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The effects of mesoscale circulation on the dispersion of pollutants (SO2) in the eastern Mediterranean, southern coastal plain of Israel
Atmospheric Environment Vol. 26B, No. 3, pp. 271-277, 1992.
0957-1272/92 $5.00+0.00 © 1992 Pergamon Press Ltd
Printed in Great Britain.
THE EFFECTS OF MESOSCALE CIRCULATION ON THE DISPERSION OF POLLUTANTS (SO2) IN THE EASTERN MEDITERRANEAN, SOUTHERN COASTAL PLAIN OF ISRAEL J. ROBINSO"N, Y. MAHRER a n d E. WAKSHAL The Seagram Centre for Soil and Water Sciences, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot 76100, Israel (First received 19 July 1991 and in final form 21 February 1992) Abstract--A three-dimensional numerical mesoscale air quality model is applied to study the effect of
pollutant recirculation in the eastern Mediterranean, southern coastal plain of Israel. The model is based on the integration of two submodels: a mesoscale submodel and a Lagrangian dispersion submodel. The model is validated using air quality data measured during a severe pollution event in the city of Ashdod. When the reeirculated portion of the pollution was separated from that directly contributed by the sources, it was found to constitute about 50% of the total concentration measured. Key word index: Numerical model, mesoscale model, air pollution, recirculation of pollutants, sea and land breezes.
INTRODUCTION
The main anthropogenic pollution sources, as well as the most densely populated areas in Israel, are concentrated in the coastal plain. The Judean mountains extend parallel to the Mediterranean coast of Israel, 20-30 km inland, and range in elevation from 600 to 800m above sea level (Fig. I). This combination of geographical features causes an increased concentration of pollutants in the coastal plain, due to circulation generated by sea and land breezes, and mountain and valley winds. The following three mechanisms can cause the recirculation of those pollutants (Segal et al., 1982): (1) The shift from land to sea breeze combined with the change from katabatic to anabatic winds during the morning hours of a typical summer day. This time of the year is characterized by a northwest to westerly synoptic flow, caused by troughing from the Persian Low. (2) A change in the coupling between offshore synoptic flow and the opposing sea breeze during the daylight hours. This often occurs during advective Sharav conditions (a low pressure system from Africa causing hot and dry easterly winds over Israel) and causes a very stable offshore structure in which pollutants become concentrated at the lower levels. (3) Pollutant transport inland by the upper return flow of the land breeze on calm winter nights. The phenomenon of increased pollutant concentration due to recirculation has been described by many authors. Lyons and Cole (1976) presented the case of increased ozone concentrations near Milwaukee at the
Lake Michigan shoreline, and attributed it to recirculation caused by the shift from a land to sea breeze. The highest ozone concentrations were measured in the morning, when the recirculated pollution plume penetrated internal thermal boundary layer (about 1.5 km from the lake's shore). Ozone concentrations measured in Israel and described by Peleg et al. (1989) were shown to be higher in the Ashdod area (30 km south of Tel Aviv, on the Mediterranean coast; Fig. 1), at noon, during periods of easterly winds. They explained that primary pollutants emitted at Ashdod were carried offshore by the easterly winds and then recirculated inland, once the sea breeze became dominant. The ozone, a secondary pollutant, was formed while the pollutants were over the water. There is general agreement that pollutants from sources located in coastal areas can be recirculated. This work deals with the more specific question of whether exceptionally high SO: concentrations are caused by the above-mentioned mechanisms, taking into account that, like most power plants, the sources are point sources. For pollution to reach the high levels dealt with here, the recirculated pollutants must be returned to the source area and added to the "freshly" emitted pollutants. Lyons (1975) and Ludwig (1983) have claimed that situations in which the recirculated pollutants return to their site of emission are extremely rare, due to the complexity of the wind field. The aim of this research is to investigate possible recirculation events associated with a weak, synopticscale easterly flow, taking into account the fact that the direction ofthe sea breeze is influenced by anabatic winds which have a constant west to northwest ori271
272
J. ROBINSOHNet al.
80.
used by McNider to correct a particle setting problem (Arritt, 1985). The Lagrangian particle model equations, discussed also in Pielke (1984), are summarized below:
F I l ' l l '
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X(KM) Fig. 1. A schematic illustration of the topography in the simulated domain. Topography contours (in m) are illustrated by solid lines; the coastline, as well as the approximate locations of Ashdod (A),Tel Aviv(T) and Jerusalem (J) are indicated;the box represents the area to which the Lagrangian dispersion model was applied.
entation (Doron and Neumann, 1977). Under these conditions there is a high probability of pollutant recirculation to the source emission area. An event of anomalously high concentrations, connected to recirculation in the Ashdod area, was investigated by simulating the meteorological and SO2 concentration fields throughout the day.
