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Ozone distribution in coastal central Taiwan under sea-breeze conditions
Atmospheric Environment 36 (2002) 3445–3459
Ozone distribution in coastal central Taiwan under sea-breeze conditions Wan-Li Cheng* Department of Environmental Science, Tunghai University, Taichung 407, Taiwan ROC Received 22 November 2001; accepted 14 April 2002
Abstract Sea-breeze circulation influences ozone concentration differently for two types of synoptic winds across the westcentral coastal plain of Taiwan. During summer months, when the synoptic flow is southerly, a strong westerly seabreeze confined to the lower 700–800 m produces a northeastward flow. Ozone concentration is low, with a core centered at about 100 m, leading to moderate ozone concentrations in the low population density northeastern foothills. In autumn (and spring), northerly synoptic winds of about the same strength combine with less energetic westerly sea breeze to produce a southeastward flow that carries higher ozone levels into the heavily populated Taichung basin. Ozone levels are high from 80 to 400 m, with a core up to 120 ppb from about 150 to 300 m, contributing to serious ozone episodes at the southern (downwind) end of the basin. Analyzing the backward trajectories and ozone concentration showed that the weak southeastward breeze is the dominant factor affecting the occurrence of high ozone events in the region. The horizontal distribution is based on 3 yr data obtained from a network of air-pollution monitoring sites in the study area, while the vertical data comes from two 2-day tethersonde experiments, measuring wind-speed, direction, temperature and humidity, NO, NO2, NMHC and O3, conducted during August and November 1999. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ozone concentration; Sea-breeze circulation; Tethersonde experiment; Ozone spatial distribution; Backward trajectory
1. Introduction Ozone (O3) in the boundary layer is the product of chemical reactions involving primary pollutants from the surface. NOx oxides and volatile organic compounds (VOCs) [specifically non-methane hydrocarbons (NMHC)] from industrial and traffic emissions are converted by strong solar radiation (lo424 nm) to secondary pollutants. NO2 is converted by strong solar radiation to NO and atomic oxygen O, after which O and O2 form O3: NO2 þ hn-NO þ O
ð1Þ
O þ O2 þ M-O3 þ M;
ð2Þ
*Fax: +886-4-235-95941. E-mail address: [email protected] (W.-L. Cheng).
where hn denotes a photon and M returns the molecule to its original state. The chemistry of the metropolitan and industrial boundary layer is much more complicated due to the presence of VOCs (alkanes, alkenes, aromatic hydrocarbons, etc.). For example: RH þ OHd -Rd þ H2 O
ð3Þ
Rd þ O2 þ M-ROd2 þ M
ð4Þ
ROd2 þ NO-ROd þ NO2 :
ð5Þ
The alkyl radical R and H atom in the alkane is susceptible to hydroxyl radical OH attack. OH is available independent of altitude. The reaction mechanism for O3 also involves NO conversion to NO2, while NO2 plays a central role in the formation of O3.
1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 3 0 7 - 2
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Ozone is a highly reactive oxide. Its cumulative concentration is influenced by solar radiation, temperature, humidity and wind. Most studies show that high solar radiation, low humidity and low wind speed all enhance ozone accumulation (Tsuang et al., 1998; Xu et al., 1997; Liu et al., 1994). Ozone concentrations at any one location also depend significantly on advected emissions and vertical mixing rates and consequently on the local variation of the wind field and parcel trajectories. Well known studies have been done in the United States (Holland et al., 1999; US EPA, 1998) and Tokyo (Wakamatsu et al., 1999), in the Yahagi basin (Kitada et al., 1986), at Los Angeles (McElory and Smith, 1986), in Southern Ontario (Hastie et al., 1999), and Athens (Kambezidis et al., 1998). Many studies indicate that the sea-breeze transports the pollution from coastal and urban areas inland and spreads ozone to an onshore distance of 20–60 km (Hastie et al., 1999; Seinfeld and Pandis, 1998; Kambezidis et al., 1998; Kitada and Kitagawa, 1990). Liu et al. (1994) have investigated the impact of the sea– land breeze on the horizontal distribution of ozone concentration in Taipei and Kaohsiung in Taiwan. They found high ozone pollution located downwind of the sea breeze. This paper analyses the area of coastal central Taiwan, having dimensions of 50 60 km2, as a part of the Central Taiwan Air-Quality Management Program (CTAMP). The study here extends work on the effect of winds on sulfur concentration (Cheng, 2001a) to their effect on ozone (Cheng, 2001b).
throughout most of the year, sea breezes occur on more than half of all days. The data for this study is based on an analysis of air quality and meteorological data from a number of sources: the horizontally distributed data comes from nine Air Quality Monitoring Stations run by Taiwan EPA, shown on the map (Fig. 1), ten stations run by Taichung Power Plant (not shown, to maintain clarity),
2. Data collection Taiwan is influenced by the northerly monsoon from autumn to spring months and the southerly one in summer months. A sea breeze is superimposed on these flows when conditions are suitable to produce daytime northwesterly or southwesterly flow, respectively. The northwesterly surface flows (autumn, denoted by N in later figures) are weak, but increase in speed from almost calm up to about 2 m s1 and remain steady for 5–6 h throughout the afternoon, with directions between 2701 and 301. The summer flows show stronger sea breeze, with the speeds increasing from o2 up to 4 m s1, and directions southwesterly confined to the sector 202–2701 (denoted by S in the later figures and tables). Since temperature contrasts between land and sea drive the sea breeze, the latter are strongest when the land is strongly heated, i.e. under cloudless skies and calm conditions (Simpson, 1994). The sea breezes generally start during late morning and may extend into late afternoon. Because of the location of coastal central Taiwan and the great number of cloudless days
Fig. 1. Location of monitoring stations in westcentral Taiwan (Tonhsiao Power Plant located 25 km north of Fengyuan).
