Forming High Ozone Concentration in the Ambient Air of Southern Taiwan under the Effects of Western Pacific Subtropical High
Due to the distinct geographical and meteorological conditions of Taiwan, air pollutants concentrations in the ambient air of it may vary with seasons. Accordingly, this study aimed to investigate the formation of high O3 concentration in the ambient air of Southern Taiwan during summers. A high O3 concentration case occurring between June 28 and July 2, 2013, was modeled and analyzed with WRF-Chem meteorological and air quality model. During the investigated period, a typical western Pacific subtropical high (WPSH) covered most East Asia, including Taiwan and its surrounding areas. The observations showed strong correlations between WPSH invasion and forming high O3 concentrations. The dispersion of air pollutants in the ambient air is not sufficient to dilute their concentrations. In the afternoon of June 30, more than 60% of the air quality monitoring stations found O3 concentrations exceeding 100 ppb, which were 2~3 times higher than their normal concentrations. Model simulation results verified that the presence of the WPSH hindered the dilution and transportation of air pollutants in ambient air. In addition, the air quality would be getting worse due to the leeward sides caused by the counter clockwise vertex formed in Southwestern Taiwan.
Well air quality is essential for healthy life. As the countries worldwide, Taiwan has established emission limitations to regulate the release of air pollutants from various air pollution sources . The release of air pollutants often causes poor air quality if they cannot be dispersed effectively in the ambient air. During the summer, the air quality in Southern Taiwan should be higher than it during other seasons. The air pollutant concentrations during summers are only about one-third of those during other seasons. Nevertheless, due to a rapidly changing global climate, increasing instances of unexpected weather conditions with high-concentrated air pollutants have been observed. The changing climate not only alters both global and local weather patterns, but also alters distribution and fates of air pollutants in the atmosphere . However, more and more instances of short-term deterioration in the ambient air quality of Southern Taiwan during summer periods have been observed [3, 4].
Other than air pollutant emissions, distinct geographical and meteorological characteristics are two critical factors dominantly affecting ambient air quality of Taiwan. Although it is a small island, Taiwan is characterized by its special geographical features. For instance, the elevation can increase from sea level to nearly 4,000 m high within a distance of 30~100 km. Because the height of the mountains exceeds 3,000 m and special local circulation systems surround high mountains, the weather patterns in Taiwan are so complex that often importantly affect air pollutant concentrations in various locations and different seasons.
During the summer, the weather patterns around Taiwan are considerably affected by the position and strength of the WPSH. Summarizing the frequency with which the WPSH occurred during the summer from 2003 to 2012, the average occurrence frequency of WPSH was approximately 40% (Table 1). Seasonal shifts and short-term variances in high-level westerly circulation in the northern hemisphere significantly determine the movement and position of the WPSH. When the WPSH is strong, it covers most region of East Asia, including the western rim of the Pacific Ocean extending from 15°N~30°N to 120°E~150°E. Besides, the WPSH may build either to the west of the Pacific Ocean or back from the west, while the westerly ridge moves east . This kind shifting between the east and west of WPSH usually has an average period of 1~2 weeks, which, however, can dominantly determine the weather patterns during its invasion period .
In addition, the southeasterly trade wind, usually located in the south and moving following a subtropical high ridge line, is another important meteorological factor interfering with the summer weather of Taiwan, but the characteristics of the southeasterly trade wind change while passing through the eastern or western side of the WPSH. For instance, as air streams flow through the eastern side of the WPSH, they subside, and their temperature rises but humidity decreases. The lower-level atmosphere near the ocean is usually separated by higher dry and warm air that sinks to the inversion layer . The temperature inversion layer, often deteriorating ambient air quality, may range from hundreds of meters to several kilometers above ground.