THE MODEL A three-dimensional numerical mesoscale model, the formulation of which is given by Mahrer and Pielke (1977) and Pielke (1984), was applied to predict the meteorological fields. The results were used as input for a Lagrangian dispersion model (Pielke, 1984; Segal et al., 1988). A 30 x 30 x 15 grid was used with a horizontal grid spacing of 5 km in the x- and ydirections, and variable grid distance in the vertical. Vertical levels were set at 5, 15, 50, 100, 300, 500, 700, 900, 1200, 1500, 2000, 3000, 4000, 5000 and 6000 m. The time step of the integration was 30 s. The initial conditions for the meteorological model included rawinsonde data of air temperature, humidity, wind speed, wind direction and pressure obtained from the Israel Meteorological Institute in Bet Dagan (about 25 km north of Ashdod). The model used a terrain-following coordinate system (Mahrer and Pielke, 1978), and was based on the equations of motion, heat, humidity and continuity in the atmosphere. When emission from a point source is considered, the vertical and spatial scales are generally beyond the resolving ability of the mesoscale model. The application of a Lagrangian particle transport and dispersion model, in which mesoscale model predictions are used to determine statistics for transport and mixing, was therefore particularly suited to this study. The approach adopted here is based on that of McNider (1981) but includes a drift correction to the vertical velocity instead of the original skewness formulation
i = 1, 2, 3,
(1)
where x~(t) is the antecedent x, y, z position of the particle and x~(t + 60 is its position after time interval fit; u~ represents the u, v and w velocity components, respectively, and u; represents the corresponding turbulence velocity fluctuations, parameterized statistically and based on the mesoscale model's boundary layer prediction. Following Hanna (1979) the turbulence velocities are given by: u',(t)= u',(t - 6t)R (60 + u',';i = 1,2,3,
(2)
where R (&) is a Lagrangian autocorrelation function and u'{ is a random component (see Wilson et al., 1981) with a Gaussian distribution of zero mean, and a variance defined by the variance in local turbulence. Particles, representing a pollutant air mass, were released at a rate of one per minute, and effective stack height was computed using the Briggs formula (Hanna et al., 1982), where recalculations were performed once an hour. Oxidation of SO 2 to H2SO 4 was assumed to occur at the maximal rate of 10% per hour, dependent upon radiation and humidity (Finlayson-Pitts and Pitts, 1986). It was further assumed that the synoptic situation remained unchanged during the pollution episode and that the pollution did not affect the mesoscale meteorological parameters. EXPERIMENTAL PROCEDURE
The combined mesoscale and dispersion model was applied to the complex topography of the Ashdod area. Taking these topographic features, the available computer resources and the need to consider the effects of sea and land breezes, mountain and valley flows and synoptic wind, a horizontal resolution of 5 km was used in the mesoscale model. The dispersion model however, was applied to a reduced area of 30 x 25 km, around the power sources. Figure 2 illustrates the locations of the power plant, refineries and monitoring stations. Measurements were taken at the Ashdod monitoring station which is located about 2.5 km southeast of the Eshkol power plant, and about 2 km southwest of Ashdod's refineries,and at the Bnaia monitoring station located 10km west of the power plant. The power plant and oil refineries are the main SO2-emitting sources in the area. The model was tested for a severe SO2 pollution episode which occurred in the city of Ashdod on 3 June 1988. On this day an average half-hour concentration of 349 ppb SO2 was measured between 11:30and 12:00LT (Fig. 3). High concentrations were not recorded at the Bnaia monitoring station on this day.
