Integral assessment of air pollution dispersion regimes in the main ...

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This paper presents a rather complete picture of conditions of stagnation, recirculation and ventilation factors in the main industrialized and urban areas in Oman ...
Arab J Geosci (2011) 4:625–634 DOI 10.1007/s12517-010-0239-6

ORIGINAL PAPER

Integral assessment of air pollution dispersion regimes in the main industrialized and urban areas in Oman Yassine Charabi & Sultan Al-Yahyai

Received: 24 August 2010 / Accepted: 7 November 2010 / Published online: 23 November 2010 # Saudi Society for Geosciences 2010

Abstract This paper presents a rather complete picture of conditions of stagnation, recirculation and ventilation factors in the main industrialized and urban areas in Oman, developed along the coastal area. This study focuses on four sites; Sohar, Muscat, Sur, and Salalah. Each site has a local emission sources from transportation, industry and energy production activities. For the calculation of the integral quantities of the ability of the atmosphere dilution, hourly data of the wind velocity measured at a height of 10 m during 5 years (2000–2005) were used in the analysis. The results show that in the northern coast of Oman, along the bay of Sea of Oman, where 56% of the total population is concentrated and the main heavy industries of the country are amassed, the atmosphere is prone to stagnations in 74.4% of the time, while in the southern and east part of Oman, they occur only 23% and 51%, respectively. The bay of sea of Oman is high affected by land–sea breeze circulation that plays a substantial role in the simultaneous occurrence of recirculation equally to stagnation. This meso-scale effect is altered gradually during the passage of the synoptic-scale flow of the southeasterly summer monsoon that enhances the occurrence of the ventilation in Salalah (24.6% of time) and Sur (15.5%). In the northern coast of Oman, where the Hajir mountains suppressed the

Y. Charabi (*) Department of Geography, Sultan Qaboos University, P.O. 42, Al-Khodh, Muscat 123, Oman e-mail: [email protected] S. Al-Yahyai Directorate General of Meteorology and Air Navigation, Department of Meteorology, Muscat International Airport, Muscat, Oman

effect of the summer monsoon, a very weak tendency towards ventilation is observed (less than 6%). The southern summer monsoon over Oman is a source of life in this arid area and as well a source of clean air. Keywords Air pollution dispersion potential . Wind regimes . Stagnation . Recirculation . Ventilation . The main industrial and urban areas . Oman

Introduction It is well known that meteorological conditions play a crucial role in determining Air Quality over urban areas where the high density of pollutant emissions represents a constant risk for human health. The determination of the risk to human health from air pollutants and the spatiotemporal variability of pollutants concentrations require the deep knowledge of the atmospheric conditions that govern the processes of transport and dispersions. The concentrations of populations and industry in the coastal area aggravate the problems of air pollution. Natural conditions for pollution dispersion in the atmosphere of the coastal area are not favorable due to the effect of the local circulation. During the last three decades, much has been written on the effect of the sea breeze circulation on coastal meteorology in general and the interaction between sea breeze and air pollutants in particular plays an essential role in determining various features of coastal environments around the world (NRC 1992, chapter 7). Comprehensive assessments of the relationship between coastal circulation and air pollutants concentrations have been provided by many researchers; e.g., Auckland, New Zealand (McKendry 1992), Kalpakkam, India (Jamima and