Table 1 Frequency of HODs during sea-breeze days in westcentral Taiwan in the period 1997–1999 Site
Sea-breeze day (N)
High-ozone day (HOD)
Frequency of occurrence (%)
Fengyuan Shalu Tali Chungming Hsitun Changhua Erhlin Nantou Chushan
324 312 319 327 322 326 329 308 273
142 88 198 149 86 65 90 189 203
43.8 28.2 63.5 45.6 26.7 19.9 27.4 61.3 74.4
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Fig. 2. Wind field and O3 concentration at (a) 08:00, (b) 11:00, (c) 14:00 and (d) 17:00 on 28 August 1999.
and three by the Central Weather Bureau. One of these stations was 1.5 km offshore and provided the sea temperature data. This was used in conjunction with the Taichung city air temperature as a sea-breeze
potential indicator. The horizontal data set covers the period 1997–1999. The vertical distribution of ozone and its precursors, and wind speed and direction were measured with a
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tethersonde system. The measurements were done up to about 1200 m, to yield data that is substantially free of immediate local sources and dry deposition. Two tethersonde experiments on 28–29 August 1999 with a typical summer southwesterly sea breeze and 5–6 November 1999 with a typical autumn northwesterly sea breeze were made at a coastal site (Longjin) and an inland site (Tsaotun) about 15 km south of Taichung city in the center of the basin. The tethersonde was manufactured by Atmospheric Instrumentation Research, Inc., USA. It consists of a balloon-borne rawinsonde for measuring pressure, temperature, humidity, wind direction and speed (Hoff et al., 1995). The sonde was supported by a spherical 3 m3 hydrogen balloon, tethered on a Kevlar line (760 lb) and controlled by an electric capstan. On the ground were additional pollutant (O3, NOx and
NMHC) analyzers. Four air samplers, each with a pair of 10-l tedlar sampling bags, were installed on the tethered line. Each of them was fitted with a timer to control the operation and duration of action of the suction pump. Once the uppermost sampler reached the designated height, all the samplers were switched on to simultaneously pump air into the bags. After the sampling was completed, the balloon was retrieved and the samplers with the bags were collected for analysis of air pollutants. A detailed description of the sampler and its use in several similar CTAMP experiments can be found in Cheng (2000) and Cheng et al. (2001). The instrumentation and the methodologies used to monitor the CTAMP vertical profile data sets have been described elsewhere (Tsuang et al., 2000; Cheng et al., 2001).
Fig. 3. Vertical profiles of virtual potential temperature monitored at Longjin and Tsaotun in westcentral Taiwan, 28–29 August 1999.
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3. Results A high ozone pollution day (HOD) is defined here as a day with a maximum ozone concentration >80 ppb. The remaining days are non-HODs (NHOD). Measurements during the 3-yr period indicate that high ozone concentrations in excess of 80 ppb are frequently reached in the spring and autumn months, especially during low wind speed days, which favor the generation of sea-breeze cells. The frequency of HODs during northwesterly sea-breeze days in 1997–1999 is shown in Table 1. Among all the stations, Chushan has the highest number of HODs with 203 out of 273 sea-breeze days (74.4%), followed by Tali (63.5%) and Nantou (61.3%). All three stations are located in the southern part of the basin, while the stations near the coast, such as Shalu (28.2%), have a very low percentage. The
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lowest corresponds to Changhua (19.9%), located in the gap between Tadu Hill and Baqua Hill. Here the wind speeds were slightly higher (20%) than adjacent stations. During the southwesterly sea-breeze days, the air quality over the whole area of coastal central Taiwan is reasonably good with O3 concentrations below roughly 40 ppb. For the tethersonde data, two consecutive days were monitored in two experiments: 28–29 August 1999 with typical summer sea-breeze conditions, and 5–6 November 1999 with typical autumn sea–breeze circulations. The meteorological conditions over the whole area for these 2-day periods were analyzed, and the vertical profiles of ozone and nitrogen oxide were examined, including their diurnal variation. Of particular interest is the difference in the evolution of the profiles in experimental periods.
Fig. 4. Vertical profiles of wind field monitored at Longjin and Tsaotun in westcentral Taiwan, 28–29 August 1999.
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3.1. Experimental campaign I During 28–29 August 1999 (summer), the synoptic pattern over Taiwan showed an extended Pacific Anticyclone. The western extension of the anticyclone directed a SE–SW wind over Taiwan. The anticyclone center was relatively weak. There were clear sea-breeze circulations. 28 August 1999 was a cloudless day with solar radiation reaching 16.65 MJ m2. The land temperature at Taichung city ranged from 24.51 to 32.11C. This gave maximum sea–land temperature differences of 4.01C (the sea temperature showed a 0.61C range over the day from a minimum of 27.81C at 04:00 h). All times hereafter are referenced to Taiwan Standard Time, which is 8 h ahead of UT. The second day was also cloudless; solar radiation reached 16.27 MJ m2, and the largest sea–land temperature difference was 3.91C at 13:00 h. On the afternoons of 28 and 29 August, the
sea-breeze inflow reached altitudes in the range of 700–800 m. At ground level, the O3 concentration was highest at the northeast corner of Fengyuan. Applying US EPA’s Mesopuff II model (US EPA, 1994), to the data from all stations mentioned previously, simulation codes for the wind field and O3 concentration were obtained. These are shown in Fig. 2 (using transform) for the horizontal distributions, and in Fig. 7 for vertical section of winds. Figs. 2 and 4 clearly show that a significant southerly land breeze dominated the region before sunrise from ground level upwards between 03:00 and 06:00 h. At ground level, the southerly land breeze abruptly died away after 06:00 h. Subsequently, a sea breeze started at coastal Longjin at 10:00 h and later reached inland Tsaotun at 12:00 noon to produce a significant westerly flow until 18:00 h. The wind speed in the entire boundary layer was below 3.0 m s1 with variable wind directions. From 03:00 to
Fig. 5. Vertical profiles of O3 concentration monitored at Longjin and Tsaotun in westcentral Taiwan, 28–29 August 1999.
W.-L. Cheng / Atmospheric Environment 36 (2002) 3445–3459
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Fig. 6. (a) Vertical profiles of NO and NO2 concentrations monitored at Longjin in westcentral Taiwan during 28–29 August 1999, (b) vertical profiles of NO and NO2 concentrations monitored at Tsaotun in westcentral Taiwan, 28–29 August 1999.
18:00 h, the surface wind direction swung clockwise from S to W to NW. This is a typical sea–land breeze circulation for the season. The pattern was similar on 29 August (Fig. 4), but of greater vertical extent. The whole region was covered by an ozone concentration field below 20 ppb (Fig. 2). At 11:00 h, once the sea-breeze developed, the concentration climbed to 40 ppb in the east half of the region, particularly in the northeast corner. This situation was maintained until 14:00 h. By 17:00 h the ozone level gradually declined to o20 ppb. The profiles of virtual potential temperature and wind field (Figs. 3 and 4) do not provide clear indices to estimate the height of the mixing layer, hence the method proposed by Holzworth (1967) has been adopted. The mixing height in the early afternoon was about 900 m at Tsaotun on 28 August and 800 m on 29 August. The vertical profiles of virtual potential temperature remained uniform throughout the lower 500 m height during the daytime. There were clear nocturnal inversions overnight both at Longjin and Tsaotun (shown in Fig. 7. Backward trajectories of Tali, Nantou and Chushan for 29 August 1999 (the numbers below the backward trajectories indicate the time at each location).
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Fig. 8. Wind field and O3 concentration at (a) 08:00, (b) 11:00, (c) 14:00 and (d) 17:00 on 5 November 1999.
