The air quality of northeastern Asia is characterized by three factors: high emission of anthropogenic air pollutants, high dust loadings, and prevailing westerly winds. Approximately one-third of the world’s population now live in this region, and the emissions of anthropogenic air pollutants have drastically increased due to the dynamic development of industries and the rapid increase in the standards of living. Emissions of SO2 in the region (about 15 000 GgS yr−1) were far higher than those of the United States (about 10 000 GgS yr−1) (Akimoto and Narita 1994; OECD 1991). Once these anthropogenic air pollutants are emitted into the atmosphere, they are transformed into acidic particles that result in acid deposition on areas both near and far from the sources of economic activities.
The dust flux originated from the arid and semiarid regions of northwestern China, the southeastern part of the former Soviet Union, and Mongolia has one of the highest concentrations in the world. The “Yellow Sand” phenomenon that occurs mostly in spring and early summer originated from northeastern Asia, and the dust is transported and deposited in the eastern parts of China, Korea, Japan, the Pacific Ocean, and as far away as Hawaii (Chung 1992; Parrington et al. 1983). It is estimated that about 20 Mton of dust are transported on the ground surface each year from source regions (Chung 1992). During the Yellow Sand phenomenon, it is common that the particle concentrations exceed 500–600 μg m−3 (Kang and Sang 1991). Although the Yellow Sand during the springtime is the best-known dust particle transport phenomenon, dust particles are transported from northeastern Asia year-round (Pye 1987). Another source contributing to the high dust concentration in the region is the local entrainment of dust from the ground surface to the atmosphere. Due to their crustal origin, the main components of dust particles are alkaline materials, such as calcium, potassium, and magnesium carbonates and/or oxides. These dust materials influence gas and aerosol chemistry while they are being transported (Kim and Seinfeld 1995).
Meteorological conditions coupled with the high rate of emissions in the wintertime frequently favor the accumulation of air pollutants near the surface layer. Since the upper-level zonal westerlies dominate eastern Asia in the winter, Korea and Japan are located downwind from China, which has large sources of air pollutants. A major concern for the people of this region, therefore, is that the acid deposition might increase drastically in the future due to the transboundary transport of air pollutants.
Cheju Island, Korea, is located about 100 km south of the Korean peninsula in the northern East China Sea (Fig. 1). Cheju Island is considered as one of the cleanest areas in Korea with low emissions of air pollutants. Thus, Cheju Island is an ideal place for investigating the long-range transport of air pollutants in this region. Few studies have been carried out at Cheju Island for both aerosol (Arimoto et al. 1996; Carmichael et al. 1996; Kim et al. 1995; Lee et al. 1995) and gaseous species (Suh et al. 1995). An intensive field study was carried out at Kosan, Cheju Island, between 11 March and 19 April 1994. In this study, total suspended particles (TSPs) were measured daily and gaseous species were measured continuously. Also, fine particles (PM1 and PM3) and particle-size distribution were measured and reported elsewhere (Kim et al. 1995).
In this report, 1) daily TSP and continuous gaseous species measurement results are shown, 2) backward trajectory analysis on the path of air pollutants measured during this period is given, and 3) the relationship between these air pollutants is investigated.
Sampling site description
The measurement site was located on the western tip of Cheju Island (33°17′N, 126°10′E, Fig. 1). A trailer containing all the gas analyzers and a particle sampler was situated about 10 m inside from a cliff of about 70 m above mean sea level. Both sampling inlets were located about 6 m above the ground. The Cheju upper-air meteorological station is located 100 m northeast of the trailer and upper-air meteorological parameters have been measured twice a day and other meteorological parameters measured routinely.
The emissions of SO2 and NOx from Cheju Island are quite low, amounting to only 0.7% and 1.1% of the total emissions from Korea, respectively, or about one-tenth of the emissions of both species from Seoul (MOE 1994). Cheju Island is popular for tourism; however, the western side of the island is less developed than other parts of the island and, therefore, has less traffic. The nearest village is about 1 km east of the site, and the nearest highway is about 2 km east of the site, with traffic flow of about 3000 vehicles per day. The effects of local emissions on gaseous measurements are discussed below.
