Impact of sea breeze air masses laden with ozone on inland surface ozone concentrations: A case study of the northern coast of Taiwan

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006jd008123, 2007 Impact of sea breeze air masses laden with ozone on inland surface ozone concentrations: A case study of the northern coast of Taiwan Ching-Ho Lin, 1 Chin-Hsing Lai, 1 Yee-Lin Wu, 2 Po-Hsiung Lin, 3 and Hsin-Chih Lai 4 Received 9 October 2006; revised 12 April 2007; accepted 8 May 2007; published 25 July [1] This work examines how ozone-laden sea breeze air masses contribute to inland surface ozone concentrations. The vertical distributions of ozone in sea breeze air masses and the characteristics of sea breezes are investigated using tethered ozonesondes and meteorological radiosondes, respectively, at a measurement site near the northern coast of Taiwan during August The investigations reveal that, initially, sea breeze air masses are stable with relatively high concentrations of ozone distributed in the upper portions of the air masses. Elevated ozone layers with concentrations of ppb were frequently observed at m. The growth of a thermal internal boundary layer (TIBL) inland that can bring ozone-rich air aloft in a sea breeze air mass into the growing TIBL subsequently increases the surface ozone concentrations farther inland. Accordingly, the surface ozone concentrations increase with distance inland, regardless of the photochemical production of ozone inland. A new conceptual model was presented to depict this pollution feature. According to a simple Lagrangian analysis, ozone-rich sea breeze air masses under the observed conditions generated a difference of as much as a 30 ppb between the surface ozone concentration at a near-coast location and that at a far inland location. TIBL development at a near-coast area can protect the area from fumigation of elevated ozone layers because the depth of the TIBL is limited there, such that the ozone in the elevated ozone layers cannot be brought to the ground. Citation: Lin, C.-H., C.-H. Lai, Y.-L. Wu, P.-H. Lin, and H.-C. Lai (2007), Impact of sea breeze air masses laden with ozone on inland surface ozone concentrations: A case study of the northern coast of Taiwan, J. Geophys. Res., 112,, doi: /2006jd Introduction [2] Serious ozone pollution has been frequently reported inland during sea breeze events in coastal regions [Grossi et al., 2000; Millan et al., 2000; Boucouvala and Bornstein, 2003; Kalthoff et al., 2005; Oh et al., 2006]. Furthermore, most inland ozone pollution is found to be related to landward transport and subsequent mixing of ozone-rich sea breeze air masses. A sea breeze air mass is initially located over a sea. The existence of an ozone-rich marine air mass is typically associated with local and long-range transport of ozone pollution. Grossi et al. [2000] and Oh et al. [2006] found that an ozone-rich marine air mass can result from the transport of an inland ozone-rich air mass by land breezes during the night. Additionally, an ozone-rich marine air mass may have been previously an ozone-rich air mass located inland, transported vertically by updrafts over a 1 Department of Environmental Engineering and Science, Fooyin University, Kaohsiung Hsien, Taiwan. 2 Department of Environmental Engineering, National Cheng-Kung University, Tainan, Taiwan. 3 Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan. 4 Center for Teacher Education, Chang Jung Christian University, Tainan, Taiwan. Copyright 2007 by the American Geophysical Union /07/2006JD sea breeze convergence zone, and, subsequently, transported horizontally by return flows over the near coast or sea area. The latter process has been supported by the observations of elevated ozone layers in the Los Angeles Air Basin [Wakimoto and McElroy, 1986; Boucouvala and Bornstein, 2003], in the lower Fraser Valley of British Columbia [McKendry et al., 1997], in the Western Mediterranean Basin [Millan et al., 1997, 2000] and in southern Taiwan [Lin et al., 2004]. Once these elevated ozone layers become parts of sea breeze air masses, they can be transported inland again and increase inland surface ozone concentrations. [3] A marine air mass over a sea is usually stable because of the large heat capacity of seawater and the frequent occurrence of atmospheric subsidence over a near coastal or sea area under developing sea breeze circulation [Atkinson, 1981; Helmis et al., 1987; Simpson, 1994]. Notably, if ozone is initially distributed in a stable marine air mass or sea breeze air mass above ground, the ozone is unable to contribute to inland surface ozone levels when no vertical mixing occurs. Therefore the extent of vertical mixing is an important factor that affects the contribution of a sea breeze air mass that is initially laden with ozone above the ground to surface ozone levels inland. In a coastal region, when a stable marine air mass is transported to a heating land surface during the day under the development of a sea breeze circulation, the bottom portion of the marine air mass (the sea breeze air mass) which is adjacent to the ground becomes unstable as heat is gained 1of16

