil-originated particles in the summertime samples (Table 1) reflect the limited air-exchange between the indoor and outdoor environments.

Nitrate-and Sulfate-containing Particles

They were also significantly encountered in subway aerosol samples, and existed mostly in the chemical forms of Ca(NO₃, SO₄), (Na, Mg) (NO₃, SO₄), and (NH₄)₂SO₄. Particles denoted as Ca(NO₃, SO₄) were the internal mixture of Ca(NO₃)₂ and CaSO₄ and were produced by the atmospheric reaction of CaCO₃ with nitrogen and sulfur oxide species (Hwang and Ro, 2006; Ro et al., 2005; Krueger et al., 2004). Similarly, (Na, Mg) (NO₃, SO₄) particles are the mixture of nitrate and sulfate formed by the reaction of sea-salts with nitrogen and sulfur oxide species in the air (Hwang and Ro, 2006; Ro et al., 2005; Laskin et al., 2003). These secondary particles containing nitrates and/or sulfates are likely from the outdoor environment. Since nitrogen and sulfur oxides are abundant gaseous species in the indoor subway microenvironment, such nitrate and sulfate particles can also be formed by the reaction of CaCO₃ and sea-salts from the outdoor with indoor gaseous nitrogen and sulfur oxides.

(NH₄)₂SO₄ Particles

They are a major chemical species in urban aerosols, especially in fine fractions. In this study, the (NH₄)₂SO₄ particles were encountered only at stage 5 (1 μm<D_p<2 μm). Nitrate-and sulfate-containing particles were encountered less frequently in the summer samples (Table 1), mainly due to less air-exchange in the summer and less introduction of outdoor particles into the subway system. Some studies suggested that subway particles are well correlated with the outdoor environment, implying that air-exchange between underground and aboveground environments is well established (Branis, 2006; Aarnio et al., 2005), whereas the others suggested that they are not correlated (Salma et al., 2007; Sitzmann et al., 1999).

The Arctic atmosphere is regarded as a unique natural laboratory to understand the occurrence, nature, origin, and transport of atmospheric aerosols and their effect on global climate change (Kawamura et al., 2007; Law and Stohl, 2007; Lohmann and Leck, 2005; Yli-Tuomi et al., 2003). It has been observed that atmospheric aerosols were often disturbed by the well-known Arctic haze, a winter/spring Arctic pollution phenomenon (Law and Stohl, 2007; Teinila et al., 2003), suggesting that the atmosphere of the Arctic region could be polluted by anthropogenic air pollutants in the winter and spring. The summertime Arctic atmosphere has been generally regarded as a clean background environment with low aerosol burden and pollution levels. Many research stations from many countries are located in Svalbard, Norway, and Korean Polar Research Center (KPRC) located at Ny-Ålesund (78°55′N, 11°56′E), Svalbard is one of them. Svalbard is an archipelago in the Arctic Ocean, including three populated islands: Spitsbergen, Bear Island, and Hopen. Ny-Ålesund is located at the remote northern settlement of Spitsbergen, where the sun is above the horizon for 24 hours a day from late April to late August. To characterize the summertime ambient Arctic aerosol particles, aerosol samples were collected at KRPC and investigated using low-Z particle EPMA technique.

16 sets of Arctic aerosol samples were collected at KPRC using a seven-stage May cascade impactor (May, 1975) on July 25-31, 2007. Sampling durations for each stage were adjusted to avoid the collection of agglomerated particles. Individual particles were measured by a JEOL JSM-6390 SEM equipped with an Oxford Link SATW ultra-thin window EDX detector. X-ray spectra were obtained using INCA software (Oxford). Overall, about 8100 individual particles (~150 individual particles on each stage) were manually investigated to obtain information on their size, secondary electron image (SEI), and chemical composition. Samples on stages 1-6 were measured due to the resolution of SEM. Particles on stage 6 were denoted as PM_0.5-1, which means the particle diameters were in the range of 0.5-1 μm. In the same way, particles on stages 2-5 were denoted as PM_8-16, PM_4-8, PM_2-4, PM_1-2, respectively.