Both aerosol and gaseous air pollutants were measured. An automatic high-volume sampler (Kimoto 195A) was used to collect the TSPs. Particles were collected on a Teflon tape filter surface that was located at a sampling probe for either 6 or 24 h and then a new clean filter surface was moved to the sampling probe on which particles were collected. The flow rate was about 170 L min−1. After 6 h of data collection, daily average values were calculated. The filters were analyzed for water soluble ions:
Results and discussion
The overall mean values, standard deviations, and ranges of daily average concentrations of water soluble ions in TSPs are given in Table 1. Also shown in Table 1 are the data for nss ions. The nss concentrations are estimated based on sodium ion and seawater composition by Horne (1969) was used for estimation.
The aerosol collected in the marine environment are subject to sampling artifacts and may not accurately represent the actual composition. One example is the evaporation of Cl− to HCl and the resultant negative concentration of nss Cl−; this could occur in the ambient air (Möller 1990) or on the filter due to the interactions among Cl−, acidic gases, and alkaline sea salts. Other volatile species, such as
Sulfate was the largest constituent of the water soluble ions in TSPs. The overall mean value was 8.79 μg m−3, which represented 52% of the total water soluble ion concentration by mass. Chloride and sodium were the next largest constituents, reflecting the fact that the site is near the sea. The arithmetic mean value of the nss sulfate concentration was 8.30 μg m−3, which represented 94% of the total sulfate concentration. Similar values had been observed at the same site by other investigators. For example, Lee et al. (1995) observed nss sulfate concentrations of 8–10 μg m−3 by the same sampler between March and April 1992, while obtaining the overall mean value of 6.53 μg m−3 for the period of March and December 1992.
These values were far higher than the average nss sulfate concentration measured at various other places in the Pacific Ocean [0.2–1.5 μg m−3 (Fitzgerald 1991)] and that of PM2 particles at remote marine regions [about 1 μg m−3 (Heintzenberg 1989)]. The range of sulfate concentrations measured in Bermuda by Wolff et al. (1986) and Hastie et al. (1988) was between 2.69 and 3.96 μg m−3. The sulfate concentrations in Japanese remote sampling sites were 3.59 μg m−3 in Oki Island for TSP (Mukai et al. 1990), and between 0.2 and 4.2 μg m−3 in the Pacific (Okita et al. 1986). In comparison, the average concentration of sulfate in Seoul, the capital of Korea, with about 10 million inhabitants, is between 5 and 10 μg m−3, which is comparable to this result (Baik et al. 1996).
The concentrations of crustal species, Ca2+, K+, and Mg2+ ions measured during this period were also higher than those measured at Oki Island (Mukai et al. 1990). However, their contribution to the total ion concentration were small. The average nss Ca2+ and nss K+ concentrations were 3.7% and 2.6% of the total ion concentration, respectively. Their mean concentrations were comparable to those reported by Carmichael et al. (1996) at the same site during the springs of 1992 and 1993.
Because the sampling site is in the marine environment, the contribution to nss sulfate concentration from biogenic sources should be assessed. The sulfur species from the biogenic sources were not measured during this study. During the “PEM-West A” measurement, however, methane sulfonic acid and nss sulfate concentrations in aerosol were measured at the Kosan site between April 1992 and February 1993 (Arimoto et al. 1996). They determined that the biogenic fraction of the nss sulfate was about 11%. Also, Luria et al. (1989) concluded that in the Gulf of Mexico area, dimethyl sulfide (DMS) emission can account for about 1 μg m−3 of sulfate. Furthermore, they concluded that in the Bermuda area less than 50% of the observed sulfate (1.9 μg m−3 for the TSPs), or 1 μg m−3 of sulfate, can be related to the natural sulfur cycle. Therefore, it was a reasonable estimation that less than 1 μg m−3 of the measured nss sulfate has originated from the biogenic sulfur cycle.