2 Figure 1. Measurement site (star), Ilan meteorological station (triangle) and two main towns (circle), Ilan and Lotung, located on the Ilan plain, and the topography of the plain and its surroundings. from the heating land surface. In this circumstance, an unstable, convective mixing layer, known as a TIBL, develops over land [Garratt, 1992]. Therefore vertical mixing in a coastal region during the development of a sea breeze circulation is controlled by the development of a TIBL. Accordingly, the development of a TIBL at a coastal region should be an important factor in determining inland surface ozone levels due to a sea breeze air mass with initially high ozone concentrations above the ground. The vertical distribution of ozone in a sea breeze air mass before it is transported inland is also important in determining the contribution of the sea breeze air mass to inland surface ozone concentrations, because the initial distribution of the ozone and the depth of the TIBL determine the total amount of ozone that can be mixed into the TIBL or the ground. [4] This work investigates the impact of the initial vertical distributions of ozone in a sea breeze air mass and the development of TIBL on inland surface ozone concentrations that are determined by this air mass. The vertical distributions of ozone in sea breeze air masses observed at a near-coast measurement site on the northern coast of Taiwan on August 2003 are presented below. Surface ozone concentrations at various distances inland that resulted from the observed sea breeze air masses were evaluated and discussed, using a simple Lagrangian analysis. 2. Experiment 2.1. Site Description [5] All measurements in this field study were made on an elementary school campus (24.39 N, E, 10 m above sea level (ASL)) (Figure 1) on August 2003, during the school summer holidays. This is a near-coast measurement site located 3.9 km from the coastline. This site is on the Ilan plain. The plain is quite flat at <100 m ASL, and is triangular with each side km long. It has the Pacific Ocean to its east, the Shiueshan Mountain Range to its northwest, and the Central Mountain Range to its southwest. These two mountain ranges rise from 500 m in the east near the coast, to over 2000 m in the west, near the end of the plain. The plain is primarily used for agriculture. Most of the pollutants on the plain are from vehicles on the roads in the plain and two main towns, Ilan and Lotung (Figure 1). For example, the annual emissions of NOx, one of the ozone precursors (NOx and VOCs), from vehicles in the two towns were only 450 and 170 tons, according to the newest version of the Taiwan Emission Data System (TEDs. 6.1) [Taiwan Environmental Protection Administration, 2006]. The corresponding NOx emission intensities for the two towns were roughly the same, 15 tons per square kilometer in one year. This emission intensity is only about 1/10 of that in Taipei City, the largest city in Taiwan. Also, the NOx emission intensity in the county of the measurement site was 3.3 tons per square kilometer in one year, which is only 1/6 of that in the two towns in Ilan plain. Therefore the emission intensity in the study plain was lower than in other areas in Taiwan. [6] When a sea breeze is prevalent, the measurement site is located upwind of the two towns. Therefore the ozone concentrations measured by the tethered ozonesondes at this near-coast site under development of a sea breeze circulation should not include the ozone that is produced by photochemical reactions of the ozone precursors that are emitted in the two towns Measurement [7] The measurements made in this field study were of surface meteorological variables, measured by an automatic weather station (MAWS 201) (Vaisala, Helsinki, Finland), and vertical profiles of meteorological variables and ozone, measured by meteorological radiosondes and tethered ozonesondes. RS-80 15G meteorological radiosondes (Vaisala, Helsinki, Finland) were used to measure the vertical profiles of winds, temperatures and humidity. The radiosondes were launched between 1400 LST on 24 August and 1400 LST on 30 August. Three radiosondes were launched daily at 0800 LST, 1400 LST and 2000 LST, respectively. Additional soundings were carried out at 1100 LST and 1700 LST on 26 and 29 August. [8] The ozone sensor of the ozonesonde was an SPC model 6A [Science Pump Corporation (SPC), 1999] Electrochemical Concentration Cell (ECC) [Komhyr, 1969; Komhyr et al., 1995]. The ECC was coupled with an RS- 80 radiosonde (Vaisala, Helsinki, Finland) to form an ozonesonde. Thus both ozone and meteorological parameters (temperature and humidity) can be measured by the ozonesonde. The ozonesonde was tethered to a balloon system, which comprised a helium-filled balloon with diameter of 3 m, a 2-km-long Kevlar line and an electric winch. The balloon was tethered to the Kevlar line, and the winch was used to control the ascent and descent of the balloon. The maximum altitude reached by the tethered balloon was generally m. In each tethered sounding, ascending and descending profiles were acquired. The time required to complete an entire ascent-descent cycle was min, depending on the wind shear aloft and the maximum sounding altitude. [9] The ECC utilized a buffered 1% potassium iodide (KI) in the cathode half-cell and a saturated KI solution in the anode half-cell, as suggested by Komhyr et al. [1995]. Each ECC was charged for more than seven days to attain a low background current and ensure that the sensor responded quickly to ozone. Additionally, several hours 2of16