Fig. 3). In this latter period the O3 concentration decreased to 10 ppb at ground level (Fig. 5). As the morning progressed, the mixed layer depth increased to around 800 m by 12 noon on 28 August while the O3
concentration steadily increased to over 40 ppb. The layer continued to thicken to 1000 m until 15:00 h. By then the sea breeze was well established, its inflow extending to a height of 800 m and to the foothills.
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Thereafter, the layer depth decreased slowly to 150 m at midnight, wind speeds fell to calm and the O3 concentration was reduced to 15 ppb. During the daytime, particularly during peak traffic hours, there was a great amount of NO and NO2 emission from both traffic and industry. This makes NO and NO2 concentrations higher at ground level, decreasing with altitude. Fig. 6(a) and (b) show that the NO and NO2 are higher during the day and lower at night. This decrease in the local maximum of ozone concentrations would be explained from the increased presence of NO and NO2 depleting the ozone. The slightly higher concentrations of NO and NO2 at Longjin compared to Tsaotun are because the former is an industrial area while the latter is a rural site. At Longjin, the average NO and NO2 on the experimental days ranged between 6 and 9 and 6 and 18 ppb, respectively, whereas at Tsaotun they ranged between 6 and 9 and 6 and 12 ppb,
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respectively. Ozone is depleted by NOx, predominantly NO: O3 þ NO-NO2 þ O2 :
ð6Þ
The NO is already present in the well-mixed atmosphere before radiative cooling of the surface starts. As this reaction proceeds and NO2 concentration increases, the reaction O3 þ NO2 -NO3 þ O2
ð7Þ
can further deplete ozone, although its reaction rate is much slower than (6). Reaction (6) then effectively depletes ozone within the lower nocturnal inversion layer (Wang, 1999; Seinfeld and Pandis, 1998). The same pattern was repeated on 29 August. Both days showed significant changes to the horizontal and vertical patterns of O3 concentration. Because the O3 concentration was not particularly high, it is suggested
Fig. 9. Vertical profiles of virtual potential temperature monitored at Longjin and Tsaotun in westcentral Taiwan, 5–6 November 1999.
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that these days were only moderately chemically active due to sea-breeze ventilation reducing the reactions between NO, NO2 and NMHC. The NMHC profiles are not presented here due to failure to meet the standard US EPA procedures of quality assurance and control. Backward trajectories provide a better picture of the sea breeze and connection between the emission source and the monitoring stations (Sirois and Bottenheim, 1995). An approach, based on the corporate Barnes objective method for interpolating spatial values and the variation-kinematical model (Yu and Chang, 2000), was adopted for correcting the effects of complex terrain, to produce hourly wind field data by using data from the 31 stations over the region. By utilizing the generated hourly wind fields at 200 m high (US EPA, 1994), backward trajectories were simulated from Tali, Nantou and Chushan. The initial time of the backward trajectories for each station was set at 15:00 h, as the hour that the maximum ozone concentration occurred.
On 29 August, the coastal wind was southwesterly and the backward trajectories show a movement within 4 h because of relative strong sea-breeze effects (Fig. 7). An additional explanation could be that the transport route for air parcels was over the southern coast and therefore along a path without many NOx or NMHC sources. The high NOx and NMHC emission inventory sources are along the northern coast and metropolitan areas of central Taiwan. 3.2. Experimental campaign II On 5 November, a continental anticyclone moved eastwards over the East China Sea with its center located northeast of Taiwan. Weak synoptic northeasterly winds covered the area for the experimental period. But over the study area, in the lee of the Central Ranges, they were light and very variable (Cheng et al., 2001). The weather was quite cloudy with solar radiation reaching
Fig. 10. Vertical profiles of wind monitored at Longjin and Tsaotun in westcentral Taiwan, 5–6 November 1999.
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10.20 MJ m2 for 5 November and 10.26 MJ m2 on the next day, about 60% of campaign I. The land temperature at Taichung city ranged from 21.81 to 29.01C. This gave maximum sea–land temperature differences of 5.61C (the sea temperature showed a 0.71C range over the day from a minimum of 22.51C at 06:00 h). Therefore, there was a relatively large temperature difference between land and sea: 5.61C compared to 4.01C in experimental campaign I. However, the wind speed was only 2.0 m s1 during the sea-breeze development, compared with 4.0 m s1 in August. The horizontal distribution of O3 and modeled wind field is shown in Fig. 8. At ground level, the early morning was calm up until 9:00 h. A sea-breeze gathered strength from 12 noon to form a clear surface westerly in the afternoon hours between 12:00 and 18:00 h. The wind speed in the entire 800 m boundary layer was below
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2.5 m s1 and wind direction varied with height: northwesterly up to 400 m at 12:00 noon, and westerly above. At 15:00 h, at the surface, it was WNW up to 800 m, and SW above. By 18:00 h, it was NW, weaker and shallower to 200 m, calm to 600 m and S above. The wind direction swung noticeably anticlockwise from NW to W to SW and S. This is a typical sea–land breeze circulation for the season. The pattern reappeared again on 6 November (Fig. 10). Ozone levels, for the period were as follows. At 08:00 h on 5 November, the entire region had an ozone concentration roughly below 20 ppb. At 11:00 h, once the sea-breeze developed, the concentration climbed to 40 ppb, particularly for two areas. One is immediately downwind of the Taichung power plant and the other possibly from coastal advection from the Tonhsiao Power Plant further north. These two areas had a concentration over 80 ppb. This indicates that the heavy
Fig. 11. Vertical profiles of O3 concentration monitored at Longjin and Tsaotun in westcentral Taiwan, 5–6 November 1999.
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ozone concentrations were directly related to the thermal power plants with heavy NO and NO2 emissions, which are the two main sources of NO and NO2 pollutants in the region. By 14:00 h, the sea-breeze transported all NO, NO2, NMHC and O3 to the downwind southern corner, resulting in ozone levels there over 110 ppb. Then the ozone concentration decreased to 50 ppb by 17:00 h (Fig. 8). This air pollution dispersion pattern was also discussed by Chang (2001) in his computer modeling results. The vertical profiles of virtual potential temperatures remained uniform up to 400 m height during the daytime. There were also significant nocturnal inversion layers both at Longjin and Tsaotun, as seen from Fig. 9. The diurnal profile changes of O3 were consistent with the diurnal cycle of the wind. The mixed layer was thin (about 150–250 m) in the early morning of both 5 and 6 November. It gradually rose to 500–600 m by 12 noon. O3 concentration climbed to over 110 ppb, with the highest value at 150–300 m. This indicates that O3 dispersion was limited by the inversion above 400–500 m (Figs. 9–11). These two days were typical for autumn HODs in the region, with the second day giving higher values than the first.