RESULTS The synoptic situation over Israel on 3 June 1988 was characterized by a warm air mass aloft and almost no pressure gradient at the land surface. At 2:00 LT
Mesoscale circulation and pollutant dispersion (0:00 GMT) the combination of a weak high-pressure cell to the north of Israel and a weak, warm lowpressure cell over Egypt (drifting slowly eastward) 25
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273
placed Israel in a weak offshore surface flow situation (Fig. 4). This was also indicated in the 0h00 LT (23:00 GMT) Bet Dagan sounding (Table 1). The weak ground-level flow allowed for more influence than usual of the mesoscale forces. During the night, under clear skies and with almost no wind, the air was subjected to intensive radiational cooling and a steep low-level inversion developed. Because of this, the land areas became much cooler than sea-surface temperature (23°C in early June) and a strong land breeze developed. In Fig. 5, the predicted horizontal wind vectors at 50m on 3 June 1988 are shown for three different hours. At 05:00 LT the winds were blowing seaward (easterly flow) as a result of the land breeze, which is enhanced in that part of Israel by katabatic flow (due to the presence of higher terrain toward the east). At 08:00 LT the mesoscale flow had completely changed direction and had begun moving to the northwest, due to the rapid daytime heating of the ground under clear skies and the very high insolation typical of the month of June. Offshore the remnants of the land breeze persisted with a continuing easterly direction. At 1h00 LT however, the sea breeze had taken complete control of low-level flow near the coast, resulting in stronger westerly wind components. Figure 6 provides a quantitative horizontal representation of the location of SO 2 particles as predicted by the model. Each point represents a volume of gas discharged during a 1-min interval, beginning at 01:00 LT. During the night, the pollutants were transported offshore to the northwest by the combined land breeze and mountain downflow. In the morning, with the diurnal veering of the wind, they were transported to the south. At 10:00 LT most of the pollutants were still offshore. At 11:00 LT, an intensifying westerly combined sea breeze and mountain upfiow caused a shift in the wind direction and at about 12:00 LT a high concentration of SO2 (depicted by the high
Table 1. Rawinsonde data, Bet Dagan, 3 June 1988 at 0h00 LT Height (m) 0 47 78 263 723 959 1019 1213 1452 1963 3104 3401 3468 5807 6233 7492 9549
P r e s s u r e Temperature (mb) (°C) 1006 1000 996 975 925 900 894 874 850 800 700 675 600 500 475 400 300
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12.7 16.3 18.8 14.3 9.3 8.0 7.7 4.3 4.3 3.4 - 3.1 - 4.9 - 10.4 - 21.4 -- 16.4 - 34.0 - 47.4
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density of points in Fig. 6) was found over the Ashdod area. The warm, dry air mass overlying the whole of Israel by day generated very stable stratification above the Mediterranean. As a result of this atmospheric stability and the weak winds, the particles were only slightly mixed while offshore and remained concentrated throughout the night in a thin, 200 m layer (Fig. 7). Predicted half-hour averaged concentrations of SO2 for the 15-50m level at 12:00 (time of maximum observed concentration at the Ashdod monitoring station) are illustrated in Fig. 8 (top). It is interesting to
note that the Bnaia monitoring station is outside the high concentration area, a fact which was verified by field observations. In order to differentiate quantitatively between recirculated particles and newly released ones, we also show the concentration due to particles released between 10:00 and 12:00 LT (Fig. 8, centre). A comparison indicates that the recirculated particles contributed about 50% to the total concentration. This can be seen in Fig. 8 (bottom)--the calculated SO2 concentration of recirculated particles initially released between 01:00 and 10:00 LT. A situation in which recirculated particles are combined with newly
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released ones from the same source can only occur when the winds are in supporting directions. Otherwise, the recirculated pollution could end up at any inland location along the coast•
CONCLUSIONS
Pollutant recirculation can be an important factor in causing anomalously high concentrations at the source area under certain meteorological conditions. This study shows that when a weak synoptic flow produces a deep low-level inversion during the night (fostering the land breeze), the shift to a sea breeze in the late morning hours can cause this phenomenon. In general, the recirculated portion does not in itself
cause such high concentrations. If, however, the wind field (combined mesoscale and synoptic) exhibits a suitable pattern, the recirculated pollutants can merge to the 'freshly' emitted ones, thereby contributing to the total observed concentration. Due to the extended period of time during which the pollutants remained in the humid offshore air, it is highly probable that the recirculated air mass also contained high quantities of oxidized sulphate, of marine origin. Unfortunately no analytical H2SO4 data were available to confirm our model findings. Results describing the effects of meteorological conditions on pollutant recirculation could be important to the timing of a switch to low-level sulphur fuel, particularly on days when stagnant meteorological conditions, such as those described here, are present.