3 Figure 2. Synoptic surface weather map at 0800 LST on 27 August 2003 (Central Weather Bureau, Taiwan). The triangle indicates the measurement site. before the launch of an ozonesonde, the flow rate, background current, zero and span calibration as well as the response time of the ozonesonde were measured or checked, following the operating guide of the ozonesonde published by SPC [1999]. The accuracy of an ECC ozonesonde is about ±6% in the lower troposphere [Komhyr et al., 1995]. [10] The tethered ozonesondes were launched from the early night of 24 August to the afternoon of 30 August. On most days, three ozonesondes were launched, one in the midmorning ( LST), one in the afternoon ( LST) and one in the early evening ( LST). No ozone sounding was performed on 27 August. The sounding on the night of 28 August was canceled because of high background current on the ECC, which failed to pass a calibration check. 3. Results and Discussion 3.1. Synoptic Flow Conditions and Surface Measurements [11] In summer, the synoptic weather over northern Taiwan is dominated by typhoons or the Pacific Anticyclone. At this time, daytime surface heating is strong and ambient environmental flow is light. These conditions favor the development of sea breeze circulations. During the experimental period, the synoptic weather in northern Taiwan was controlled by the western edge of the Pacific Anticyclone. Figure 2 shows an example of a typical weather pattern on 27 August. A light synoptic flow came from the east. However, on the first two days of the experimental period, synoptic flow was more southerly, because of the influence of the outer edge of a typhoon in the South China Sea. [12] Figures 3 and 4 plot variations from 24 to 30 August in surface wind direction, wind speed, temperature and humidity, as measured at the automatic weather station installed at the measurement site and at the Ilan meteorological station, maintained by the Central Weather Bureau of Taiwan. The diurnal variations in temperature and humidity at both stations indicated strong surface heating during daytime. Moreover, easterly (onshore) and westerly (offshore) winds dominated during the day and night, respectively, indicating that sea and land breezes developed throughout the experimental period. The daily onset of sea breezes during the experimental periods occurred at LST, as revealed by the variations in wind direction Sea Breeze Circulations [13] Figure 5 presents wind profiles that were measured by the meteorological radiosondes at the measurement site on August. Standard wind bars are used to express wind speeds and directions. The daily local circulations can be roughly identified, below synoptic flows, from the wind profiles in Figure 5. A southerly synoptic flow prevailed at 2200 m to over 4000 m from the afternoon of 24 August to the morning of 25 August. On 25 August, the upper level synoptic flow changed. From the morning of 26 August to the end of the experiment (30 August), easterly synoptic flows began to dominate at m. Easterly, onshore sea breeze layers were present at m and occasionally extended upward to 1200 m, according to daily 1400 LST soundings or the 1100 LST soundings on 26 and 29 August (Figure 5). The sea breeze layers were evident on the afternoons of 24, 25, 27 and 30 August. Strong westerly offshore flows at m were detected each afternoon on August (Figure 5). [14] The tops of an afternoon sea breeze and the return sea breeze were defined as the altitude at which the first offshore breeze was present above the ground and as the altitude at which an easterly wind first presented above the main offshore flow, respectively. Accordingly, the tops of the sea breeze layer and return sea breeze layer on each day 3of16

4 Figure 3. Variations of the surface wind speed and direction and temperature and humidity at the measurement site during August Data during the period from midnight of 24 August to the morning of 25 August were missing because of a failed data logger. were identified (Figure 6). The tops of the sea breeze layers and return sea breeze layers were at m and at m, respectively. However, the tops of these sea breeze layers in the afternoon on some days may have been several hundred meters above those identified in Figure 6, because of the likely presence of a transition layer above each identified sea breeze layer (Figure 6). Transition layers were evident on 25 August (Figure 6a), 28 August (Figure 6d), 29 August (Figure 6e) and 30 August (Figure 6f). In these transition layers, the flows were weak and fluctuated in the flow directions. Therefore the lower portion of each transition layer can also be identified as a part of a sea breeze layer. In this situation, each identified sea breeze layer depth (Figure 6) may be underpredicted by m. Additionally, each identified sea breeze return flow (Figure 6) is likely a combined sea breeze and mountain return flow. Previous flow studies by Lu and Turco [1995] and Millan et al. [2000] in other complex coastal environments suggest that the combined flow is easily produced. [15] Figure 6 also plots the temporal, vertical variations of the water-mixing ratio and potential temperature. Comparing the daily differences between water mixing ratios and potential temperatures between morning (0800 LST) and afternoon (1400 LST) soundings yielded a daily increasing water-mixing ratio layer (MRL) and a daily decreasing potential temperature layer (PTL) at m and at Figure 4. Hourly variations of surface wind speed and direction and temperature and humidity at the Ilan meteorological station during August of16

5 Figure 5. Wind profiles measured by meteorological radiosondes launched at the measurement site during August m, respectively. Notably, the top of the daily MRL and the top of the daily PTL were almost identical. The presence of the MRLs and PTLs is most likely related to the vertical transport that is associated with the sea breeze circulation. Before the start of the daily sea breeze circulation, the water-mixing ratio always decreased as the altitude increased, as revealed by the daily morning soundings (Figure 6). After the sea breeze circulation started, air masses initially at lower altitudes were transported upward at the convergent zone of sea breeze or at a mountain slope, given a combined sea breeze and mountain flow (or even atop the mountain when the combined flow is sufficiently strong). The ascent of the air masses can cause a local increase in the water-mixing ratio. Subsequently, the ascending air masses become part of the return sea breeze and are farther transported to the near-coast area. This process can explain the formation of the MRLs that were observed at the measurement site. The MRL was present daily, implying that it was formed by a regular diurnal process. This finding further supported the claim that daily MRL was related to the circulation of the sea breeze. Furthermore, the probability that ambient environmental flow with a high water-mixing ratio is the cause of the MRLs is quite low because ambient environmental flows cannot vary diurnally. [16] The same process can explain the existence of PTLs (Figure 6). Surface heating can increase the potential temperature of air that remains near the ground or in the mixing layer. However, potential temperature in the mixing layer should always be lower than that above the mixing layer because the atmosphere is stable. Consequently, when an air mass that is initially at a relatively low altitude is transported vertically in the sea breeze convergence zone, the potential temperature drops locally. Notably, the initial vertical distribution of the potential temperature before the start of sea breeze circulation can also be determined from the morning sounding (Figure 6). Finally, the tops of the daily MRL and PTL are the same because both represent the upper limit of the height of the ascending air mass in the sea breeze convergence zone. [17] Finally, the top of the daily MRL or PTL was, except on 27 August, a little lower than the top of the daily return sea breeze (Figure 6), suggesting that the part of the return sea breeze that is above the top of the MRL or PTL was not upwardly transported air mass in the sea breeze convergence Figure 6. Profiles of westerly wind component (WS), potential temperature (PT) and water-mixing ratio (MR), based on soundings at 0800 LST and 1400 LST at the measurement site during August 2003, (a) for 25 August, (b) for 26 August, (c) for 27 August, (d) for 28 August, (e) for 29 August, and (f) for 30 August. Positive and negative wind speeds indicate offshore and onshore flows, respectively. Vertical arrows associated with MRL and PTL indicate the layer of increasing water-mixing ratio and the layer of decreasing potential temperature, respectively. Horizontal solid lines represent the top of the afternoon sea breeze (SB) and the tops of the MRL and PTL. 5of16