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Lakshminarasimhan 2004), Huston, (Nielsen-Gammon 2000), Athens (Clappier et al. 1999), Busan, South Korea (Oh et al. 2006); Hong Kong (Liu et al. 2001), Eastern Tunisian Coast (Bouchlaghem et al. 2007); the Iberian peninsula (e.g., Millan et al. 1984, 1987). All those studies shown that air pollutants concentrations are seriously affected by coastal circulations. In Oman, as elsewhere in the world, rapid economic development, improvement in standards of living and increased population density have brought in their wake pollutant emissions from both stationary and mobile sources. Oman’s specific conditions—concentration of population and industry in the coastal area and the topoclimatology of the mains industrial and urban areas— aggravate the problems of air pollution. The most important sources of air pollution in Oman are energy production, transportation and industry. In view of the fact that these are mainly concentrated in the coastal area, the highest levels of pollution have been detected in these locations. In this regard, natural conditions for pollution dispersion in the atmosphere of the coastal area are not favorable and, until recently, relatively high concentrations of sulfur dioxide (SO2), emitted for the most part by oil refineries, have been recorded in Sohar city. Intense industrial activity in Sohar city, coupled with difficult atmospheric dispersion conditions caused by the influence of the Sea of Oman and the complex topography of Hijar Mountains, make this area one of the most problematic in terms of air pollution. The rapid emergence of industrial plants in the vicinity of urban centers has exacerbated air pollution problems throughout Oman. Pollution sources include cement plants, chemical and petrochemical plants. Dense vehicular traffic is also a major contributor to air pollution, causing high nitrogen oxide (NOx) concentrations, especially in the capital (Muscat), which houses 843,760 inhabitants in 2008, constituting 29.4% of the total populations in Oman According to the published literatures, climatological studies of the dilution efficiency of the atmosphere are nonexistent for Oman, despite the spectacular socioeconomic development since 1970, stimulated by oil exploration and production. As a first step towards an effective knowledge of atmospheric conditions of transport and dispersion a complete picture of the recirculation phenomena is needed in Oman for the emplacement and development of urban and industrial areas. This paper aims to provide a quantitative assessment of the occurrence of stagnations, recirculations and ventilations in the atmosphere of the main urban and industrial areas in Oman based on a method presented by Allwine and Whiteman (1994). This approach offers the possibility to compare site/ regions, or to examine the frequency of the integrals

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quantities of the atmospheric dilution over long periods of time.

Methodology and data sets Quantitative assessment of air mass stagnation, recirculation and ventilation Allwine and Whiteman (1994), proposed an objective quantitative assessment of air mass stagnation, recirculation and ventilation on the basis of the measurement of wind speed (U) and wind direction (θ). This approach considers a time series of N data pairs (U; θ) (where θ is the direction from which the wind is blowing, measured clockwise from north) and it proposes to resolve the wind vector into east– west (positive towards the east) and north–south (positive towards the north) components, respectively, in the following manner: ui ¼ Ui sinðq i  180Þ vi ¼ Ui cosðqi  180Þ Where i=1,….,N. By summing the above two equations over 24 h, we can calculate the transportation distance of the effluents in the directions of east–west and north–south, respectively. Xi ¼ t Yi ¼ t

iþp P j¼1 iþp P

uj vj

j¼1

where t is the sampling intervals and p=24 h/t. The straight-line distance (Li) from the release point is calculated as given below: qffiffiffiffiffiffi Li ¼ Xi2 þ Yi2 The real transport distance which is also known as the wind run (Si) is calculated as given below: Si ¼ t

iþp X

Ui

j¼1

The recirculation factor (Ri) is computed at each time step ti by using the following equation: Ri ¼ 1 

Li Si

The wind run is the total distance a parcel would travel, regardless of direction, over the transport time t. The wind run is calculated to assess stagnation. Total stagnation subsists when S is equal to 0. During those

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situations, the air flow velocity is reduced or stopped, allowing pollutants concentrations to increase in the surrounding area of the source of emissions (Venegas and Mazzeo 1999). The resultant transport distance represents the net distance a parcel will pass through over the transport time t. The consequential transport direction (measured clockwise from north) represents the direction a parcel will move during the transport time t (Nankar et al. 2009). The recirculation factor provides an indication of the presence of local recirculations on the time scales comparable with t. When R is equal to 0, straight-line transport will occur and when R is equal to 1, 0 net transport will happen over time interval t and as a result there will be total recirculation. In other terms, recirculations are situations in which contaminated air is primarily pushed away from the source of emission, but it come back later. Ventilation is distinguished by high values of S and low values of R. The ventilation events are characterized by the dilution of the polluted atmosphere by uncontaminated air (Allwine and Whiteman 1994). Allwine and Whiteman (1994) suggested an approach for classifying the atmosphere of different sites by comparing the mean values of the wind run (S) and of the recirculation factor (R) with predetermined critical values. If the mean value of S is lower than the critical value, the local atmosphere shows a tendency towards stagnation of the air. If the mean value of R is greater than the corresponding critical value, the local air flow will have a tendency towards recirculation. For a site prone to ventilation, the mean value of S is greater than a critical value of S and the mean value of R is lower than a critical value of R. A second procedure for classifying stagnation, recirculation and ventilation potential is based on computations of the percent occurrence of Si ≤Sc (stagnation), Ri ≥Rc (recirculation) and simultaneously Si ≥Scv and Ri ≤ Rcv (ventilation), where Sc, Rc are the average daily critical transport indices (CTIs) for stagnation and recirculation, respectively, and Scv and Rcv are the average daily CTIs for ventilation. Data sets The Sultanate of Oman occupies the south-eastern corner of the Arabian Peninsula and is located between latitudes 16°40′ and 26°20′ north and longitudes 51°50′ and 59°40′ east (Fig. 1). It has a coastal line extending almost 1,700 km, from the Strait of Hormuz in the North to the borders of the Republic of Yemen, overlooking three seas; the Arabian Gulf, Sea of Oman, and the Arabian Sea. The total area of the Sultanate of Oman is 309,500 km2, and it is the third largest country in the Arabian Peninsula.

