Introduction

Emissions of air pollutants are on a steady rise with economic and industrial development in the East Asian region (Streets and Waldhoff 2000; Seinfeld et al. 2004). The rapid development of China is responsible for the consumption of 2.91 billion tons of fossil fuel (Korea Energy Economics Inst. 2012). In 2005, the combustion of fossil fuels emitted 2774, 1824, and 994 kt of TSP, PM10, and PM2.5, respectively (Zhao et al. 2008). It resulted in extremely severe and persistent haze occurrences in eastern and central China in January 2013 which was reported as the haziest month in the past 60 years in Beijing (Lu et al. 2013). In addition, eastern China was shrouded in prolonged fog and haze during February 2014. On the other hand, dust storms generally occur in northern China and Mongolia in the spring and are transported to Korea, Japan, and as far as the USA (McKendry et al. 2001; Kurosaki and Mikami 2003; Wang and Fang 2006).

Many model1ing approaches and techniques have been used to forecast dust air pollution, including dust storm and anthropogenic dust pollution (Carnevale et al. 2012; Tan et al. 2012; Wang and Niu 2013; Sun et al. 2013a, b). At the same time, studies have been conducted on the long-range transport of dust air pollution related to synoptic scales in the East Asian region (Kurosaki and Mikami 2003; Wang and Fang 2006; Kim et al. 2014a). Dust air pollution in East Asia affects air quality in the leeward side in varying ways depending on the origin and the passing route. Air quality standards of dust air pollution in Korea are defined as PM10 (100 μg m−3 day−1) and PM2.5 (50 μg m−3 day−1). Therefore, it is necessary to conduct a quantitative evaluation of the impact of dust pollution cases on air quality.

Operational monitoring makes it possible to observe the different origins of air pollution, distinguish aerosol types, and compare satellite data with surface mass concentrations (Chung and Kim 2008; Kim and Chung 2008; Logan et al. 2013). The variations of TSP, PM10, and PM2.5 and the causes were analyzed through long-term observations (Kim et al. 2014b). The purpose of this observational study is to analyze the impact of dust air pollution on the air quality of some of the typical dust pollution cases which occurred in central Korea during the first 5 months of 2014.

Measurements and analyses

Operational monitoring has been conducted since 1993 at the Korea Centre for Atmospheric Environment Research (KCAER) in Cheongju, a rural location in central South Korea (Fig. 1). The geographical environment and instruments of KCAER were introduced in previous studies (Kim and Chung 2008; Kim et al. 2014a). The observatory is located approximately 120 km south of Seoul and 120 km east downwind of the Tae-ahn Peninsula on the west coast of the Korean Peninsula. In the case of moderate winds, it takes roughly 3–5 h for the airflow from Tae-ahn to reach the observatory. There are no large cities or industrial complexes between the Tae-ahn Peninsula and the observatory.

Fig. 1
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Map of the East Asian region and annual PM2.5 exposure from 2008 to 2010 (van Donkelaar et al. 2010). Tae-ahn (T) and Korea Centre for Atmospheric Environment Research (KCAER) are the background monitoring stations in central Korea

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The mass concentrations of TSP, PM10, and PM2.5 particulate matters were measured by using the TEOM Series 1400a by Rupprecht & Patashnick, which is designed to measure the mass of dust collected in a filter by means of the tapered element oscillating microbalance (TEOM) method. The RGB-composite images of National Oceanic and Atmospheric Administration (NOAA) satellites are created using three channels, including 1, 2, and 4, from the Advanced Very High Resolution Radiometer (AVHRR) data. It is possible to distinguish the particles according to the sensitivity of visible rays and near-infrared rays from dust storms and anthropogenic dust pollution in the atmosphere, and by the difference in the absorption of thermal infrared rays (Chung and Le 1984; Chung and Kim 2008). RGB-composite images display the seas and lakes in blue, typically without air pollution due to its low reflection rate. The clouds appear in white and the land in green. Anthropogenic dust pollution images of the seas and lakes appear in magenta, and dust storms appear in purplish yellow. With NOAA satellites, of which 18 and 19 are now operational, the occurrence and transport of large-scale air pollution can be observed by directly receiving AVHRR data twice a day.

Results and discussions

Mass concentration measurements at the rural site in central Korea

The long-range transport of severe haze and dust storms during winter and spring is classified based on measurements of mass concentrations of different particle sizes and satellite observation. Figure 2 shows the day-to-day variation of daily maximum mass concentrations measured at KCAER between January and May 2014. The average values of daily TSP, PM10, and PM2.5 were 68 ± 32, 49 ± 25, and 29 ± 17 μg m−3, respectively. Mass concentrations of PM10 measured in downtown Cheongju, which is east of the observation site, reached 88 ± 58 μg m−3. Between January and May 2014, there were five anthropogenic dust pollution cases (8 days) and three natural dustfall cases (5 days). Also, TSP mass concentration increased sharply to 344 μg m−3 on May 2, 2014, where pine pollen dispersed into the atmosphere in that area.

