Meteorology and Photochemical Air Pollution in West.pdf

March 15, 2019 | Author: Nadji Chi | Category: Atmosphere Of Earth, N Ox, Wound, Smog, Volatile Organic Compound
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 Hamoudi et al Journal of Environment Environment (2014), Vol. 03, Issue 04, pp. 57-64

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Research Paper

Meteorology and Photochemical Air Pollution in Western Mediterranean Mediterranean Basin; Focused Analysis on Algerian Coast Benameur Hamoudi1, Fethi Saidi 1*, Boumedienne Beladjine 1 and Abbes Azzi 1 1

 Laboratory of Naval Aero-Hydrodynamic, University of Sciences and Technology of Oran, Mohamed Boudiaf, Al geria *E-Mail: [email protected]

Abstract In this study a summer air pollution episode from 31 st of July to 1 st of August 2013 over the Western Mediterranean Basin (WMB) is investigated and analysed through advanced atmospheric modelling. This episode was reported from the air quality simulations performed during this period for the Transport and Transformation of air pollutants from South Europe to North Africa. The present study is concentrated on ground level Ozone (O 3), oxide nitric (NO), dioxide nitrogen (NO 2) concentrations variability in order to determine the role of large scale atmospheric transport on t he behaviour of tropospheric ozone and its precursors. During this episode the synoptic condition showed by the meteorological WRF model favours the development of mesoscale circulation, which enhances the long range transport of polluted air masses toward the Algerian Coast. Moreover, the photochemical model results of both days simulated are compared with observed data and it shows a good concordance about the O3  and NOx, while a similarity in temporal variation of the concentrations of compounds studied is detected during this episode  Keywords: CAMx, O3-NO-NO2 , South Europe, North Africa, Algerian Coast

1. Introduction The increasing incidence of photochemical smog and its irritating and damaging effects spurred a flurry of research activities in the early 1980 that continued at a record pace through the 1990 (Le Bras, 1993). Millan et al (1997) is credited with reporting that the most injurious effect of smog could be attributed to oxidants (mainly ozone) produced in the lower atmosphere by a complex, photochemically initiated process involving oxides of nitrogen and various hydrocarbons. More recently, the factors affecting photochemical air quality [NOx, CO, CH4 and Volatile Organic Compounds (VOCs)] were reviewed by Jiménez & Baldasano (2004), this case, requisite two refers to a stagnant air condition that must confine and maintain reactant levels for a stagnant air condition that solar irradiation to take place and to initiate the smog process. Temperature must be included as a parameter because of its effect on the rate of the reaction. It was shown long

ago that the photochemical process essentially stops at ambient temperatures below the 15 °C (Gerasopoulos et al, 2006). Highly populated Western Mediterranean Basin (WMB) is surrounded on three sides by mountains (Alps, Atlas etc.) up to 2500 m in elevation. Millions of automobiles and various industrial sources (South Europe) release large quantities of various reactive organic gases and oxides of nitrogen in the atmosphere (Kallos, 1997 and K allos et al, 1998). In addition, the air pollution climatology of the Western Mediterranean Basin should be a major consideration (Millan et al, 2002). Particular interested on the average wind speed and direction in order to provide a rough estimate of pollutant dispersion patterns. The effect of topography and local wind systems, such as land sea breezes and mountain-valley winds, should be investigated with respect to dis-

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 Hamoudi et al Journal of Environment (2014), Vol. 03, Issue 04, pp. 57-64

