1. Introduction

ACS

Atmospheric and Climate Sciences

2160-0414

Scientific Research Publishing

10.4236/acs.2022.122024

ACS-116482

Articles

Earth&Environmental Sciences

Analysis of Concentration Levels of Atmospheric Pollutants in Warri, Nigeria

Ifeanyi

Innocent Onwosi

¹Emmanuel

Iruka Njoku

²Emmanuel

Fartiyahcha Nymphas

Department of Physics, University of Ibadan, Ibadan, Nigeria

Department of Physics and Engineering, Delaware State University, Dover, DE, USA

10022022

120240942018, February 20229, April 2022 12, April 2022

2014

This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/

A critical environmental problem facing the Niger Delta region is Air Pollution. This study therefore analy se s concentration levels of atmospheric pollutants in the region. Statistical analysis of CH₄ and O₃ concentrations for the period of 2003 to 2012 and NO₂ and CO₂ 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 (O₃) 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 CO₂ within the period of investigation. This steady increase in CO₂ can be traced to the alarming increase in anthropogenic activities which appreciably increases the amount of CO₂ in the atmosphere. Methane (CH₄) had higher standard deviation values than carbon dioxide (CO₂), meaning that on a per molecule basis, a proportional rise in CH₄ is much more effective as a greenhouse gas than a similar increase in CO₂. However, CO₂ has a greater effect than CH₄ 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.

Air Pollution Atmospheric Pollutants Mann-Kendall Rank Statistics

1. 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, SO₂: 0.01 ppm and NO₂: 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 PM₁₀ annual mean and 24-h mean (PM_2.5: 10 μg/m³ annual mean, 25 μg/m³ 24-h mean; PM₁₀: 20 μg/m³ annual mean, 50 μg/m³ 24-h mean). Furthermore, [7] undertook an assessment of the atmospheric levels of PM₁₀ in Port Harcourt. The study revealed that the trend in the seasonal PM₁₀ 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.

2. Study Station, Materials and Method2.1. 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.

2.2. Materials

Data description

The daily methane (CH₄), carbon dioxide (CO₂), nitrogen dioxide (NO₂) and tropospheric ozone (O₃) concentrations data used in this study were obtained from the National Aeronautics and Space Administration (NASA).

2.3. Method

Monthly and annual averaging of the daily pollutant concentrations (NASA data) within the period of investigation were carried out. Statistical analysis of CH₄ and O₃ concentrations for the period of 2003 to 2012 and NO₂ and CO₂ 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

t i = ∑ 1 i n i (1)

● The distribution of the test statistic t_i has a variance and a mean as follows

var t i = i ( i − 1 ) ( 2 i + 5 ) 72 (2)

E ( t i ) = i ( i − 1 ) 4 (3)

● The values of the statistic u(t_i) in sequence are then calculated as

u ( t i ) = [ t i − E ( t i ) ] var t i (4)

● Likewise, the values of u'(t_i) are calculated backward starting from the end of the series.

3. Results and Discussion3.1. Average Monthly Concentration of the Atmospheric Pollutants

Table 1, Table 2 show the values of average monthly concentration of CH₄ (ppmv) and O₃ (ppmv) respectively for the period of 2003 to 2012, while Table 3, Table 4 show the values of average monthly concentration of NO₂ (ppmv) and CO₂ (ppmv) respectively for the period of 2011 to 2014.

Table 1 Values of average monthly concentration of CH4 (ppmv) for the period of 2003 to 2012

