ABSTRACT
Vehicular exhaust emissions are a dominant source of air pollution and contribute significantly to greenhouse gases and human health problems. An improved dispersion model designed to estimate pollutant concentrations near travel ways of urban streets was developed and its performance evaluated. The model has four inbuilt parameters namely; the aerodynamic coefficient, the wind speed offset, the effective release height and the street wind turbulence coefficient. The model was calibrated with field data collected in Mobil-Bosso Road in Minna using a gas analyzer with detector tubes. The model’s performance was evaluated and compared with Calder’s and OSPM models. The model adequately simulated the concentrations of the pollutants and resulted in a percent error margin less than 10 for all the pollutants considered. The model was validated with data collected in a Copenhagen (Denmark) study for Nitrogen dioxide and was found to over predict the measured concentrations by 4%, which was considered adequate. The model requires less user inputs and modeller expertise and could be used by local authorities or air quality monitoring units in a variety of applications including, but not limited to, air quality and traffic management, urban planning and population exposure studies.
CHAPTER ONE
INTRODUCTION
1.1 Background to the Study
Air pollution is the contamination of the atmosphere by the discharge or emissions of undesirable substances and gases, or their formation from the emissions by chemical reactions in the atmosphere. This definition identifies primary and secondary pollutants. The primary pollutants are emitted directly into the atmosphere, while the secondary pollutants are formed by the chemical reactions involving the primary pollutants. Primary pollutants include nitrogen oxides, volatile organic compounds and carbon monoxide. Tropospheric ozone is an example of a secondary pollutant which is photo chemically produced (Aghedo, 2007).
Agriculture is the principal source of employment and income for most poor people in Africa. Despite the slow economy, rapid urbanization occurred in some regions of Africa, for example Nigeria, caused by rural–urban migration due to a combination of factors serving as incentives to migrants, which include social amenities, industry, schools and health-care facilities (Afolayan, 1985). The population of Lagos, Nigeria has increased from 1.7 million in 1975 to about 10.9 million in 2005, and it is projected to become the first and the eleventh most populous city in Africa and the world, respectively by 2015 (UN Urbanization Prospects, 2005), with about 16 million inhabitants. The rapid urbanization has resulted in increasing air pollution emissions, typically arising from transportation, energy production and industrial activities, concentrated in densely populated areas (Gujar et al., 2008) and surpassing the limits of the cities’ physically occupied areas, thus contributing significantly to air quality on a global scale through the long range transport of air pollutants (Gujar and Lelieveld, 2005; Butler and Lawrence, 2009). The low economic power implies that most vehicles in Africa are old or used ones imported from Europe and elsewhere. There have not been corresponding infrastructure development and economic improvements to stem the production of anthropogenic pollutants. It has been reported by Savile (1993) that nearly 50% of global carbon monoxide, hydrocarbon and nitrogen oxides emissions from fossil fuel combustion come from gasoline and diesel powered engines. He further stated that in the city centres, especially on highly congested streets, motor traffic is responsible for as much as 90-95% of ambient carbon monoxide levels, 80-90% of the nitrogen oxides and hydrocarbons and a large portion of particulates, posing significant threats to humans and natural resources. Urban areas are characterized by high population density and economic development. The resulting pollutant emissions place an increasing pressure on the air quality of these areas. Borrego et al., (2000) wrote that the major current air pollutants come from road traffic, whereas in the past the major reasons for poor air quality were industrial activity and domestic heating.
Although, African anthropogenic emissions are currently much lower than those in other parts of the world (Aghedo, 2007). This may change in the future, due to industrial advancement, transfer of old technologies from Western world and continued increase in urbanization. Biomass burning and natural sources dominate emissions in Africa, while the emissions of industrialized countries are mainly anthropogenic in origin. This is due to higher energy consumption and industrial activities in industrialized countries than in Africa. For example the total energy consumed by the Organization for Economic Cooperation and Development (OECD) countries in the year 2003 was about 56% of the total energy consumed by the world, despite the fact that these countries are just 18% of the world population (International Energy Outlook, 2006). Natural emissions in Africa are mainly from vegetation and soil, lightning NOx emissions. Africa contributes a significant amount to the global emissions from these three sources, while emissions from fossil fuel combustion are important only on the regional scale (Aghedo, 2007).