persion patterns and nearby residential and industrial areas (Max Planck Society for the Advancement of Science, 2002). Additionally, great consideration should be given to the problem of trans boundary long-range ozone and its precursors within WMB during the summer (Bouchlaghem et al, 2012). As noted in previous studies i.e. Kallos et al (2002) and Millan et al (2002), the synoptic and regional circulation during the summer favours and promotes the transport of aged air masses maintained within the within the entire Mediterranean Basin for a long-distance released from the South-East Europe and Central Mediterranean to North Africa. The factors and processes spatially and temporally have been the subject of intense modelling efforts for more than three decades. The model currently in favour with various projects at Mediterranean Region is a massive computer program, the Comprehensive Air quality Model with Extension (CAMx) (Kallos et al, 2002 and Tesche et al, 2006). The issue of air quality conditions and atmospheric pollution in Western North African countries, more specifically Algerian Coast, has been addressed through consideration of its relativity to Europe Region. Global, regional and local sources of air pollution in the Western Mediterranean Region (WMR) have been considered. Indeed, the air pollution was highest during times of this episode when Meteorology factors like average wind speed and direction, also average temperature are favour for accumulation and transport of pollutants mainly O 3, NO and NO2  from South Europe to Western North Africa. Typically, these conditions are identified with the combined systems of advanced atmospheric modelling WRF/CAMx (Saidi et al, 2014).

2. Re-Circulation and Transport over the WMR The air pollution re-circulation and transport was the subject of many studies during the last 15 years (Gangoiti et al, 2001), such as MECAPIP and T-TRAPEM gave information about the re-circulation mechanisms, the layering, the paths and transformation processes, mainly the photo-oxidants, (Millan et al, 2000). Similarly, Max Planck Society for the Advancement of Science, 2002, investigates atmospheric pollution transport over the Mediterranean region resulting in large scale decrease of air quality and precipitation (MINOS). These investigations were included in the BEMA, MAMCS, SUB-AERO and ADIOS projects. The T-TRAPEM and MINOS projects developed two paths of transport as: •

From SE Europe and Black Sea towards the Middle East and North Africa, across the Aegean Sea.

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From Western Mediterranean towards Eastern with two branches i.e. towards NE Mediterranean and towards SE Mediterranean, Middle East and Africa.

During the cold period of the year, the washout mechanisms are important. Photochemical processes are not at their peak due to limited insulation and cloud formations. During the warm period, the wet removal processes are very limited and insulation helps photochemical processes, moreover, they affect the weather at higher latitudes (Varinou et al, 2001). The air masses coming towards North Africa are highly influenced by strong sea-land breeze (differencing in temperature) and the intensity of the Azores high-pressure system, which surrounds the Western Mediterranean Basin (Millan et al, 2000). The western North Africa especially the Algerian Coast is mainly located under the influence of topography and climate of WMB, surrounded by high coastal mountains and characterised by dry hot summers and mild winters. Precipitations are poor, irregular and sometimes drought situateion. Annual average temperature is 18 oC, annual average precipitation around 450 mm. Nevertheless, there is a diverse climate, with strong contrasts through the year (United Nations Environment Program, Regional Office for West Asia, League of Arab states, 2006). The sun provides the energy to drive the winds by heating the surface of the earth and in turn the air above it. This heat affects the Algerian Coast most of the time. There are three major factors that influence the Algerian Coast weather and air quality: •

The differential heating of the land and Mediterranean masses

The large scale seasonal weather regimes

The local topography.

3. Model Systems A short description of the modelling system used in this study is provided below. Figure 1 shows the computational domain for the meteorological (WRF) and the p hotochemical (CAMx) models. The Weather Research and Forecasting (WRF) Model is a next-generation mesoscale numerical weather prediction system designed to serve both atmospheric research and operational forecasting needs. The Comprehensive Air Quality Model with Extensions (CAMx) is an Eulerian photochemical model that allows integrated assessment of air pollution over many scales

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 Hamoudi et al Journal of Environment (2014), Vol. 03, Issue 04, pp. 57-64

ranging from urban to regional. The CAMx performance running is summarised on Table 1 and Table 2.

