Inventory of Greenhouse Gases Emissions from Gasoline and Diesel Consumption in Nigeria

*Corresponding author’s e-mail address: sologiwa2002@yahoo.com doi: http://dx.doi.org/10.4314/njtd.v14i1.1 ABSTRACT: Emissions from fossil fuel combustion are of global concern due to their negative effects on public health and environment. This paper is an inventory of the greenhouse gases (GHGs) released into the environment through consumption of fuels (gasoline and diesel) in Nigeria from 1980 to 2014. The fuel consumption data for the period in view were sourced from bulletins released by Nigeria National Petroleum Corporation, (NNPC) and were utilized for GHGs estimation based on default emission factors (69300 kg/TJ (CO2; gasoline), 74100 kg/TJ (CO2; diesel), 18 kg/TJ (CH4; gasoline), 3.85 kg/TJ (CH4; diesel), 1.9 kg/TJ (N2O; gasoline) and 2.25 kg/TJ (N2O; diesel). In addition, the uncertainty and sensitivity analyses associated with the inventory were carried out. Total amount of GHGs emitted into the environment for the period under consideration was 7.30 x 10 tCO2 e (5.20 x 10 tCO2 e and 2.10 x 10 tCO2 e of gasoline and diesel, respectively). It is worth noting that gasoline consumption accounted for 71.23% of the total amount of GHGs with CO2 making up 98.72 % (CH4 = 1.39 % and N2O = 0.61 %) of the emissions. For this study, uncertainty of estimate was between -80.93 % and 78.36 % while volume of diesel is more sensitive than the volume of gasoline of the input parameters. National policy and enforcement on low or neutral emission fuels utilization are amongst the recommended actions toward reducing GHG emissions in the country.


I. INTRODUCTION
Man-made emissions of greenhouse gases (GHGs) have increased by 70 % (29 Gtons of CO2 equivalent (tCO2 e) in 1970 to 49 GtCO2 e in 2004), of which 25.8 Gtons came from CO2 emissions from the combustion of fossil fuels (OCED/ITF, 2009).In the developing world, automotive air pollution is mostly a problem in large cities with high levels of traffic, such as Mexico City, Bangkok, New Delhi and Lagos (Nigeria).In other cities, power plants, factories, and other stationary sources still constitute the greatest threat to air quality.However, the share of emissions from developing countries is expected to rise in the future because of the growing sizes of motor vehicle fleets and the use of less efficient fuel-burning technologies (IPCC, 1995).
One of the most important human impacts on the environment is the rapid increase in greenhouse gas (GHG) which includes carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFCs), perfluorocarbons (PFCs), and hydrocarbons (HFCs) (Giwa, 2014).Levels of these gases are increasing as a direct result of human activity.Apart from global warming, GHGs are also responsible for the phenomenon known as ozone layer depletion.The rise in GHG is more rapid than at any time in the past because of the increase in industrial activities (Houghton et al., 2001).Emission of GHGs is due to an increased dependence on machines and equipment that burn fossil fuels; such as automobiles and generators, as well as enhanced chemical processes carried out in factories and power plants.Another source is fire, from firewood cooking, bush burning, and incineration of refuse.Emission of CO2 is largely due to human activities; transportation, industry, and power plants and its release into the atmosphere can result in increase in ambient temperature, and consequent climatic changes (OCED/ITF, 2009).
The negative impact of GHGs on agriculture and food security, especially in tropical and subtropical regions is expected to increase the risk of hunger by additional 80 million people by 2080 in Africa and Southern Asia (Odjugo et al., 2001).Odjugo (2009) revealed that GHGs which caused climate change have led to a shift in crops cultivated in Northern Nigeria.The study reported that as at 1978, the preferred crops the farmers cultivated were guinea corn followed by groundnut and maize, but due to increasing temperature and decreasing rainfall amount and duration occasioned by climate change, the farmers as a means of adaptation in 2007 shifted to the production of millet followed by maize and beans.Another major problem to agriculture in Nigeria due to climate change is the reduction of arable lands.As the sea incursion is reducing the arable land of the coastal plains, the desert encroachment is depriving farmers of their grazing rangelands and agricultural farmlands (Odjugo et al., 2003).Moreover, the frequent droughts and lesser rains have started shortening the growing season thereby causing crops' failure and food shortage.
The Intergovernmental Panel on Climate Change (IPCC) has developed a set of common guidelines for national GHG inventories by sources and sinks in relation to national obligations under the United Nation Framework Committee on Climate Change (UNFCCC).The GHG emissions inventory is an accounting of the amount of GHGs emitted to or removed from the atmosphere over a period of time.It also provides information on the activities that cause emissions and removals, as well as background on the methods used to make the calculations.Researchers use GHG inventories as inputs to atmospheric and economic models (Aderogba, 2011).Policy makers use GHG inventories to track emission trends, develop strategies and policies and assess progress (Aderogba, 2011).The total GHG emissions in Nigeria increased in 2000 to 135 % of that in 1990, implying considerable increase in the socioeconomic activities (National Communication on Climate Change, 2014).Energy-related activities have the major share of emissions.
The energy sector recorded emissions of 155.34 MtCO2 e, representing 70.4% of the country's total emission in 2000 with CO2 as the largest contributor (114.72 MtCO2 e) (National Communication on Climate Change, 2014).It is worth noting that the diesel or gasoline-powered electricity generator sets currently in monumental use in Nigeria, is an addition to national sources of GHGs emission and this has contributed significantly to the choking air in cities like Abuja and Lagos, which are beleaguered by smog shrouding the skyline of the central cities (Ndoke and Jimoh, 2005).
This study aimed at providing an inventory of the emission of GHGs released into the atmosphere through the combustion of fossil fuels (gasoline and diesel) made available for consumption over a period of 35 years (1980 -2014).It involved the estimation of the amounts of CO2, CH4 and N2O and total GHGs emitted through the combustion of gasoline and diesel in Nigeria with the view of proffering possible solutions to reducing the GHGs which are devastating to the environment and human health.

