Ambient Air Quality Assessment in the Grand Casablanca Area (Morocco): Impact of Road Traffic Emissions for the 2013-2016 Period

Air Pollution is a serious hazard worldwide especially in urban areas. Road traffic is the main cause of pollution in agglomerations that are confronted to an excess of pollutants due to traffic intensity and the dominance of diesel cars. This paper presents the assessment of road traffic pollution in the Grand Casablanca area. Data used are the result of simultaneous measurements at thirteen sites located in the Grand Casablanca. Available data cover 4 years period (2013-2016). Traffic-related air pollutants are reviewed in order to assess their impact on the local air quality. It include nitrogen dioxide (NO2), particulate matter (PM10), carbon monoxide (CO) and Benzene (C6H6). Annual evolutions are presented and compared to national air quality standards; NO2 annual trends are also evaluated. The [NO]/[NO2] emissions ratio calculation allows then to characterize the measurement sites against road traffic. The paper focuses on determining the contribution of road traffic emissions on air quality modifying; we found spatial variability in traffic pollutants. The results pointed out that road traffic and conditions are the main causes of air pollution in the area and the analysis provide a quick view of the relatively critical areas that need more action to reduce this pollution.

activities that adversely affect environment, public health, well-being, and quality of life. Population increase is accompanied with proportional increase in emissions while higher urbanization rate and lower average household size increase emissions as well (Cole & Neumayer, 2004). The ever-changing transport sector is exacerbating the phenomenon of pollution and contributing to the overall balance of gas emissions. The transition from massively industrial pollution to a pollution where transports have a predominant share has gradually been accompanied by a major transformation of monitoring systems, while industrial pollution has significantly declined in recent years, transport pollution has been steadily increasing, with the main cause being the increase of cars traffic and the vehicles fleet. Air pollution parameters demonstrates a strong correlation with the traffic intensity. The highest correlation indicators established are from CO and NO 2 pollution (Zariņš, 2014). Moreover, it is in cities that the situation is most sensitive, because while the pollution depends on topography and weather conditions, the consumption of a car in the city is four times higher than on the highway, and the highest pollution levels globally occurs in winter and summer (Cichowicz, Wielgosiński, & Fetter, 2017;Dadhich, Goyal, & Dadhich, 2017). A study on the urban population exposure to air pollution during their travels in the Paris agglomeration highlighted that the highest exposure levels are obtained in the passenger cars (Delaunay, Goupil, Ravelomanantsoa, & Person, 2012). Air pollutants are known or suspected to cause harmful effects on human health, traffic-related pollution proved to be significantly associated with respiratory symptoms in children (Kim et al., 2004), moreover, many studies showed that pollution increases mortality and pointed out the causal link between short-term exposure to air pollution and mortality (L. Pascal, 2009), (Dab et al., 2001). The Grand Casablanca , Morocco's biggest agglomeration and economic capital, is currently affected by year-round air pollution (Khatami, Ponche, Jabry, & Mirabel, 1998).
This study has made the first attempt to define the possible influence of road traffic emissions in ambient air quality over a large urban agglomeration from Morocco: the coastal area of the Grand Casablanca. The work carried out in this study focuses on: -Presenting the main indicators of air pollution originating from road traffic (NO x , PM 10 , CO and C 6 H 6 ), in particular through their effects on health and the environment; -Present the main associated air quality standards.

Study Area
The Grand Casablanca area ( Figure 1) is located on the Atlantic coast in the north-west of the country, with an area of 1 615 km² and a population of about 4 million inhabitants (RGPH 2014). The Grand Casablanca hosts different activities, a large industrial environment (the thermal power station, Mohammedia refinery and Ain Sebaa-Bernoussi industrial zone) characterizes the North-East part, the urbanized part is bordered by the Bouskoura forest in the south and rural agricultural lands in the South-East and South-West, the southern parts are less urbanized. It is also a major transport hub for the whole country; railway lines and several major roads and highways connecting the city with the rest of the country cross it: the N1 and N11 National Roads, South Ring Road, Coastal Road, Expressway, R315 Regional Road and the A7, A3 and A5 Highways.

