The relative contribution of individual volatile organic compounds (VOC) species to photochemical ozone formation depends on their atmospheric concentrations and their oxidation mechanism. In an attempt to evaluate the ozone creation potential of ambient VOCs captured in an urban settlement of Benin City, Nigeria, the VOCs concentrations data collected in field studies at nine measurement sites of different air quality in the city and a background site were analysed. Air samples were collected at human breathing height of 1.5 meters from ground level at each site. Active sampling method using the low volume sampling pump (Acuro, Drager, Lubeck, Germany) was used to drawn the air into the tube; the absorbent was Chromosorb 106. The sampling periods were between May 2010 and June 2011; the period covered both dry and wet seasons. The adsorbed gases were desorbed using solvent extraction method with carbon disulphide as solvent. The extracted solutions were analyzed with gas chromatography and mass spectrometer. The observed concentrations of individual VOCs were determined and maximum incremental reactivity (MIR) coefficient along with rate constants of VOC-OH reactions were applied to assess the ozone formation potential of individual VOC in the ambient atmosphere. Sixteen VOC species were observed at various sites with mixing height in decreasing order: toluene (5.82), mp-xylene (3.58), ethylbenzene (3.46), benzene (2.29), and n-butane (0.84). The ozone formation potential study revealed that, ranking by propyl-equivalent, the alkanes included in this study account for 58% of the total propyl-equivalent concentration. The total ozone creation potential in the atmosphere of the Benin City was calculated to be 281.1
Monitoring of volatile organic compounds (VOCs) is a key piece in understanding photochemical air quality in urban atmosphere. VOCs degradation in the atmosphere contributes significantly to the generation of secondary air pollutants, such as aldehydes and peroxyacetyl nitrate (PAN) secondary aerosol [
Several studies have provided information on reactivity and ozone creation potentials of nonmethane hydrocarbons in some cities of the world [
Benin City, Southern Nigeria, is one of the urban settlements in the country’s oil-rich zone. It is the administrative headquarter of Edo states and a transitory city leading to the southern, southwestern, and northern parts of the country. The city has a population of about 1.3 million inhabitants [
As part of our efforts in understanding the behaviors of VOCs in the atmosphere of Nigerian cities, this paper is aimed at identifying key VOC species and assessing the reactivities of each VOC in the atmosphere of Benin City, Southern Nigeria. The study also includes evaluation of the relative ozone-forming potentials of the captured VOC species in the atmosphere of this urban settlement of Southern Nigeria from data obtained between May 2010 and June 2011. Hopefully, the study will provide data that would assist in understanding the processes of reaction rate and degradation of atmospheric VOCs and helpful in better policy formulation towards reduction of the impacts of ozone in the areas.
Benin City, Southern Nigeria, is located between longitude 6.20°N and latitude 5.31°E. It is situated within the equatorial climatic belt (Af Koppen’s climatic classification) and is one of the urban centers in southern part of the country with about 1.3 million habitants [
Table
The coordinate and description of the sampling locations.
