Evaluation of Pollutants Along the National Road N2 in Togo using the AERMOD Dispersion Model

Background. Air pollution has become a major problem around the world and is increasingly an issue in Togo due to increased vehicular traffic. Gaseous pollutants are released by engines and are very harmful to human health and the environment. The fuels used on the major road in Togo, the N2, are adulterated with unknown contents and are of poor quality. Many of the vehicles come from neighboring countries, such as Benin, Ghana and Nigeria. Objectives. The present study aims to evaluate the pollution rate in Togo through the estimation of the concentrations of sulfur dioxide (SO2), nitrogen oxides (NOx), and particular matter (PM) on the international road, the National Road N2, in Lomé, compared to the World Health Organization's (WHO) standard limit. Methods. The simulations of pollutant concentration were performed using the Industrial Source Complex Short Term Version 3 model, which is included in the United States Environmental Protection Agency Regulatory Model (USEPA) AERMOD View software. The meteorological averages data were obtained from the local station near the National Road N2 in Togo in 2018. Hourly averages were calculated according to the European Monitoring Evaluation Programme/European Environmental Agency air pollutant emission inventory guidebook 2016 and were processed using AERMET View and a terrain pre-processor, AERMAP. For the model, the sources of pollution were the vehicles traveling on the road segment. The source was a line volume with 20 m of width and 2 m of height. The estimation methodology covered exhaust emissions of NOx, SO2 and PM contained in the fuel. Results. The simulations provided average hourly, daily and annual concentrations of the different pollutants: 71.91 μg/m3, 42.41 μg/m3,11.23 μg/m3 for SO2; 16.78 μg/m3, 9.89 μg/m3, 2.46 μg/m3 for NOx and below the detection limit, 0.62 μg/m3, 0.15 μg/m3 for PM, respectively. These results indicate that on the National Road N2 in Togo, the concentrations of SO2 were high compared to those of NOx and PM. The daily average concentration of SO2 was twice the permissible limits set by the WHO. Conclusions. Emissions obtained from the AERMOD for NOx and PM were less than the permissible limits set by the WHO, while the rate of SO2 was twice the permissible limit. The fuels used on this road were very rich in sulfur. The sulfur level in fuels must be monitored by stakeholders in Togo. Competing Interests. The authors declare no competing financial interests.