Fig. 12(a) and (b) show that the concentrations of NO and NO2 were higher during the daytime and decreased at nighttime both at Longjin and Tsaotun. As a whole, coastal Longjin is higher than the inland site Tsaotun, which has been already discussed in the experimental campaign I in Autumn. The vertical profiles of NMHC during the campaign period ranged between 0.4 and 4.8 ppm at Longjin and between 0.4 and 1.0 ppm at Tsaotun, are well mixed and slightly higher in nighttime than daytime (Fig. 13). With regard to NO, NO2 and O3 concentrations, NMHC began to decrease from late morning to afternoon. Their inter-relation can be seen in Reactions (1)–(5). The existing NMHC, with abundant NO and NO2, in the boundary layer could cause high ozone pollution in metropolitan and downwind areas. It is noted that the extremely high NO, NO2 and NMHC at a height of 400–500 m at 15:00 h at Longjin were due to wind blowing the pollutants from nearby stacks (250 m tall) of the Taichung power plant. Following the approach of Milford et al. (1994), no consistent association was found between the sensitivity of ozone to reductions in NMHC versus NOx emissions in this study. The ratios of NMHC/NOx at Longjin are
Fig. 12. (a) Vertical profiles of NO and NO2 concentrations monitored at Longjin in westcentral Taiwan, 5–6 November 1999, (b) vertical profiles of NO and NO2 concentrations monitored at Tsaotun in westcentral Taiwan, 5–6 November 1999.
W.-L. Cheng / Atmospheric Environment 36 (2002) 3445–3459
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Fig. 13. Vertical profiles of NMHC concentration monitored at Longjin and Tsaotun in westcentral Taiwan, 5–6 November 1999.
widely scattered between 5 and 30 with R2 ¼ 0:240; and at Tsaotun between 5 and 65 with R2 ¼ 0:016: This inconsistency could be due to high air pollutants as well as complicated meteorological conditions in the Taiwan region, as discussed in a recent study by Lu and Chang (1998). It is interesting to find that the NO had a maximum of 66 ppb from the ground up to 200 m, NO2 had a maximum of 36 ppb at 100 m and NMHC had a high maximum of 4.8 ppm from the ground up to 100 m at Longjin on the early morning of 5 November. The noon monitored high ozone concentration at Longjin and even higher over 110 ppb at downwind Tsaotun. Since NO, NO2 and NMHC are precursors of O3, the vertical profiles of NO, NO2, NMHC and O3 show a high correlation in this experiment. This concurs with the results of Aneja et al. (2000), Gusten . et al. (1998) and Neu et al. (1994), in which ozone and its precursors may be stored aloft at nighttime and mixed downward to the
ground in the morning as the surface is heated and the nocturnal inversion breaks up. The vertical profiles of the wind show that a significant sea breeze started at 12 noon on both days. The height of the sea-breeze inflow was about 400– 500 m, lower than that of the summer experiment. Under autumn conditions, ozone concentration increased by up to 60 ppb within 2–3 h. These increases occurred simultaneously with the sea-breeze development [identified from both ground (Fig. 8) and high altitude winds (Fig. 10)], which indicate that polluted air masses from industrial and traffic sources are transported inland by the sea breeze. The air parcels travel long distance from the northern coast and metropolitan areas where the main NOx and NMHC sources are located. With the weak sea breeze, the polluted air parcels have sufficient time to stagnate in the high anthropogenic activity areas and form high ozone concentrations. Additionally, the backward
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(61.3%) and Chushan (74.4%). The results of this study indicate the importance of advection patterns and inversions when formulating pollution abatement strategies for sites with frequent sea breezes, such as the west-central coastal plain of Taiwan.
Acknowledgements
Fig. 14. Backward trajectories of Tali, Nantou and Chushan for 5–6 November 1999 (the numbers below the backward trajectories indicate the time at each location).
trajectories show long transport times of two days (Fig. 14).
4. Conclusions Seasonal sea breezes have different influences on the horizontal distribution of ozone concentration in coastal central Taiwan, due to their different directions and speeds. In summer months, the sea-breeze arrives from the Taiwan Strait, heading inland (eastwards) across central Taiwan. Due to its relatively high speed and thick vertical structure, it is mixed vertically up to 700– 900 m. The clear air from the sea entrains polluted air and moves eastwards, bringing clean air to the region, although the northeastern corner at Fengyuan occasionally gets some ozone, where the flow is constrained by the Central Ranges. During autumn, the sea-breeze enters central Taiwan from NNW of the Taichung basin. It is characterized by low wind speed and a shallow depth; this breeze tends to be steered more by the terrain and not as well mixed vertically, being capped by a stronger inversion. It also shown by the analyses of backward trajectories that the weak northwesterly sea-breeze plays an important role in producing high ozone concentrations in the downwind areas, such as Tali (the frequency of occurrence 63.5%), Nantou
The author would like to thank the National Science Council of Taiwan ROC (NSC-89-EPA-029-001), the Environmental Protection Administration of Taiwan ROC (EPA-89-FA11-03-231) and the Environmental Protection Bureau of Taichung city for funding this research and the Taiwan Power Company and the Central Weather Bureau for their co-operation in providing air quality and meteorological data. Special thanks go to Prof. L. Leslie of University of New South Wales and Prof. B.-J. Tsuang of ChungHsing University of Taichung for their stimulating ideas. The author is grateful to Dr. J. Bennett of Flinders University of south Australia for reading the draft of this paper. My appreciation also extends to Miss J.-L. Bai, Dr. G. Cheng and Mr. K. Cheng for their effective preparation and execution of the research, as well as their kind secretarial assistance.