	2003	2004	2005	2006	2007	2008	2009	2010	2011	2012
January February March April May June July August September October November December	1775.0 1745.0 1748.1 1735.6 1735.1 1724.2 1714.3 1711.1 1732.6 1757.4 1761.3 1752.1	1745.4 1740.2 1737.6 1732.4 1743.9 1721.0 1717.0 1726.7 1748.2 1750.9 1754.7 1739.3	1741.1 1738.3 1745.0 1737.5 1733.4 1731.9 1727.9 1729.0 1738.1 1750.8 1739.9 1735.0	1727.7 1727.5 1730.5 1731.4 1722.5 1733.2 1719.9 1718.6 1725.0 1739.4 1755.6 1739.9	1739.3 1730.9 1743.3 1735.0 1738.7 1737.1 1724.0 1718.1 1740.5 1759.0 1768.9 1753.9	1748.1 1746.5 1739.9 1746.0 1746.2 1745.5 1737.1 1710.1 1735.8 1749.7 1760.4 1758.2	1759.8 1747.2 1749.6 1750.4 1751.7 1744.4 1740.1 1735.6 1746.8 1754.8 1769.2 1752.8	1741.9 1740.8 1741.6 1747.0 1747.9 1746.9 1740.9 1736.0 1744.8 1757.5 1755.1 1755.5	1751.4 1746.9 1746.8 1749.4 1745.6 1732.5 1748.3 1758.2 1771.9 1784.7 1796.9 1786.5	1780.9 1787.8 1777.3 1785.0 1775.8 1766.9 1754.9 1757.7 1761.4 1769.2 1779.9 1788.5

Table 2 Values of average monthly concentration of O3 (ppmv) for the period of 2003 to 2012

	2003	2004	2005	2006	2007	2008	2009	2010	2011	2012
January February March April May June July August September October November December	53.083 53.020 54.475 57.050 56.903 57.453 58.435 59.857 58.779 56.736 54.428 54.918	54.087 54.610 57.048 57.351 57.944 59.799 60.004 60.014 59.561 56.904 55.315 54.402	53.102 53.378 53.807 54.234 54.311 54.931 56.209 56.808 56.176 55.092 53.042 52.921	51.841 52.988 54.723 57.257 58.751 60.413 61.434 61.170 60.357 58.134 55.519 53.095	52.296 52.977 54.523 56.259 56.352 55.851 56.572 57.225 56.329 55.752 54.990 54.133	54.010 54.592 56.948 59.102 60.760 61.497 61.865 61.961 61.958 58.921 56.241 54.814	55.492 53.956 55.433 57.521 57.939 58.897 60.130 60.363 59.632 58.306 55.710 53.729	53.562 52.588 54.360 56.053 56.856 57.872 60.783 61.713 62.794 61.593 59.955 58.268	57.025 57.328 58.274 58.216 58.588 59.459 60.032 60.609 60.485 58.873 56.316 54.621	53.947 55.076 55.733 56.920 57.495 58.238 58.957 59.552 59.249 57.000 55.523 53.915

Table 3 Values of average monthly concentration of NO2 (ppmv) for the period of 2011 to 2014

	2011	2012	2013	2014
January February March April May June July August September October November December	176.4286 143.4444 121.8750 100.9412 98.7500 101.2857 127.8333 76.0000 68.4444 123.4118 228.1905 261.7308	143.0500 129.9615 110.0556 99.4286 93.6429 93.2857 61.3333 102.4000 89.5833 107.9444 251.7391 247.7727	178.4762 115.2105 127.9500 121.3500 99.8333 86.0000 127.8333 79.5000 92.9375 127.5000 197.9048 252.7083	172.6957 144.6875 124.3529 86.1111 89.8462 119.1429 86.4286 82.1250 81.5882 150.1176 204.5789 251.2857

Table 4 Values of average monthly concentration of CO2 (ppmv) for the period of 2011 to 2014

	2011	2012	2013	2014
January February March April May June July August September October November December	388.2671 386.9302 384.7967 382.8482 384.6600 376.1516 376.1521 380.1761 378.7995 377.4204 383.4106 388.8309	389.7453 386.7494 384.8546 387.1007 385.4377 382.8430 385.2328 379.6688 381.1844 379.7802 387.5586 391.1420	394.1395 393.6935 391.1232 390.5237 387.5442 385.2102 382.4967 384.8706 380.6630 384.0695 389.4768 393.5256	393.8462 394.1328 391.8489 390.1177 389.6226 391.4982 392.9677 391.7415 389.4396 391.4127 393.5066 396.0969

3.2. Average Annual Concentration of the Atmospheric Pollutants

Table 5 shows the values of average annual concentration of CH₄ (ppmv) and O₃ (ppmv) for the period of 2003 to 2012 while Table 6 shows the values of average annual concentration of NO₂ (ppmv) and CO₂ (ppmv) for the period of 2011 to 2014.