6 Figure 6 6of16

7 zone. Rather, this part of the return sea breeze was probably an upper level ambient flow, which became part of the return sea breeze because of the wind shear that was produced by the upward transported airflow. Tijm et al. [1999] demonstrated this possibility. Their numerical simulation of sea breeze circulations indicated that upper level ambient flow can account for as much as 30% of the return sea breeze flow flux Vertical Distributions of Ozone Initially in Sea Breeze Masses [18] Figures 7 9 present the ozone, potential temperature and water-mixing ratio profiles measured by tethered ozonesondes at the measurement site in midmorning, afternoon and early evening, respectively, during the experimental period. Notably, no tethered ozonesonde was launched on 27 August. Each depth of the TIBL plotted in these figures represents the average of ascending and descending measurements. The top of a TIBL was defined in terms of the marked changes in the temperature and humidity gradients within the sea breeze layer [Garratt, 1992]. Notably, the tethered ozonesondes did not measure winds. Therefore the sea breeze depths were determined from wind data obtained by meteorological soundings around the time of each ozonesonde measurement. [19] Measurements made using tethered ozonesondes at LST revealed that sea breezes prevailed in midmorning because they had started at LST (see Figures 3 and 4). However, the upper boundary of each sea breeze layer in midmorning could not be determined, except on 26 and 29 August, because additional meteorological soundings were conducted at 1100 LSTonly on these two days. The depths of the sea breeze layers on 26 and 29 August were 900 and 850 m, respectively (Figures 7b and 7d). The depths of the TIBLs on 26 and 29 August in the midmorning were 200 and 500 m, respectively, and were, therefore, considerably less than the depths of the sea breeze layers (Figures 7b and 7d). Ozone concentrations in the TIBLs were roughly uniform because of convective mixing within the TIBLs (Figure 7). The ozone concentrations in the TIBLs were lower than those above the TIBLs (Figure 7). On 29 August, distinct ozone-rich layers were observed at m, and distributed in the upper portion of the sea breeze layer and in the return flow layer (Figure 7d). [20] In the afternoon, measurements by tethered ozonesondes revealed that the depths of the TIBLs were m (Figure 8). All identified TIBL depths were markedly lower than the tops of the sea breeze layers that were identified from wind profiles obtained by meteorological soundings taken at 1400 LST. Like those in midmorning, the lowest ozone concentrations were still present in the TIBLs, and the relatively ozone-rich air layers were located above the TIBLs (Figure 8). The ozone distributions in afternoon TIBLs were more uniform than those in midmorning hours, because relatively strong mixing occurred in the afternoon. Distinct ozone-rich layers with ozone concentrations of ppb were observed on the afternoons of 29 and 30 August (Figures 8d and 8e). These ozone-rich air layers were distributed at m, again in the upper portions of the sea breeze layers and in the return flow layers (Figures 8d and 8e). These ozone-rich air layers were associated with relatively high water-mixing ratios, indicating that they had previously remained near the ground or within a mixing layer where they gained additional water vapor. [21] At night, surface cooling stabilized the lower atmosphere. Consequently, the ozone distributed in layers because of a lack of vertical mixing. The surface ozone concentrations in most cases dropped to below 20 ppb (Figure 9), because of the loss by dry deposition at the ground or the titration of small NO emissions close to the ground. Elevated ozone layers were frequently detected at night (Figure 9). These ozone layers were distributed at m and associated with relatively high watermixing ratios. [22] Elevated ozone-rich air layers was repeatedly observed in midmorning (Figure 7d), the afternoon (Figures 8d and 8e), the early evening and at night (Figures 9b 9e) during the period of experimentation. These ozone-rich layers may have previously formed inland in the sea breeze and, subsequently been transported toward the coastal region aloft in the return sea breeze. However, as mentioned in section 2.1, the Ilan plain produced only light anthropogenic emissions of ozone precursors. Therefore the probability that the local emissions produced the ozone-rich air layers should be low. Alternatively, the ozone-rich air layers may have been transported horizontally a considerable distance, and were some of the mixing layers in a more polluted area, such as Taipei city, located just to the west of the Shiueshan Mountain Range, which is the natural barrier that separates the study area (Ilan plain) and Taipei city. Other meteorological process may still account for the formation of the observed elevated ozone layers, such as the offshore advection of pollutants, lofting of pollutants in the sea breeze front and the injection of convective air mass into the inversion layer, advective venting, undercutting of the mixed layer by the advancing sea breeze, injection of pollutants into inversion layers by slop flows, mountain venting and evening stabilization. McKendry and Lundgren [2000] reviewed in detail possible mechanisms of formation of elevated pollutant layers. The formation of the elevated ozone layers in the study areas was expected to be quite complicated. However, further study is necessary to improve our understanding of the behavior of the ozone pollution therein. A sophisticated photochemical model associated with detail flow simulations should be a powerful tool in such work. Finally, the altitudes of the nighttime elevated ozone layers are higher than those observed in midmorning or the afternoon (Figures 7 and 8), possibly because of the development of a relatively largescale sea breeze or a combination of a sea breeze and mountain circulations during the late afternoon or evening. Figure 7. Ozone (O 3 ), potential temperature (PT) and water-mixing ratio (MR) profiles measured by tethered ozonesondes at the measurement site in the midmornings during August 2003, (a) for 25 August at LST, (b) for 26 August at LST, (c) for 28 August at LST, (d) for 29 August at LST, and (e) for 30 August at LST. A and D indicate ascending and descending measurements. The horizontal dashed lines represent TIBL depths; horizontal solid lines on 26 and 29 August represent the tops of sea breeze layers that were identified from wind profiles obtained from 1100 LST meteorological soundings. 7of16