Based on the X-ray spectra and SEI data of all the analyzed individual particles, 13 particle types were identified. Their characteristics and relative abundances in different stage samples are described below.

Sea-salt Aerosols (SSA)

They are ubiquitous in the marine boundary layer (MBL) (Athanasopoulou et al., 2008). Shown in Fig. 12 are secondary electron images of Arctic aerosol particles. The size dependency of major particle types was observed, i.e., the frequent observation of soil-derived particles is in coarse fraction (PM_2-4) and carbonaceous particles in fine fraction (PM_0.5-1). Most of sea-salt particles were found in PM_1-2. The genuine (or fresh) sea-salt particles were cubic in shape and looked bright on SEI (e.g., particles #16 and #17 in Fig. 12b). In general, genuine sea-salt particles contained small amount of oxygen, which likely came from the NaOH shell, an alkaline hygroscopic coating around the NaCl (Laskin et al., 2003). Occasionally, MgCO₃ and MgSO₄, which might originate from the evaporation of sea salts, were observed.

Fig. 12. 
Typical secondary electron images of aerosol particles collected at Ny-Ålesund, Svalbard. Droplets rich in C, N, O, and S are abbreviated as “(C, N, O, S)”, which are likely the mixture of elemental/organic carbon and H₂SO₄/NH₄HSO₄/(NH₄)₂SO₄. “Aluminosilicate*” is for aluminosilicate particles containing Fe, for example, particles no. 9, 11, and 35. Reprinted from permission from ref. Geng et al., 2010. Copyright 2010 American Chemical Society.

In the atmosphere, the reacted (or aged) sea-salts containing NaNO₃ and/or Na₂SO₄ are produced when genuine sea-salts react with nitrogen and sulfur oxides species, which results in chlorine loss, sometimes without remaining any chloride if the reaction is complete (Hopkins et al., 2008; Laskin et al., 2002). Hence, SSA often acts as a sink for anthropogenic gases such as nitrogen and sulfur oxides species. In this study, the reacted sea-salts were classified into three types based on their X-ray spectral data. The first type is for those containing nitrates such as Na(Cl, NO₃) and (Na, Mg) (Cl, NO₃), which are the reaction products of genuine sea-salts and NO_x or HNO₃, e.g. particles #6, #10, #15, #18, #21, #29, #31, #40, and #43 in Fig. 12. Particles internally mixed with NaCl, MgCl₂, NaNO₃, and Mg(NO₃)₂ were denoted as (Na, Mg)(Cl, NO₃). The second type is for those containing methanesulfonate (CH₃SO₃^-) and non-sea-salt sulfate (nss-SO₄^2-) such as Na₂SO₄ and (Na, Mg)SO₄ (e.g., particle #46 in Fig. 12), which resulted from the reaction of genuine sea-salts with SO₂/H₂SO₄ or methylsulfonic acid (MSA)-a product of dimethylsulfide (DMS) (Hopkins et al., 2008). The third type is for those containing both NO₃^- and CH₃SO₃^-/nss-SO₄^2-, e.g. particles #23, #24, and #38 in Fig. 12. Sometimes, the reacted sea-salts were found as a mixture with mineral dust particles, which were classified into the “reacted sea-salt & mixture” group, e.g. particles #19, #26, and #51 in Fig. 12.