Introduction
Atmospheric pollution refers to the presence of pollutants (gaseous or particles) in the atmosphere. Air pollution can have harmful effects on the environment and human health. 1 The sources of this pollution can be either natural or related to human activities, especially combustion processes (motor vehicles, industrial installations, energy production, etc.). 2, 3 Many studies have shown a correlation between the degradation of the environment and human health and the presence of pollutants in the atmosphere. 4 Exposure to air pollution is responsible for respiratory and lung diseases and leads to premature deaths worldwide (4.2 million in 2016 according to the World Health Organization (WHO)). 5-7 Pollution contributes to climate change and the phenomena of acid rain which has a very harmful effect on vegetation and global warming. 1,8 Sulfur dioxide, emitted from fossil fuel consumption or industrial activities, is an acidifying gaseous pollutant that forms sulfuric acid in the presence of water. It contributes to the phenomenon of acid rain that disrupts the composition of air, surface water and soil, and could damage plants and vegetation and kill animal species. 1, 9 At higher concentrations, sulfur dioxide can have serious health effects and impact pulmonary function. Apart from industrial processes, road traffic is Research an important source of atmospheric pollution in the world, especially in developing countries, due to the age of vehicles used and poor quality of fuel. 10 Nitrogen oxides constitute very toxic odorant gases, and are formed by the oxidation of nitrogen in the air, from fuels with oxygen or during the combustion phenomena in engines. They have harmful effects on human health and the environment with the formation of ozone (related to the greenhouse effect), and increased sensitivity of the bronchi to microbial infections for children. 11 For nitrogen dioxide (NO 2 ), a traffic-related pollutant, short-term exposure causes significant inflammation of the respiratory tract, reduces lung function and increases symptoms of bronchitis in those with asthma. 7 Particulate matter (PM) is comprised of ultrafine particles that impact human health. These small aerosols can penetrate deep into the lungs and into the alveoli. 12, 13 Nuclei condensation can be formed where moisture and pollutants (lead, sulfur dioxide) can absorb, making them even more toxic. Therefore, PM is an important vector of respiratory tract intoxications in areas of high traffic. 14 Diallo and coworkers demonstrated the correlation between air quality and its impact on respiratory diseases due to PM in Lomé, Togo. 15 Air pollution in urban areas due to road traffic is an important issue in developing countries. Globally, many countries have little or no access to low-sulfur fuel, and do not have standards for vehicle emissions. 16 The sulfur content in fuel in most developed countries is currently 50 ppm sulfur or less; however high sulfur content can be found in many low-and middleincome countries from ranges of 500-5000 ppm. 16 Transport traffic is estimated to grow rapidly by 2050, which will double global fuel demand. 17 Almost 50% of the fuels imported to West Africa come from Amsterdam, Rotterdam and Antwerp, and trade statistics showed that 80% of the diesel exported to Africa has a sulfur content at least 100 times above the European standard. 18 In Togo, more than 50 000 motorcycles and taxis travel on traffic roads daily, polluting the atmosphere, and as a result the population has respiratory problems due to the poor quality of fuels that these vehicles consume. 19 The N2 is an international road not only used by vehicles from Togo, but cars and buses from Nigeria and Benin travel through to Togo, Ghana and the Côte d'Ivoire and vice versa. In Togo and along the National Road N2, even in Benin and Nigeria, the most used fuels are adulterated with unknown contents of sulfur (locally called "Boudé" or "Kpayo" in Togo and Benin). It is thus necessary to determine air pollution in this area.
Five West African countries including Togo introduced standards to regulate emissions and lower the levels of sulfur diesel in their fuels in 2016. 18 Many studies have been carried out in countries using AERMOD simulation for the measurement and control of air quality and assessment of their impact on human health and the environment. using AERMOD and CALPUFF models across seasons. The obtained maximum daily concentrations were higher during the heavy rainy season than minor rainy and dry seasons (37.7 µg/m 3 for SO 2 , 9.6 µg/m 3 for NO 2 , and 38.8 µg/m 3 for PM 2.5 ), respectively. 12,22 The National Road N2 of Togo, from Lomé to the Aného toll, covers a distance of 15.4 km and heavily trafficked by vehicles from the Gulf of Guinea countries. Particulate matter, SO 2 and NOx, are the main pollutants from road traffic and industries. Few studies have been conducted on air quality and fuel quality in Togo. 15,23,24 It is thus necessary to evaluate the concentration of these pollutants in the air along the National Road N2. The results will be useful for the assessment of air pollution impacts on users, the health of nearby residents and the surrounding ecosystem using the most predictable modeling system, AERMOD.

Research
The objective of the present study was to estimate the concentration of SO 2 , NO x and PM emitted along the N2 in Togo in order to determine the pollution rate compared to international standards set by the WHO. This study could be useful for decision makers setting air quality policies for the future, in order to monitor the emissions of atmospheric pollutants, and for future studies as a baseline on the health of the population in the country.

Methods
This study was carried out on the national road of Togo, the National Road N2, from Lomé to the Aného toll, which travels over a distance of 15.4 km (Figure 1). The National Road N2 (Lomé-Aneho), about 50 km long, lies along the coast of Gulf of Guinea. The present case study focused on the evolution of air quality in the highly trafficked area for pollutants (SO 2 , NO x , and PM emissions). Figure 1 presents a road map of south Togo and a Google map photo of the study area along the National Road N2.