In summary, the observed water soluble ion concentrations during this period were higher than those measured at remote marine areas but were comparable to those at Seoul, a highly urbanized area. Since the local emission rates of air pollutants are quite low, it is likely that the observed high values of aerosol ion concentrations can be accounted for by the transport of air pollutants from outside Cheju Island, and this will be discussed later.
The overall mean values, standard deviations, and ranges of daily average concentrations of gaseous species, such as SO2, O3, NO, and NOx are also given in Table 1. The NO concentration during the period was mostly below the detection limit of the analyzer. The overall mean value of NOx was 3.5 ppb. The mean value of NOx was far higher than that at Oki Island (Jaffe et al. 1996).
The maximum hourly concentration of SO2 during the period was less than 10 ppb, the maximum daily average value was 3.5 ppb, and the overall mean value was 0.97 ppb. These values were far lower than those observed at Seoul. For example, the yearly mean concentration of SO2 in Seoul during 1993 was 23 ppb. Similar or slightly higher values for SO2 were observed by other researchers at the same site. During March and April 1992, the average concentration of SO2 was about 1 ppb (SERI 1993) and between February and December 1992 it was 1.42 ppb (Suh et al. 1995). The SO2 concentration at Kosan, however, was higher than other clean areas. The SO2 concentration in Oki Island in November 1992 was about 0.2 ppb (Hatakeyama 1994) and in Bermuda in spring of 1985 it was 0.27 ppb (Hastie et al. 1988).
On the contrary, the ozone concentration at Cheju was quite high. The overall mean value of O3 was 55 ppb and the maximum daily average concentration was 73 ppb. The O3 concentration at this site is one of the highest in Korea. Similarly, high ozone concentrations had also been observed in Japanese mountainous regions (Sunwoo et al. 1994) and in the Atlantic Ocean with the altitude of somewhere between 35° and 40°N (Thompson 1994) during the springtime. In Oki Island, Japan, lower values than this study, the mean values of 36.1 ppb in September and 42.2 ppb in October 1991, were observed (Akimoto et al. 1996). This discrepancy between two sites might be caused by two factors. First, Kosan is nearer to the Asian continent than Oki Island and thus more affected by photochemical reactions among air pollutants from it. Second, ozone concentrations in the region are highest during the springtime due to the mixing of the stratospheric ozone to the troposphere caused by the jet stream (Sunwoo et al. 1994). Since the measurements at Oki (Akimoto et al. 1996; Jaffe et al. 1996) were carried out during the fall, while this study was carried out during the spring, the ozone concentrations at the two sites might be different.
Since the effects of local emissions of SO2 and NOx to the concentrations of gaseous species have not been studied yet, those should be analyzed to interpret the trend of the gaseous species concentrations. During the period, three distinctive patterns of O3 and NOx concentration variations have been observed, as shown in Fig. 2. A typical case showing variations over less than 1 h is shown in Fig. 2a. During part of the period, the NOx concentration increased sharply, while the O3 concentration decreased accordingly. This can happen when a locally emitted NO plume is titrated with high O3 concentration (Finlayson-Pitts and Pitts 1986; Sexton and Westberg 1979). Since this kind of phenomenon occurs independent of the time of day and SO2 concentration did not change, it can be concluded that this is caused by a local NO source located very near to the site, probably automobile traffic.
Figure 2b shows another type of variation that is similar to the previous case, but it lasted a few hours longer. The SO2 concentration over this period sometimes increased, but generally remained constant. It is likely that this phenomenon is influenced by NO emissions from other areas of Cheju. Figure 2c shows a case of both NOx and O3 concentrations increasing simultaneously with a time span of about 10 h. This case occurred mainly during the daytime and can be interpreted as the transport of polluted air mass from the outside of the Cheju Island. The SO2 concentration shows no apparent trends in concentration during the period.