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the staffat the Israeli Meteorological Institute for their useful suggestions and assistance.
REFERENCES
10
't
277
10
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20
25
X (KM) Fig. 8. Model-calculated isoplates of SO2 for the 15-50 m level, on 3 June 1988, (top) at 12:00 LT (particle release starting at 01:00 LT, (centre) contribution of particles released between 10:00 and 12:00 LT, (bottom) contribution of particles released between 01:00 and 10.00 LT.
Acknowledoements--This research was supported by the Israel Academy of Science, by the World Meteorological Organization and by the S. A. Schonbrunn Research Endowment Fund, The Hebrew University of Jerusalem• The authors are grateful to David N. Deker, Doron Lahav and
Arritt R. W. (1985) Numerical studies of thermally and mechanically forced circulation over complex terrain. Ph.D. dissertation, Department of Atmospheric Sciences, Colorado State University, Fort Collins, CO. Doron E. and Neumann J. (1977) Land and mountain breezes with special attention to Israel's Mediterranean coastal plain. Israel Met. Res. Papers 1, 109--122. Finlayson-Pitts B. J. and Pitts J. N. (1986) Atmospheric Chemistry: Fundamental and Experimental Techniques. Wiley, New York. Hanna S. R. (1979) Some statistics of Lagrangian and Eulerian wind fluctuation. J. appl. Met. 18, 518-531. Hanna S. R., Briggs C. A. and Hosker R. (1982) Plume rise. In Handbook on Atmospheric Diffusion, pp. 11-17. Technical Information Center, U.S. Department of Energy, U.S.A. Ludwig F. L. (1983) A review of coastal zone meteorological processes important to the modeling of air pollution• In Air Pollution Modeling and its Application in Challenges of Modern Society, Vol. 7, pp. 225-257. Lyons W. A. (1975) Turbulent diffusion and pollution transport in shoreline environments• In Lectures on Air Pollution and Environmental Impact Analyses, pp. 136-208. American Meteorological Society, Boston,• MA. Lyons W. A. and Cole H. S. (1976) Photochemical oxidant transport: mesoscale lake breeze and synoptic scale aspects. J. appl. Met. 15, 733-743. Mahrer Y. and Pielke R. A. (1977) A numerical study of the airflow over irregular terrain. Beitrag Phys. Atmos. 50, 98-113, Mahrer Y. and Pielke R. A. (1978) A test of an upstream spline interpolation technique for the advective terms in numerical mesoscale models. Mon. Weath. Rev. 106, 818-830. McNider R. T. (1981) Investigation of the impact of topographic circulations on the transport and dispersion of air pollutants. Ph.D. dissertation, Dept. of Environmental Sciences, University of Virginia, Charlottesville, VA. Peleg M., Berner D., Lahav D. and Luria M. (1989) Ozone levels in central Israel during May 1988. In Proc. Fourth lnt. Conf. Israel Society for Ecological and Environmental Quality Sciences, pp. A37-A44, Jerusalem, Israel. Pielke R. A. (1984) Mesoscale Meteorological Modelin 0. Academic Press, Orlando, EL. Segal M., Pielke R. A. and Mahrer Y. (1982) Evaluation of onshore pollutant recirculation over the Mediterranean coast area of central Israel. Israel J. Earth Sci. 31, 39--46. Segal M., Yu C. H., Arritt R. W. and Pielke R. A. (1988) On the impact of valley/ridge thermally induced circulations on regional pollutant transport. Atmospheric Environment 22, 471--486. Wilson J. D., Thurtell G. W. and Kidd G. E. (1981) Numerical simulation of particle trajectories in inhomogeneous turbulence,If: Systems with a variable turbulent velocity scale. Boundary-Layer Met. 21, 423-441.