A. Emission estimation method
The Tier 1 (sectoral) approach was employed in this study for the estimation of GHGs from the consumption of fuels (gasoline and diesel) according to the 2006 guidelines on National GHG Inventories (IPCC, 2006).Tier 1 approach is fuel-based, since emissions from all sources of combustion can be estimated on the basis of the quantities of fuel combusted (usually from national energy statistics) and default emission factors (as presented later in this section).The quality of these emission factors differs between gases.The emission factors for CO2, CH4 and N2O mainly depend upon the carbon content of the fuel.Combustion conditions (combustion efficiency, carbon retained in slag and ashes etc.) are relatively unimportant.Therefore, GHGs emissions can be estimated accurately to an extent based on the total amount of fuels combusted and the averaged carbon content of the fuels (IPCC, 2006).The Tier 1 approach calculates CO2, CH4 and N2O emissions by multiplying estimated fuel sold with a default CO2, CH4 and N2O emission factor.The approach is represented using the following expressions (IPCC, 2006): where: i = Specific GHG (CO2, CH4 and N2O) released from combustion of gasoline or diesel; E i,G = Emission of a specific GHG from combustion of gasoline in tons; E i,D = Emission of a specific GHG from combustion of diesel in tons; ρ G = Density of gasoline (0.745 kg/l); ρ D = Density of diesel (0.832 kg/l); V G = Volume of gasoline in liters; V D = Volume of diesel in liters; HHV D = Higher heating value of diesel (45.77MJ/kg); HHV G = Higher heating value of gasoline (46.54 MJ/kg); EF G = Emission factor of CO2 for gasoline engine (69300 kg/TJ); EF D = Emission factor of CO2 for diesel engine (74100 kg/TJ); EF G = Emission factor of CH4 for gasoline engine (18 kg/TJ); EF D = Emission factor of CH4 for diesel engine (3.85 kg/TJ); EF G = Emission factor of N2O for gasoline engine (1.9 kg/TJ); EF D = Emission factor of N2O for diesel engine (2.25 kg/TJ); The emission factors and the Equations used in this study were obtained from the literature as provided by IPCC (2006).As an example, the emission values for CO2, CH4, N2O and total GHGs was evaluated for the year 1980 using data (from NNPC) on volumes of gasoline and diesel consumed.For year 1980, 2.
Where: M = mass (kilogram); L = length (meter) and T = time (seconds).However, Blacksmith Institute (2007) and IPCC (2007) present and assert that the contributions of the GHGs depend on their global warming potentials (GWPs) which measures the absorption of infrared radiation emitted back into the atmosphere, and these actually form major GHGs that contribute to the thermal imbalance of the earth.