In summary, NOx concentrations are affected by local emissions and, thus, are higher than that at Oki Island. The effects of the local emissions to SO2 concentration were small. The aerosol ion concentrations, especially sulfate, at Kosan were high, comparable to those at Seoul, while the concentrations of SO2 and NOx at Kosan were much lower than those at Seoul. It is a typical characteristic of long-range transport of air pollutants, and a similar observation was made in remote islands in Japan (Akimoto et al. 1996).
Backward trajectory analysis
Since the air pollutant concentrations strongly suggest these are transported from outside Cheju Island, it is important to determine their transport path. Four-day backward trajectories of air parcels arriving at Kosan at 2100 LST were prepared using the Gridded Atmospheric Multilevel Backward Isobaric Trajectories model of NOAA (National Oceanic and Atmospheric Administration) (Harris 1982). The meteorological data used for this analysis were gridpoint values from the Japanese Meteorological Agency. The reason that trajectories at 2100 LST were calculated was that they corresponded approximately to the midpoint of the filter sampling period. Trajectories at the 700-, 850-, and 1000-hPa levels were estimated.
The northeastern Asia region was divided into five sections, as shown in Fig. 3. Section I corresponds to air parcels traveling from the Korean peninsula, and section II is from northern China. Section III corresponds to middle and southern China, including Taiwan;section IV is from the Pacific Ocean. Finally, section V is from Japan. In Table 2, the number of days and the location of air parcels represented by the 850-hPa trajectories at one, two, three, and four days before arriving at Kosan at each section are given. About 55% of air parcels at 850 hPa have traveled from section II, 30% from section III, 10% from section I, 7% from section IV, and none from section V. There was a strong time dependence of air parcel paths. During March, more than 80% of air parcels moved from section II. In April, about half of the air parcels moved from section III, while about 20% of air parcels moved from section II. Dominance of air parcels from sections II and III (more than 80% of the total days) is due to the westerlies, the prevailing winds for this region in spring. Thus, high concentrations of anthropogenic air pollutants, such as nss sulfate, can be attributed to the transport of air pollutants from the Asian continent.
The average concentrations of nss
Relationship between air pollutant concentrations
The correlation coefficients between the ions and gaseous species concentrations are shown in Table 4. The nss sulfate, which is considered an indicator of anthropogenic air pollutants in the region (Carmichael et al. 1996), showed a strong relationship with nss K+ (r = 0.873),
In this study nss
It is generally assumed that main sources of K+ in the region are natural sources, mostly dust particles (Carmichael et al. 1996). The nss K+ in this study showed a strong relationship with both nss
The trends of nss
To estimate the contributions from anthropogenic and natural sources to the concentration of nss K+, scattergrams of nss K+ and nss Ca2+, and nss K+ and nss
The long-range transport of air pollutants in northeastern Asia is an important factor in understanding the air quality of the region. Measurements of TSPs and gaseous species were carried out between 11 March and 19 April 1994 at Kosan, Cheju Island, Korea, in which emissions of air pollutants are among the lowest in Korea.
It was shown that in water soluble ion concentrations of TSP particles, sulfate concentrations were higher than other reported remote marine areas and Oki Island, a Japanese background measurement site. Furthermore, the sea salt fraction of sulfate was about 6%, that is, for the overall mean nss sulfate concentration of 8.30 μg m−3, the contribution from biogenic sources is small (less than 1 μg m−3). The concentrations of crustal species were also higher than those measured at Oki Island, but nss sulfate concentrations were comparable to those of Seoul. In contrast, the concentration of SO2 was quite low compared to those of Seoul and other urban areas in Korea. Therefore, it is suggested that the air pollutants measured at the site were transported from outside the island.
Concentrations of nitrogen oxides are thought to be affected by local emissions. The concentrations of NOx and SO2 were lower than those measured at the urban areas in Seoul but higher than in clean areas of the region. The overall mean concentration of ozone was 55 ppb, comparable to or slightly higher than the values reported from the mountainous sites in Japan and marine areas with similar latitude.