8 Figure 7 8of16

9 Figure 8. As Figure 7 but measured in the afternoon and the tops of sea breeze layers that are identified from the wind profiles that were obtained from daily 1400 LST meteorological soundings, (a) for 25 August at LST, (b) for 26 August at LST, (c) for 28 August at LST, (d) for 29 August at LST, and (e) for 30 August at LST. 9of16

10 Figure 9. As Figure 7 but measured at night. Vertical lines represent the ranges of altitudes of elevated ozone layers, (a) for 24 August at LST, (b) for 25 August at LST, (c) for 26 August at LST, (d) for 29 August at LST, and (e) for 29 August at LST. 10 of 16

11 Banta [1995] reported on this situation in a study of sea breeze on the California Coast. In his investigation, the sea breeze developed initially from the ground up to m before 1200 PST. However, in the afternoon, a deeper sea breeze developed and extended over 2000 m. Banta [1995] related the development of the deeper sea breeze to the superimposition of a regional circulation on a local one. [23] The initial vertical distributions of ozone in sea breeze air masses before they were transported inland during the experimental period can be revealed from the ozone profiles that were measured at the near-coast measurement site (Figures 7 and 8). Regardless of the lower part of the profiles within the TIBL developed at the measurement site, which were mixed by the development of the TIBL between the coastline and the measurement site, these ozone profiles (Figures 7 and 8) suggest that relatively high concentrations of ozone were usually distributed in the upper levels of sea breeze air masses before they were transported inland. Moreover, the ozone distributions in the lower and upper parts of the sea breeze air mass differ. In the lower part of the sea breeze air mass, below 400 m, the concentrations of ozone usually increased with altitude (Figures 7 and 8). In the upper part of a sea breeze air mass, above 500 m, elevated ozone layers were occasionally present (Figures 7d, 8d, and 8e). The increase of ozone concentration with altitude in the lower part of the sea breeze air mass is probably related to the loss of ozone by dry deposition at the sea surface and after transportation inland. Typical ozone dry deposition velocities are 0.4 and 0.07 cm/s over land and water, respectively [Seinfeld and Pandis, 1998; Wesely and Hicks, 2000]. Although the deposition velocity over land greatly exceeds that over the sea surface, the overall amount of ozone deposited may still be dominated by the air mass remaining over the surface of the sea, because the air mass is expected to stay much longer time over the sea surface than over the land. The ozone at the upper levels of the air mass cannot quickly replenish the ozone lost at the surface of the sea because the air mass over the sea is stable, and vertical mixing is forbidden. The elevated ozone layers in the upper parts of the sea breeze air masses, such as those observed during the day on 29 and 30 August (Figures 7d, 8d, and 8e) may be formed by a range of possible mechanisms, as discussed above. [24] Notably, since the air masses over the sea are stable before they are transported inland, and ozone is necessarily lost at the sea surface by dry deposition, the vertical derivative of ozone concentration in the lower part of a sea breeze air mass before it is transported inland is expected to be positive frequently in a coastal region. Moreover, elevated ozone layers, frequently detected during this experimental period, were found in other coastal environments, as mentioned in the Introduction. Therefore the relatively high concentrations of ozone in the upper levels of a sea breeze air mass before the air mass is transported inland, is probably a feature that is common to coastal environments Effect of Ozone-Rich Sea Breeze Air Masses on Inland Surface Ozone Concentrations [25] The development of a TIBL in a coastal region under sea breeze circulation is a common feature of coastal regions [Garratt, 1992]. Additionally, as discussed in the preceding section, the relatively high concentrations of ozone in the upper levels of a sea breeze air mass before the air mass is transported inland may also be a feature that is common to coastal regions during the development of a sea breeze circulation. For example, Millan et al. [2000] demonstrated that ozone reservoir layers frequently form above the coastal areas of the Western Mediterranean Basin, and the Mediterranean Sea, in summer. The fumigation of the ozone reservoir layers during the development of sea breeze circulations or other mesoscale atmospheric circulations strongly influenced the ozone exposure in these coastal areas. The coupling of these two features can produce an interesting inland ozone distribution that inland surface ozone concentrations due to the contribution of the ozone-rich sea breeze air mass increase with the distance inland. Figure 10 presents a conceptual model of the development of the ozone distribution. The conceptual model shows that when a sea breeze air mass with relatively high concentrations of ozone in its upper levels is transported inland, the depth of the TIBL developed in the lower part of the air mass increases. Subsequently, the increase in the depth of the TIBL brings relatively ozone-rich air aloft into the growing TIBL and, subsequently, increases surface ozone concentrations farther inland. Notably, this conceptual model is not limited to sea breeze conditions. It applies whenever onshore flow occurs over relatively cool coastal waters. Under these conditions, the onshore air mass is initially stable when marching on the land, and, subsequently, a TIBL develops in its lower part adjacent to the ground, because the land surface is relatively warm. The surface ozone concentrations at various distances inland, governed by the ozone-rich sea breeze air masses, which were observed at the near-coast measurement site on five afternoons during the experiment, are analyzed to demonstrate the conceptual model that is presented in Figure 10. [26] Table 1 lists the analyzed sea breeze air masses. The afternoon sea breeze air masses are given because the daily ozone inland peaked usually in the afternoon. Furthermore, the sea breeze depths are important parameters in subsequent analysis and were detected daily during the afternoons in the experimental period. The TIBL depth is an important parameter in determining the extent of vertical mixing in the analyzed ozone-rich sea breeze air mass. Therefore the prediction of TIBL depth inland is first presented. The depth of the TIBL developed in a sea breeze air mass at a particular distance inland is estimated using the simple semiempirical equation, presented by Garratt [1992], H b ðþ¼ax x 1=2 ; where x denotes the fetch in meters, which is the distance a given sea breeze air mass traveled from the coastline; H b (x) is the estimated TIBL depth at fetch x, and a is an empirical coefficient. The coefficient varies inversely with sea breeze speed and is related to the initial stability of the sea breeze air mass [Garratt, 1992]. Equation (1) indicates that the predicted TIBL depths increase with the square root of the distance inland. [27] Coefficient a in equation (1) for predicting the TIBL depth in each analyzed sea breeze air mass can be obtained by fitting H b and the fetch measured at the measurement site associated with each analyzed sea breeze air mass. Each ð1þ 11 of 16

