The study investigates the concentrations of CO, NO2, SO2, CO2 and HC arising mainly from the activities of motor vehicles on the ambient air quality of selected sites in Kaduna metropolis. The sites are situated in the Central market area, the Stadium Roundabout, and Kawo area. Others include Bakin Ruwa Junction, Abuja Junction, Sabon Tasha and a control site at the Angwa Rimi G.R.A. Furthermore, sites situated about a distance of 100m from each of the traffic sites were investigated. The sampling was carried out over both the dry and wet season. Results from dry season survey indicate that the average CO concentrations at the Stadium Roundabout peaked at 29.04ppm. The site also recorded highest concentrations for NO2, SO2, CO2 and HC at 0.042ppm, 0.040ppm, 370.92ppm and 0.030ppm respectively. In the wet season, the Stadium Roundabout recorded highest CO concentrations at 18.72ppm. NO2 was highest at 0.03ppm in Sabon Tasha. Both Stadium Roundabout and Sabon Tasha area recorded highest SO2 concentration at 0.032ppm. Sabon Tasha recorded highest concentrations for both CO2 and HC at 370.92ppm and 0.028ppm respectively. Results from comparison of the average CO concentration with the National Ambient Air Quality Standard (NAAQS), showed that CO concentrations in virtually all sites exceeded the 10ppm for an averaging time of 1 hour in both seasons. The same was true for SO2, which exceeded the 0.01ppm limit for an averaging time of 1 hour. NO2 limit of 0.04ppm for a 1 hour averaging time was exceeded at Stadium Roundabout in the morning hour, Central Market area in the afternoon, and in the evening hours at Central Market, Stadium Roundabout and Bakin Ruwa all in the dry season. All sites were within limit in the wet season. An assessment of the air quality status during the dry season as adjudged using the AQI of the United States indicates that CO concentration at all sites (except Abuja Junction) was “very unhealthy”. During the wet season however, CO concentrations was “very unhealthy” in Kawo, Central Market area and Stadium Roundabout, Sabon Tasha and Bakin Ruwa were “unhealthy” while Abuja Junction was “moderate”. NO2 concentrations were “good” in both seasons for all sites. SO2 in the dry season was “moderate” in all sites but was “good” in Abuja junction. In the wet season, all sites were adjudged “good. Result from the model showed a decrease in all pollutants concentrations with increased distance away from the traffic sites. The model developed is therefore useful for planning of residential and other facilities in Kaduna metropolis and beyond.

The air we breathe is a mixture of gases and particulate solid and liquid matter. Some of these substances come from natural sources while others are caused by human activities such as our use of motor vehicles, domestic activities, industries and businesses. Air pollution occurs when the air contains substances in quantities that could harm the comfort or health of humans and animals, damage plants and materials. These substances are called air pollutants and can be either particles, liquids or gaseous in nature (Alias et al., 2007). Keeping the air quality acceptable has become an important task for decision makers as well as for non-governmental organizations.

As many cities around the world become more congested, concerns increase over the level of urban air pollution being generated and in particular its impact on localized human health. The more this relationship is understood, the better chance there is of controlling and ultimately minimizing such effects. Urban air quality is an issue that is currently on top of air pollution agendas around the world (Colvile et al., 2001). Estimate worldwide show that nearly one billion people in urban environments are continuously being exposed to health hazards from air pollutants (Ahrens, 2003).
Air pollutants are airborne substances that occur in concentrations high enough to cause adverse effects on health, the environment and/or outdoor structures. Air pollutants can

affect health in different ways and in varying degrees of severity ranging from minor irritation through serious illness, to premature death (Dickey, 2000).

Air pollutant emissions come from both natural (biogenic emissions) and anthropogenic sources. Although emissions from natural sources can be substantial, and are indeed the dominant source in non-urban areas, this study specifically investigates (anthropogenic) road traffic emissions in an urban area. The emission of air pollutants has led to several air quality issues such as photochemical smog, acid rain, visibility degradation and nuisance. Although major efforts have been made over the past decades to reduce air pollution and improve air quality, these issues have proven to be quite persistent and continue to exist, despite the implementation of several air quality strategies. A major factor in this is the strong and continued growth in road traffic.

Road transport emits air pollutants from the combustion of liquid or gaseous fossil fuels. Although thousands of air pollutants from road traffic can be identified, most of them can be classified in the following major groups according to their origins and formation processes:

a). Products of incomplete combustion, including carbon monoxide (CO), particulate matter (PM) and hydrocarbons (HCs):

b). Products of high-temperature combustion processes, including nitrogen oxides (NOx);
c). By-products of combustion due to impurities in the fuel, including heavy metals and sulphur oxides (SOx);

d). Non-combustion products, including evaporative emissions;

e). Secondary air pollutants such as photochemical oxidants, including tropospheric ozone (O3) and peroxyacetyl nitrate (PAN); and

f). Greenhouse gases, including carbon dioxide (CO2) and methane.

Around the world, and particularly for CO, NOx and HC, road traffic is the dominant, if not the most important, anthropogenic source of air pollution in urban areas (Fenger, 1999). This is not only because of the magnitude of its emissions, but also because pollutants are emitted in close proximity to human receptors, which enhances exposure levels.

With the emission concentration in traffic being some 104 – 105 times above typical ambient background and released only a few tens of centimeters above ground level, excellent dispersion is essential for dilution of the pollutants in the ambient air (Colls, 2002; Abhishek and Colls, 2010). Thus the meteorological conditions, in addition to concentration of activities, which generates emission, account for the spatial and temporal variations. The emission of vehicle pollutants into the atmosphere is an increasingly important health issue that affects nearly everyone (Rouphail et al, 2001).

The health challenges faced by road users, passers-by, residents and business operators in traffic flash points, having high concentration of vehicular traffic during some periods of the day are worrisome issues (Utang and Peterside, 2011). A comparison of the monitored and inventory emissions with acceptable standards (threshold) is useful in determining the extent of safety of road side business operators and hawkers in traffic intersection and congested traffic points.

Generally, exhaust gas emission concentrations vary, depending on the engine operating mode (idling, accelerating, cruising and decelerating) (Colls, 2002). Deceleration and idle are characteristics of peak traffic at road intersections, while high acceleration and cruise are common at off peaks. Thus a spatio-temporal variation in rate and type of air pollutant exists between and within peak and off peak periods of urban traffic (Utang and Peterside, 2011). Once these emissions are released into the atmosphere, dispersion processes transport and dilute these emissions. In addition to dispersion, pollutants can also undergo (chemical) transformation and deposition. Depending on the ground level location, these processes results in certain ambient concentration levels, which are referred to as immissions (Harssema, 1987), the extent of which is a function of meteorological conditions, topographical characteristics and distance between source and receptor.

The level of exposure to air pollutants depends on ambient concentration levels and where sensitive receptors (e.g. population) are situated in time and place. Health effects then depend on dose-effect relationships, which may be obtained from epidemiological of clinical studies. The magnitude of the effects subsequently determines the economic effects (cost) of air pollution.

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