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The Sultanate of Oman is composed of varying topographic areas consisting of plains, wadis, and mountains. The most important area is the plain overlooking the Sea of Oman and the Arabian Sea with an area of about 3% of the total. The mountain ranges occupy about 15% of the total, the most important of which are “Al-Hajr mountains”, extending in the form of an arch from Ras Musandam in the North to Ras Al-Had and Dhofar mountains in the South-Western corner of Oman. The remaining area is mainly sand and desert which includes part of ArRub–Al-Khali occupying about 82% of the total area (Fig. 1). The main urban and industrial areas of Oman are developed along the costal plane (Fig. 1). This study focuses on four sites; Sohar, Muscat the capital of Oman, Sur and Salalah. Each site has a local emission sources from transportation, industry and energy production activities. Sohar (24°22′N, 56°45′E) is the most developed city in Oman outside the capital Muscat with 124,000 inhabitants in 2008. Located 150 km north of Muscat and distanced some 120 km from the mountain ridge. The Omani government has paid special consideration to the city of Sohar, and placed it in the main concern of the future plan of the Omani economy in 2020. The target of the Omani government is to make Sohar a business and industrial hub and help the Omani economy diversify away from oil. In order to achieve this economic diversification, the Omani government is investing in a number of projects in the industrial area of Sohar. The city holds steel, petrochemical, aluminum industries, and a refinery with a capacity of 116,000 barrels per day (Statistical Year Book 2009). Muscat city (23°36′N, 58°35′E) is the largest metropolitan area in Oman with its associated mobile emission sources, power plant and a refinery with a capacity of 85,000 barrels per day (Statistical Year Book 2009). The city is situated in a very complex topographical setting of mountains and a narrow valley. The built-up area of Muscat is about 200 km2 looks currently as a ribbon meandering throughout the narrow coastal prone areas as well as across hill slopes and Wadi valleys (Mokhtar 2010; Fig. 1). Sur city (22°34′N, 59°31′E) is located at 150 km southeast Muscat with a population of 82,152 inhabitants in 2008 and holds petrochemical industries and liquefied natural gas industry (Statistical Year Book 2009; Fig. 1). Salalah is located in the extreme southern coast of Oman (17°2′N, 54°9′E) in a narrow costal plane bounded from the north by a mountain ridge with heights up to 800 m above the sea level at a distance of 40–60 km from the coast (Fig. 1). Salalah is the second largest city in Oman with 195,640 inhabitants in 2008 and holds cement and petrochemicals industries (Statistical Year Book 2009). At each of those main cities, there is a weather station installed and equipped with different weather parameter measurements. Wind masts and installed at 10 m above the

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Fig. 1 Localization of the study areas

ground and wind data (velocity and direction) are reported hourly. This study is based on the analysis of 5 years (2000–2005) of hourly wind data at the selected sites. The selected sites are occupying a coastal position so, they will probably affected by the occurrence of the local circulation induced by diurnal heating. For this reason, a transport time (t) of 24 h is considered.

Synoptic features of wind climatology over Oman By virtue of its position astride the Tropic of Cancer, the sultanate of Oman belongs to the arid area of the earth according to the Koppen classification, but it is influenced by air masses from four different directions—the Mediterranean, Central Asia, the tropical maritime regime of the Indian Ocean and tropical Africa. The four atmospheric

flow patterns operate at different times of the year, bringing a variable degree of seasons: Winter season [December–February] (Northeast Monsoon) The low-level northeast wind is the dominating flow during the winter season. This flow is controlled by the Siberian high which follows the general circulation system (Al-Baz and Makharita 1994; Charabi and Al-Hatrushi 2010). The prevailing northeasterly airflow is interrupted by the succession of troughs and ridges moving from west (Africa and Mediterranean). Due to the increase in surface pressure over the northern parts of the Arabian Peninsula (Fig. 2) after the passage of the upper air trough, north or northwesterly winds develop over northern Oman (locally known as Shamal). Shamal, which causes strong dust storms, can last from few hours to several days and reduce surface visibility to less