Fig. 2
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Variations of daily maximum TSP, PM10, and PM2.5 measured at KCAER from January to May in 2014 (AD refers to a anthropogenic dust pollution case, ND refers to a natural dustfall case, and BG refers to a background airflow case)

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Anthropogenic dust pollution cases from severe haze in eastern China

Eastern China was covered with severe prolonged haze from February 21 to 26, 2014, and then the haze moved to the Korean Peninsula on a synoptic scale (Fig. 3). The total areas under haze reached 2.07 million km2 at the height of its dispersion, covering 11 provinces in northern China (NCC 2014). On February 21 (Fig. 3a), severe haze developed on a large scale in eastern China, located at the rear of a high-pressure system over the Yellow Sea. A NOAA satellite RGB-composite image (1619 LST, February 21) also shows haze occurring in the eastern part of China on a large scale and air pollution originating from north of the Shandong Peninsula expanding eastward with the west-northwesterly airflow. Additionally, the north-northeasterly wind was blowing over toward the Korean Peninsula. The cumulus developed sequentially in the direction of north to south in the southern parts of both the Yellow Sea and Jeju Island. TSP, PM10, and PM2.5 decreased to 43, 31, and 19 μg m−3, respectively, at 1300 LST on February 21 (Fig. 4).

Fig. 3
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a–e Surface meteorological charts (left) and NOAA satellite RGB-composite images (right) showing haze in eastern China and large-scale transport of anthropogenic air pollution (LSTAP) over the Yellow Sea from February 21 to 26, 2014

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Fig. 4
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Variations of hourly TSP, PM10, and PM2.5 mass concentrations measured at KCAER between February 20 and 28, 2014. Gray areas are the background airflow case and anthropogenic dustfall case

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On February 22 (Fig. 3b), a high-pressure system over the Yellow Sea slowly expanded toward the Korea East Sea, subjecting the Korean Peninsula to the influence of a stagnant high-pressure system. Cumulus that developed over the Yellow Sea had dissipated in the NOAA satellite RGB-composite image by 1607 LST, February 22. Furthermore, haze was observed in eastern China at the rear of a high-pressure system, and air pollution in the north of the Shandong Peninsula affected the Korean Peninsula due to northwesterly winds. Under the influence of a continuous stagnant high-pressure system on February 23 (Fig. 3c), haze developed in eastern China, and large-scale air pollution was distributed over the Yellow Sea and the Korean Peninsula. The mass concentrations of TSP, PM10, and PM2.5 gradually increased between February 22 and 23 (Fig. 4).

Eastern China, the Yellow Sea, and the Korean Peninsula were located at the rear of a high-pressure system as the other high-pressure system developing in Mongolia expanded southeastward between February 24 and 25 (Fig. 3d). Haze continuously occurred in eastern China, and large-scale air pollution was distributed from the north of the Shandong Peninsula to the Korean Peninsula in the NOAA satellite RGB-composite image (1358 LST, February 25). On February 26 (Fig. 3e), a low-pressure system moved from southern China to the south of Jeju Island. This resulted in precipitation in the southern part of the Korean Peninsula and caused south-southeasterly airflow into central Korea. The clouds covered as far as central Korea in the NOAA satellite RGB-composite image (1347 LST, February 26). Between February 24 and 26 (Fig. 4), mass concentrations gradually increased and peaked at 1100 LST on February 26. Daily averages of PM10 and PM2.5 exceeded the air quality standard in Korea between February 24 and 26. However, mass concentrations rapidly decreased on the afternoon of February 26 with a south-southeasterly wind.

The observatory was located ahead of, in the centre of, and at the rear of high pressure on February 21, between February 22 and 23, and between 24 and 26, respectively (Fig. 5). In Fig. 4, daily variations of mass concentrations peaked between February 22 and 26 at 1000∼1100 LST as a result of the large-scale transport of anthropogenic air pollution from eastern China. As a rule, cities having more air pollutant emissions show a high level of air pollutants at dawn when the mixing layer height stays low. The mixing layer height shows daily variations where the height starts to rise at 0900 LST, peaking at 1500 LST. Mass concentrations peaked on the surface due to a vertical mixture in the morning when air pollution from eastern China flowed into central Korea. However, after mass concentration reached its peak at 1100 LST on February 23, it sharply dropped at 1500 LST when the height of the mixing layer reached its maximum. It can be argued that the continuous transport of air pollution from eastern China located at the rear of the high-pressure system was delayed since KCAER was located in the centre of a high-pressure system on February 23, and the concentrations again increased on February 24.