Studied Parameters and Their Health Impact
Air pollution parameters considered here (NO 2 , PM 10 , CO and C 6 H 6 ) are the main indicators of road traffic pollution especially in urban areas. The exhaust (fuel combustion), evaporation, abrasion of vehicle equipment (brakes, tires, etc.) as well as road wear and the resuspension emit these pollutants.
Nitrogen dioxide (NO 2 ) is a motor traffic tracer (L. Smith et al., 2015), it forms during the combustion of fossil fuels from the reaction of Nitrogen monoxide (NO) with oxygen or ozone in the air. NO 2 causes a range of harmful health effects, a short-term exposure to NO 2 is associated with adverse health effects and increases hospital admissions and mortality especially for susceptible populations (children, elderly, and asthmatics) and long-term exposure is associated with adverse health effects (respiratory symptoms/diseases, hospital admissions, mortality, etc.) (Latza, Gerdes, & Baur, 2009 and automobile traffic and the smaller they are, the more dangerous they are (Ropeik & Gray, 2002).
According to a public health study in France, fine particles pollution causes 48,000 deaths per year, which is as much as alcohol, and reduces life expectancy by up to two years (M. Pascal et al., 2016).
Ultra-fine particles provoke alveolar inflammations, with release of mediators capable, in susceptible individuals (elderly, diabetics, or those with known coronary artery disease) of causing exacerbations of lung disease and increasing blood coagulability, thus also explaining the increases in cardiovascular deaths associated with urban pollution episodes (Martinelli, Olivieri, & Girelli, 2013;Seaton, MacNee, Donaldson, & Godden, 1995). Carbon monoxide (CO), an odorless, colorless and flammable gas, results from the incomplete combustion and motor vehicles is the absolutely dominant source (Fenger, 1999).
Numerous studies showed that prolonged exposure to CO has health consequences: headaches and dizziness (Raaschou-Nielsen, Nielsen, & Gehl, 1995) and elderly hospitalizations for cardiovascular disease (Hoek, Brunekreef, Goldbohm, Fischer, & Van Den Brandt, 2002). Benzene (C 6 H 6 ) is a volatile organic compound belonging to the family of hydrocarbons that mainly comes from the residential and tertiary sector, in particular from woods combustion, and road transport. Benzene has effects on the nervous system, blood cells and platelets that can cause loss of consciousness and the major health risk associated with exposure to low concentrations has been shown to be cancers risk and leukemia

Data Collection
Hourly data were obtained from the air quality monitoring network of the Grand Casablanca (13 stations) (see Figure 2) from January of 2013 to December of 2016, except for the PM 10 that are daily data. The fixed stations are generally located in public places and measures continuously and in real time the concentrations of a number of ambient air pollutants including those quoted above, as well as the meteorological parameters allowing the transformation and dispersion of the pollutants (wind speed and direction, ambient temperature, atmospheric pressure and relative humidity). Air pollution parameters studied here are subject to specific regulations in Morocco (Table 1). In order to perform a relevant statistical analysis, basic data are analyzed according to the requirements of the ADEME

Data Analysis
For each site of the monitoring network, the mean annual of the traffic pollution indicators: NO 2 , PM 10 , CO and C 6 H 6 were calculated and compared to the Moroccan legislated air quality standards, for the entire period 2013-2016. Temporal trends of NO 2 concentrations were then evaluated for stations with a minimum data coverage of 75% of valid data per year for at least three out of the 4-years period. The non-parametric method proposed by Thiel (1950) and Sen (1968) for estimating the slope of a linear trend was used. This approach involves computing slopes for all the pairs of ordinal time points and then using the median of these slopes as an estimate of the overall slope. Since Sen's slope is robust against outliers, it is widely used for the estimation of trending magnitudes of climate series (Deo, (Mintz, Fitz-Simons, & Wayland, 1997). We represent the evolution of the minimum, 25th percentile (P25), median, mean, 75th percentile (P75) and the maximum.
Meteorological data are used for the interpretation of the pollutant concentrations observed at the urban-traffic station representing a traffic pollution hot-spot. Meteorological parameters analyzed here include wind direction (°) and wind speed (m/s).

Nitrogen Dioxide NO 2
The evolution of NO 2 annual averages (Figure 3 (a)) allows the differentiation of the thirteen study sites; it shows that the NO 2 levels respect relatively the sanitary standard. A prioritization seems to be in place with three sites recording the highest values and exceedances of the limit value for the protection of         results or to study the evolution of annual concentrations. However, we found that the recorded levels are all below 10 μg/m 3 , the annual limit value.