sSite | Site code | Coordinates | Site description |
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1 | BNAR |
N06° 20′ 52.1′′ |
Created along Ramat park, close to a petroleum depot and two breweries |
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2 | BNUG | N06° 24′ 10.1′′E005° 36′ 32.2′′ | Ugbowo monitoring site created at the University of Benin main gate and close to fuel dispensing stations along the express way to the western part of the country |
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3 | BNNB | N06° 20′ 57.3′′ |
Site three created at New Benin bus terminal. A location with very high traffic density and petroleum products dispensing stations |
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4 | BNRR | N06° 38′ 45.1′′ |
Created at the King’s square with many road intersections and high traffic density |
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5 | BNSP | N06° 17′ 44.6′′ |
Created along Sapele road very close to the Santana market |
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6 | BNEN | N06° 19′ 28.4′′ |
Ekenwan site close to the University of Benin postgraduate student hostel |
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7 | BNOL | N06° 19′ 17.8′′ |
3rd East Circular road site with many road intersections |
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8 | BNAU | N06° 19′ 42.6′′ |
Upper Adesuwa in GRA, close to Word of Faith secondary school |
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9 | BNSP | N06° 18′ 28.4′′ |
Upper Sakponba junction, a bus stop with many shops |
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10 | BNEK | N06° 38′ 45.1′′ |
Odighi site, a background site, created close village square |
Samples of ambient air were collected using active sampling method. The air was withdrawn by low volume pump at controlled air flow and adsorbed into Chromosorb 106 as adsorbent. The sensitivity and reliability of the sampling approach have been reported [
After sampling, adsorption tubes were labeled and closed with special caps to avoid contamination. The desorption process, chemical extraction, and analysis have been discussed elsewhere [
A quality assurance program was implemented in the framework of which all analytical systems related to the analysis of VOCs have been checked for their performance. Laboratory and field blanks analysis were carried out during each sampling period. Extraction solvent (CS2) was also analyzed. The blank activated carbon tubes as well as process blank were analyzed to determine if there was any contamination in the activated carbon tubes. None of the compounds included in this study were detected in CS2 and in process blanks. The efficiency of the adsorbent was tested by using a backup tube after a sampling. Sampling efficiency of acetone and acetate was found to be 87% and 73%, respectively.
Various quality control tools were used in order to ensure that adequate laboratory performance was maintained. These included control charts for standard solutions and analysis of control standards as unknowns. The calibration curve was found to show good linearity, with determination coefficients (
Recently, different methods are used to define the reactivity for ozone forming potential from nonmethane hydrocarbon. In this study, the contribution of individual VOC to ozone formation was also studied according to the propylene-equivalent concentration [
Propylene-equivalent is a measure of the concentration of hydrocarbon on an OH-reactivity based scale normalized to the reactivity of propene. The propylene-equivalent is defined as
The ozone formation potential is evaluated as product of the concentration of each VOC and the maximum incremental reactivity coefficient (MIR). The MIR is defined as
Ambient VOCs data for this study were the measured data generated between May, 2010, and June, 2011. In our previous report [
Figure
Plot of mean values of TVOCs of different classes in atmosphere of Benin City.
Table
Composition and descriptive statistic of the observed VOCs.