1.2 Problem Statement
Present day urban environment are mostly dominated by traffic emissions. Economic crisis in most developing countries has led to a fall in the purchasing power of many middle class citizens. This has led to a geometric aging of the vehicular fleet. Even when the populace wishes to buy cars, low income levels have been an incentive to import older used vehicles in recent years, to use cheap two-wheelers and to postpone maintenance (Faiz et al., 1994). In the urban environments and especially in those areas where population and traffic density are relatively high, human exposure to hazardous substances is expected to be significantly increased. This is often the case near busy traffic axis in city centres where urban topography and microclimate may contribute to the creation of poor air dispersion conditions giving rise to contamination hot spots. Traffic density in most city streets is high. The combination of traffic congestion and poor road network can lead to large vehicle emissions and to high levels of pollution within the street. It is at kerbside locations where the general public suffers common exposures to the highest concentration of pollutants (Baumbach et al., 1995; Buckland and Middleton, 1997). It is therefore quite expedient that the levels of concentrations of the pollutants from motorized traffic be measured and monitored. A model will be beneficial in its application in situations where no measurements can be obtained. Local authorities through the use of a model, will assess air quality in their areas against targets set by the government and propose proper traffic management policies.
1.3 Aim and Objectives
The aim of this work is to provide modelling tools for the determination of traffic generated air pollution in streets in Minna metropolis in Nigeria.
The specific objectives include:
1. to develop, calibrate and validate an air pollution model for motorized traffic in streets in Minna metropolis.
2. to investigate and determine in volumetric terms, the nature and quantity of some vehicular exhaust pollutants.
3. to measure and obtain emission factors for vehicular traffic in Minna from the analysis of local driving patterns.
1.4 Justification
Transport is contributing more and more significantly to a number of environmental and human health problems, particularly climate change, acidification, ground level ozone formation, local air pollution and noise (Saija and Romano, 2002). Emissions from transport are an often dominant source of air pollution and contribute significantly to greenhouse gases and energy use. Transport has been said to account for 16% of greenhouse gases linked to human activities worldwide (Westmoreland et al., 2007). Some of these pollutants contribute to global warming and climate change. Global warming and climate change are two issues that have preoccupied scientists as the Earth is facing a potential danger from natural and manmade sources of pollution. The study of the contribution of the most potent anthropogenic source (motorized traffic) to pollution especially greenhouse gases like Carbon dioxide is quite apt. Carbon dioxide represents more than 95% of the greenhouse gases in the transport sector (Nicolas and David, 2009). At present scientists posit that the increase in the Earth’s temperature is due partly to human activities, the chief cause being the burning of fossil fuels like coal, oil and natural gas which release Greenhouse gases and other substances to the atmosphere. The potential consequences of global warming are so great that many of the world’s leading scientists have called for international cooperation and immediate action to counteract the problem. To assess the present and future state of emissions from transport and to evaluate different policies for reducing emissions require the development and application of emission models which are accurate, reliable, consistent and credible.
No locally developed modelling tools exist for environmental protection agencies like the National Environmental Standards and Regulations Enforcement Agency (NESREA) in charge of pollution management and control in Nigeria. There is also no State or Federal regulation as regards traffic air pollution. In more developed climes effective control and efficient reduction of pollutant emissions from automobiles are major focuses of environmental scientists and legislation. Many have argued that the development of air quality regulations and advances in motor vehicle emission reduction technology can curb the environmental pollution. Although these may be true, yet these, in many places are often offset by the increase in the number of automobiles.
Most city roadside traders and dwellers are not aware of the dangers of being exposed to these pollutants. The adverse health effects of roadway-generated pollution have remained major environmental and public concerns. This work would provide awareness for the city dwellers as regards dangers involved in exposing themselves to these pollutants.
1.5 Scope and Limitation of the Work
This work was limited to the development of an improved finite line source model. This is because air pollution dispersion from road traffic has been found to be Gaussian and treatment of the source had been in the form of a line source. Other pollution sources were not included in the study because the focus of the work was modelling traffic air pollution. Minna was chosen as a case study because it is not home to industries which may constitute a great proportion of background pollution. Minna is a diverse, poorly planned urban sprawl that forces higher rates of motor vehicle use which in turn is expected to increase levels of pollutant emitted. Furthermore the study did not take into consideration roads in the rural areas because of the low population density and economic activity. High population density and economic activities are characteristic of urban areas where the resulting pollutant emissions place an increasing pressure in the air quality of the areas. The work was further limited to the study of gaseous pollutants since most of the vehicles in city streets in Nigeria are cars and motor cycles, whose main pollutants are gaseous as they use premium motor spirit (PMS) or petrol different from the particulate matter discharged from diesel engine vehicles. The work was also limited to measuring concentration of the pollutants and not a study of the chemistry of their formation and deposition since concentrations are influenced primarily by meteorological parameters such as wind speed and direction.
The work also involved a study of the local driving patterns and development of emission factors for the local traffic. This was necessary because the variables used in the calibration and validation of some existing line source models were obtained for local traffic in the United States and Europe where there are enough traffic regulations as regards air pollution.
This material content is developed to serve as a GUIDE for students to conduct academic research
MATHEMATICAL MODELLING OF URBAN TRAFFIC AIR POLLUTION IN MINNA METROPOLIS, NIGERIA
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