B. Data source and processing
The data used in this present study was sourced from the bulletins released on the official website of the national agency (Nigeria National Petroleum Corporation (NNPC)) in charge of petroleum related matters in the country.Data available and garnered were from 1980 to 2014, spanning a period of 35 years.Necessary conversion of units and all estimations were carried out using Microsoft Excel (2013).It is assumed that the data obtained from NNPC and subsequently used in this study were correct.

i. Uncertainty Analysis Procedure
Volumes of fuels (gasoline and diesel) consumed in the country were used as the input parameters for modelling the outputs (CO2, CH4, N2O and total GHGs) in order to estimate the uncertainty associated with the emission inventory.Empirical formulae as given in Equations (1 -3) were used in establishing the model.For this study, EasyFit® 5.6 (evaluation version) was utilized to fit the input data (VG (volume of gasoline) and VD (volume of diesel)) into the appropriate probability distribution function while Analytica® (4.5) software was used for modelling the uncertainty of the emission estimate.Thereafter, the probability distribution models of the input parameters (VG and VD) were developed as model inputs.The procedure involved the input models, propagations of uncertainty from input parameters to model outputs using Latin hypercube sampling (LHS), which is a Tier 2 method recommended for national GHG inventories (IPCC, 2006).Finally, the quantitative uncertainty associated with GHGs released from the consumption of gasoline and diesel was determined.

ii. Methods for Simulating Uncertainty Propagation
Currently, both Monte Carlo Simulation (MCS) and LHS are the most generally used numerical simulation methods.The benefit of using MCS is that it can afford an excellent approximation of the output distribution with a sufficient sample size.However, the disadvantage is that it may be necessary to use large sample sizes to obtain a smooth approximation of the probability distribution function.In this present study, LHS -a numerical simulation method -was used for simulating the propagation of probability distributions of all inputs using a model based on simulated random sampling.Minimal Standard which is the default method in Analytica® was used as random number generator while median Latin hypercube is preferred to random Latin hypercube as the default sampling method due to its high accuracy.Using LHS, the values of each uncertain input are not randomly generated.Instead, the probability distribution is first divided into ranges of equal probability, and then one sample is taken from each range (Lu et al., 2013).For some applications with a given simulation sample size, LHS is a more precise numerical simulation method than MCS (Lu et al., 2013).

D. Sensitivity Analysis
The most significant source contributing to the uncertainty associated with the emission inventory from the model inputs were identified using sensitivity analysis.The result of the analysis can assist decision-makers to verify the main sources that make most contributions to the uncertainty in the model output, and to decide where additional data collection are needed for reducing uncertainty in the model inputs.In this study, sensitivity analysis was carried out on the input models to determine the parameter which considerably influences the emission of GHGs.(1997; 1998; 2008-2014)

A. Consumption of gasoline and diesel
Tables 1 and 2 present the quantities of fuels (gasoline and diesel) and emissions (CO2, CH4, N2O and total GHGs) for years from 1980 to 2014.The total volume of gasoline and diesel consumed in the country for the period under consideration was 2.13 x 10 11 litres and 7.37 x 10 10 litres, respectively.This implies that on volumetric basis, 65.4 % of the fuel consumed was gasoline.Figure 1 gives an illustration of the consumption pattern of gasoline and diesel.It clearly shows that gasoline was consumed more than diesel, as this is evident in both the total volume of the products consumed and the number of ICEs using the products.Though national statistics to this effect is not available but this assertion is based on our personal observation.
A combination of Table 1 and Figure 1 gives a clear view of the consumption of gasoline in the country.As can be seen in Figure 1, the volume of gasoline consumed increased gradually from 3.87 x 10 9 litres in 1980 to 7.62 x 10 9 litres in 1994 and then witnessed a rapid reduction in quantity from 1994 to 1998 (3.70 x 10 9 litres).From 1998, the volume of gasoline consumed in the country increased sharply to 8.73 x 10 9 litres in 2003, which was relatively steady thereafter with a peak in 2007 (8.86 x 10 9 litres).A significant reduction in quantity from 8.86 x 10 9 litres in 2007 to 3.82 x 10 9 litres in 2013 was recorded (Figure 1).This was followed by a slight increase in quantity (3.97 x 10 9 litres) in 2014.As observed in Figure 1, the trend of gasoline consumed has its minimum and maximum values in the year 1998 and 2007 which corresponds to 3.70 x 10 9 litres and 8.86 x 10 9 litres, respectively.
As could be observed in Figure 1, diesel consumption showed a gradual reduction pattern from 2.32 x 10 9 litres in 1980 to 3.92 x 10 8 litres in 2014.The highest quantity of diesel consumed was recorded in the year 1993 with a value of 4.02 x 10 9 litres while the lowest volume was in the year 2014 (3.92 x 10 8 litres).A correlation coefficient of 0.167 was obtained between the data for gasoline and diesel consumed in the country.This shows a positive and weak relationship between these sets of data.It is therefore evident that both data are independent of another.