The majority of reacted sea-salt (& mixture) particles were nitrate-containing. Their round or semispherical shape on SEIs (Fig. 12) indicates that they were liquid droplets at the time of collection. The presence of water greatly enhances the reactivity of NaCl with gaseous materials such as HNO₃, ClONO₂, and N₂O₅. If hygroscopic nitrates were formed, due to the low deliquescence and efflorescence points of the secondary SSA particles, they remain as liquid droplet (Hoffman et al., 2004; Laskin et al., 2002). When these liquid droplets were collected on the substrate foil, water started to evaporate and fractionally crystallized particles were formed as illustrated in Fig. 12b. The abundances of nitrate-containing sea-salt particles greatly outweighed those of the CH₃SO₃^- /nss-SO₄^2--containing ones for all the samples except the sample collected on July 27, 2007. Why were such a large number of nitrate-containing sea-salt particles formed and where did NO_x come from in the summertime Arctic region? The possible explanations are described as follows: (1) Sunlight or solar radiation provides favorable conditions for the enhancement of the reaction of NO_x (or HNO₃) and sea-salt aerosols. Gard et al. (1998) observed that active heterogeneous chemical reaction of sea-salt particles and NO_x (or HNO₃) primarily occurred during the daytime. It was also reported that sea-salt particles were almost unprocessed during the nighttime, while they were completely converted mostly into NaNO₃ particles during the daytime (Laskin et al., 2002). At Ny-Ålesund, the allday-long daytime during the sampling period should enhance photochemical activities, consequently resulting in the production of nitric acid that readily reacts with sea-salt particles. (2) Levels of NO_x and some precursors of secondary aerosols are increasing in the Arctic Ocean because of the rapid increase of shipping traffic in the region (Xie et al., 2007; Xie et al., 2006). It was reported that the high level of nitrate was observed over and downwind of shipping lanes, as its relatively low deposition rate of high level of NO_x species emitted from travelling ships provided ample time for rate-limited mass transfer for the displacement of chloride (Athanasopoulou et al., 2008). In Russian, Canadian, and U.S. Arctic regions, shipping activities such as the running of icebreakers, transportation of crude oil, and deliveries of general cargo and petroleum products are extensive. Air pollutant emissions from shipping traffic and fuel usage along the main navigation routes in the Arctic Ocean should have an impact on chemical compositions of the Arctic aerosols in the marine boundary layer (Xie et al., 2007; Xie et al., 2006). (3) Anthropogenic air pollutants such as nitrogen and sulfur oxides from industrial emissions in northern Europe and western Siberia can be transported to Ny-Ålesund during the summer (Yli-Tuomi et al., 2003; Ohta et al., 1996). (4) Renoxification (i.e., reduction of NO₃^- to NO_x and HONO) may be one of the most important mechanisms (Morin et al., 2008; Nyeki et al., 2005). During the continuous daytime in the Arctic region, photochemically induced reactions are active. Photochemical reactions occurring within the snowpack or on the snow/ice surface where NO_x and NO₃^- were accumulated or enriched in the winter and spring (particularly when the Arctic haze occurred) can produce high level of NO_x from nitrate (Ianniello et al., 2002). Under solar radiation, the deposited NO₃^- on the Arctic snowpack or ice can be reactivated or reduced to nitrite (NO₂^-) or to NO₂ and hydroxyl radicals (OH), which may eventually lead to reemission of nitrogenous compounds to the atmosphere (Beine et al., 2003), resulting in elevated reacted (aged) sea-salt particles by the reaction of NO_x or HNO₃ with sea salts. The nitrogen cycling rates in the Arctic were reported to increase as shrub growth is stimulated by increasing summer temperature and nitrogen supply (Chapin et al., 2005). Also, the acceleration of summer warming in the Arctic region can result in the advancing of snowmelt and the declining of summer sea ice. (5) The decomposition of peroxyacetyl nitrate (PAN) in the Arctic atmosphere during the polar night period also contributes to the formation of NO₂ (Beine and Krogne, 2000). The thermal decomposition of PAN to NO₂ in the Arctic troposphere (e.g., at Svalbard) occurs mostly in the summer, although the amounts of PAN and NO₂ from PAN are small (Morin et al., 2008).

Mineral Dust Particles

They include aluminosilicate (AlSi-containing), quartz (SiO₂), calcite (CaCO₃)/dolomite (CaMg(CO₃)₂), gypsum (CaSO₄∙2H₂O), Fe-rich (mostly iron oxides), and rutile (TiO₂). They were popular in coarse fraction and appeared irregular and bright on their SEIs (see Fig. 12a). Fe-rich particles look angular on their SEIs and it is well known that they can act as a nutrient for biota in marine environments with their depositions (Xie et al., 2007). Reacted mineral dust particles were classified mainly as “reacted CaCO₃/CaMg (CO₃)₂” and “aluminosilicate+(N, S)” types in which (N, S) notation represented compounds containing either nitrates, sulfates, or both. They are produced when aluminosilicate (especially Ca-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 (Sullivan et al., 2007; Hwang and Ro, 2006; Lan et al., 2005). They can be formed either through the reaction of aluminosilicate and CaCO₃/CaMg(CO₃)₂ particles with H₂SO₄ and/or HNO₃, or from adsorption of NH₄NO₃ or (NH₄)₂SO₄/NH₄HSO₄ on particle surface (Sullivan et al., 2007), e.g., particles #2, #8, and #39 in Fig. 12.