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4. Case Study  4.1. Synoptic Situation during the Episode

Figure 1. Computing Domain for Resolution of (12*12 km) Grid1/(5*5 km) Grid-2 (longitude*latitude) Table 1. CAMx Specifications



The temperature distribution over the WMB was associated with a weak Coast flow moving slowly eastwards, this situation induced weak cyclonic circulation over the Western and the Centre of Algeria and a tropospheric anticyclonic circulation above these regions. Under this weak synoptic forcing, strong insulation may promote the development of mesoscale flows associated with the local topography (mountain and valley), while the difference in temperature between the sea and the land enhances the development of sea-land breezes. Later in the day, the surface air temperature decreases over the WMB, land begins to cool, while the air temperature above the Mediterranean Sea is higher to generate an air flow between sea and land, after the sunset on July 31 st as well as the next days (August 1st). A relatively stationary high pressure area on North Africa provides an easterly flow of cool upper air which subsides over the basin causing an inverseng in surface temperature. The resulting inversion, combined with a light sea breeze and the surrounding mountains. An additional effect of the Azore high pressure cell dominates the WMB with clear skies.


CAMx v5.2


Linux, Ifort v.11.0-0

CPU type

USTO Cluster

Emissions source

EMEP Inventory

 4.1.1. Surface Temperature

Initial conditions

camx.v 5.2. inst. 5.2

Boundary conditions

camx.v 5.2.BC

Meteorological data

WRF Meteorological Model

The general meteorological conditions simulated are characterised by maximum surface air temperature at 1200 UTC (Coordinated Universal Time) ranging between 34 and 42 ºC over the North Africa at 31 st of July, while the next days (1st August) it varied between 34 and 40 ºC over the same area (Figure 2). During the nights, the land cools faster than the sea mass, causing a land breeze where the air flows slowly down the Algeria coastal areas into the Mediterranean basin. The increase in temperature according to the altitude result an inversion coupled with stagnation conditions near the surface of the Alps and the Atlas Mountains during the noon period drive to the formation of a box over this both region. This last aid in traps the emitted pollutants and provides a massive photochemical

Table 2. CAMx Run-Time Options

WRAP PSAT Speciation


Advection Solver


Chemistry Parameters


Chemistry Solver




Figure 2. Mean Sea Level Pressure and Surface Air Temperature at 12 UTC for the 31 st of July and the 1 st August, 2013

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 Hamoudi et al Journal of Environment (2014), Vol. 03, Issue 04, pp. 57-64

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reaction vessel. Wind at 1500 m

 4.1.2. Wind Field

The wind field at 850 hPa shows the synoptic wind circulation. Figure 4, shows the 850 hPa wind field on the 31 st of July, and 1st of August. As for the wind at 10 m, the wind at 850 hPa blows out from Gulf of Lion towards the coastal line of Algeria, while the wind from Gibraltar Strait blows towards North Africa. This flow pattern is maintainned during the whole period simulated.

The two days of the episode, 31 st of July and 1 st of August 2013, shows that the first and second layer of WRF model up to 850 hpa are in roughly similar for the wind direction. The air block '' very slow air flow'' extends from 10 m to 850 hpa (1500 m). Whereas, the wind coming by the Gulf of Lion breaks this wall (air block) to exit over the Algerian Coast giving rise to a re-circulation over this area.

5. Results and Discussion Wind at 10 m

 5.1. Model Evaluation

For the two days simulated, the wind field is characterised by a North-Westerly direction from the Gulf of Lion towards Tyrrhenian Sea with the maximum o ffshore of the Tunisian Coast. By 0600 UTC the situation over the Algerian Coast was almost stagnant with very low winds apart from an area offshore where the wind has its maximum (5 ms -1). At 1200 UTC the wind flow is similar as in the previous period with slightly higher winds inland of Algeria. At the early afternoon the air flow over the Mediterranean Sea and the nearby coastal areas was enhanced as a result of the synoptic conditions (Figure 3).