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transition period. Due to the heat low over Pakistan, the anticyclonic flow over Arabian Sea is replaced by cyclonic circulation. The pre-monsoon transition period ends by the onset of the southwesterly trade wind (Walters and Sjoberg 1988). During the transition period OCZ moves northwesterly due to the intensifying southwesterly flow. Occasionally, convective activity can be found over high terrains. Summer season [May–September]

Fig. 2 The direction and cause of winter Shamal that persists over Arabian Gulf (Al-Baz and Makharita 1994)

than 1 km (Al-Maskary 2006). Influenced by the Zagros Mountains, Shamal can blow very strongly over the Arabian Gulf. Winter Shamal is characterized by its cold and dry flow. Occasionally, when low-level northwesterly flow coming from Saudi Arabia meets the northeasterly flow that has been deflected inland due to strong sea breeze circulation, a convergence zone develops over the area south of the Hajar Mountains. This zone is called Oman convergence zone (OCZ; Pedgley 1970) as shown on Fig. 3. Spring transition season [March–April] Due to the increase in solar radiation, the northeasterly flow starts losing its strings on the region during the spring Fig. 3 Mean January (left), April (right) position for OCZ (Walters and Sjoberg 1988)

Southwest Monsoon (locally knows as Khareef) is characterized by winds blowing from the southwest over the Arabian Sea. It starts over the southern part of Oman from the last week of June and ends around the mid September (Al-Maskary 2006). Persistent drizzle and rare thunderstorm activity are the main characteristics of Khareef over Oman. The monsoon spreads inland to reach as far north as Fahud (Oman map) before it is deflected to a southeasterly (Pedgley 1970) due to the extension of high pressure over the Arabian Sea. Pedgley (1970) also showed (using pilot balloon data) that the monsoon, on many days, reaches as far north as the Arabian Gulf. Based on these pilot balloon data, Pedgley suggested that the depth of the monsoon at Salalah is between 1 and 2 km. As an extension of the western Pakistan thermal low, massive thermal low pressure trough dominates the season with a secondary center over south central Saudi Arabia. These thermal lows contribute to the dynamics of the southwesterly circulation over the Indian Ocean and Arabian Sea (Walters and Sjoberg 1988). During this season the inter-tropical convergence zone (ITCZ) is located along the Omani coast from Dhofar to Sharqyia where desert and marine air converge. Monsoon ITCZ absorbs the Omani Convergence Zone during

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summer period. Figure 4 shows that ITCZ reaches its most northern position over the Hajar Mountains during July and August, where the low-level, relatively moist southeasterly flow from the Arabian Sea meets a relatively dry northerly/ northwesterly flow from the Arabian Gulf. Shamal winds in the summer affect the northern part of the Arabian Peninsula, which is characterized by dry air flow (Fig. 5). It is a result from combination of flow around Pakistani low pressure area and the Saudi Arabia heat low and the barrier effect of the Zagros Mountains. The summer Shamal strength is less significant than the winter Shamal (Rao et al. 2001). Due to maximum radiation cooling at night (clear sky conditions) and a strong temperature inversion, lowlevel Shamal is often strong during daytime but typically decreases at night (Al-Maskary 2006). Even a weak Shamal reaching the Hajar Mountains affects summer convection development where dry air suppresses convection over the mountains. Fall transition season [October–November] Like spring, the fall transition period is a major synoptic pattern reversal. The massive heat located over southern Saudi Arabia is replaced by weak low-level continental anticyclonic flow (northeast trades) because of the development of the Siberian high.

Fig. 4 Mean July surface position of monsoon ITCZ (Pedgley 1970)

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Local circulations The northern part of Oman is affected by two different sea breezes. One originates from the Oman Sea where the coast runs almost parallel to the Hajar Mountains. The second one originates from the Arabian Gulf. Sea breeze (especially from Oman Sea) contributes to the summer orographic convection over the Hajar Mountains but not as important as the moist monsoon flow. Breeze from the Arabian Gulf is also influenced by the dominating drier northwesterly flow. Due to the rapid heating of the Hajar Mountains, the minimum land–sea temperature difference needed to trigger the sea breeze is reached earlier on the Oman Sea than over the Arabian Gulf. Therefore, sea breeze starts earlier on the Oman Sea side of the Hajar Mountains. On the other hand the Hajar Mountains blocks the sea breeze from deep inland penetration compared to the Arabian Gulf side. Figure 6 shows numerical weather prediction (NWP) model simulation of a typical sea breeze case (July 14th 2002). High-resolution model which is hydrostatic limited-area numerical weather prediction model is used for this simulation. It shows the onset of the sea breeze at 05Z along the Omani coast. The onset of the sea breeze is marked by the formation of the convergence zone near the Omani coast and the change in the wind direction from off-shore to on-shore. At the UAE coast, the sea