0957-1272/92 $5.00+0.00 © 1992 Pergamon Press Ltd
Printed in Great Britain.
THE EFFECTS OF MESOSCALE CIRCULATION ON THE DISPERSION OF POLLUTANTS (SO2) IN THE EASTERN MEDITERRANEAN, SOUTHERN COASTAL PLAIN OF ISRAEL J. ROBINSO"N, Y. MAHRER a n d E. WAKSHAL The Seagram Centre for Soil and Water Sciences, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot 76100, Israel (First received 19 July 1991 and in final form 21 February 1992) Abstract--A three-dimensional numerical mesoscale air quality model is applied to study the effect of
pollutant recirculation in the eastern Mediterranean, southern coastal plain of Israel. The model is based on the integration of two submodels: a mesoscale submodel and a Lagrangian dispersion submodel. The model is validated using air quality data measured during a severe pollution event in the city of Ashdod. When the reeirculated portion of the pollution was separated from that directly contributed by the sources, it was found to constitute about 50% of the total concentration measured. Key word index: Numerical model, mesoscale model, air pollution, recirculation of pollutants, sea and land breezes.
INTRODUCTION
The main anthropogenic pollution sources, as well as the most densely populated areas in Israel, are concentrated in the coastal plain. The Judean mountains extend parallel to the Mediterranean coast of Israel, 20-30 km inland, and range in elevation from 600 to 800m above sea level (Fig. I). This combination of geographical features causes an increased concentration of pollutants in the coastal plain, due to circulation generated by sea and land breezes, and mountain and valley winds. The following three mechanisms can cause the recirculation of those pollutants (Segal et al., 1982): (1) The shift from land to sea breeze combined with the change from katabatic to anabatic winds during the morning hours of a typical summer day. This time of the year is characterized by a northwest to westerly synoptic flow, caused by troughing from the Persian Low. (2) A change in the coupling between offshore synoptic flow and the opposing sea breeze during the daylight hours. This often occurs during advective Sharav conditions (a low pressure system from Africa causing hot and dry easterly winds over Israel) and causes a very stable offshore structure in which pollutants become concentrated at the lower levels. (3) Pollutant transport inland by the upper return flow of the land breeze on calm winter nights. The phenomenon of increased pollutant concentration due to recirculation has been described by many authors. Lyons and Cole (1976) presented the case of increased ozone concentrations near Milwaukee at the
Lake Michigan shoreline, and attributed it to recirculation caused by the shift from a land to sea breeze. The highest ozone concentrations were measured in the morning, when the recirculated pollution plume penetrated internal thermal boundary layer (about 1.5 km from the lake's shore). Ozone concentrations measured in Israel and described by Peleg et al. (1989) were shown to be higher in the Ashdod area (30 km south of Tel Aviv, on the Mediterranean coast; Fig. 1), at noon, during periods of easterly winds. They explained that primary pollutants emitted at Ashdod were carried offshore by the easterly winds and then recirculated inland, once the sea breeze became dominant. The ozone, a secondary pollutant, was formed while the pollutants were over the water. There is general agreement that pollutants from sources located in coastal areas can be recirculated. This work deals with the more specific question of whether exceptionally high SO: concentrations are caused by the above-mentioned mechanisms, taking into account that, like most power plants, the sources are point sources. For pollution to reach the high levels dealt with here, the recirculated pollutants must be returned to the source area and added to the "freshly" emitted pollutants. Lyons (1975) and Ludwig (1983) have claimed that situations in which the recirculated pollutants return to their site of emission are extremely rare, due to the complexity of the wind field. The aim of this research is to investigate possible recirculation events associated with a weak, synopticscale easterly flow, taking into account the fact that the direction ofthe sea breeze is influenced by anabatic winds which have a constant west to northwest ori271
272
J. ROBINSOHNet al.
80.
used by McNider to correct a particle setting problem (Arritt, 1985). The Lagrangian particle model equations, discussed also in Pielke (1984), are summarized below:
F I l ' l l '
60: 40>-
x~(t+cSt)=x~(t)+[ut(t)+u~(t)]6t;
2O: 0
,,
4O
,
, , i , , , , ,
80
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120
, , , , , ,
160
X(KM) Fig. 1. A schematic illustration of the topography in the simulated domain. Topography contours (in m) are illustrated by solid lines; the coastline, as well as the approximate locations of Ashdod (A),Tel Aviv(T) and Jerusalem (J) are indicated;the box represents the area to which the Lagrangian dispersion model was applied.
entation (Doron and Neumann, 1977). Under these conditions there is a high probability of pollutant recirculation to the source emission area. An event of anomalously high concentrations, connected to recirculation in the Ashdod area, was investigated by simulating the meteorological and SO2 concentration fields throughout the day.
