Analysis of Concentration Levels of Atmospheric Pollutants in Warri, Nigeria

A critical environmental problem facing the Niger Delta region is Air Pollution. This study therefore analyses concentration levels of atmospheric pollutants in the region. Statistical analysis of CH 4 and O 3 concentrations for the period of 2003 to 2012 and NO 2 and CO 2 concentrations for the period of 2011 to 2014 were carried out. The results showed that concentration levels of the pollutants were lower during the rainy season than during the dry year time. This is due to higher occurrences of atmospheric instability during the rainy season. On the other hand, ozone (O 3 ) concentration reached its peak value during the peak period of the rainy season unlike the other pollutants. In all likelihood, some of the ozone-depleting substances such as aerosols and atmospheric hydrogen chloride become soluble in water and are being washed off by precipitation during rainy season, thereby leading to increased tropospheric ozone concentration during the rainy season. The study also revealed a steady increase in the concentration of CO 2 within the period of investigation. This steady increase in CO 2 can be traced to the alarming increase in anthropogenic activities which appreciably increases the amount of CO 2 in the atmosphere. Methane (CH 4 ) had higher standard deviation values than carbon dioxide (CO 2 ), meaning that on a per molecule basis, a proportional rise in CH 4 is much more effective as a greenhouse gas than a similar increase in CO 2 . However, CO 2 has a greater effect than CH 4 on climate change owing to its higher atmospheric concentration. The Mann-Kendall rank statistics of the atmospheric pollutants revealed that the standardization variables U(t i ) and U'(t i ) have a sequential fluctuating behavior around a zero level.


Introduction
Air pollution is the addition of harmful substances known as air pollutants to the atmosphere, resulting in damage to the natural or built environment, human health, and quality of life. The major sources of air pollution in the Niger Delta area are gas flaring, traffic emissions and industrial emissions [1].
Since Nigeria's discovery of oil in the 1950's, the country (especially the Niger Delta region) has been suffering the undesirable environmental repercussions of oil development [2]. Nigeria is accountable for about 46% of Africa's total gas flared per tonne of oil produced and has the highest record (19.79%) of natural gas flaring globally [3]. [4] carried out a comparison of concentrations of ambient air pollutants in Lagos and in the Niger Delta region. He concluded that concentration levels of the pollutants were highest in the Niger Delta region. [5] undertook an air quality assessment of the Niger Delta. The study revealed that the levels of volatile oxides of carbon, sulphur and nitrogen exceed existing Federal Environmental Protection Agency (FEPA) limits for CO: 10 ppm, SO 2 : 0.01 ppm and NO 2 : 0.04 -0.06 ppm. Also, [6] examined air samples obtained from 16 communities in the Niger Delta region for their suspended particulate matter (SPM) composition. The study showed that the particulate load was above the World Health Organization (WHO) specification for both PM 2.5 and PM 10 annual mean and 24-h mean (PM 2.5 : 10 μg/m 3 annual mean, 25 μg/m 3 24-h mean; PM 10 : 20 μg/m 3 annual mean, 50 μg/m 3 24-h mean). Furthermore, [7] undertook an assessment of the atmospheric levels of PM 10 in Port Harcourt. The study revealed that the trend in the seasonal PM 10 concentration levels was dry > transition > wet. Even though some amount of work has been done on the air quality assessment of some other parts of the Niger Delta area, not much work has been undertaken on the analysis of emission levels of atmospheric pollutants in Warri which is one of the major hubs of petroleum activities in the Niger Delta region. Understanding the extent of the emission of atmospheric pollutants in Warri could assist in the mitigation of air pollution in the Niger Delta area.

