Modeling and Simulation of Air Quality in Bamako

Abdoulaye Samaké, Faculty of Science and Technology (FST), Bamako University of Science, Techniques and Technology

and

Christophe Prud’homme, Centre for Modelling and Simulation in Strasbourg (Cemosis), Strasbourg University

1. Background and rationale

The city of Bamako is built in a basin surrounded by hills that trap heavier-than-air gases. It extends from west to east over 22 km and from north to south over 12 km, for a surface area of 267 km\(^2\). Bamako, the economic capital of Mali, is home to more than 70% of the country’s economic activities. The development of its road transport sector is characterised by the predominance of second-hand vehicles and motorcycles, as well as the poor state of the road infrastructure. This sector is the main factor promoting air pollution in urban areas, far ahead of factory emissions, waste incineration, domestic and bush fires, and suspended dust (due to the wind or lifted by various machines). Urban transport alone consumes more than four times the volume of fuel reserved for the industrial sector. The liberalised importation of vehicles has led to the acquisition of a significant number of used vehicles compared to new vehicles. As a result, the road transport fleet is obsolete, with nearly 75% of vehicles more than 15 years old. The large number of two-wheeled vehicles, the advanced age of the majority of four-wheeled vehicles and the increase in the population are among the factors responsible for air pollution. The numerous exhaust fumes from vehicles, which are carbon monoxide, carbon dioxide, sulphur dioxide, nitrogen dioxide, lead and aromatics are the main pollutants in Bamako’s air. This often results in poor visibility due to vehicle exhaust fumes (mostly diesel-fuelled) which dissipate little in the large Bamako basin.

A study on air quality in Bamako was conducted by the Ministry of Environment and Sanitation from September 2009 to January 2010 to examine the current situation of air quality and emissions in Bamako. The study focused on current regulations, criticism of existing air quality data, meteorology and its impact on air quality, the country’s energy balance and its application in Bamako, the vehicle fleet and its fuel consumption, the situation of the urban road network and the analysis of emissions other than those from motor traffic. The results of the study reveal that air pollution is mainly due to automobile traffic (including two-wheeled vehicles), either through direct emissions from combustion engines or through the suspension of particles caused by traffic on dusty roads and pollution from domestic wood-burning stoves. Benzene in vehicle exhaust is carcinogenic and could cause an excess collective risk of about 249 cancers per year in Bamako. This excess could rise to 686 cancers in 2020.

In the city of Bamako, the annual average concentration of PM10 particles has been estimated at 333 ug/m3 , with daily peaks exceeding 600 ug/m3 , whereas the WHO daily recommendation is 50 ug/m3 , not to be exceeded for more than 3 days. Exposure of the air pollution load mainly affects the respiratory, circulatory, olfactory and optical systems with a particular emphasis on the respiratory system, the main route of entry of pollutants into the body. These pollutants introduced into the body can end up in the blood, urine, liver, marrow, bone, hair and teeth and initiate a certain but slow destruction of the body. DNA abnormalities, the risk of cancer development, hematological disturbances, the risk of cardio-vascular, kidney and nervous system disorders are not to be overlooked. Although the results of statistics on diseases related to air pollution alone are not available, it can be noted that the people most sensitive to this pollution load are children. It is estimated that the mortality rate in Bamako due to air quality has increased by an average of 23%. Since pollution from car exhausts is effective, it seems useful to identify the main pollutants found in emissions. These are oxides of carbon, nitrogen, sulphur, suspended and smoked particles, ozone, lead, hydrocarbons, hydrogen sulphide (H2S), mercaptans and aldehydes. The removal of these pollutants from the body can take years and is not always easy. The different pollutants act each on its own but also interact. The most frequently noted conditions in the population of Bamako, and which could be related to air quality are: Pneumonia, chronic bronchitis (dry cough, breathing difficulties, asthma, pulmonary emphysema (dilation of the alveoli), cancer, etc. of the lungs, cardio-vascular disorders, hyperglobulism, anemia due to traces of carbon monoxide, eye inflammations, eye inflammation, and ORL conditions, food poisoning, food poisoning, eating disorders, and breathing.

Faced with this public health and environmental challenge, the air quality in the city of Bamako, in order to assess risks and environmental health of populations, is today a Mali’s population is growing every year, and this is a necessity for the country. his transport sector.

Proposal

We propose the development of a national monitoring system and efficient, robust and reliable air quality prediction in the city of Bamako through mathematical modeling and numerical simulation. This system will provide a better understanding of the quality index trend, anticipate the consequences and take appropriate action.

2. Project Objectives

The main objective of this project is the development of a model of prediction and measurement of air pollution levels in the city of Bamako. This model is described by partial differential equations advection-diffusion-reaction type depending on several parameters physics and measurements, including meteorological conditions and the chemistry of the atmosphere. The main specific objectives pursued are:

  • To develop a mathematical model to predict the quality index of the air in Bamako;

  • To contribute to the establishment of a warning and assistance system for the decision;

  • To assess the impact of measures taken to improve the quality of the air;

  • To build capacity in environmental science research and development at the national level.