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variation in near-surface ozone concentration—a case study. Boundary Layer Meteorology 69, 221–247. Seinfeld, J.H., Pandis, S.N., 1998. Atmospheric Chemistry and Physics. Wiley, New York, 1326pp. Simpson, J.E., 1994. Sea Breeze and Local Winds. Cambridge University Press, Cambridge, 234pp. Sirois, A., Bottenheim, J.W., 1995. Use of backward trajectories to interpret the 5-yr record of PAN and O3 ambient air concentrations at Kejimkujik National Park, Nova Scotia. Journal of Geophysical Research 100, 2867–2881. Tsuang, B.-J., Cheng, W.-L., Lin, M.-D., 1998. The formation of ozone pollution in Nantou. EPB Report, Nantou County, 210pp. Tsuang, B.-J., Chen, C.-L., Tu, C.-Y., 2000. Vertical profiles of air pollutants measured from sampling bags by tethered balloon in central Taiwan. Proceeding of the Australia– Taiwan Joint Symposium on Environment Modelling and Management, National Science Council of Taiwan ROC, pp. 7–13. US Environmental Protection Agency, 1994. A revised user’s guide to Mesopuff II (V.5.1). Technical Support Division (MD-14). Research Triangle Park, NC. US Environmental Protection Agency, 1998. Clean Air Status and Trends Network (CASTNet) deposition summary report (1987–1995). Research Triangle Park, NC, EPA600/R-98/027. Wakamatsu, S., Uno, I., Ohara, T., Schere, K.L., 1999. A study of the relationship between photochemical ozone and its precursor emissions of nitrogen oxides and hydrocarbons in Tokyo and surrounding areas. Atmospheric Environment 33, 3097–3108. Wang, M., 1999. Atmospheric Chemistry. 2nd Edition. China Meteorological Press, Beijing, 467pp. Xu, J., Zhu, Y., Li, J., 1997. Seasonal cycles of surface ozone and NOx in Shanghai. Journal of Applied Meteorology 36, 1424–1429. Yu, T.-Y., Chang, L.-F.W., 2000. Selection of the scenarios of ozone pollution at southern Taiwan area utilizing principal component analysis. Atmospheric Environment 34, 4499–4509.
Ozone distribution in coastal central Taiwan under sea-breeze conditions Wan-Li Cheng* Department of Environmental Science, Tunghai University, Taichung 407, Taiwan ROC Received 22 November 2001; accepted 14 April 2002
Abstract Sea-breeze circulation influences ozone concentration differently for two types of synoptic winds across the westcentral coastal plain of Taiwan. During summer months, when the synoptic flow is southerly, a strong westerly seabreeze confined to the lower 700–800 m produces a northeastward flow. Ozone concentration is low, with a core centered at about 100 m, leading to moderate ozone concentrations in the low population density northeastern foothills. In autumn (and spring), northerly synoptic winds of about the same strength combine with less energetic westerly sea breeze to produce a southeastward flow that carries higher ozone levels into the heavily populated Taichung basin. Ozone levels are high from 80 to 400 m, with a core up to 120 ppb from about 150 to 300 m, contributing to serious ozone episodes at the southern (downwind) end of the basin. Analyzing the backward trajectories and ozone concentration showed that the weak southeastward breeze is the dominant factor affecting the occurrence of high ozone events in the region. The horizontal distribution is based on 3 yr data obtained from a network of air-pollution monitoring sites in the study area, while the vertical data comes from two 2-day tethersonde experiments, measuring wind-speed, direction, temperature and humidity, NO, NO2, NMHC and O3, conducted during August and November 1999. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ozone concentration; Sea-breeze circulation; Tethersonde experiment; Ozone spatial distribution; Backward trajectory
1. Introduction Ozone (O3) in the boundary layer is the product of chemical reactions involving primary pollutants from the surface. NOx oxides and volatile organic compounds (VOCs) [specifically non-methane hydrocarbons (NMHC)] from industrial and traffic emissions are converted by strong solar radiation (lo424 nm) to secondary pollutants. NO2 is converted by strong solar radiation to NO and atomic oxygen O, after which O and O2 form O3: NO2 þ hn-NO þ O
ð1Þ
O þ O2 þ M-O3 þ M;
ð2Þ
*Fax: +886-4-235-95941. E-mail address: [email protected] (W.-L. Cheng).
where hn denotes a photon and M returns the molecule to its original state. The chemistry of the metropolitan and industrial boundary layer is much more complicated due to the presence of VOCs (alkanes, alkenes, aromatic hydrocarbons, etc.). For example: RH þ OHd -Rd þ H2 O
ð3Þ
Rd þ O2 þ M-ROd2 þ M
ð4Þ
ROd2 þ NO-ROd þ NO2 :
ð5Þ
The alkyl radical R and H atom in the alkane is susceptible to hydroxyl radical OH attack. OH is available independent of altitude. The reaction mechanism for O3 also involves NO conversion to NO2, while NO2 plays a central role in the formation of O3.
1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 3 0 7 - 2
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Ozone is a highly reactive oxide. Its cumulative concentration is influenced by solar radiation, temperature, humidity and wind. Most studies show that high solar radiation, low humidity and low wind speed all enhance ozone accumulation (Tsuang et al., 1998; Xu et al., 1997; Liu et al., 1994). Ozone concentrations at any one location also depend significantly on advected emissions and vertical mixing rates and consequently on the local variation of the wind field and parcel trajectories. Well known studies have been done in the United States (Holland et al., 1999; US EPA, 1998) and Tokyo (Wakamatsu et al., 1999), in the Yahagi basin (Kitada et al., 1986), at Los Angeles (McElory and Smith, 1986), in Southern Ontario (Hastie et al., 1999), and Athens (Kambezidis et al., 1998). Many studies indicate that the sea-breeze transports the pollution from coastal and urban areas inland and spreads ozone to an onshore distance of 20–60 km (Hastie et al., 1999; Seinfeld and Pandis, 1998; Kambezidis et al., 1998; Kitada and Kitagawa, 1990). Liu et al. (1994) have investigated the impact of the sea– land breeze on the horizontal distribution of ozone concentration in Taipei and Kaohsiung in Taiwan. They found high ozone pollution located downwind of the sea breeze. This paper analyses the area of coastal central Taiwan, having dimensions of 50 60 km2, as a part of the Central Taiwan Air-Quality Management Program (CTAMP). The study here extends work on the effect of winds on sulfur concentration (Cheng, 2001a) to their effect on ozone (Cheng, 2001b).
throughout most of the year, sea breezes occur on more than half of all days. The data for this study is based on an analysis of air quality and meteorological data from a number of sources: the horizontally distributed data comes from nine Air Quality Monitoring Stations run by Taiwan EPA, shown on the map (Fig. 1), ten stations run by Taichung Power Plant (not shown, to maintain clarity),
2. Data collection Taiwan is influenced by the northerly monsoon from autumn to spring months and the southerly one in summer months. A sea breeze is superimposed on these flows when conditions are suitable to produce daytime northwesterly or southwesterly flow, respectively. The northwesterly surface flows (autumn, denoted by N in later figures) are weak, but increase in speed from almost calm up to about 2 m s1 and remain steady for 5–6 h throughout the afternoon, with directions between 2701 and 301. The summer flows show stronger sea breeze, with the speeds increasing from o2 up to 4 m s1, and directions southwesterly confined to the sector 202–2701 (denoted by S in the later figures and tables). Since temperature contrasts between land and sea drive the sea breeze, the latter are strongest when the land is strongly heated, i.e. under cloudless skies and calm conditions (Simpson, 1994). The sea breezes generally start during late morning and may extend into late afternoon. Because of the location of coastal central Taiwan and the great number of cloudless days
Fig. 1. Location of monitoring stations in westcentral Taiwan (Tonhsiao Power Plant located 25 km north of Fengyuan).