Carbonaceous Particles

They are generated mainly from incomplete combustion of fossil fuels and biomass, are ubiquitous in the tropospheric atmosphere, and have strong radiative-forcing effects (Chen et al., 2005). Carbonaceous particles are generally differentiated into two groups: elemental carbon (EC, termed “carbon-rich particle” herein) and organic carbon (OC). In this study, the particle was classified as carbon-rich (or EC) when the carbon content was 3-fold greater than the oxygen content and no other elements were present (Ro et al., 2005). Other carbonaceous particles which have more than 90% atomic concentration of C and O (sometimes including N) were classified as OC.

The OC particles were categorized into four different types based on their morphological and X-ray spectral data. The first type was of primary or water-insoluble secondary organic particles, which look bright and angular on their SEI and have high levels of C, N, and O. It has been termed (C, N, O)-rich particles or nitrogen-containing organic compounds (NOC) by other researchers (Laskin et al., 2009), and may be generated mostly by biomass burning. The second type was of biogenic particles (e.g., pollen, spore, algae, bacteria, and plant or insect fragments), a special type of organic particles. They have a “biogenic fingerprint”, i.e., the presence of N, K, P, S and/or Cl together with C and O (e.g. particles #47 and #48 in Fig. 12d) (Geng et al., 2009a, b). Most of the biogenic particles were likely from the seawater of the Arctic Ocean or the surrounding seas nearby the sampling site. The third type was of liquid droplet particles (probably containing water-soluble organic carbon (WSOC)), which looked dark with round shape and had high levels of C, N, and O, often together with a trace amount of Na and Mg elements indicating their marine origin. These (C, N, O)-rich droplets were possibly composed of organic matter and NH₄NO₃. Oxidation of volatile organic compounds, humic substances or “humic-like substances (HULIS)” in the marine environment, and crude oils spilled on the sea surface from ship tankers are perhaps contributory to the organic matter (Geng et al., 2009b). The fourth type of OC particles was of droplet particles rich in C, N, O, and S, represented as (C, N, O, S)-rich droplets, which were also significantly observed in the summertime aerosol samples collected at Tokchok Island, Korea (Geng et al., 2009a). They look dark on their SEIs (e.g., many particles in Fig. 12d) and are likely internally mixed with (NH₄)₂SO₄/NH₄HSO₄ and organic matter. Ambient sulfate is often neutralized by ammonia (mainly from animal waste, fertilizer application, soil and industrial emissions, and direct emissions from vegetation and oceans) and its most common form is (NH₄)₂SO₄. However, if ammonia is scarce, sulfate can remain in more acidic forms such as NH₄HSO₄ or H₂SO₄ (Ansari et al., 1998). Ammonia is usually scarce in the Arctic region being without major ammonia sources (Teinila et al., 2003), so that the (C, N, O, S)-rich droplet particles might include more acidic NH₄HSO₄ (or H₂SO₄) rather than (NH₄)₂SO₄. There are three possible sources of SO₂ and SO₄^2- in the Arctic region: the first is from ship emissions around the Arctic Ocean (Xie et al., 2007, 2006); the second is likely from industrial emissions in western Siberia (Russia) and northern Europe; the third is from DMS in seawater, as high biological activity in ocean margins results in a significant production of DMS, which can be oxidized to form various sulfur containing products, such as MSA, SO₂, nss-SO₄^2- (Yang et al., 2009; Hopkins et al., 2008). The organic matter in the (C, N, O, S)-rich droplet particles is likely similar to that in the (C, N, O)-rich droplets. Perhaps, some products of DMS are involved (Hopkins et al., 2008).