Backward trajectory analysis was carried out in order to estimate the travel path of air parcels that arrived at Kosan during the measurement period. About 85% of the air parcels passed over the Asian continent, while only about 10% of air parcels traveled over the Korean Peninsula. A half of the air parcels during the period were from northern China and about 30% of the air parcels were from southern China. The main difference of air pollutant levels between the two areas was higher crustal species and lower nss sulfate concentrations for northern China.
This work was supported by the Korea Institute of Science and Technology.
Akimoto, H., and H. Narita, 1994: Distribution of SO2, NOx and CO2 emissions from fuel combustion and industrial activities in Asia with 1° × 1° resolution. Atmos. Environ.,28, 213–226.
——, and Coauthors, 1996: Long-range transport of ozone in the East Asian Pacific Rim region. J. Geophys. Res.,101, 1999–2010.
Arimoto, R., and Coauthors, 1996: Relationships among aerosol constituents from Asia and the North Pacific during PEM-West A. J. Geophys. Res.,101, 2011–2023.
Artaxo, P., W. Maenhaut, H. Storms, and R. van Grieken, 1990: Aerosol characteristics and sources for the Amazon Basin during the wet season. J. Geophys. Res.,95, 16 971–16 985.
Baik, N. J., Y. P. Kim, and K.-C. Moon, 1996: Visibility study in Seoul, 1993. Atmos. Environ.,30, 2319–2328.
Carmichael, G. R., Y. Zhang, L.-L. Chen, M.-S. Hong, and H. Ueda, 1996: Seasonal variation of aerosol composition at Cheju Island, Korea. Atmos. Environ.,30, 2407–2416.
Chung, Y.-S., 1992: On the observations of Yellow Sand (dust storm) in Korea. Atmos. Environ.,26A, 2743–2749.
Finlayson-Pitts, B. J., and J. N. Pitts, Jr., 1986: Atmospheric Chemistry, Fundamentals and Experimental Techniques. John Wiley and Sons, 967 pp.
Fitzgerald, J. W., 1991: Marine aerosols: A review. Atmos. Environ.,25A, 533–545.
Harris, J. M., 1982: The GMCC atmospheric trajectory program. NOAA Tech. Memo. ERL ARL-116, 30 pp. [Available from NOAA/ARL, Boulder, CO 80303.].
Hastie, D. R., H. I. Schiff, D. M. Whelpdale, R. E. Peterson, W. H. Zoller, and D. L. Anderson, 1988: Nitrogen and sulfur over the western Atlantic Ocean. Atmos. Environ.,22, 2381–2391.
Hatakeyama, S., 1994: Databook of ’92 IGAG/APARE/PEACAMPOT Survey. National Institute for Environmental Studies (NIES), Japan, 132 pp.
Heintzenberg, J., 1989: Fine particles in the global troposphere: A review. Tellus,41B, 149–160.
Horne, R. A., 1969: Marine Chemistry. Wiley-Interscience, 71 pp.
Jaffe, D. A., and Coauthors, 1996: Measurements of NO, NOy, CO, and O3 and estimation of the ozone production rate at Oki Island, Japan, during PEM-West. J. Geophys. Res.,101, 2037–2048.
Kaneyasu, N., S. Ohta, and N. Murao, 1995: Seasonal variation in the chemical composition of atmospheric aerosols and gaseous species in Sapporo, Japan. Atmos. Environ.,29, 1559–1568.
Kang, K. H., and S. E. Sang, 1991: Influence of yellow sand on TSP in Seoul. Proc. Second IUAPPA Regional Conf. on Air Pollution, Vol. II, Seoul, Korea, Korea Air Pollution Res. Assoc., 1–7.
Kim, Y. P., and J. H. Seinfeld, 1995: Atmospheric gas–aerosol equilibrium III. Thermodynamics of crustal elements Ca2+, K+, and Mg2+. Aerosol Sci. Technol.,22, 93–110.