Fig. 5
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Variations of hourly pressure and mixing layer height calculated at KCAER between February 20 and 28, 2014. Gray areas are the background airflow case and anthropogenic dustfall case

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Natural dustfall cases due to dust storms

The background airflow case was observed with a surface weather chart and an RGB-composite image on March 13 before the occurrence of the dustfall case in central Korea on March 16∼18, 2014 (Fig. 6). The chart illustrates that the low-pressure system developing along the Yangtze River in southern China moved northeastward toward Japan on March 13 (Fig. 6a). This caused precipitation of 26.0 and 1.0 mm in central Korea on March 12 and March 13, respectively. In addition, north-northeasterly winds swept across the Yellow Sea and the Korean Peninsula on the forward side of an anticyclone expanding over northern China all the way down to southern China. A NOAA satellite RGB-composite image shows that cumulus developed to the south of the Yellow Sea (Fig. 6b). Daily averages of TSP (27 μg m−3), PM10 (22 μg m−3), and PM2.5 (16 μg m−3) were low due to the precipitation and the north-northeasterly airflows on March 13 (Fig. 8).

Fig. 6
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a A surface meteorological chart (1500 LST) and b a NOAA satellite RGB-composite image (1425 LST) showing the north-northeasterly airflow over the Yellow Sea region and rainfall in central Korea on March 13, 2014

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On the other hand, Fig. 7 illustrates meteorological charts and NOAA satellite RGB-composite images showing dust storm observations for March 16∼18, 2014. On March 16 (Fig. 7a), central Mongolia was located on the forward side of an intense anticyclone and at the rear of a low-pressure system. The surface wind was over 10 m s−1 due to a strong pressure gradient. The NOAA RGB-composite image shows dust being blown upward into the atmosphere in central Mongolia. On March 17 (Fig. 7b), a dust storm was located at the rear of a cold front of a cyclone that moved toward the north-northwest of the Korean Peninsula and ahead of an expanding anticyclone. On March 18 (Fig. 7c), as the centre of low pressure was moving toward the northeast of the Korean Peninsula, the cold front lying northeast-southwest passed through the Korean Peninsula to the Korean East Sea. Dust storms moved into the central part of the Korean Peninsula ahead of the high-pressure system with the north-northeast airflow.

Fig. 7
figure 7

a–c Surface meteorological charts (left) and NOAA satellite RGB-composite images (right) showing the occurrence and transport of dust storms (DS) over the East Asian region between March 16 and 18, 2014

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A natural dustfall case lasted 13 h on March 18, and TSP and PM10 mass concentrations rose to 508 and 410 μg m−3, respectively (Fig. 8). However, PM2.5 mass concentrations of 65 μg m−3 did not rise as high as those on other days. The dust loadings on the dusty day show that most of the dust in the atmosphere was distributed with coarse dust particulates larger than 2.5 μm. Daily averages of TSP and PM10 were 211 and 160 μg m−3, respectively, on March 18, thus exceeding the air quality standard of PM10 in Korea.

Fig. 8
figure 8

Variations of hourly TSP, PM10, and PM2.5 mass concentrations measured at KCAER between March 9 and 19, 2014. Gray areas are the background airflow case and natural dustfall case

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Observations of background mass concentrations

The northern part of the Korean Peninsula has less air pollutant emissions than the eastern part of China (Fig. 1). Accordingly, north-northwesterly airflow resulted in a background airflow case when the Korean Peninsula was located in the forward side of an anticyclone. The background mass concentrations at KCAER refer to those cases when particulate matter in the atmosphere has been washed out or rained out by precipitation due to a low-pressure center. Background mass concentrations could be a criterion by which to quantitatively estimate low air quality.

Table 1 shows the daily mass concentrations and a synoptic feature in the background airflow cases within 10 days right before an anthropogenic dust pollution case and a natural dustfall case occurred at KCAER. Daily averages of TSP, PM10, and PM2.5 of the background airflow case prior to the anthropogenic dust pollution case registered 49, 33, and 19 μg m−3, respectively, and the daily averages of TSP, PM10, and PM2.5 prior to the natural dustfall case reached 41, 30, and 18 μg m−3, respectively.

