Levels of Nitrogen Dioxide and Sulphur Dioxide Measured Around Roadside Gardens in Port Harcourt Metropolis

Port- Harcourt being the capital and major city of Rivers State, with rapid urbanization and an associated growth in industry and vehicle use, an increase in the emissions of sulphur dioxide, hydrocarbons, nitrogen oxide, and other industrial and automobile emissions are bound to be expected (Ideriah, 1996). If these pollutants exceed the recommended levels, then there is no doubt that they will become harmful to higher animals including humans through the soil, water and plants since their existence is in close proximity.

 The study area, Port Harcourt, is a highly industrialized city in Nigeria; it has a large number of multinational firms as well as other industrial concerns, particularly businesses relating to the petroleum industry. Port Harcourt lies within latitude 40 43' and 40 54' N and longitude 60 56' and 70 03' E, 18meters (59ft) above sea level and with a mean annual rainfall of over 2000mm and mean annual temperature of about 290C. (NMS, 1998) (Fig. 1). 

As air quality is slowly improving in developed countries, it is rapidly deteriorating in developed countries. A rapidly multiplying population and the growth of industries and car use are the main causes of air pollution today. In Asia, rapid urbanization, with the associated growth in industry and vehicle use has increased emissions of sulphur dioxide (SO2) and nitrogen dioxide (NO2) (PEPU, 2009). 

The two main sources of pollutants in urban areas are transportation (predominantly automobiles) and fuel combustion in stationary sources, including residential, commercial and industrial heating and cooling and coal-burning power plants. Motor vehicles produce high levels of carbon monoxide (CO) and constitute a major source of hydrocarbon (HC) and nitrogen oxides (NOx), whereas, fuel combustion in stationary sources is the dominant source of SO2 (Socha, 2007). 

Slanina and Howard (2008) reported that the transport sector is responsible for 60 to 70% of NO2 emissions in Europe. 

Nitrogen oxide (NO or NO2) is a brown, odourless gas which is a by-product of combustion, when oxygen is used to oxidize nitrogen instead of hydrocarbon. Transportation accounts for 45-50% of total emission of nitrogen oxides (Jean-Paul, 2009). NO2 is a gas that is emitted into the atmosphere as a result of various human activities. An excess of NO2 is mainly from power plants in major cities and the burning of fuels due to various motor vehicles (Kashimira, 2009). Nitrogen oxides are known to prevent the growth of crops and thus reduce agricultural yields (Jean-Paul, 2009).

Air pollution affects plants and wildlife. This is why it can be more difficult for plants to thrive in city centres. SO2 and NOx can make water and soil more acidic and therefore harmful to some plants and animals (Direct Gov., 2010). 

Air pollution injury to plants can be evident in several ways; injury to foliage may be visible in a short time and appear as necrotic lesions (dead tissues), or it can develop slowly as a yellowish or chlorosis of the leaf (Heather, 2009). There may be reduction in growth of various portions of a plant, plants may be killed outright, but they usually do not succumb until they have suffered recurrent injury (Heather, 2009). Agricultural crops can be injured when exposed to high concentration of various air pollutants. Injury ranges from visible markings on the foliage to reduced growth and yield, to premature death of the plant (Heather, 2009). The development and severity of the injury extends not only on the concentration of the particular pollutant, but also on a number of other factors. These include the length of exposure to the pollutants, the plant species and its stage of development as well as the environmental factors conducive to a build-up of the pollutants and the preconditioning of the plants, which makes it either susceptible or resistant to injury (Heather, 2009). Different plant species and varieties and even individuals of the same species may vary considerably in their sensitivity to SO2. These variations occur because of the differences in geographical locations, climate, stage of growth and maturity (Heather, 2009). 

According to Jean-Paul (2009), air pollutant has serious harmful effects on plants. SO2 causes chlorosis resulting in the death of cells and tissues, also, forest trees are worst affected by SO2 pollutants. SO2 is a heavy, colourless gas with a strong odour, it is the result of the combustion of fossil fuels like coal (particularly bituminous coal and hydrocarbons). Transportation accounts for about 5% of total SO2 emissions. Sulphur is an essential nutrient for plants, but SO2 is regarded as an inhibitor of physiological activities. Most affected plants are those having a high physiological activity like crops and commercial timber forests (Jean-Paul, 2009). SO2 is converted to sulphuric acid in the atmosphere, which can be poisonous to both plants and animals (PEPU, 2009). 

Pollutants such as SO2, NOx and ozone cause direct damage to leaves of crop plants and trees when they enter leaf pores (stomata). Chronic exposures of leaves and needles to air pollutants can also break down the waxy coating that helps to prevent excessive water loss and damage from diseases, pest, drought and frost (Miller, 1990).

Heather (2009) reported that SO2 enters the leaves mainly through the stomata (microscopic openings) and the resultant injury is classified as either acute or chronic. He further explained that acute injury is caused by absorption of high concentrations of SO2 in a relatively short time. The symptoms appear as a two sided (bifacial) lesions that usually occur between the veins and occasionally along the margins of the leaves. The colour of the necrotic area can vary from a light tan or near white to an orange-red or brown, depending on the time of the year, the plant species affected and the weather conditions. Recently expanded leaves are usually the most sensitive to acute SO2 injury, the youngest and oldest being somewhat more resistant. Chronic injury is cursed by long-term absorption of SO2 at sub-lethal concentrations. The symptoms appear as a yellowing or chlorosis of leaf and occasionally as a bronzing on the under surface of the leaves (Heather, 2009).

 Changes in the physical appearance of vegetation are an indication that the plants’ metabolism is impaired by the concentration of SO2 (Carter, 2003). Harm caused by SO2 is first noticeable on the leaves of the plants, injury can occur within hours or days of being exposed to high levels of sulphur dioxide and it is the leaves in the mid-growth that are most vulnerable, while the older and youngest leaves are more resistant (Carter, 2003). The effects of SO2 are influenced by other biological and environmental factors such as plant type, age, sunlight levels, temperature, humidity and the presence of other pollutants (Ozone and NO2) (Carter, 2003). 

Ideriah, (1996) reported that mounting population pressure and urbanization has resulted in ever increasing smaller farm sizes and in some cases the complete clearing of farms for other uses. Small scale farmers are left with the option of raising home gardens where annual and perennial vegetables are grown along with the common short duration fruit crops (banana, paw-paw, pineapple, etc). Other fruit crops such as mango, guava, avocado, etc are also grown along roadsides and around the house. It is not uncommon to find people plucking and eating these unwashed raw fruits, or food hawkers displaying their stuff openly and unwrapped alongside heavy traffic roads which are apparently contaminated with air pollutants (Ideriah, 1996). The aim of this study is to determine the levels of sulphur dioxide and nitrogen around gardens along major road junctions in Port Harcourt, a fast developing city in the Niger Delta, Nigeria.