12 Figure 10. Conceptual model for describing the vertical variation of ozone in a sea breeze air mass during transport of the air mass from the sea area (1) to different inland locations (2 4). The model demonstrates that when relatively high concentrations of ozone distributed at upper altitudes of the sea breeze air mass before the mass is transported inland, the surface ozone concentration, because of the contribution of the ozone-rich sea breeze air mass, will increase as distance inland increases, C 1 <C 2 < C 3 < C 4. This situation results from the growth of the TIBL inland, H b1 <H b2 <H b3 <H b4, bringing relatively high concentrations of ozone aloft into the growing TIBL and, subsequently, increasing the surface ozone concentration farther inland. fetch was calculated from the location of the measurement site, 3.9 km inland, and the direction traveled by each air mass at the measurement site. This direction can be determined from the time series of surface wind directions, plotted in Figure 3. The calculated fetches for the five analyzed sea breeze air masses were (Table 1). These fetches were approximately equal to the distance from the coastline to the measurement site, 3.9 km, because the five analyzed sea breeze air masses approached almost directly normal to the coastline. [28] The TIBL depths and fetches at the measurement site for each analyzed sea breeze air mass were substituted into equation (1) to determine the empirical coefficients for each analyzed air mass, which were (Table 1). The theoretical analysis by Hsu [1986] suggested that a reasonable range for the empirical coefficient is Garratt [1992] stated that most observations yield coefficients of between 2 and 5. Therefore the coefficients from 2.9 to 6.4 (Table 1), are quite consistent with those determined by Garratt [1992] and Hsu [1986]. Finally, each acquired empirical coefficient was substituted in equation (1) to estimate the depth of the TIBLs that had developed in each sea breeze air mass at various distances inland. [29] A sea breeze air mass is regarded as a single entity with complete mixing in the TIBL, which develops in the lower part of each sea breeze air mass over land. Additionally, the ozone that is initially present in an analyzed sea breeze air mass before it is transported inland is assumed to be a passive tracer. Then, the inland surface ozone concentration contributed by the ozone-rich sea breeze air mass at a given distance inland can be estimated from the ozone concentration after complete mixing of the ozone in the TIBL in the sea breeze air mass, Cx ð Þ ¼ 1 H b ðþ x Z z¼hb ðþ x z¼0 Cz ðþdz; where x is the distance inland; C(x) is the predicted ozone concentration in the TIBL or at the ground surface due to complete mixing in the TIBL; H b (x) is the TIBL depth Table 1. Time Period for Each Sea Breeze Air Mass Arriving at the Measurement Site, the Observed TIBL Depth and Calculated Fetch During Each Time Period, and the Derived Empirical Coefficient, a, Defined in Equation (1) for Each Air Mass Date Time, LST TIBL, m Fetch, km Coefficients, a 25 Aug Aug Aug Aug Aug ð2þ 12 of 16