3.3. Descriptive Statistics of the Atmospheric Pollutants

Table 7 shows the descriptive statistics (Minimum and Maximum values, Mean and Standard Deviation) of annual averages of CH₄, O₃, NO₂ and CO₂ concentrations within the period of investigation.

3.4. Time Series Plot of the Atmospheric Pollutants Concentration

Figures 2(a)-(d) show the graph of average monthly concentration levels of the atmospheric pollutants within the period of investigation, while Figures 3(a)-(d)

Table 5 Values of average annual concentration of CH4 and O3 for the period of 2003 to 2012

Year	Mean CH₄ (ppmv)	Mean O₃ (ppmv)
2003 2004 2005 2006 2007 2008 2009 2010 2011 2012	1740.994 1738.120 1737.331 1730.918 1740.740 1743.628 1750.178 1746.312 1759.923 1773.787	56.262 57.253 54.501 57.139 55.272 58.556 57.259 58.033 58.319 56.800

Table 6 Values of average annual concentration of NO2 (ppmv) and CO2 (ppmv) for the period of 2011 to 2014

Year	Mean NO₂ (ppmv)	Mean CO₂ (ppmv)
2011 2012 2013 2014	135.695 127.516 133.934 132.747	382.370 385.108 388.111 392.186

Table 7 Descriptive statistics of annual averages of CH4, O3, NO2 and CO2 concentrations within the period of investigation

	Minimum	Maximum	Mean	Standard Deviation
CH₄ (ppmv) O₃ (ppmv) NO₂ (ppmv) CO₂ (ppmv)	1730.918 54.501 127.516 380.139	1773.787 58.556 135.695 392.186	1746.193 56.939 132.473 386.944	12.500 1.300 3.519 4.208

show the graph of average annual concentration levels of the atmospheric pollutants within the period of investigation.

3.5. Mann-Kendall Trend Validation

Table 8 shows the Mann-Kendall rank statistics for CH₄ and O₃ for the period of 2003 to 2012, while Table 9 shows the Mann-Kendall rank statistics for NO₂ and CO₂ for the period of 2011 to 2014.

Figures 4(a)-(d) show the graph of Mann-Kendall trend validation for CH₄, O₃, NO₂ and CO₂ respectively within the period of investigation. The results of the Mann-Kendall trend validation for the atmospheric pollutants showed that the standardization variables U(t_i) and U'(t_i) have a sequential fluctuating behavior around a zero level.

3.6. Discussion

The results of the descriptive statistics of the annual averages of CH₄, O₃, NO₂

Table 8 Mann-Kendall rank statistics for CH4 and O3 for the period of 2003 to 2012

Year	CH₄ (ppmv)	U(t_i)	U'(t_i)	O₃ (ppmv)	U(t_i)	U'(t_i)
2003 2004 2005 2006 2007 2008 2009 2010 2011 2012	1740.994 1738.120 1737.331 1730.918 1740.740 1743.628 1750.178 1746.312 1759.923 1773.787	0.000 3.977 1.920 1.920 2.057 1.783 2.194 1.646 −0.549 1.371	−1.920 −2.880 −0.274 −1.646 −0.960 −1.509 −1.920 −0.274 −1.920 −1.371	56.262 57.253 54.501 57.139 55.272 58.556 57.259 58.033 58.319 56.800	4.114 3.291 4.526 2.469 2.606 3.977 3.566 0.137 4.663 4.526	−2.469 −3.017 −2.880 −2.469 −2.606 −2.743 −2.606 −2.331 −3.017 −2.880

Table 9 Mann-Kendall rank statistics for NO2 and CO2 for the period of 2011 to 2014