Study Station
The city of Warri (5.52˚N, 5.75˚E) is a major center of petroleum activities in southern Nigeria. It has a population of over 311,970 (2006 census) [8]. The climate is marked by two different seasons: the rainy season (May to October) and the dry season (November to April). Figure 1 is the map of Delta state showing gas flaring sites and highlighting study station (Warri). The area is characterized with annual rainfall amount of about 2768.8 mm with rainfall periods varying from January to December. Over the course of the year, temperature typically varies from 20.56˚C to 31.11˚C and is rarely below 16.11˚C or above 33.33˚C.  The daily methane (CH 4 ), carbon dioxide (CO 2 ), nitrogen dioxide (NO 2 ) and tropospheric ozone (O 3 ) concentrations data used in this study were obtained from the National Aeronautics and Space Administration (NASA).

Method
Monthly and annual averaging of the daily pollutant concentrations (NASA data) within the period of investigation were carried out. Statistical analysis of CH 4 and O 3 concentrations for the period of 2003 to 2012 and NO 2 and CO 2 concentrations for the period of 2011 to 2014 were carried out. The sequential version of the Mann-Kendall rank statistics was then used to analyze the atmospheric pollutants data in order to identify long-term trends. The effective application involves the following steps in sequence: • The values x i of the initial series are substituted by their ranks y i , set up in ascending order.
• The magnitudes of y i , (i = 1, ..., N) are compared with y j , (j = 1, ..., i − 1). At each comparison, the number of cases y i > y j is counted and represented by n i . • A statistic t i is given as follows (1) • The distribution of the test statistic t i has a variance and a mean as follows ( )( ) 1 • The values of the statistic u(t i ) in sequence are then calculated as • Likewise, the values of u'(t i ) are calculated backward starting from the end of the series. Table 1, Table 2 show the values of average monthly concentration of CH 4 (ppmv) and O 3 (ppmv) respectively for the period of 2003 to 2012, while Table 3, Table   4 show the values of average monthly concentration of NO 2 (ppmv) and CO 2 (ppmv) respectively for the period of 2011 to 2014.   Table 5 shows the values of average annual concentration of CH 4 (ppmv) and O 3 (ppmv) for the period of 2003 to 2012 while Table 6 shows the values of average annual concentration of NO 2 (ppmv) and CO 2 (ppmv) for the period of 2011 to 2014.   show the graph of average annual concentration levels of the atmospheric pollutants within the period of investigation.  Table 9 shows the Mann-Kendall rank statistics for NO 2

Discussion
The results of the descriptive statistics of the annual averages of CH 4         is much more effective as a greenhouse gas than a similar increase in CO 2 [9].
However, CO 2 has a greater effect than CH 4 on climate change owing to its higher atmospheric concentration.
The results from the analysis of the average monthly concentration of the at- September and begins to increase as the dry season sets in. Therefore, concentration levels of the atmospheric pollutants were lower during the rainy season than during the dry yeartime. This is due to higher occurrences of atmospheric instability during the rainy season. This finding is in agreement with the result of [7]. On

Conclusions
The results of the analysis of concentration levels of the air pollutants showed that concentration levels were lower during the rainy season than during the dry yeartime. This is due to higher occurrences of atmospheric instability during the rainy season. On the other hand, ozone (O 3 ) concentration reached its peak value during the peak period of the rainy season unlike the other pollutants. In all likelihood, some of the ozone-depleting substances such as aerosols and atmospheric hydrogen chloride become soluble in water and are being washed off by precipitation during the rainy season, thereby leading to increased tropospheric The study also revealed a steady increase in the concentration of CO 2 within the period of investigation. This steady increase in CO 2 can be traced to the alarming increase in anthropogenic activities (such as combustion of fossil fuels, industrial emissions, gas flaring and deforestation) which appreciably increases the amount of CO 2 in the atmosphere. Methane (CH 4 ) had higher standard deviation values than carbon dioxide (CO 2 ), meaning that on a per molecule basis, a proportional rise in CH 4 is much more effective as a greenhouse gas than a similar increase in CO 2 . However, CO 2 has a greater effect than CH 4 on climate change owing to its higher atmospheric concentration. The Mann-Kendall rank statistics of the pollutants showed that the standardization variables U(t i ) and U'(t i ) have a sequential fluctuating behavior around a zero level.