The West of North Africa, more precisely Algeria, has a great lack of data in air quality. In this context, this study will give a clear sight on air quality of the WMR afterwards on the whole Algerian coast. CAMx produce excellent simulation results, as shown in Figure 5. It presents ozone and two important precursor NO and NO2 exactly at South of France: AIR LanguedocRoussillon, Pyrénées measure Station situated at (latitude= 43’6; longitude= 3’88). The predicted results from a CAMx simulation of the Pyrénées Metropolitan area are

Figure 3. 10m Wind Field at 1200 UTC on July 31 st and 1st August, 2013

Figure 4. Wind Field at 850 hpa at 1200 UTC on July 31 st and 1st August, 2013

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 Hamoudi et al Journal of Environment (2014), Vol. 03, Issue 04, pp. 57-64

compared with hourly monitoring data. A good concordance between CAMx model and measure station is noticed. The light differences between the model results and measured of the concentration are due to the grid refinement. Bright and clear skies combined with an early morning temperature in the range of 32-40 °C. These factors produce a typical smog day at Algerian Coast. Figure 5 presents typical levels of NOx and ozone as the smog process progresses at Western North Africa. Both simulated days show a similarity on the day cycle of the three species studied (O3, NO, NO2).

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Region where the daytime peaks are consistently over 0.07 ppm in all simulated days, reaching more than 0.09 ppm coincides with a steep decease in the precursor NO values (Figures 6a and 6b), not only the max values of ozone are associated with minimum values in NO, but the whole diurnal cycle of the two species is more or less opposite, from West, Central, East and South Europe, across the North Africa more precisely across of the Algerian Coast. During this episode, the first day presents a similarity on the Ground-level ozone, in which from 31 st of July to 1 st of August the relatively higher concentrations extend from Southern

Figure 5. Hourly Average of Modelled and Measured Concentration for O 3, NO2, NO at 31 st of July and 1 st of August, 2013

 5.2. Hourly Average Concentration during this Episode During the night of 31 st of July, from roughly 0000 to 0600 UTC, the concentration of oxides of nitrogen remain at coastal levels. As traffic and industrial builds to reach a maximum at around 0800 UTC, there is an attendant rapid increase in the concentration of NO occurs first, as these emitted from sources directly. The increase in NO 2 gaps by a short period because it is formed by atmospheric reactions. Shortly after sunrise and the photochemical reaction sequence, the NO concentration begins a rapid decrease and the oxidant predominantly ozone concentration increase at a similar rate. By 0900 UTC the photochemical process oscillate over the equilibria. If there is a noticeable pungent odour of ozone in the air, one’s eyes begin to water and burn and the panoramic view of the city begins to fade in a brownish aerosol haze that will increase until about 1100 to 1300 UTC (noon). At 1800 UTC the intensity of solar radiation has decreased to a low level and the photochemical process has dropped. Oxidant levels turn down rapidly as their production rates go to zero. By 0000 UTC the concentration of oxides of nitrogen have again reached normal coastal levels (stagnant air conditions), and are ready to participate in more smog formation reactions the next day (1st of August).  5.3. Transport and Transformation during this Episode The diurnal cycle is most pronounced in the Mediterranean

Europe to Northern Algeria. Pockets of higher concentrateons exist within this band where the means reach to 0.062 ppm. For much of the region the mean modelled; ozone concentrations are between 0.045-0.060 ppm. Over much of North Algerian, South France, East Iberian Peninsula and Italy the maximum ozone concentrations varies between 0.062-0.082 ppm. During this episode, several potential mechanisms for recirculating O3 and its precursors have been shown by meteorological pattern over North Algeria, including the hot surface air temperature and wind reversals along the coast caused by thermal low circulation and formation of pollution in South France (Gulf of Lion) and East Spain, subject to long-range transport toward North Algeria. A loft caused by the polluted layer system reaches the areas surrounding the Alps as well as Gibraltar, whereas, part of these pollutants could be above the trade winds dominating these regions (Azores high pressure). The processes could be favoured by a high relief injection level provided by the South Europe facing slopes of Alps, also of Sierra Nevada (South Spain) range (3000 m peaks). The pollutants, however, remaining in the lower layers could travel toward and along the Algerian Coast with the trade winds. This transport mechanism could explain some high O 3 levels at the Algerian Coast background occurred in this episode. Its formation takes place mainly above the Mediterranean Sea where primary pollutants have been accumulating until early. The photochemical reactions are more

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 Hamoudi et al Journal of Environment (2014), Vol. 03, Issue 04, pp. 57-64

pronounced this day over the domain study. The mechanisms occurred during this day for the transformation and transport of pollutants was similar to the latest day. Those processes (mechanisms transformations) lead to the layering of the aged pollutants, that is being trapped in strata which are essentially uncoupled from each other and from the surface, this layers are generated directly along the coastal mountains ranges (all over WMB) Figure 6b, and convection toward the sea. The photochemical formation of ozone at the Algerian Coast is influenced by ambient temperature, solar radiation, NO and NO 2 concentration.