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breeze front has not started yet and that winds are still blowing off-shore (A). On 9 UTC (B), the onset of the sea breeze along the UAE coast just started while the sea breeze front on the Omani side is already well developed.

Application to major industrial and urban areas in Oman

Fig. 5 Synoptic view of surface summer Shamal showing the location of surface high pressure and surface Low pressure zones that usually persist over the region (Pedgley, 1970)

The wind run (Si) was found to vary from 59.5 to 110.5 km, clearly indicating the weakness in the horizontal wind speed in the regions. This is due to weak pressure gradient that is maintained by the subtropical high pressure system. The lowest mean of wind run occurs at Seeb (average of Si 59.5 km, daily wind speed 0.70 m/s) and Sohar (average of Si 62.4 km, daily wind speed 0.73 m/s) situated in the northern coast of Oman. The highest values are observed at Salalah (average of Si 72.8 km, daily wind speed 0.85 m/s) and Sur (average of Si 110.5 km, daily wind speed 1.30 m/s) situated in the extreme south and the east corner of Oman, respectively (Table 1). The analysis of the wind speed

Fig. 6 NWP model simulation of sea breeze, case of July 14th 2002, 5 UTC (a), 9 UTC (b), 13 UTC (c), 16 UTC (d). Pink color shade represents breeze front (conversion zone), arrows represent wind direction

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Table 1 Daily variation of wind run in km (Si) of the selected sites during 2000–2005

Arithmetic mean Max Min Median Std. dev Lower quartile Upper quartile

Sohar

Seeb

Sur

62.4 159.8 1.0 59.7 14.9 1.0 159.8

59.5 172.9 24.4 55.9 18.5 24.4 172.9

110.5 351.8 8.2 97.4 55.3 8.2 351.8

Salalah 72.8 286.1 2.6 66.9 28.9 2.6 286.1

observed at Seeb and Sohar are very weak, their findings show that, on the average, the wind speed exceeds 6 m/s only 5.65% and 3.77%, respectively from the annual wind rose (Fig. 7). On the other hand, in Sur and Salalah the annual frequency of wind speed greater than 6 m/s is 31.5%

and 13.47% respectively. In Sur, the occurrence of wind speed greater than 6 m/s is much more frequent in summer (46.6%) than in winter season (16.9%). This is mainly due to the Tropical Continental air masses which are formed over the hot and dry land surfaces of middle of Sudan in late spring and early summer when surface heating is extreme. This synoptic wind (locally called Al-Gharbi) is reinforced by the extreme heat of the ArRub–Al-Khali desert (Empty Quarter) and the wind speed can reach 12 m/s. For Salalah, the occurrence of wind speed above 6 m/s is largest in winter with 23.6% than in summer (13.5%) due to the effect of the winter northeasterly monsoon. During the summer season, the wind regime over Salalah is controlled by the summer monsoon which is characterized by a speed average less than 6 m/s (Fig. 7). According to the above consideration of wind speed and following Allwine and Whiteman (1994), we may assume a critical wind run value Sc =70 km (daily average wind

Fig. 7 Annual and seasonal wind rose of the selected sites during 2000–2005

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633 80 Winter Spring Summer Automn