Table 1 Frequency of HODs during sea-breeze days in westcentral Taiwan in the period 1997–1999 Site
Sea-breeze day (N)
High-ozone day (HOD)
Frequency of occurrence (%)
Fengyuan Shalu Tali Chungming Hsitun Changhua Erhlin Nantou Chushan
324 312 319 327 322 326 329 308 273
142 88 198 149 86 65 90 189 203
43.8 28.2 63.5 45.6 26.7 19.9 27.4 61.3 74.4
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Fig. 2. Wind field and O3 concentration at (a) 08:00, (b) 11:00, (c) 14:00 and (d) 17:00 on 28 August 1999.
and three by the Central Weather Bureau. One of these stations was 1.5 km offshore and provided the sea temperature data. This was used in conjunction with the Taichung city air temperature as a sea-breeze
potential indicator. The horizontal data set covers the period 1997–1999. The vertical distribution of ozone and its precursors, and wind speed and direction were measured with a
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tethersonde system. The measurements were done up to about 1200 m, to yield data that is substantially free of immediate local sources and dry deposition. Two tethersonde experiments on 28–29 August 1999 with a typical summer southwesterly sea breeze and 5–6 November 1999 with a typical autumn northwesterly sea breeze were made at a coastal site (Longjin) and an inland site (Tsaotun) about 15 km south of Taichung city in the center of the basin. The tethersonde was manufactured by Atmospheric Instrumentation Research, Inc., USA. It consists of a balloon-borne rawinsonde for measuring pressure, temperature, humidity, wind direction and speed (Hoff et al., 1995). The sonde was supported by a spherical 3 m3 hydrogen balloon, tethered on a Kevlar line (760 lb) and controlled by an electric capstan. On the ground were additional pollutant (O3, NOx and
NMHC) analyzers. Four air samplers, each with a pair of 10-l tedlar sampling bags, were installed on the tethered line. Each of them was fitted with a timer to control the operation and duration of action of the suction pump. Once the uppermost sampler reached the designated height, all the samplers were switched on to simultaneously pump air into the bags. After the sampling was completed, the balloon was retrieved and the samplers with the bags were collected for analysis of air pollutants. A detailed description of the sampler and its use in several similar CTAMP experiments can be found in Cheng (2000) and Cheng et al. (2001). The instrumentation and the methodologies used to monitor the CTAMP vertical profile data sets have been described elsewhere (Tsuang et al., 2000; Cheng et al., 2001).
Fig. 3. Vertical profiles of virtual potential temperature monitored at Longjin and Tsaotun in westcentral Taiwan, 28–29 August 1999.
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3. Results A high ozone pollution day (HOD) is defined here as a day with a maximum ozone concentration >80 ppb. The remaining days are non-HODs (NHOD). Measurements during the 3-yr period indicate that high ozone concentrations in excess of 80 ppb are frequently reached in the spring and autumn months, especially during low wind speed days, which favor the generation of sea-breeze cells. The frequency of HODs during northwesterly sea-breeze days in 1997–1999 is shown in Table 1. Among all the stations, Chushan has the highest number of HODs with 203 out of 273 sea-breeze days (74.4%), followed by Tali (63.5%) and Nantou (61.3%). All three stations are located in the southern part of the basin, while the stations near the coast, such as Shalu (28.2%), have a very low percentage. The
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lowest corresponds to Changhua (19.9%), located in the gap between Tadu Hill and Baqua Hill. Here the wind speeds were slightly higher (20%) than adjacent stations. During the southwesterly sea-breeze days, the air quality over the whole area of coastal central Taiwan is reasonably good with O3 concentrations below roughly 40 ppb. For the tethersonde data, two consecutive days were monitored in two experiments: 28–29 August 1999 with typical summer sea-breeze conditions, and 5–6 November 1999 with typical autumn sea–breeze circulations. The meteorological conditions over the whole area for these 2-day periods were analyzed, and the vertical profiles of ozone and nitrogen oxide were examined, including their diurnal variation. Of particular interest is the difference in the evolution of the profiles in experimental periods.
Fig. 4. Vertical profiles of wind field monitored at Longjin and Tsaotun in westcentral Taiwan, 28–29 August 1999.
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3.1. Experimental campaign I During 28–29 August 1999 (summer), the synoptic pattern over Taiwan showed an extended Pacific Anticyclone. The western extension of the anticyclone directed a SE–SW wind over Taiwan. The anticyclone center was relatively weak. There were clear sea-breeze circulations. 28 August 1999 was a cloudless day with solar radiation reaching 16.65 MJ m2. The land temperature at Taichung city ranged from 24.51 to 32.11C. This gave maximum sea–land temperature differences of 4.01C (the sea temperature showed a 0.61C range over the day from a minimum of 27.81C at 04:00 h). All times hereafter are referenced to Taiwan Standard Time, which is 8 h ahead of UT. The second day was also cloudless; solar radiation reached 16.27 MJ m2, and the largest sea–land temperature difference was 3.91C at 13:00 h. On the afternoons of 28 and 29 August, the
sea-breeze inflow reached altitudes in the range of 700–800 m. At ground level, the O3 concentration was highest at the northeast corner of Fengyuan. Applying US EPA’s Mesopuff II model (US EPA, 1994), to the data from all stations mentioned previously, simulation codes for the wind field and O3 concentration were obtained. These are shown in Fig. 2 (using transform) for the horizontal distributions, and in Fig. 7 for vertical section of winds. Figs. 2 and 4 clearly show that a significant southerly land breeze dominated the region before sunrise from ground level upwards between 03:00 and 06:00 h. At ground level, the southerly land breeze abruptly died away after 06:00 h. Subsequently, a sea breeze started at coastal Longjin at 10:00 h and later reached inland Tsaotun at 12:00 noon to produce a significant westerly flow until 18:00 h. The wind speed in the entire boundary layer was below 3.0 m s1 with variable wind directions. From 03:00 to
Fig. 5. Vertical profiles of O3 concentration monitored at Longjin and Tsaotun in westcentral Taiwan, 28–29 August 1999.
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Fig. 6. (a) Vertical profiles of NO and NO2 concentrations monitored at Longjin in westcentral Taiwan during 28–29 August 1999, (b) vertical profiles of NO and NO2 concentrations monitored at Tsaotun in westcentral Taiwan, 28–29 August 1999.