Ambient air quality is determined by both emissions of air pollutants and their dispersion patterns in the atmosphere. For investigating their effects on the ambient air quality, many meteorological and air quality models, such as CAMX, SAQM, CALGRID, MODEL3, and MM5, have been applied [8–13]. The influences of physical or chemical processes on both transport and fates of air pollutants in the atmosphere can be evaluated, if both models can be employed effectively. Nevertheless, some transferring efforts are usually found while these models combined with each other and some errors often occur while the simulated results transferred from one model to another. Hence, a new mesoscale weather forecasting model, weather research and forecasting (WRF) model, was developed by the US government and academic groups for reducing the potential problems of model combination and data transformation [14, 15]. Furthermore, considering the pollutants with chemical reactions in the atmosphere, WRF-Chem model was developed for simulating and diagnosing transport and fates of reactive air pollutants [16–19]. Essentially, WRF-Chem is an accurate weather/air quality simulation model that is portable, simple to maintain, extensible, efficient, and convenient to operate [20, 21].
Accordingly, this study aimed to investigate the phenomenon of rapid accumulation of O3 in the ambient air of Southern Taiwan during summer periods. Ground-level ozone (O3), formed by photochemical reactions in the atmosphere, is one of the most concerned pollutants in the ambient air for metropolitan cities. A high O3 concentration case occurring between June 28 and July 2, 2013, was modeled and analyzed. During this period, a typical WPSH covered most of East Asia, including Taiwan and its surrounding areas. The WRF-Chem model was used to simulate the accumulation of O3 in the ambient air of particular locations in Southern Taiwan. The simulated results were also verified with the O3 concentrations measured at air quality monitoring stations. This paper discusses the formation mechanisms for O3 accumulation during the summer in the ambient air in the investigated area, which is beneficial for conducting air quality management policies.
2. Materials and Method
2.1. Weather Systems
This study modeled and analyzed a typical case of a high O3 concentration from June 28 to July 2, 2013. Figure 1 shows weather maps of East Asia during this period, which illustrates the major weather systems around Taiwan. A WPSH system was centered in the atmosphere over Japan and extended down to Taiwan on June 28, 2013. Although a tropical cyclone, Rumbia, was active to the east of the Philippines, the whole Taiwan was mainly under the influence by the WPSH. The west to the west ridge of WPSH covered most area of the East China Sea, including Taiwan, causing the weather in Taiwan to be dry and with little wind. On June 30, Taiwan was covered by the western Pacific high, and poor air quality was observed due to strong subsidence inversion and stable weather. Figure 2 depicts the distribution of observed O3 concentrations in the ambient air of Northeast Asia on June 29 and July 2, respectively. High O3 concentrations persisted in East Asia from June 29 to July 1. After July 1, the WPSH gradually moved away from Taiwan, no longer influencing the weather patterns around the island.
2.2. Initial Data and Model Setting for Simulation
In this study, the WRF-Chem model V3.1  was used to simulate the O3 concentration in the ambient air of Taiwan from June 28 to July 2 (China Standard Time). The initial 48-hour period was set as a preintegration stage for model calculation. Both domain settings and configuration options for WRF-Chem model operation are summarized in Table 2 and illustrated in Figure 3. Three nesting domains were defined by performing Lambert projection for the model operation. The outermost domain (D1), mainly covering east China and centered at 25°N, 125°E, had a grid spacing of 27 km and an overall grid area of 115 × 91. The secondary domain (D2), centered at 16°N, 121°E and featuring grid spacing of 9 km to form an area of 73 × 73, covered the southern part of China and the whole island of Taiwan. The tertiary domain (D3) mainly covered Kaohsiung and Pingtung Counties and was centered at 14.5°N, 120.25°E with an overall grid area of 67 × 58 and grid spacing of 1 km. All grid meshes comprised 28 vertical sigma layers from ground level to the top pressure of 100 hPa (about 10 km high). The initial meteorological fields and boundary conditions were obtained from NCEP FNL. Operational global analysis data was based on 1° × 1° resolution grids prepared operationally for every 6 hr.
2.3. Pollutant Emission Inventories and Air Quality Data
Air pollutant emission inventory data, referred to as TEDS V7.1 obtained from the national emission inventories of the Taiwan Emission Data System, was used for the model simulation. The inventory was conducted to ascertain potential pollution sources in Taiwan, including fixed, mobile, point, face, line, and natural sources . In addition, hourly O3 concentrations from the air quality monitoring stations located in both Kaohsiung and Pingtung Counties were used to verify the simulated results. Kaohsiung City located in the southwest of Taiwan and neighboring Pingtung County. Metropolitan Kaohsiung has the largest population in Southern Taiwan and the largest heavy industries of Taiwan.