13 Table 2. Predicted Surface Ozone Concentrations at the Near-Coast Measurement Site and the Inland Sea Breeze Penetrated Location, and the Concentration Difference in Predicted Surface Ozone Concentrations Between the Two Places for Each Analyzed Case Date Analyzed Case Measuring Site Penetrated Location Sea Breeze Depth, m Fetch, km Ozone, ppb Fetch, km Ozone, ppb Ozone Concentration Difference, ppb 25 Aug Aug Aug Aug Aug developed at the lower part of the sea breeze air mass at a distance x; z is the altitude, and C(z) is the initial vertical distribution of ozone in the sea breeze air mass before the mass was transported inland. Notably, the best C(z) for use in equation (2) is that measured at the coastline. In this situation, the deviation in the prediction of the surface ozone concentration by equation (2) from the actual value due to the production or loss of ozone in the sea breeze air mass before it moves over land can be completely eliminated. Additionally, equation (2) reveals that the initial vertical distributions of ozone in the sea breeze air mass before it is transported inland, C(z), and the TIBL depth developed at a given distance inland in the lower part of the sea breeze air mass, H b (x), are the only two factors that are required to determine inland surface ozone concentrations, C(x). [30] In the subsequent analysis, the ozone profile measured at the near-coast measurement site is used as C(z) in the right side of equation (2). Accordingly, the surface ozone concentrations predicted at a given location between the coastline and the measurement site are approximately equal to those predicted at the measurement site because the lower part of the ozone profile used in equation (2) has been mixed by the TIBL that developed at the measurement site. Notably, if the ozone profile used in equation (2) was the same as that in the sea breeze air mass before it is transported inland, as shown in Figure 10, in which the ozone concentrations increase with altitude, then the predicted surface ozone concentrations should increase with distance inland from the coastline to the measurement site. Additionally, the surface ozone concentrations obtained via equation (2) should be limited to the inland distances that are smaller than the inland penetration of the analyzed sea breeze air mass, which for each analysis, can be approximated using the required distance at which the TIBL depth given by equation (1) approaches the sea breeze depth (Table 2). This principle was originally proposed by Garratt [1992]. [31] Figure 11 presents the inland sea breeze penetrations, which are indicated by vertical lines, estimated for the five analyzed cases. Table 2 also lists the estimated inland sea breeze penetrations. The penetrations were km (Figure 11 and Table 2). These distances, with the exception of that on 25 August (Table 2), are in the study plain or the surrounding mountain ranges less than 10 km from the inland plain border. However, the inland penetrations of sea breezes estimated for the five analyzed cases were probably underpredicted by km because of the associated underprediction of sea breeze depths. As discussed above, sea breeze depths may be underpredicted by several hundred meters. [32] Figure 11 plots the predicted surface ozone concentrations at various distances inland that are due to the ozonerich sea breeze air masses that were observed at the near-coast measurement site in the afternoons on the five days of the experiment. Table 2 lists the predicted surface ozone concentrations at the measurement site and in the inland sea-breeze-penetrated position in each analyzed case. In each case, the predicted depth of the TIBL increases with the distance inland (Figure 11). This increase in TIBL depth is expected on the basis of the predictive formula, equation (1). The predicted inland surface ozone concentration also increases with distance inland (Figures 11b and 11d), except near the coast, where the predicted ozone concentrations are relatively invariant because the ozone profiles that were used in equation (2) were measured at the measurement site, as discussed above. Notably, the five analyzed events are associated with similar surface ozone concentrations in the near-coast region, ranging between 20 and 30 ppb, but quite different surface ozone concentrations farther inland (Figure 11). The surface ozone concentrations are relatively high farther inland are on 29 and 30 August. Table 2 also demonstrates that surface ozone concentrations at the near-coast measurement site in the five analyzed cases are similar but those farther inland vary. The ozone concentrations at the measurement site were ppb and those at the inland sea-breezepenetrated locations were ppb. Differences in surface ozone concentrations between the measurement site and the sea-breeze-penetrated location were Additionally, on 29 and 30 August, the difference between the surface ozone concentration at the measurement site and that at the inland sea-breeze-penetrated location was relatively large. Notably, since the sea breeze depths that are used to predict inland sea breeze penetration may be underpredicted by several hundred meters, as discussed above, the sea breeze penetrations may be underpredicted by km. When underprediction of the inland sea breeze penetration is considered, the predicted surface ozone concentrations at the inland sea-breeze-penetrated locations for the two cases may be as high as 60 ppb (Figures 11d and 11e), and the differences between the ozone concentration at the measurement site and that at the inland sea-breeze-penetrated location in the two cases may be as high as 30 ppb (Figures 11d and 11e). [33] The variation in predicted surface ozone concentrations, which increase with distance inland, was consistent with the conceptual model (Figure 10). The growing inland TIBL also affects the increase in the surface ozone concentration with distance inland in the five cases. This increase 13 of 16