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 5.4. Transect Along the Western Mediterranean Basin Figure 7 shows the O3 concentrations from South Europe to North Africa at several times of the day. In Pyrénées, located on the valley at slight of 400 m (Near the Gulf of Lion), O3 is still available from the reservoir layers (Costal hills) that it has not been depleted fully from the drainage flow and the concentrations do not drop below 0.4 ppm during the night. While the O 3 concentration at the Western North Africa, more precisely at the Algerian Coast, is low as compared to the Pyrénées area. At the 0700 UTC,

Figure 6a. Hourly Average O 3, NO2 and NO Concentration respectively at 06, 12 and 20 UTC for 31 st of July, 2013

Figure 6b. Hourly Average O3, NO2 and NO Concentration respectively at 06, 12 and 20 UTC for 1 st of August, 2013

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 Hamoudi et al Journal of Environment (2014), Vol. 03, Issue 04, pp. 57-64

the O3 increases and reaches 0.6 ppm and the same behaviour has been seeing the next day. This increase occurs with a weak wind speed. By 1000 UTC the wind speed at sea surface near ten North Africa areas increase to 3 ms -1. These two factors (rise of O 3 and wind field) create the transport of O 3 within the marine boundary layer in the reservoir layers above the sea. The O 3 reaches its maximum value of 0.85 ppm at 1200 UTC, at this time the situation is calm and stagnant. At this time the O 3 keeps increasing and when the maximum wind speed is attained the polluted air masses (O3, NO, NO2) are dispersed and transported to release a new photochemical production and accumulated above the sea. It reaches a maximum around 1400 UTC with a well-developed sea breeze at the Algerian Coast.

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The polluted air reaching the areas surrounding the Alps, whereas part of these pollutants mainly O 3  and NOx could be present above the trade winds dominating these two regions (Azores high).

The pollutants remaining in the lower layers could travel through the marine boundary layer towards and along the Algerian Coast with trade winds, especially the air flow in the lowest layers near the free sea surface.

Acknowledgement We would like to express our special thanks of gratitude to

Figure 7. Hourly Average O 3 Concentration along Distance between South French (AIR Languedoc-Roussillon [0 km]) and North Algeria (Algiers) [700 km] at 06, 12 and 22 UTC Respectively for 31st of July (Top) and 1st of August (Bottom), 2013

6. Conclusion During this study, the investigation and the analysis of ground level concentration combined with meteorological conditions show a direct relationship between primary (NO) and secondary (O3, NO2) pollutants over the WMB. The analysis illustrate a continuous stream of pollutants (O 3 and NO2) from South France by the Gulf of Lyon and East Spain released into a steady wind in the atmosphere, then bend over the mean wind, which will carry them toward WM Sea and North Algeria. In summer the air quality in various locations in the WMR, more especially near the coast, defined mainly by thermal circulations (diurnal cycle) but the long-range component is considered as significant too. In addition, the synoptic situation (meteorological conditions) associated with topography surrounding the WMB generate two mechanisms of long-range transport towards the Westerly North Africa were identified; from West Spain, South France. These mechanisms could explain some high O 3  concentrations occurred in this period at the West of the North Africa especially Algerian Cost:

the director of the Aero-hydrodynamics laboratory, Mechanical Faculty, University of Sciences and technology of Oran, Mohamed Boudiaf, as well as the whole team for giving us the golden opportunity to achieve this piece of work on the topic of air pollution in the Mediterranean Basin, focussing on the Algerian Coast, which also helped us in digging deep into the topic and we came to draw such a conclusion.

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