As Figure 3 shows, the Taiwan Environmental Protection Agency (EPA) established several air quality monitoring stations within this area, including one background ambient air quality station in Qiaotou (B-1); one industry ambient air quality station in Qianzhen (O-3); two traffic air quality stations in Fengshan (B-4) and Fuxing (B-6); and eight general ambient air quality stations in Meinong (M-1), Nanzi (B-2), Renwu (B-3), Zuoying (O-1), Qianjin (O-2), Xiaogang (O-4), Dailao (B-5), and Linyuan (O-5). In addition, the Pingtung monitoring station (M-2) and the Chaozhou (M-3) monitoring station are located near Kaohsiung City. Among these air quality monitoring stations, the M-1 station, approximately 300–600 m above sea level, is situated in the northeast side of Kaohsiung City and faces Meinong Mountain that belongs to the southern extension of the Yushan Mountain range.
In this study, as shown in Table 3, the air quality monitoring stations in Kaohsiung were divided into three groups according to their location either near the ocean or close to major mountains. Because the sea-land breeze usually affects the regions within a 10~20 km distance from the ocean [25, 26], the air quality monitoring stations located west of 120.24°E, including O-1~O-5, were classified as the “ocean group.” Monitoring stations located east of 120.30°E, including M-1~M-3, were classified as the “mountain group” because they are near major mountains. The third group, the rest of the stations located between 120.24°E and 120.30°E, included B-1~B-6. The O3 concentrations in the ambient air detected by the monitoring stations were compared to evaluate the synergistic effect of the leeward side and the WPSH exerted on forming high O3 concentrations. In addition to geographical factors affecting the air quality at the monitoring stations, nearby human activity also may contribute the air pollutant concentrations at some stations near major industrial parks, such as B-2, B-3, O-3, O-4, O-5, and B-5.
3. Results and Discussion
3.1. O3 Concentration Variation during the Investigated Period
Figures 4–6 show the observed O3 concentration variation profiles and ground wind speeds detected at the ambient air quality monitoring stations from June 28 to July 2, 2013. The O3 concentration peaked at noon daily and progressively increased, beginning on June 28 and reaching peak value on June 30 (the 60th–66th hours). In the afternoon of June 30, more than 60% of the air quality monitoring stations (B-1–B-5, O-1, and O-2) found O3 concentrations exceeding 100 ppb. Stations O-1 and B-4 recorded the highest O3 concentrations of 124 ppb and 116 ppb, respectively, which were approximately 2~3 times higher than the usual O3 concentration. These observations indicated that the mixing conditions of air pollutants in the ambient air were not strong enough to disperse air pollutants while WPSH invading.
As matter of fact, another possible reason for forming high O3 concentration in the ambient air for this case was that Taiwan was close to the saddle field between the front in the north and the typhoon in the south. The saddle field was a weak area where local thermal circulations (land sea breeze) were active. The invasion of WPSH provided air quality getting worse because it depressed the vertical convection which was caused by heated ground. Besides, the southeast wind near the surface caused a clockwise vortex in Southern Taiwan on the event day. The vortex basically limited pollutants moving out of Southwestern Taiwan. That is, the synergistic phenomena contributed by both WPSH and lee-ward side effects resulted in poor air quality within this area.
After July 1 (the 84th~90th hours), the WPSH began to decrease in strength and shifted northeast, coinciding with declining O3 concentrations measured at most air quality monitoring stations. The weather patterns of Southern Taiwan returned to a normal summer weather pattern after July 2. Thus, these observations suggested a strong correlation between the presence of the WPSH over Taiwan and forming high O3 concentrations in the ambient air of its southwest side.