Year	NO₂ (ppmv)	U(t_i)	U'(t_i)	CO₂ (ppmv)	U(t_i)	U'(t_i)
2011 2012 2013 2014	135.695 127.516 133.934 132.747	0.000 1.509 0.686 −1.097	−2.057 0.137 −2.194 −1.920	382.370 385.108 388.111 392.186	1.509 0.960 1.783 0.411	−1.509 −0.960 −0.137 −0.411

and CO₂ concentrations within the period of investigation revealed that CH₄ had the highest standard deviation value of 12.500 ppmv while O₃ had the lowest standard deviation value of 1.300 ppmv. NO₂ and CO₂ had standard deviation values of 3.519 ppmv and 4.208 ppmv respectively. Therefore CH₄ concentration values were the most dispersed or spread out around the mean of 1746.193 ppmv,

while O₃ concentration values were the least dispersed around the mean of 56.939 ppmv. Methane (CH₄) had higher standard deviation values than carbon dioxide (CO₂), meaning that on a per molecule basis, a proportional rise in CH₄ is much more effective as a greenhouse gas than a similar increase in CO₂ [9]. However, CO₂ has a greater effect than CH₄ on climate change owing to its higher atmospheric concentration.

The results from the analysis of the average monthly concentration of the atmospheric pollutants within the period of investigation indicated that methane (CH₄) concentration had the lowest value of 1710.1 ppmv in August, 2008 and the highest value of 1796.9 ppmv in November, 2011. Tropospheric ozone (O₃) concentration had the lowest value of 51.841 ppmv in January, 2006 and the highest value of 62.794 ppmv in September, 2010. Nitrogen dioxide (NO₂) concentration had the lowest value of 61.3333 ppmv in July, 2012 and the highest value of 261.7308 ppmv in December, 2011. Carbon dioxide (CO₂) concentration had the lowest value of 376.1516 ppmv in June, 2011 and the highest value of 396.0969 ppmv in December, 2014. Hence, concentration of CH₄, NO₂ and CO₂ has minimum values during the peak of the rainy season between June and 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 the other hand, ozone (O₃) 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 an increase in tropospheric ozone (O₃) concentration during the rainy season. This finding is also in concurrence with the result of [10].

The results from the analysis of the average annual concentration of the atmospheric pollutants within the period of investigation showed that Methane (CH₄) concentration had the lowest value of 1730.918 ppmv in 2006 and the highest value of 1773.787 ppmv in 2012. Tropospheric ozone (O₃) concentration had the lowest value of 54.501 ppmv in 2005 and the highest value of 58.556 ppmv in 2008. Nitrogen dioxide (NO₂) concentration had the lowest value of 127.516 ppmv in 2012 and the highest value of 135.695 ppmv in 2011. Carbon dioxide (CO₂) concentration experienced a steady increase with time, having its lowest value of 380.139 ppmv in 2011 and its highest value of 392.186 ppmv in 2014. This steady increase in CO₂ 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 CO₂ in the atmosphere. The lifespan of a CO₂ molecule in the atmosphere is of the order of a century or more. This is more than enough time for the billions of tons of anthropogenic CO₂ to uniformly envelop the planet like a blanket [11].

The results of the Mann-Kendall trend validation for CH₄, O₃, NO₂ and CO₂ revealed that the standardization variables U(t_i) and U'(t_i) have a sequential fluctuating behavior around a zero level as shown in Figures 4(a)-(d), this therefore confirms the validity of the trends [12] [13].

4. 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 (O₃) 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 ozone concentration during the rainy season.

The study also revealed a steady increase in the concentration of CO₂ within the period of investigation. This steady increase in CO₂ 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 CO₂ in the atmosphere. Methane (CH₄) had higher standard deviation values than carbon dioxide (CO₂), meaning that on a per molecule basis, a proportional rise in CH₄ is much more effective as a greenhouse gas than a similar increase in CO₂. However, CO₂ has a greater effect than CH₄ 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.

Acknowledgements

I. I. Onwosi gratefully thanks Dr. Chinonyelum Vivian Onwosi for being his pillar of support.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

Cite this paper

Onwosi, I.I., Njoku, E.I. and Nymphas, E.F. (2022) Analysis of Concentration Levels of Atmospheric Pollutants in Warri, Nigeria. Atmospheric and Climate Sciences, 12, 409-420. https://doi.org/10.4236/acs.2022.122024

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