14 Figure 11. Predicted depths of the TIBL at different distances inland and the predicted surface ozone concentrations at different distances inland contributed by ozone-rich sea breeze air masses observed at the measurement sites on the afternoons (a) for 25 August, (b) for 26 August, (c) for 28 August, (d) for 29 August, and (e) for 30 August. The horizontal line indicates the sea breeze depth for each analyzed case; the vertical line indicates the estimated location of the inland sea breeze penetration for each case. 14 of 16

15 can bring the upper level ozone-rich air into the growing TIBL, increasing surface ozone concentration with distance inland. On 29 and 30 August, the predicted surface ozone concentrations at inland locations were relatively high. Such situations probably result from the presence of quite ozonerich air in the upper portions of the sea breeze air masses, the elevated ozone layers, on 29 and 30 August (Figures 8d and 8e). Therefore an elevated ozone layer can increase surface ozone contributions inland, and, subsequently, increase the differences between the surface ozone concentrations near the coast and inland. However, the two cases with elevated ozone layers do not have significantly larger contributions to surface ozone concentrations in the nearcoast area than other cases without elevated ozone layers; all analyzed cases have similar ozone contributions in the near-coast area (Table 2). Notably, an elevated ozone layer can contribute only to surface ozone concentrations at a location inland that the TIBL has reached. Conversely, the elevated ozone layer should contribute little to the surface ozone concentration near the coast because the depth of the TIBL is limited there such that the ozone in the elevated ozone layer cannot be brought to the ground. Therefore the development and limited depth of TIBL in at a near-coast area may protect the area, eliminating fumigation of the elevated ozone layers. 4. Summary [34] This work employed tethered ozonesondes and meteorological radiosondes to measure vertical distributions of ozone at a site near the northern coast of Taiwan during August Experimental data indicate that relatively high concentrations of ozone are always initially distributed in the upper portions of sea breeze air masses before they are transported inland. This distribution probably results partially from the loss on ozone by dry deposition at the sea surface before the sea breeze air mass is transported inland, and partially from the transport of elevated ozone layers. The initial profile of ozone in a sea breeze air mass before the mass is transported inland, and the depth of the TIBL that develops inland in the lower part of the sea breeze air mass are two important factors that determine the surface ozone concentrations contributed by the ozone-rich sea breeze air mass. The relatively high concentration of ozone in the upper altitudes in a sea breeze air mass before it is transported inland, coupled with the growth of TIBL inland, can produce an interesting ozone pollution feature, in which surface ozone concentrations increase with the distance inland. This pollution feature is formed because when the sea breeze air mass is transported farther inland, the ozonerich air aloft in the sea breeze air mass is brought into the growing TIBL inland and subsequently increases the surface ozone concentration inland. This pollution feature was reproduced in five analyzed cases, on the basis of the vertical distributions of ozone and sea breezes that were observed at the near-coast measurement site. Although the model used to analyze the inland surface ozone concentration due to the contribution of ozone-rich sea breeze air masses is simple, using a more detailed model should produce the same pollution features. The pollution feature supposes that relatively serious ozone pollution inland, usually observed in coastal regions, can be generated in part by the landward transport and subsequently downward mixing of the ozone-rich air that was initially aloft in a sea breeze air mass, and is not completely attributable to the inland photochemical production of ozone. Apparently, the vertical ozone gradient dominates the difference between the concentration in a near-coast area and those in areas far inland. On the days of this study, the difference reached as high as 30 ppb, but on other days or other situations, the difference may be much higher. [35] Elevated ozone layers were frequently observed during the experimental period. Similar elevated ozone layers have been also found in other coastal environments, as described in the Introduction. This work demonstrated that an elevated ozone layer can increases surface ozone concentrations only beyond a certain distance inland when the depth of the TIBL has reached the ozone layer. Conversely, an elevated ozone layer contributes little to the surface ozone concentration near the coast because the depth of the TIBL is limited in the near-coast area, such that the ozone in the elevated ozone layer cannot be mixed downward toward the ground. Therefore the development of the TIBL and its limited depth in a near-coast area may protect that area from the fumigation of elevated ozone layers. Finally, vertical ozone profile measurements in a coastal region are useful in evaluating the contribution of ozone-rich sea breeze air masses to surface ozone concentrations, because the surface ozone measurements in a nearcoast area do not reveal high concentrations of ozone in the upper levels of a sea breeze air mass. [36] Acknowledgments. The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contracts NSC E and NSC 93-EPA-Z References Atkinson, B. W. (1981), Mesoscale Atmospheric Circulations, Elsevier, New York. Banta, R. M. (1995), Sea breezes shallow and deep on the California Coast, Mon. Weather Rev., 123, Boucouvala, D., and R. Bornstein (2003), Analysis of transport patterns during a COS97-NARSTO episode, Atmos. Environ., 37, S73 S94. Garratt, J. R. (1992), The Atmospheric Boundary Layer, Cambridge Univ. Press, New York. Grossi, P., P. Thunis, A. Martilli, and A. Clappier (2000), Effect of sea breeze on air pollution in the Greater Athens Area. Part II: Analysis of different emission scenarios, J. Appl. Meteorol., 39, Helmis, C. G., D. N. Asimakopoulos, D. G. Deligiorgi, and D. P. Lalas (1987), Observations of sea-breeze fronts near the shoreline, Boundary Layer Meteorol., 38, Hsu, S. A. 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