70

% stagnation

60 50 40 30 20 10 0

Muscat

Sohar

Sur

Salalah

80 Winter Spring Summer Automn

% Recirculation

70 60 50 40 30 20 10 0

Muscat

Sohar

Sur

Salalah

80 Winter Spring Summer Automn

70

% Ventillation

speed of 0.8 m/s). Figure 8 show the frequencies of occurrences of Si ≤Sc for each site. These results show that Seeb and Sohar have 74.4% of stagnations, while Salalah has 51.3% and Sur 23%. The seasonal distribution of stagnation potential shows a bimodal distribution for all sites with two peaks during summer and winter (Fig. 9). Assuming Rc =0.4 (Allwine and Whiteman 1994; Table 2), Sohar and Seeb show high deposition towards recirculation with a frequency above 60% (Fig. 8). The relatively high frequency of recirculation in these areas during the summer season is the direct consequences of the land–sea breeze circulations (Fig. 9). Because of the deferential heating between sea and land, the northern coast of Oman is frequently subjected to the influence of sea land breeze circulation. According to the statistical analyses of meteorological data of Seeb meteorological station over a 25-year period (1980–2005), sea and land breeze occurs on an average of 157 and 109 days per year, respectively. For Sur, the frequency of recirculation is reduced to 52%. In the this coastal area of the corner east of Oman where the Arabian Sea begins, the land–sea breeze circulation is reduced under the effect of the summer monsoon that reach Sur from the southern west. The strength of the southwesterly summer monsoon decrease gradually when they reach the land, but they still enough vigor when they reach Sur to alter partially the onset of the local circulation. For Salalah, the frequency of recirculation falls to 30.2%, due to the direct effect of the summer monsoon, which swept the region from middle of June to the end of September (Figs. 8 and 9). This direct exposition to this sustained synoptic-scale wind contributes efficiently to the occurrence of ventilation in Salalah (24.6%). Figure 8 shows that the percent of ventilation events generally decrease with latitude. The frequency of occurrence of ventilation at Sur is reduced to 15.5% and less than 6% in Seeb and Sohar. It is observed that the sum of

60 50 40 30 20 10 0

Muscat

Sohar

Sur

Salalah

Fig. 9 Seasonal frequency of occurrence of stagnation, recirculation and ventilation of the selected sites during 2000–2005

100 Stagnation

90

Recirculation

80

Ventillation

70

Table 2 Daily variation of recirculation factor (Ri) of the selected sites during 2000–2005

%

60 50 40 30 20 10 0 Muscat

Sohar

Sur

Salalah

Fig. 8 Frequency of occurrence of stagnation, recirculation, and ventilation of the selected sites during 2000–2005

Arithmetic mean Max Min Median Std. dev Lower quartile Upper quartile

Sohar

Seeb

Sur

0.476 0.995 0.011 0.503 0.244 0.011 0.995

0.447 0.976 0.000 0.439 0.201 0.000 0.976

0.423 0.980 0.001 0.431 0.234 0.001 0.980

Salalah 0.261 0.988 0.000 0.161 0.243 0.000 0.988

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percentage occurrence of stagnation, recirculation and ventilation was found to be over 100%. The sum of the observed percentage occurrence of stagnation, recirculation, and ventilation is not equal to 100%, because some wind flow conditions did not show any of these features and in some cases, the stagnation and recirculation phenomenon was observed simultaneously.

Conclusions The integral assessment of air pollution dispersion regimes in main industrialized and urban areas in Oman is diagnosed based on the concepts of stagnation, recirculation and ventilation developed by Allwine and Whiteman (1994). The analysis of the values of the integral quantities shows that the atmosphere of the four sites has a great tendency towards stagnation. The average of the wind run value are very small in Sohar (62.4 km), Seeb (59.5 km) and Salalah (72.8 km). The highest wind run average is obtained in Sur (110.5 km). In the northern coast of Oman, along the bay of the Sea of Oman, where 56% of the total population is concentrated and the main heavy industries of the country are amassed, stagnations are presents in 74.4% of the time, while in the southern and eastern part of Oman, they occur only 23% and 51%, respectively. In all sites, the atmospheric situations leading to stagnation are more frequent during the long summer season stretched over 5 months from May to September, with frequency of occurrence around 40%. During the short winter season, the frequencies of occurrence of stagnation for all sites ranged between 25% to 30%. Large frequency of recirculation factor is observed in Sohar and Seeb (≥60%), with high occurrence during the summer season. This finding could confirm that stagnation and recirculation in this area occur simultaneously. The most ventilated area is Salalah (24.6% of the time) and Sur (15.5% of the time) due to the effect of the summer monsoon. Based on that, we can assume that ventilation is reduced from South to North according to the gradually decrease of the summer monsoon effects. In the northern coast of Oman a very low tendency towards ventilation is observed during the whole year with less than 6% of the time. It is clear that summer monsoon does not bring only rainfall to the southern of Oman, but also it allows better dilution to the atmosphere. Acknowledgments This research was funded by His Majesty Annual Trust Fund (Sultan Qaboos University). The authors are grateful to the Directorate General of Meteorology and Air Navigation, Sultanate of Oman for providing access to its PC Cluster to run the NWP models and providing wind hourly data for this research.

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