Alternatively, unlike most of the air quality monitoring stations, where decreased O3 concentrations were observed after June 30, the M-1 station continuously detected high O3 concentrations exceeding 80 ppb for an additional 1~2 days until July 2 (see red line and wind vector details in Figure 5) due to the synergistic effects of the leeward side and WPSH. High O3 concentration is often found in the Meinong (M-1) valley on sunny afternoons. During summer, when prevailing southwesterly winds blow, a strong sea breeze tends to push air pollutants from Kaohsiung (southwest) to the Meinong valley (northeast), and air pollutants accumulate in the valley during the late afternoon, known as the sea-land breeze that results from uneven temperature changes in the ocean and land when the sun radiates on them . During the day, temperature increase on land is typically much faster than that in the ocean, creating a strong sea breeze (see details of wind vector in Figure 4) that blows air pollutants inland until blocked by high mountains or hills. By contrast, at night, long-wave radiation cools the land, and the temperature decrease on land is more rapid than that in the ocean. Near-surface winds blow toward the ocean, causing land breeze.
The daytime sea breeze begins with an easterly breeze from the South China Sea generated by the local land-water thermal contrast, and then 1~2 h later, a stronger southerly to southeasterly breeze across the Taiwan Strait prevails. Sea-breeze circulations propelled by the thermal contrast at the coast assume two possible forms including producing a steady 24 h rotation of the wind vector around the larger-scale gradient wind and/or generating rotation manifesting in low-level winds . When the gradient wind is light to moderate, the wind perturbation associated with the diurnal sea-breeze cycle can temporarily counteract the gradient wind to produce a few hours of stagnant wind conditions. When the gradient flow is offshore (from the north or northwest), this stagnant period occurs during the afternoon hours. Generally, in Southern Taiwan, the sea breeze begins at approximately 10 a.m. and shifts to a land breeze near 10 p.m.
The synergistic effects of the leeward side and the WPSH engender low-speed southwesterly winds near Meinong, particularly from June 30 to July 2 (Figure 5). The presence of this type of slow wind speed typically occurs with stable weather and poor air quality. In comparison of the wind speeds recorded at other monitoring stations (see color wind vector, Figures 4 to 6), the wind speed at M-1 was lowest from June 28 to July 2, 2013, when the WPSH had begun to slowly leave Taiwan. Thus, the WPSH caused poor ambient air quality, which deteriorated while coupled with the leeward side effect caused by the particular geographical conditions of the Southern Taiwan.
3.2. Modeling Results
In this study, the WRF-Chem model (D3 grid data) was applied to simulate O3 concentrations in the ambient air from June 28 to July 2, 2013. Figure 7 shows the simulated O3 concentrations and those detected at air quality monitoring stations, indicating that the model simulated results fit the observed data well for most cases, although it also can find that the simulated O3 concentration may be overestimated when observed O3 concentration is low and may be underestimated when observed O3 concentration is very high. Figure 8 shows the wind field maps and O3-concentration contours (according to color) collected in Southern Taiwan during the investigated period. When the WPSH was present, hindering air pollutant dispersion, high O3 concentrations formed in the ambient air across most of Southern Taiwan, particularly in areas strongly affected by the leeward side and WPSH. The O3 concentrations peaked above 100 ppb in the afternoon of June 30, exceeding the concentrations throughout the rest of Kaohsiung (Figure 9(c)). When the WPSH moved gradually east after July 1, diffusion and transportation of air pollutants improved, causing the O3 concentration to slowly decrease to 35~40 ppb (Figure 9(d)) after July 2. The O3 concentration decreased by more than 50% once the WPSH shifted away from Taiwan (Figure 9(e)).
(a) 8 a.m., June 28, 2013
(b) 8 a.m., June 29, 2013
(c) 8 a.m., June 30, 2013
(d) 8 a.m., July 1, 2013
(e) 8 a.m., July 2, 2013
(a) 2 p.m., June 28, 2013
(b) 2 p.m., June 29, 2013
(c) 2 p.m., June 30, 2013
(d) 2 p.m., July 1, 2013
(e) 2 p.m., July 2, 2013
In addition, it was found that a closed subtropical high formed below the geopotential height (gpm) of 5,880 m in the atmosphere above the western Pacific Ocean (Figures 8(a)~8(e)). Southeasterly winds formed when the subtropical high edge formed to the west of Taiwan near the East Sea (June 29). By contrast, when the edge of the subtropical high was situated in the atmosphere above Taiwan, southerly winds blew (June 30). Because of warming caused by a descending draft, the weather was dry, humidity was low, and the baric gradient was so weak with nearly no wind. The temperature was sufficiently high to cause subsidence inversion . After June 30, the WPSH began to leave Taiwan, shifting northeast. Only the east side of Kaohsiung remained affected by the subtropical high, and therefore, the weather conditions in the east were stable and dry with low humidity and weak wind (Figures 6 to 8).