18:00 h, the surface wind direction swung clockwise from S to W to NW. This is a typical sea–land breeze circulation for the season. The pattern was similar on 29 August (Fig. 4), but of greater vertical extent. The whole region was covered by an ozone concentration field below 20 ppb (Fig. 2). At 11:00 h, once the sea-breeze developed, the concentration climbed to 40 ppb in the east half of the region, particularly in the northeast corner. This situation was maintained until 14:00 h. By 17:00 h the ozone level gradually declined to o20 ppb. The profiles of virtual potential temperature and wind field (Figs. 3 and 4) do not provide clear indices to estimate the height of the mixing layer, hence the method proposed by Holzworth (1967) has been adopted. The mixing height in the early afternoon was about 900 m at Tsaotun on 28 August and 800 m on 29 August. The vertical profiles of virtual potential temperature remained uniform throughout the lower 500 m height during the daytime. There were clear nocturnal inversions overnight both at Longjin and Tsaotun (shown in Fig. 7. Backward trajectories of Tali, Nantou and Chushan for 29 August 1999 (the numbers below the backward trajectories indicate the time at each location).
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Fig. 8. Wind field and O3 concentration at (a) 08:00, (b) 11:00, (c) 14:00 and (d) 17:00 on 5 November 1999.
Fig. 3). In this latter period the O3 concentration decreased to 10 ppb at ground level (Fig. 5). As the morning progressed, the mixed layer depth increased to around 800 m by 12 noon on 28 August while the O3
concentration steadily increased to over 40 ppb. The layer continued to thicken to 1000 m until 15:00 h. By then the sea breeze was well established, its inflow extending to a height of 800 m and to the foothills.
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Thereafter, the layer depth decreased slowly to 150 m at midnight, wind speeds fell to calm and the O3 concentration was reduced to 15 ppb. During the daytime, particularly during peak traffic hours, there was a great amount of NO and NO2 emission from both traffic and industry. This makes NO and NO2 concentrations higher at ground level, decreasing with altitude. Fig. 6(a) and (b) show that the NO and NO2 are higher during the day and lower at night. This decrease in the local maximum of ozone concentrations would be explained from the increased presence of NO and NO2 depleting the ozone. The slightly higher concentrations of NO and NO2 at Longjin compared to Tsaotun are because the former is an industrial area while the latter is a rural site. At Longjin, the average NO and NO2 on the experimental days ranged between 6 and 9 and 6 and 18 ppb, respectively, whereas at Tsaotun they ranged between 6 and 9 and 6 and 12 ppb,
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respectively. Ozone is depleted by NOx, predominantly NO: O3 þ NO-NO2 þ O2 :
ð6Þ
The NO is already present in the well-mixed atmosphere before radiative cooling of the surface starts. As this reaction proceeds and NO2 concentration increases, the reaction O3 þ NO2 -NO3 þ O2
ð7Þ
can further deplete ozone, although its reaction rate is much slower than (6). Reaction (6) then effectively depletes ozone within the lower nocturnal inversion layer (Wang, 1999; Seinfeld and Pandis, 1998). The same pattern was repeated on 29 August. Both days showed significant changes to the horizontal and vertical patterns of O3 concentration. Because the O3 concentration was not particularly high, it is suggested
Fig. 9. Vertical profiles of virtual potential temperature monitored at Longjin and Tsaotun in westcentral Taiwan, 5–6 November 1999.
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that these days were only moderately chemically active due to sea-breeze ventilation reducing the reactions between NO, NO2 and NMHC. The NMHC profiles are not presented here due to failure to meet the standard US EPA procedures of quality assurance and control. Backward trajectories provide a better picture of the sea breeze and connection between the emission source and the monitoring stations (Sirois and Bottenheim, 1995). An approach, based on the corporate Barnes objective method for interpolating spatial values and the variation-kinematical model (Yu and Chang, 2000), was adopted for correcting the effects of complex terrain, to produce hourly wind field data by using data from the 31 stations over the region. By utilizing the generated hourly wind fields at 200 m high (US EPA, 1994), backward trajectories were simulated from Tali, Nantou and Chushan. The initial time of the backward trajectories for each station was set at 15:00 h, as the hour that the maximum ozone concentration occurred.
On 29 August, the coastal wind was southwesterly and the backward trajectories show a movement within 4 h because of relative strong sea-breeze effects (Fig. 7). An additional explanation could be that the transport route for air parcels was over the southern coast and therefore along a path without many NOx or NMHC sources. The high NOx and NMHC emission inventory sources are along the northern coast and metropolitan areas of central Taiwan. 3.2. Experimental campaign II On 5 November, a continental anticyclone moved eastwards over the East China Sea with its center located northeast of Taiwan. Weak synoptic northeasterly winds covered the area for the experimental period. But over the study area, in the lee of the Central Ranges, they were light and very variable (Cheng et al., 2001). The weather was quite cloudy with solar radiation reaching
Fig. 10. Vertical profiles of wind monitored at Longjin and Tsaotun in westcentral Taiwan, 5–6 November 1999.
W.-L. Cheng / Atmospheric Environment 36 (2002) 3445–3459
10.20 MJ m2 for 5 November and 10.26 MJ m2 on the next day, about 60% of campaign I. The land temperature at Taichung city ranged from 21.81 to 29.01C. This gave maximum sea–land temperature differences of 5.61C (the sea temperature showed a 0.71C range over the day from a minimum of 22.51C at 06:00 h). Therefore, there was a relatively large temperature difference between land and sea: 5.61C compared to 4.01C in experimental campaign I. However, the wind speed was only 2.0 m s1 during the sea-breeze development, compared with 4.0 m s1 in August. The horizontal distribution of O3 and modeled wind field is shown in Fig. 8. At ground level, the early morning was calm up until 9:00 h. A sea-breeze gathered strength from 12 noon to form a clear surface westerly in the afternoon hours between 12:00 and 18:00 h. The wind speed in the entire 800 m boundary layer was below
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2.5 m s1 and wind direction varied with height: northwesterly up to 400 m at 12:00 noon, and westerly above. At 15:00 h, at the surface, it was WNW up to 800 m, and SW above. By 18:00 h, it was NW, weaker and shallower to 200 m, calm to 600 m and S above. The wind direction swung noticeably anticlockwise from NW to W to SW and S. This is a typical sea–land breeze circulation for the season. The pattern reappeared again on 6 November (Fig. 10). Ozone levels, for the period were as follows. At 08:00 h on 5 November, the entire region had an ozone concentration roughly below 20 ppb. At 11:00 h, once the sea-breeze developed, the concentration climbed to 40 ppb, particularly for two areas. One is immediately downwind of the Taichung power plant and the other possibly from coastal advection from the Tonhsiao Power Plant further north. These two areas had a concentration over 80 ppb. This indicates that the heavy
Fig. 11. Vertical profiles of O3 concentration monitored at Longjin and Tsaotun in westcentral Taiwan, 5–6 November 1999.