Model set up
The Industrial Source Complex Short Term Version 3 model, which is included in the United States Environmental Protection Agency (USEPA) Regulatory Model, AERMOD View software, was used to perform the dispersion simulations of pollutants. 25, 26 The model was used to predict air concentrations and ambient impacts around the point/area and volume sources. The emission rates and the meteorological conditions were used as model inputs. Meteorological data, such as air temperature, wind speed, wind direction, ceiling height, cloud cover, pressure, relative humidity, and precipitation were obtained from the local station at Lomé (Togo) for the The model is composed of three parts as described by Yadav and coworkers: AERMOD meteorological preprocessor (AERMET) to extract meteorological data and to assess data quality; AERMOD terrain preprocessor (AERMAP) to merge all data available over a 24-hour period and record them in a single file; and AERMOD Gaussian plume model that reads the merged meteorological data and estimates the boundary layer parameters for the dispersion calculations. 20, 26 The main program was AERMOD, while data were preprocessed in AERMET and AERMAP.
Meteorological data such as wind speed, wind direction, temperature, and cloud cover were input to AERMET to determine the boundary layer parameters. AERMAP uses geological data to calculate the terrain height scale and to create receptor grids before passing receptor characteristics to AERMOD for final processing. Lastly, the source data were sent directly to AERMOD for processing. In addition, the wind rose plot for the most predominant wind direction and the pollutant contents were collected.

Equation 1 E i = ∑ j (∑ m (FC j,m × EF i,j,m ))
where, E i is the emission of pollutant "i" (g); FC j,m is the fuel consumption of vehicle category "j" using fuel "m" (kg); and EF i,j,m is the fuel consumptionspecific emission factor of pollutant "i" for vehicle category "j" and fuel "m" (g/kg).
Vehicles were categorized as passenger cars, light commercial vehicles, heavyduty vehicles or L-category vehicles (which includes two or three wheelers, quadricycles and micro cars) and the considered fuels included petrol, diesel and natural gas. Examples of fuel sulfur content periods can be found in Table 2.
The emissions of SO 2 per fuel-type m can be estimated by assuming that all sulfur in the fuel was transformed completely into SO 2 , using Equation 2. 28

Equation 2 E SO2,m = 2K s,m * FC m
where, E SO2,m is the emissions of SO 2 per fuel "m" (g); K s,m is the weight related to sulfur content in fuel of type "m" (g/g fuel); and FC m is the fuel consumption of fuel "m" (g).
In the case of Togo, the sulfur content limit in 2018 is given in Table 3 from the Fuel Quality and Emission Standard Developments in Africa. 24 Our investigations on transportation in Togo provide a characterization of traffic levels along the N2 (Table 4). We identified almost 10,000 vehicles in 24 hours.

Results
A wind rose diagram illustrates the speed, direction and frequency of winds of a given location using a center coordinate system. Meteorological pre-processed data were used to determine the corresponding wind rose plot ( Figure  3), which shows the most predominant wind direction. The wind rose presented a main wind direction of south-west with an annual probability up to 38% and average wind speed between 3.6 -5.7 m/s. Secondary directions were mainly west and northwest with a probability up to 12% and wind speed up to 8.8 m/s.

AERMOD dispersion modeling results
Hourly, daily and annual averages of concentrations of pollutants (SO 2 , NO x , and PM) were investigated along the National Road N2, by AERMOD simulations. The maximal concentrations of emitted SO 2 , NO x and PM obtained through AERMOD were compared with permissible limits  Table 3

Table 4 -Vehicle type by time of day
Research of the WHO in Table 5. 29

Concentration distribution of Nitrogen oxides
Simulations were performed for the concentration of NO x along the National Road N2. Figures 4-6 present estimations of the maximum hourly, daily and annual concentrations of NOx on the National Road N2, respectively. The AERMOD simulation showed that the maximum hourly ( Figure 4) and daily ( Figure 5) N (Figure 6).

Concentration distribution of sulfur dioxide
Concentration simulations performed for SO 2 emitted on the National Road N2 are presented in Figures  7-9 for hourly, daily and annually concentrations, respectively. The maximum average concentrations of SO 2 emitted on the road were 71.91 μg/m 3 for hourly concentrations (Table  5 and Figure 7), 42.41 μg/m 3 for daily concentrations (Figure 8) and 11.23 μg/m 3 for annual concentrations (Figure 9) at the UTM coordinates of 313734 m E and 681400 m N.

Concentration distribution of particulate matters
For PM, the model did not yield any valuable graphical representations, due to the very low obtained concentrations. In all AERMOD simulations, the PM concentration was low, as shown in Table 5, and

Discussion
The Research contained in fuels to avoid harmful impacts of sulfur dioxide on human health and the environment. We recommend point monitoring on this road by measuring gas pollutants concentrations and an assessment on impact to local population health.