THE MODEL A three-dimensional numerical mesoscale model, the formulation of which is given by Mahrer and Pielke (1977) and Pielke (1984), was applied to predict the meteorological fields. The results were used as input for a Lagrangian dispersion model (Pielke, 1984; Segal et al., 1988). A 30 x 30 x 15 grid was used with a horizontal grid spacing of 5 km in the x- and ydirections, and variable grid distance in the vertical. Vertical levels were set at 5, 15, 50, 100, 300, 500, 700, 900, 1200, 1500, 2000, 3000, 4000, 5000 and 6000 m. The time step of the integration was 30 s. The initial conditions for the meteorological model included rawinsonde data of air temperature, humidity, wind speed, wind direction and pressure obtained from the Israel Meteorological Institute in Bet Dagan (about 25 km north of Ashdod). The model used a terrain-following coordinate system (Mahrer and Pielke, 1978), and was based on the equations of motion, heat, humidity and continuity in the atmosphere. When emission from a point source is considered, the vertical and spatial scales are generally beyond the resolving ability of the mesoscale model. The application of a Lagrangian particle transport and dispersion model, in which mesoscale model predictions are used to determine statistics for transport and mixing, was therefore particularly suited to this study. The approach adopted here is based on that of McNider (1981) but includes a drift correction to the vertical velocity instead of the original skewness formulation
i = 1, 2, 3,
(1)
where x~(t) is the antecedent x, y, z position of the particle and x~(t + 60 is its position after time interval fit; u~ represents the u, v and w velocity components, respectively, and u; represents the corresponding turbulence velocity fluctuations, parameterized statistically and based on the mesoscale model's boundary layer prediction. Following Hanna (1979) the turbulence velocities are given by: u',(t)= u',(t - 6t)R (60 + u',';i = 1,2,3,
(2)
where R (&) is a Lagrangian autocorrelation function and u'{ is a random component (see Wilson et al., 1981) with a Gaussian distribution of zero mean, and a variance defined by the variance in local turbulence. Particles, representing a pollutant air mass, were released at a rate of one per minute, and effective stack height was computed using the Briggs formula (Hanna et al., 1982), where recalculations were performed once an hour. Oxidation of SO 2 to H2SO 4 was assumed to occur at the maximal rate of 10% per hour, dependent upon radiation and humidity (Finlayson-Pitts and Pitts, 1986). It was further assumed that the synoptic situation remained unchanged during the pollution episode and that the pollution did not affect the mesoscale meteorological parameters. EXPERIMENTAL PROCEDURE
The combined mesoscale and dispersion model was applied to the complex topography of the Ashdod area. Taking these topographic features, the available computer resources and the need to consider the effects of sea and land breezes, mountain and valley flows and synoptic wind, a horizontal resolution of 5 km was used in the mesoscale model. The dispersion model however, was applied to a reduced area of 30 x 25 km, around the power sources. Figure 2 illustrates the locations of the power plant, refineries and monitoring stations. Measurements were taken at the Ashdod monitoring station which is located about 2.5 km southeast of the Eshkol power plant, and about 2 km southwest of Ashdod's refineries,and at the Bnaia monitoring station located 10km west of the power plant. The power plant and oil refineries are the main SO2-emitting sources in the area. The model was tested for a severe SO2 pollution episode which occurred in the city of Ashdod on 3 June 1988. On this day an average half-hour concentration of 349 ppb SO2 was measured between 11:30and 12:00LT (Fig. 3). High concentrations were not recorded at the Bnaia monitoring station on this day.