Trends in Annual NO 2 Concentrations Average
Trends in data for nitrogen dioxide (NO 2 ) were investigated over the period 2013 to 2016 at the Grand Casablanca monitoring stations.   the lowest values are observed at night (23:00-06:00). CO daily profile follows the same trend as for NO 2 but with less intense peaks. For all temporal profiles, the Bouskoura site seems to be intermediate to the other sites for all the studied pollutants.

Identification of Typical Traffic Sites: [NO]/ [NO 2 ] Ratio
Considering the results, the[NO]/[NO 2 ]ratio differs greatly from one site to another (Table 5). The resulting values allows to draw a distinction between the sites and to group them in three categories: •   The box-plot representation of the average cycles allow analyzing the dispersion of the concentration data against the average. It is therefore necessary to analyze the representativity of the average cycles over the considered period. For the Bouskoura site, which represents the highest NO/NO 2 ratio, considering the figure, the data are more dispersed in winter than in summer, during the weekly peak (midweek) and peak hours (09:00 and 20:00). The dispersion of the NO 2 data seems to be better than the CO and PM 10 data. This observation is illustrated, in part, by the difference between the median and the mean of the extracted cycles (represented by the curve in Figure 7). Indeed, for the NO 2 data these are relatively close, whereas for the CO and PM 10 data the means are sometimes below the median values. The CO and PM 10 means appear accordingly smoothed by the low extreme values and the NO 2 extreme values are higher than the high border. A specificity is noted with regard to NO 2 data for the daily cycle, the interquartile gap is larger for hours from 7am to 10am. This can be explained quite simply by the alternation of the road attendance that can vary according to peak hours.  Casablanca suggest that the predominant winds come from the northern sector with a velocity mostly between 0 and 2 m/s, followed by the westerly winds; this situation is favorable to the accumulation and stagnation of pollution plumes loaded with air pollutants. The site was therefore influenced by the emissions of pollutants from the center of the agglomeration and the neighboring road network.

Discussions
The purpose of this study is to examine the role of road traffic in modifying air quality in a large urban agglomeration, the Grand Casablanca. Over all measurement sites, the measured concentrations result from different contributions: background pollution, transregional contribution (imported via air mass flows), urban contribution mainly related to traffic and meteorological factors impact. NO 2 Annual means showed a significant increase for some monitoring sites. The absence of statistical significant tendencies in time series for some monitoring sites is due to the shortness of the period used for this study. The high annual value of NO 2 in Ain Harrouda (67.51 μg / m³) can be explained by the exposure of the site to the prevailing winds of the SAMIR oil refinery of Mohammedia, and the decrease is probably due to the significant decline of refining activities and their definitive cut by the end of 2014.
As for PM 10 , the majority of sites experienced limit values exceedances most of the time; a large proportion of the particulate pollution can result from road dust resuspension (Amato et al., 2009) , and rise with the vehicles speed (Nicholson, Branson, Giess, & Cannell, 1989 pollution levels by their transport or transmission (Mayer, 2015), a low wind speed (less than 3 m/s) favors the accumulation of the pollutants (Latini, Grifoni, & Passerini, 2002). The analysis of the wind roses in the Bouskoura site reveals that the site is mainly subject to the prevailing northern winds, which turns the pollution plumes formed downtown to this site. That can explain the strong values recorded.

Conclusion
Urban air quality is getting worse; road traffic in the Grand Casablanca is largely responsible for the local emissions of nitrogen oxides and particles, and thus contributing to the degradation of its air quality. Some air pollutants are present in high concentrations, often above the recommended guidelines. In addition to emissions, air pollution levels also depend on weather conditions. This study allowed us to understand the spatio-temporal distribution of pollutants, particularly those emitted by road traffic in the Grand Casablanca and to identify the most affected areas by this type of pollution.
This work suggests that road traffic plays an important role in the air quality modification, especially in areas with heavy road traffic or close to major traffic routes. The Improvement of local air quality requires therefore additional controls and the application of reduction strategies for the vehicular emissions to reduce the alarming pollution levels.

Declaration of interest
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article.