——, S. G. Shim, K. C. Moon, N. J. Baik, S. J. Kim, C. G. Hu, and C. H. Kang, 1995: Characteristics of particles at Kosan, Cheju Island: Intensive study results during March 11–17 (in Korean). J. Korea Air Poll. Res. Assoc.,11, 263–272.
Lee, H.-G., K.-Y. Park, M.-S. Suh, K.-M. Jang, C.-H. Kang, and C.-G. Hu, 1995: Chemical analysis of water soluble aerosols at Kosan, Cheju Island (in Korean). J. Korea Air Poll. Res. Assoc.,11, 245–252.
Luria, M., C. C. van Valin, J. N. Galloway, W. C. Keene, D. L. Wellman, H. Sievering, and J. F. Boatman, 1989: The relationship between dimethyl sulfide and particulate sulfate in the mid-Atlantic Ocean atmosphere. Atmos. Environ.,23, 139–147.
MOE, 1994: Korea Environmental Yearbook (in Korean). Korean Ministry of Environment, Seoul, Korea.
Möller, D., 1990: The Na/Cl ratio in rainwater and the seasalt chloride cycle. Tellus,42B, 254–262.
Mukai, H., and Coauthors, 1990: Long-term variation of chemical composition of atmospheric aerosol on the Oki Islands in the Sea of Japan. Atmos. Environ.,24A, 1379–1390.
OECD, 1991: OECD environmental data. Organization for Economic Co-operation and Development, Paris, France.
Okita, T., and Coauthors, 1986: The characterization and distribution of aerosol and gaseous species in the winter monsoon over the western Pacific Ocean. J. Atmos. Chem.,4, 343–358.
Parrington, J. R., W. H. Zoller, and N. K. Aras, 1983: Asian dust:Seasonal transport to the Hawaiian Islands. Science,220, 195–197.
Pye, K., 1987: Aeolian Dust and Dust Deposits. Academic Press, 198 pp.
SERI, 1993: Investigation on long-term changes of atmospheric environment, III (in Korean). System Engineering Research Institute, Korea, 182 pp.
Sexton, K., and H. Westberg, 1979: Ambient air measurements of petroleum refinery emissions. J. Air Poll. Control Assoc.,29, 1149–1152.
Suh, M.-S., K.-Y. Park, H.-G. Lee, K.-M. Jang, C.-H. Kang, C.-G. Hu, and Y.-J. Kim, 1995: A study on the characteristics of rural and urban surface ozone concentrations (in Korean). J. Korea Air Poll. Res. Assoc.,11, 253–262.
Sunwoo, Y., G. R. Carmichael, and H. Ueda, 1994: Characteristics of background surface ozone in Japan. Atmos. Environ.,28, 25–37.
Thompson, A. M., 1994: Oxidants in the unpolluted marine atmosphere. Environmental Oxidants, J. O. Nriagu and M. S. Simmons, Eds., Advances in Environmental Science and Technology, Vol. 28, John Wiley and Sons, 31–61.
Wolff, G. T., M. S. Ruthkosky, D. R. Stroup, P. E. Korsog, M. A. Ferman, G. R. Wendel, and D. H. Stedman, 1986: Measurement of SOx, NOx, and aerosol species on Bermuda. Atmos. Environ.,20, 1229–1239.
Zhang, Y., Y. Sunwoo, V. Kotamarthi, and G. R. Carmichael, 1994:Photochemical oxidant processes in the presence of dust: An evaluation of the impact of dust on particulate nitrate and ozone formation. J. Appl. Meteor.,33, 813–824.
Summary of aerosol and gaseous air pollutant measurements.*
Distribution of air parcel trajectories arriving in Kosan by sector.
Mean concentrations of nss
Correlation coefficients among ions and gaseous air pollutant concentrations.
Correlation coefficients among ions with different air parcel paths.