The aforementioned meteorological phenomena verify that the presence of the WPSH substantially influences the dilution and transportation of air pollutants in ambient air, particularly in the locations affected by the leeward side and the WPSH. The simulated results from gpm below 500 hpa (Figures 8(a)~8(e)) and concentration distributions of O3 (Figures 9(a)~9(e)) indicated that the peak O3 concentration occurred in the western coastal area of Kaohsiung in the afternoon of June 30 (see details in Table 3), during which time a southeasterly blew toward Eastern Taiwan, resulting in a subsidence temperature inversion in western Taiwan (see blue color pattern in Figures 9(b)~9(c)). Thus, poor air quality was found.
3.3. Spatial Distribution of O3 in the Ambient Air While WPSH Moving
The presence of WPSH is typically accompanied by weather conditions for the accumulation of air pollutants, such as, stream downdraught, temperature inversion, rare cloud coverage, short-wave radiation, low relative humidity, and slow wind speed. Some of them were shown in the afternoon of June 30, 2013. WPSH was more likely to accumulate O3. In this investigated case, peak O3 concentrations were observed at every monitoring station except the M-1 station where they occurred in the afternoon of June 30. Therefore, the 24 h O3 concentration monitoring data collected throughout the day were compared using a correlation coefficient comparison (see Table 4). During the period for the O3 concentration reaching peak high, the correlation coefficients between O-1 and the other stations were usually higher than 0.6, suggesting being highly related. For example, at O-2 on June 28, the highest O3 concentration was 102 ppb. Because the wind blew from the west and northwest, the highest O3 concentrations at O-2 and B-6, which were located downwind, were observed an hour later. On the other hand, for station O-5, relatively poor correction was observed, which may result from the brink of a river to the sea. Sea breeze effect is more obvious than other locations.
Regarding the M-1 monitoring station in a rural area, after analysis, high correlation coefficients remained among urban monitoring stations (Table 4). However, most correlation coefficients of O-5 and some stations were below 60%. M-2 exhibited the lowest coefficient of 0.48.
The sea breeze affected the wind speed (3 m/s) at the O-5 station, and a swift wind blew from the south and southwest preventing O3 from accumulating. However, data showing O3 concentrations exceeding 100 ppb at each station on June 30 suggested that, under the effects of WPSH, the high temperature, low relative humidity, and lack of clouds caused the surface to receive a higher rate of short-wave radiation . A temperature inversion occurred to cause accumulation of O3 in the near-surface ambient air. Checking the skew-temperature diagrams as shown in Figure 10, strongly dry downdraft for high-level atmosphere and temperature inversion (see Figure 10 O-1) in the near ground ambient air of the O-1 station in the afternoon of June 30, which can result in the accumulation of air pollutants in the near-surface ambient air, can be found. On the other hand, in the atmosphere above the O-5 station, although it is also strongly dry downdraft in high-level streams, its temperature inversion is not so significant, causing its O3 concentration to be not so high in the afternoon of June 30 (see Figure 10 O-5).
(1) O3 Concentration Variation in the Ambient Air near Ocean. Table 3 summarizes the O3 concentrations in Kaohsiung and Pingtung at B-5, O-1, B-2, and O-4, respectively. The lowest O3 concentration was observed in 48 hours before June 30, less than 30 ppb, and occurred in the afternoon on the west and northwest sides of Kaohsiung (B-6, B-3). 48 h later, the lowest concentration occurred on the southwest side (O-5, O-4). This phenomenon was caused by both sea-breeze effects and the weakening of WPSH. The wind blew from the west during the day, but it blew from the south during the night, resulting in high O3 concentrations formed for the stations downwind. The stations located downwind and near industrial and urban districts detected substantial O3 diffusion.