3. Methodology

3.1. Model Description

The study and forecasting of air quality is based on traditionally on modeling and numerical simulation. The evolution of the concentration of pollutants is given by an equation to partial derivatives (PDEs) of the advection-diffusion-reaction type. Advection corresponds to transport through the wind field, and the diffusion describes the turbulent mixture. The reaction corresponds to physico-chemical processes of transformation of pollutants.

Let \(c\) a vector of concentration fields, where each stem element:[c_i] corresponds to the scalar concentration field of the chemical species (pollutant) \(i\) in the air. Evolution spatio-temporal concentration of this stem concentration:[c_i] for the species \(i\) is as follows:

\[\label{eq:1} \left\{ \begin{aligned} &\dfrac{\partial c_i}{\partial t} = - \underbrace{\nabla \cdot \left ( \mathbf{u} c_i \right)}_{\text{advection}} + \underbrace{\nabla \cdot \left ( \rho K \nabla \left ( \dfrac{c_i}{\rho} \right) \right )}_{\text{diffusion turbulente}} + \underbrace{\chi_i (c,t)}_{\text{chimie}} + \underbrace{S_i(x,t)}_{\text{sources volumiques}} - \underbrace{\Lambda_i(x,t)c_i}_{\text{lessivage}}\, \\[4pt] & \text{+Conditions aux limites et conditions initiales} \end{aligned} \right.\]

where

\(\rho\) Air density;

\(\chi_i\): the balance of production and reaction losses…​ chemical of the stem species:[i];

\(\Lambda_i c_i\): the leaching losses of the pollutant stem: [i];

\(S_i\): sources of the species \(i\);

\(\mathbf{u}=\mathbf{u}(x,t)\): Wind speed;

\(K\): the matrix of turbulent diffusion coefficients;

\(x\): the space variable;

\(t\): time.

the wind speed field \(\mathbf{u}\) is the solution of the turbulent Navier-Stokes equation with boundary conditions provided by meteorological data:

\[\left\{ \begin{aligned} & \rho \frac{\partial \mathbf{u}}{\partial t} - 2 \nabla \cdot (\mu \mathbf{D}(\mathbf{u})) + \rho (\mathbf{u} \cdot \nabla) \mathbf{u} + \nabla p = \mathbf{f} \\ & + \mbox{Conditions initiales} \\ & + \mbox{Conditions aux limites météo} \end{aligned} \right.\]

where

\(\rho\): Air density;

\(\mu\): the dynamic viscosity of air;

\(\mathbf{u}\): Wind speed;

\(p\): wind pressure;

\(\mathbf{D}(\mathbf{u})\): the deformation tensor.

The equation #eq:1 above, for which the field of concentration \(c_i\) is subject to transport and chemical reactions, describes the life cycle of the chemical species \(i\). This model is based on the assumption that there is no feedback between chemical species and flow fields (wind speed,turbulent diffusivity, temperature). These flow fields are pre-processed by meteorological calculations or parameterizations.

Data

A great deal of data is required to initiate a simulation of the air quality. These data must be available before the simulation, that is to say, a priori. They play an important role in the quality of the results.

The main data are:

  • meteorological data

  • topographical data

  • traffic data

  • emissions

  • boundary and initial conditions.

Meteorological data are essential for our model. They are the main input data, with a minimum set at to provide consists of the wind fields \(\mathbf{u}\), of temperature, humidity and pressure. These measured data will come from the Weather Centres, for example Mali Météo, or from the satellite observations.

The emissions correspond to the space-time evolution of the different pollutants in the atmosphere. To determine them, we will use the COPERT utility, which is the European standard for determine vehicle emissions.

Traffic will be simulated using a map of Bamako obtained via the OpenStreetMap project and the PTV tool Vissim, which is a software for road traffic simulation that models, simulates and analyses all road traffic modes of transport and their interactions.

4. Collaboration

The project will be a collaboration between the research team at A. Samaké and C. Prud’homme Director of the Centre de Modélisation and Simulation of the University of Strasbourg (Cemosis).

  1. Samaké is a member of the Mathematics and Applications Laboratory. (LAMA) of the University of Sciences, Techniques and Technologies from Bamako. LAMA’s research activities are centred around the modelling and scientific computing, optimal control and research operational. LAMA is a member of the EDP Modelling and Control network. (EDP-MC), which brings together academic institutions from Mali, the Senegal and Burkina Faso.

Cemosis is an expert in modeling simulation high performance computing. and data science. Cemosis is working on similar projects and develops the Feel++ software chain for simulation multi-physics, high-performance computing and data mining.

References

  • [WHO] W.H.Organization, Air pollution, www.who.int/phe, 2020.

  • [Kumar] S.Kumar, R.Kumar, Air Quality: Monitoring and Modeling, BoD-Books on Demand, 2012.