Soot agglomerates, which are abundant in urban atmosphere, were not observed in the summertime Arctic samples, possibly due to the absence of combustion sources of diesel oil or coal in Ny-Ålesund. A few tar balls and chars were observed, which have irregular or spherical morphology, quite different from that of soot aggregates. They can be generated by gas-to-particle conversion within smoke plumes and are particularly abundant in slightly aged biomass smoke (Pósfai et al., 2004).

Relative Abundances of Various Types of Particles

The relative abundances of particle types observed in 16 summertime Arctic samples are shown in Fig. 13. Arctic aerosols were classified into three groups according to their origins: marine-originated, soil-derived, and carbonaceous. Marine-originated particles contain genuine and aged (reacted) sea-salt particles. The aged sea-salts including nitrates and/or sulfates were produced by the reaction between sea-salts and NO_x/SO_x. In Arctic aerosol samples, sea-salt particles were the most frequently encountered with more than 50% relative abundance, and most of them were observed as aged (reacted) species. However, the relative abundances of reacted sea-salt depended on the passage of air mass. Similar observation was reported by Friedman et al. (2009).

Fig. 13. 
Relative abundances of various particle types in summertime Arctic samples. Reprinted from permission from ref. Geng et al., 2010. Copyright 2010 American Chemical Society.

Soil-originated mineral dust particles contained aluminosilicate, CaCO₃/(Ca,Mg)CO₃, SiO₂, CaSO₄, and Fe-rich particles, among which aluminosilicate particles were the most frequently encountered. The relative abundances of mineral dust particles (including reacted ones) were in the range of 25-35% and they were observed mainly in coarse fraction (PM_8-16 and PM_4-8). The non-reacted mineral dust particles outnumbered the reacted ones (only 5.3%, 6.4%, and 7.0% on average for PM_4-2, PM_2-1, and PM_1-0.5 fractions, respectively). Many of the mineral dust particles should be of local origin, mostly from snow-free mountains during summertime (Teinila et al., 2003). The possible reasons for the predominance of the genuine mineral dust particles over the reacted ones are: (i) anthropogenic emissions were not strong in the remote Arctic region so that mineral dust particles had no chance to react with air pollutants; (ii) the reactivity of mineral dust particles with NO_x and SO₂ was much slower than that of sea-salts (Kang et al., 2009). Like the reacted sea-salt particles, the reacted mineral dust particles containing nitrates were predominant over those containing sulfates, indicating that NO_x or HNO₃ contributed to the formation of reacted mineral dust particles more than SO₂ or H₂SO₄ did.

Most of organic carbon (OC) particles were primarily composed of liquid droplets rich in (C, N, O) and (C, N, O, S), and it was hard to find carbon-rich ones. It implies that the summertime Arctic atmosphere was more affected from the mixture of organic matter and secondary anthropogenic pollutants such as NH₄NO₃ and NH₄HSO₄/(NH₄)₂SO₄ than EC particles, which are produced from smoke plumes of forest fires or from oil combustion by ships around the periphery of the Arctic Ocean (Xie et al., 2007). The relative abundances of (C, N, O, S)-rich droplets were much higher than those of (C, N, O)-rich droplets. On average, (C, N, O)-rich droplets were encountered with relative abundances of 1.9%, 1.7%, and 3.4% in PM_4-2, PM_2-1, and PM_1-0.5 fractions, respectively; whereas (C, N, O, S)-rich droplets were significantly encountered in PM_4-2, PM_2-1, and PM_1-0.5 fractions for almost all the samples, peaking at 50-60% of relative abundances for PM_4-2 fraction of samples collected at 20:40-8:40, 7/30-31/2007. Possible reasons for this observation are (i) NH₄HSO₄/(NH₄)₂SO₄ was easier to be formed in the summer than NH₄NO₃ and/or (ii) the air masses that originated from continental European countries and passed over the south Arctic Ocean and Barents Sea carried more NH₃ and SO₂. It has been reported that air masses from western Siberia can be contaminated by anthropogenic pollutants such as SO₂ from smelting factory in Norilsk (Ohta et al., 1995). And nss-sulfate concentrations in precipitations at Ny-Ålesund were reported to be high when air masses had passed over Scandinavia and western Siberia regions (Yamagata et al., 2009).