(2) O3 Concentration Variation in the Ambient Air near Mountains. During the afternoons of June 29 and 30 and July 1, the O3 concentration observed at the M-1 monitoring station reached 80 ppb (20% higher than average), because the station was located within leeward affecting area (refer to Figures 9(b)~9(c) M-1 point wind vector). In the afternoon of Jun. 29, the highest O3 concentration was 68 ppb (Figure 9(b)) because the WPSH shifted east. The weather remained warm with low relative humidity (below 50%) in the afternoon of July 1, and a weak wind blew from the west (approximately 1 m/s). Meanwhile, because of the wind direction, the highest O3 concentration at the station was 81 ppb in day time, which is 15 times higher than the lowest O3 concentration (5.4 ppb; Table 2). From 3:00 p.m. to 4:00 p.m. on July 2, stronger winds (exceeding 4 m/s) blew from the southwest, enabling greater diffusion; therefore, the highest O3 concentration was 56 ppb, which is 19 times higher than the lowest O3 concentration that day (5.6 ppb).
The delayed decrease in the O3 concentration at M-1 resulted from the synergistic effect of the leeward side and the WPSH. High O3 concentrations occur frequently in the Meinong valley in sunny afternoons. During a typical summer, when southwesterly winds blow, a strong sea breeze tends to deliver air pollutants from Kaohsiung (west) to the Meinong valley (east), and O3 accumulates in the valley during the late afternoon. A strong sea breeze blows air pollutants inland until blocked by high mountains (Figure 5). By contrast, at night, near-surface winds blow toward the ocean, causing land breeze (Figure 9). Again, the simulated skew-temperature diagram for M-1 station indicated the occurrence of temperature inversion in the near-surface below 1 km in the afternoon of June 29 (see Figure 10 M-1). On July 1, high level with significant turning of winds in 5000 m caused near-surface winds blowing toward the mountains and resulted in the accumulation of O3 in ambient air (Figure 9(e)).
(3) O3 Concentration Variation in the Ambient Air between Ocean and Mountains. At B group stations (B-1–B-6), which are located in middle Kaohsiung areas, the O3 concentrations in the Kaohsiung and Pingtung areas were recorded during the investigation period (Table 2). 48 hours before June 30, the lowest O3 concentration, below 30 ppb, was recorded in the afternoon on the west and northwest sides of the Kaohsiung area (Figure 6). On the same day, the highest O3 concentration for B-4 was 116 ppb, which is 23 times greater than the lowest concentration (5 ppb, see details in Table 3). The temperature was high, the relative humidity was low (less than 65%), and the wind blew weakly from the west (<1.5 m/s). As the WPSH prevailing, O3 formed in the ambient air of both Kaohsiung and Pingtung due to strong photochemical reactions (Figure 9(e)).
This study demonstrated that WPSH is typically accompanied by weather conditions for the accumulation of air pollutants to result in poor air. This study used WRF-Chem model to analyze a high O3 concentration case occurring in the ambient air of Southern Taiwan between June 28 and July 2, 2013. During this period, a typical WPSH covered most of East Asia, including Taiwan and its surrounding areas. Some important findings in the study are summarized as below.
During WPSH invasion period, more than 60% of the air quality monitoring stations of Southern Taiwan found O3 concentrations above 100 ppb, suggesting strong correlations between the presence of the WPSH over Taiwan and forming high O3 concentrations in the ambient air of its southwest side. The model simulated results verified the presence of the WPSH hindering the dispersion of air pollutants in ambient air. Air quality gets worse when the leeward side effects are strong. The synergistic effects of the leeward side and the WPSH engender low-speed southwesterly winds near Meinong, particularly from June 30 to July 2 (Figure 5). The presence of this type of slow wind speed typically occurs with stable weather but poor air quality. Under WPSH weather conditions, the formation of strongly dry downdraft for high-level atmosphere and near ground temperature inversion resulted in the accumulation of air pollutants in the ambient air. The O3 concentration during the WPSH invasion may reach 2~3 times higher than its concentration in summer without WPSH invasion.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank both the Ministry of Science and Technology, Taiwan, and the National Kaohsiung First University of Science and Technology for financially supporting this research under Contract no. NSC 102-2221-E-327-002-MY3 and A23200.
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