RESULTS The synoptic situation over Israel on 3 June 1988 was characterized by a warm air mass aloft and almost no pressure gradient at the land surface. At 2:00 LT
Mesoscale circulation and pollutant dispersion (0:00 GMT) the combination of a weak high-pressure cell to the north of Israel and a weak, warm lowpressure cell over Egypt (drifting slowly eastward) 25
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273
placed Israel in a weak offshore surface flow situation (Fig. 4). This was also indicated in the 0h00 LT (23:00 GMT) Bet Dagan sounding (Table 1). The weak ground-level flow allowed for more influence than usual of the mesoscale forces. During the night, under clear skies and with almost no wind, the air was subjected to intensive radiational cooling and a steep low-level inversion developed. Because of this, the land areas became much cooler than sea-surface temperature (23°C in early June) and a strong land breeze developed. In Fig. 5, the predicted horizontal wind vectors at 50m on 3 June 1988 are shown for three different hours. At 05:00 LT the winds were blowing seaward (easterly flow) as a result of the land breeze, which is enhanced in that part of Israel by katabatic flow (due to the presence of higher terrain toward the east). At 08:00 LT the mesoscale flow had completely changed direction and had begun moving to the northwest, due to the rapid daytime heating of the ground under clear skies and the very high insolation typical of the month of June. Offshore the remnants of the land breeze persisted with a continuing easterly direction. At 1h00 LT however, the sea breeze had taken complete control of low-level flow near the coast, resulting in stronger westerly wind components. Figure 6 provides a quantitative horizontal representation of the location of SO 2 particles as predicted by the model. Each point represents a volume of gas discharged during a 1-min interval, beginning at 01:00 LT. During the night, the pollutants were transported offshore to the northwest by the combined land breeze and mountain downflow. In the morning, with the diurnal veering of the wind, they were transported to the south. At 10:00 LT most of the pollutants were still offshore. At 11:00 LT, an intensifying westerly combined sea breeze and mountain upfiow caused a shift in the wind direction and at about 12:00 LT a high concentration of SO2 (depicted by the high
Table 1. Rawinsonde data, Bet Dagan, 3 June 1988 at 0h00 LT Height (m) 0 47 78 263 723 959 1019 1213 1452 1963 3104 3401 3468 5807 6233 7492 9549
P r e s s u r e Temperature (mb) (°C) 1006 1000 996 975 925 900 894 874 850 800 700 675 600 500 475 400 300
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12.7 16.3 18.8 14.3 9.3 8.0 7.7 4.3 4.3 3.4 - 3.1 - 4.9 - 10.4 - 21.4 -- 16.4 - 34.0 - 47.4
0 360 145 150 255 360 350 310 300 310 330 320 290 250 250 230 250
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density of points in Fig. 6) was found over the Ashdod area. The warm, dry air mass overlying the whole of Israel by day generated very stable stratification above the Mediterranean. As a result of this atmospheric stability and the weak winds, the particles were only slightly mixed while offshore and remained concentrated throughout the night in a thin, 200 m layer (Fig. 7). Predicted half-hour averaged concentrations of SO2 for the 15-50m level at 12:00 (time of maximum observed concentration at the Ashdod monitoring station) are illustrated in Fig. 8 (top). It is interesting to
note that the Bnaia monitoring station is outside the high concentration area, a fact which was verified by field observations. In order to differentiate quantitatively between recirculated particles and newly released ones, we also show the concentration due to particles released between 10:00 and 12:00 LT (Fig. 8, centre). A comparison indicates that the recirculated particles contributed about 50% to the total concentration. This can be seen in Fig. 8 (bottom)--the calculated SO2 concentration of recirculated particles initially released between 01:00 and 10:00 LT. A situation in which recirculated particles are combined with newly
J. R O B ~ S o n N et al.
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released ones from the same source can only occur when the winds are in supporting directions. Otherwise, the recirculated pollution could end up at any inland location along the coast•
CONCLUSIONS
Pollutant recirculation can be an important factor in causing anomalously high concentrations at the source area under certain meteorological conditions. This study shows that when a weak synoptic flow produces a deep low-level inversion during the night (fostering the land breeze), the shift to a sea breeze in the late morning hours can cause this phenomenon. In general, the recirculated portion does not in itself
cause such high concentrations. If, however, the wind field (combined mesoscale and synoptic) exhibits a suitable pattern, the recirculated pollutants can merge to the 'freshly' emitted ones, thereby contributing to the total observed concentration. Due to the extended period of time during which the pollutants remained in the humid offshore air, it is highly probable that the recirculated air mass also contained high quantities of oxidized sulphate, of marine origin. Unfortunately no analytical H2SO4 data were available to confirm our model findings. Results describing the effects of meteorological conditions on pollutant recirculation could be important to the timing of a switch to low-level sulphur fuel, particularly on days when stagnant meteorological conditions, such as those described here, are present.