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ozone concentrations were directly related to the thermal power plants with heavy NO and NO2 emissions, which are the two main sources of NO and NO2 pollutants in the region. By 14:00 h, the sea-breeze transported all NO, NO2, NMHC and O3 to the downwind southern corner, resulting in ozone levels there over 110 ppb. Then the ozone concentration decreased to 50 ppb by 17:00 h (Fig. 8). This air pollution dispersion pattern was also discussed by Chang (2001) in his computer modeling results. The vertical profiles of virtual potential temperatures remained uniform up to 400 m height during the daytime. There were also significant nocturnal inversion layers both at Longjin and Tsaotun, as seen from Fig. 9. The diurnal profile changes of O3 were consistent with the diurnal cycle of the wind. The mixed layer was thin (about 150–250 m) in the early morning of both 5 and 6 November. It gradually rose to 500–600 m by 12 noon. O3 concentration climbed to over 110 ppb, with the highest value at 150–300 m. This indicates that O3 dispersion was limited by the inversion above 400–500 m (Figs. 9–11). These two days were typical for autumn HODs in the region, with the second day giving higher values than the first.
Fig. 12(a) and (b) show that the concentrations of NO and NO2 were higher during the daytime and decreased at nighttime both at Longjin and Tsaotun. As a whole, coastal Longjin is higher than the inland site Tsaotun, which has been already discussed in the experimental campaign I in Autumn. The vertical profiles of NMHC during the campaign period ranged between 0.4 and 4.8 ppm at Longjin and between 0.4 and 1.0 ppm at Tsaotun, are well mixed and slightly higher in nighttime than daytime (Fig. 13). With regard to NO, NO2 and O3 concentrations, NMHC began to decrease from late morning to afternoon. Their inter-relation can be seen in Reactions (1)–(5). The existing NMHC, with abundant NO and NO2, in the boundary layer could cause high ozone pollution in metropolitan and downwind areas. It is noted that the extremely high NO, NO2 and NMHC at a height of 400–500 m at 15:00 h at Longjin were due to wind blowing the pollutants from nearby stacks (250 m tall) of the Taichung power plant. Following the approach of Milford et al. (1994), no consistent association was found between the sensitivity of ozone to reductions in NMHC versus NOx emissions in this study. The ratios of NMHC/NOx at Longjin are
Fig. 12. (a) Vertical profiles of NO and NO2 concentrations monitored at Longjin in westcentral Taiwan, 5–6 November 1999, (b) vertical profiles of NO and NO2 concentrations monitored at Tsaotun in westcentral Taiwan, 5–6 November 1999.
W.-L. Cheng / Atmospheric Environment 36 (2002) 3445–3459
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Fig. 13. Vertical profiles of NMHC concentration monitored at Longjin and Tsaotun in westcentral Taiwan, 5–6 November 1999.
widely scattered between 5 and 30 with R2 ¼ 0:240; and at Tsaotun between 5 and 65 with R2 ¼ 0:016: This inconsistency could be due to high air pollutants as well as complicated meteorological conditions in the Taiwan region, as discussed in a recent study by Lu and Chang (1998). It is interesting to find that the NO had a maximum of 66 ppb from the ground up to 200 m, NO2 had a maximum of 36 ppb at 100 m and NMHC had a high maximum of 4.8 ppm from the ground up to 100 m at Longjin on the early morning of 5 November. The noon monitored high ozone concentration at Longjin and even higher over 110 ppb at downwind Tsaotun. Since NO, NO2 and NMHC are precursors of O3, the vertical profiles of NO, NO2, NMHC and O3 show a high correlation in this experiment. This concurs with the results of Aneja et al. (2000), Gusten . et al. (1998) and Neu et al. (1994), in which ozone and its precursors may be stored aloft at nighttime and mixed downward to the
ground in the morning as the surface is heated and the nocturnal inversion breaks up. The vertical profiles of the wind show that a significant sea breeze started at 12 noon on both days. The height of the sea-breeze inflow was about 400– 500 m, lower than that of the summer experiment. Under autumn conditions, ozone concentration increased by up to 60 ppb within 2–3 h. These increases occurred simultaneously with the sea-breeze development [identified from both ground (Fig. 8) and high altitude winds (Fig. 10)], which indicate that polluted air masses from industrial and traffic sources are transported inland by the sea breeze. The air parcels travel long distance from the northern coast and metropolitan areas where the main NOx and NMHC sources are located. With the weak sea breeze, the polluted air parcels have sufficient time to stagnate in the high anthropogenic activity areas and form high ozone concentrations. Additionally, the backward
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(61.3%) and Chushan (74.4%). The results of this study indicate the importance of advection patterns and inversions when formulating pollution abatement strategies for sites with frequent sea breezes, such as the west-central coastal plain of Taiwan.
Acknowledgements
Fig. 14. Backward trajectories of Tali, Nantou and Chushan for 5–6 November 1999 (the numbers below the backward trajectories indicate the time at each location).
trajectories show long transport times of two days (Fig. 14).
4. Conclusions Seasonal sea breezes have different influences on the horizontal distribution of ozone concentration in coastal central Taiwan, due to their different directions and speeds. In summer months, the sea-breeze arrives from the Taiwan Strait, heading inland (eastwards) across central Taiwan. Due to its relatively high speed and thick vertical structure, it is mixed vertically up to 700– 900 m. The clear air from the sea entrains polluted air and moves eastwards, bringing clean air to the region, although the northeastern corner at Fengyuan occasionally gets some ozone, where the flow is constrained by the Central Ranges. During autumn, the sea-breeze enters central Taiwan from NNW of the Taichung basin. It is characterized by low wind speed and a shallow depth; this breeze tends to be steered more by the terrain and not as well mixed vertically, being capped by a stronger inversion. It also shown by the analyses of backward trajectories that the weak northwesterly sea-breeze plays an important role in producing high ozone concentrations in the downwind areas, such as Tali (the frequency of occurrence 63.5%), Nantou
The author would like to thank the National Science Council of Taiwan ROC (NSC-89-EPA-029-001), the Environmental Protection Administration of Taiwan ROC (EPA-89-FA11-03-231) and the Environmental Protection Bureau of Taichung city for funding this research and the Taiwan Power Company and the Central Weather Bureau for their co-operation in providing air quality and meteorological data. Special thanks go to Prof. L. Leslie of University of New South Wales and Prof. B.-J. Tsuang of ChungHsing University of Taichung for their stimulating ideas. The author is grateful to Dr. J. Bennett of Flinders University of south Australia for reading the draft of this paper. My appreciation also extends to Miss J.-L. Bai, Dr. G. Cheng and Mr. K. Cheng for their effective preparation and execution of the research, as well as their kind secretarial assistance.
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