Table 1 Daily average mass concentrations of TSP, PM10, and PM2.5 and major synoptic and weather conditions on the day of background airflow before the anthropogenic dust pollution case and the natural dustfall case occurred from January to May in 2014
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Impact of dust air pollution case on air quality

Table 2 shows daily averages of TSP, PM10, and PM2.5 in an anthropogenic dust pollution case and a natural dustfall case between January and May 2014. The average mass concentrations of TSP, PM10, and PM2.5 reached 120, 93, and 70 μg m−3, respectively, in the anthropogenic dust pollution case. However, average mass concentrations of TSP, PM10, and PM2.5 in dustfall cases registered 162, 118, and 52 μg m−3, respectively. In case 6, the maximum hourly values reached as high as 271, 183, and 89 μg m−3 though daily averages of TSP, PM10, and PM2.5 were low. The case 7 shows higher TSP and PM10 mass concentration levels compared with cases 6 and 8, and the PM2.5 mass concentration levels turned out lower. Accordingly, the PM2.5/TSP daily mass concentration ratio reached 20 %, a level lower than in cases 6 and 8 rated at 45 and 35 %, respectively. The dust storm of case 7 was transported into the Korean Peninsula with north-northwest airflow; thus, this dust storm did not pass through eastern China. While TSP and PM10 mass concentrations in the anthropogenic dust pollution case originating from eastern China were lower than those in the natural dustfall case, the PM2.5 mass concentration was higher in the anthropogenic dust pollution case. The average PM2.5/TSP mass concentration ratio in the anthropogenic dust pollution case reached 58 %, a level higher than 32 % of the natural dustfall case.

Table 2 Daily average mass concentrations of TSP, PM10, and PM2.5 and major synoptic features for anthropogenic dust pollution cases and natural dustfall cases observed at Cheongju from January to May in 2014
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Table 3 compares the anthropogenic dust pollution case and the natural dustfall case with regard to the air quality standard of PM10 and PM2.5 in Korea between January and May 2014. The observed daily PM10 concentrations of 8 days exceed the upper limit value defined in the air quality standard of PM10. The number of days exceeding PM10 concentration standards in anthropogenic dust pollution cases and natural dustfall cases is 4 days each. Furthermore, of the 18 days where the air quality standard of PM2.5 was exceeded, anthropogenic atmospheric pollution cases were observed on 12 days, and natural dustfall cases on 3 days. Air quality in central Korea is affected by long-range large-scale transport of air pollution from China.

Table 3 Number of days when daily mass concentrations exceeded the air quality standards of PM10 and PM2.5 in Korea with the large-scale transport of anthropogenic air pollution and dust storms from January to May 2014
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Conclusions

After an extremely severe haze persisted in eastern and central China in January 2013, another severe haze occurred and continued on a large scale over eastern China during February 2014. Air pollution transported on a wide scale in East Asia was observed through satellites and air quality measurements taken at a background observation site in central Korea. This study observed the occurrence and transport of five anthropogenic dust pollution cases and three natural dustfall cases between January and May 2014, and analyzed the impact of those cases on air quality.

Background airflow cases can be observed in consideration of the distribution of air pollutants and the transport of airflow of synoptic scales. Background airflow cases were observed when the Korean Peninsula was located ahead of an anticyclone and in the presence of a northwesterly airflow, and in the presence of precipitation due to low pressure. The mass concentration in the background airflow cases can be referred to in order to compare dust air pollution cases between January and May 2014.

During this period, the numbers of episode days exceeding the air quality standards of PM10 and PM2.5 were 8 and 18 days, respectively. These episodes were all affected by dust pollution cases. Anthropogenic air pollution occurred when eastern China and the Yellow Sea were located at the rear of a stagnant anticyclone and a migrating anticyclone. This anthropogenic air pollution moved to the Korean Peninsula on a large scale. An anthropogenic dust pollution case with a daily average of PM2.5 concentrations at 70 μg m−3 ran higher than that of PM2.5 in a natural dustfall case. In an anthropogenic dust pollution case, the air quality standard of PM10 was exceeded on 3 days, and the air quality standard of PM2.5 was exceeded on 12 days. Anthropogenic dust pollution cases generally loaded PM2.5 fine dust in large quantities in the atmosphere in central Korea.

On the other hand, natural dust cases that occurred ahead of an anticyclone developed in Mongolia and northern China and moved toward the Korean Peninsula. In natural dust cases, daily averages of TSP, PM10, and PM2.5 registered 162, 118, and 52 μg m−3, respectively, and all exceeded the air quality standards in Korea. In natural dust cases in particular, the number of days was 4 when the PM10 air quality standard was exceeded, while the number of days was 4 when the PM2.5 air quality standard was exceeded. While dust loading varied depending on the strength of dustfall occurrences, the case of January 20 that passed the industrial region in eastern China the PM2.5/TSP mass concentration ratio ran higher than the cases of March 18 and May 27∼20.