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the staffat the Israeli Meteorological Institute for their useful suggestions and assistance.
REFERENCES
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277
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20
25
X (KM) Fig. 8. Model-calculated isoplates of SO2 for the 15-50 m level, on 3 June 1988, (top) at 12:00 LT (particle release starting at 01:00 LT, (centre) contribution of particles released between 10:00 and 12:00 LT, (bottom) contribution of particles released between 01:00 and 10.00 LT.
Acknowledoements--This research was supported by the Israel Academy of Science, by the World Meteorological Organization and by the S. A. Schonbrunn Research Endowment Fund, The Hebrew University of Jerusalem• The authors are grateful to David N. Deker, Doron Lahav and
Arritt R. W. (1985) Numerical studies of thermally and mechanically forced circulation over complex terrain. Ph.D. dissertation, Department of Atmospheric Sciences, Colorado State University, Fort Collins, CO. Doron E. and Neumann J. (1977) Land and mountain breezes with special attention to Israel's Mediterranean coastal plain. Israel Met. Res. Papers 1, 109--122. Finlayson-Pitts B. J. and Pitts J. N. (1986) Atmospheric Chemistry: Fundamental and Experimental Techniques. Wiley, New York. Hanna S. R. (1979) Some statistics of Lagrangian and Eulerian wind fluctuation. J. appl. Met. 18, 518-531. Hanna S. R., Briggs C. A. and Hosker R. (1982) Plume rise. In Handbook on Atmospheric Diffusion, pp. 11-17. Technical Information Center, U.S. Department of Energy, U.S.A. Ludwig F. L. (1983) A review of coastal zone meteorological processes important to the modeling of air pollution• In Air Pollution Modeling and its Application in Challenges of Modern Society, Vol. 7, pp. 225-257. Lyons W. A. (1975) Turbulent diffusion and pollution transport in shoreline environments• In Lectures on Air Pollution and Environmental Impact Analyses, pp. 136-208. American Meteorological Society, Boston,• MA. Lyons W. A. and Cole H. S. (1976) Photochemical oxidant transport: mesoscale lake breeze and synoptic scale aspects. J. appl. Met. 15, 733-743. Mahrer Y. and Pielke R. A. (1977) A numerical study of the airflow over irregular terrain. Beitrag Phys. Atmos. 50, 98-113, Mahrer Y. and Pielke R. A. (1978) A test of an upstream spline interpolation technique for the advective terms in numerical mesoscale models. Mon. Weath. Rev. 106, 818-830. McNider R. T. (1981) Investigation of the impact of topographic circulations on the transport and dispersion of air pollutants. Ph.D. dissertation, Dept. of Environmental Sciences, University of Virginia, Charlottesville, VA. Peleg M., Berner D., Lahav D. and Luria M. (1989) Ozone levels in central Israel during May 1988. In Proc. Fourth lnt. Conf. Israel Society for Ecological and Environmental Quality Sciences, pp. A37-A44, Jerusalem, Israel. Pielke R. A. (1984) Mesoscale Meteorological Modelin 0. Academic Press, Orlando, EL. Segal M., Pielke R. A. and Mahrer Y. (1982) Evaluation of onshore pollutant recirculation over the Mediterranean coast area of central Israel. Israel J. Earth Sci. 31, 39--46. Segal M., Yu C. H., Arritt R. W. and Pielke R. A. (1988) On the impact of valley/ridge thermally induced circulations on regional pollutant transport. Atmospheric Environment 22, 471--486. Wilson J. D., Thurtell G. W. and Kidd G. E. (1981) Numerical simulation of particle trajectories in inhomogeneous turbulence,If: Systems with a variable turbulent velocity scale. Boundary-Layer Met. 21, 423-441.