VOC species | Mean ± SD | Morning | Afternoon | Evening |
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n-Butane | 0.84 ± 0.01 | 0.79 ± 0.04 | 0.75 ± 0.16 | 0.98 ± 0.42 |
Isopentane | 0.1 ± 0.01 | 0.1 ± 0.04 | 0.1 ± 0.02 | 0.1 ± 0.08 |
2-Methylpentane | 0.95 ± 0.67 | 0.8 ± 0.07 | 0.98 ± 0.02 | 1.09 ± 0.08 |
3-Methylpentane | 0.09 ± 0.33 | 0.1 ± 0.01 | 0.08 ± 0.02 | 0.1 ± 0.01 |
Undecane | 0.54 ± 0.13 | 0.87 ± 0.08 | 0.62 ± 0.07 | 0.14 ± 0.01 |
Benzene | 2.29 ± 1.07 | 2.03 ± 0.16 | 2.71 ± 0.19 | 2.15 ± 0.07 |
Toluene | 5.82 ± 0.67 | 5.6 ± 0.96 | 5.87 ± 0.54 | 6.01 ± 1.03 |
Ethylbenzene | 3.46 ± 1.33 | 3.84 ± 0.53 | 3.08 ± 0.05 | 3.47 ± 1.14 |
m,p-Xylene | 3.58 ± 1.11 | 3.72 ± 1.03 | 3.81 ± 0.92 | 3.21 ± 0.79 |
o-Xylene | 2.23 ± 0.33 | 2.24 ± 0.58 | 2.26 ± 0.14 | 2.2 ± 0.05 |
Naphthalene | 0.88 ± 0.06 | 0.73 ± 0.11 | 1.03 ± 0.08 | 0.9 ± 0.04 |
Methylene chloride | 0.79 ± 0.01 | 0.67 ± 0.02 | 1.02 ± 0.04 | 0.69 ± 0.02 |
Tetrachloroethane | 0.96 ± 0.12 | 0.89 ± 0.06 | 0.97 ± 0.10 | 1.03 ± 0.08 |
Carbon tetrachloride | 0.03 ± 0.01 | 0.04 ± 0.01 | 0.06 ± 0.02 | 0.01 ± 0.01 |
Chloroform | 0.24 ± 0.03 | 0.28 ± 0.09 | 0.27 ± 0.04 | 0.18 ± 0.03 |
Acetone | 0.92 ± 0.11 | 0.85 ± 0.16 | 1.02 ± 0.30 | 0.9 ± 0.12 |
A cursory look at the distribution pattern of VOC species in Benin City revealed that aromatic hydrocarbons (BET) were ranked among the VOCs species with high concentrations. The sources of aromatics in the atmosphere cities are fuel combustion and evaporation of fuel and solvents [
Among the alkanes, isopentane and n-butane have highest mean concentrations of 0.35
Table
According to Wang et al. [
Table
Reactivity and ozone formation potential of the measured VOCs in atmosphere of Benin City.
VOC species | Group |
|
OH reactivity | Prop-equiv. | MIR | OFP |
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n-Butane | Aliphatic | 2.54 | 0.86 | 0.03 | 1.08 | 0.367 |
Isopentane | 5.6 | 1.96 | 0.08 | 1.36 | 0.476 | |
2-Methylbutane | 2.59 | 0.96 | 0.04 | 1.11 | 0.411 | |
3-Methylpentane | 3.7 | 0.05 | 1.7 | 0.595 | ||
Undecane | ||||||
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Benzene | Aromatic | 11.2 | 26.43 | 1.01 | 0.55 | 1.298 |
Toluene | 1.23 | 2.90 | 0.11 | 0.69 | 1.628 | |
Ethylbenzene | 5.96 | 12.28 | 0.47 | 3.88 | 7.993 | |
m,p-Xylene | 7.1 | 26.70 | 1.02 | 2.93 | 11.02 | |
o-Xylene | 19 | 32.11 | 1.22 | 15.21 | 25.7 | |
Naphthalene | 13.7 | 7.12 | 0.27 | 7.44 | 3.869 | |
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Methylene chloride | 3.24 | 0.194 | ||||
Carbon tetrachloride | 0.036 | 0.006 | ||||
Chloroform | 0 | 0 | ||||
Acetone | 0.02 | 0.002 | ||||
0.35 | 0.147 |
OH reactivity: product of mean individual VOC concentration and OH reaction rate coefficient.
The total ozone creation potential in the atmosphere of the Benin City was calculated to be 281.1
Figure
Spatial variation of ozone in sampling sites of Benin City.
A comparison of total ozone formation potential (OFP) in our study with results from other cities of the world revealed that the total concentration of ozone production in our study is 3-fold lower than the value of 863.4
The VOC concentration in ambient air of an urban settlement of Benin City was studied to understand the distribution patterns. The TVOCs were found to show significant spatial variation among the nine sampling sites of Benin City. The diurnal trend of TVOCs concentrations showed bimodal peaks with morning and evening peaks which followed the traffic pattern of the study centers and suggest the importance of vehicular emission. The prevalence of low wind speed in the studied centers was also observed to be responsible for the poor dilution and dispersion of the emitted VOCs. Evaluation of ozone formation potentials of the four classes of hydrocarbons detected showed that aromatics play important roles in ozone formation in the centres.
The author declares that there is no conflict of interests regarding the publication of this paper.
The author wishes to express gratitude to ETF fund for the sponsorship of this research work.