B. Greenhouse gases emitted from gasoline consumption
The emission factors employed in this study for the estimation of CO2, CH4, and N2O were given in Sub-section 2.1 as obtained in literature (IPCC, 2006).The choice of emission factors for both stationary and mobile engines (combustors) was informed by the fuels under consideration in this study.Moreso, due to the unavailability of information in the country concerning the percent or amount of engines and machineries using a type of fuel and under the mobile and stationary category, this led to the use of average emission factor for each GHG under each fuel type and combustor category.
From Table 1, the quantities of CO2, CH4 and N2O released into the atmosphere from gasoline consumption for the period in view are presented.Total amount of GHGs emitted was 5.20 x 10 8 tons of CO2 equivalent (t CO2e) which translates to emission of 1.33 x 10 5 tons of CH4, 1.41 x 10 4 tons of N2O and 5.13 x 10 8 tons of CO2.Based on these values, yearly average of 3.80 x 10 3 tons, 401.7 tons and 1.47 x 10 7 tons of CH4, N2O and CO2, respectively, were released into the environment.For the year 2014, 9.67 x 10 6 t CO2e of GHGs were emitted into the environment which translates to $145.07 million (N44.97 billion at N305 to $US 1) based on $15/t CO2e (N4725) carbon tax.As seen in Figure 2, the same pattern was observed for the amounts of CO2, CH4 and N2O

Year
Gasoline Diesel *Corresponding author's e-mail address: olubunmimokuolu@yahoo.comemitted which is similar to the pattern noticed in Figure 1 for the volume of gasoline consumed.This can be attributed to the linear nature of the mathematical expressions used for the estimation of the quantities of GHGs.It is worth noting that 98.62 % of the total GHGs (CO2, CH4 and N2O) released into the atmosphere as a result of gasoline burning in combustors was CO2.Significantly small amounts of N2O (4.36 x 10 6 t CO2e) and CH4 (2.80 x 10 6 t CO2e) was emitted compared to CO2 (5.13 x 10 8 tons) for the year span (1980 to 2014) under consideration.

C. Greenhouse gases emitted from diesel consumption
The amounts of CO2, CH4 and N2O emitted during the consumption of diesel for 35 years are provided in Table 2. Total amount of GHGs released into the atmosphere for using diesel was 2.10 x 10 8 t CO2 e.This quantity comprises of 1.08 x 10 4 tons of CH4, 6.32 x 10 3 tons of N2O and 2.08 x 10 8 tons of CO2.On yearly average, 308.7 tons, 180.4 tons and 5.94 x 10 6 tons of CH4, N2O and CO2, respectively, were released into the atmosphere.It is apparent that the same trend of emissions (amount of CO2, CH4 and N2O) noticed in Figure 3 was observed in Figure 1 for the volume of diesel consumed in the country.Again, the quantity of CO2 was significantly higher than other gases as shown in Figure 2 and as given in Table 2.This supports the fact that CO2 is a major global warming contributor despite its low global warming potential of 1, compared to those of CH4 (21) and N2O (310) in terms of CO2e.

D. Total GHGs emitted from gasoline and diesel consumption
The estimated amount of GHGs emitted from the consumption of gasoline and diesel was 5.20 x 10 8 t CO2e and 2.10 x 10 8 t CO2e, respectively (Tables 1 and 2).In terms of carbon tax at the present rate of $15/t CO2e (N4725), these values sum up to $7.80 billion (N2.38 trillion) and $3.15 billion (N960.75),respectively.From Figure 4, it is clear that the quantity of GHGs released as a result of the consumption of gasoline is more than that of diesel.This can be linked with the volume of gasoline consumed during the period in view compared to that of diesel.A total of 7.30 x 10 8 t CO2e of GHGs was estimated to be released into the environment due to the consumption of 2.15 x 10 11 litres of gasoline and 7.45 x 10 10 litres of diesel for the 35-year period in the country.From Tables 1 and 2, it was estimated that 71.23 % of the total GHGs was as a result of the consumption of gasoline as fuel.Of the estimated total amount of GHGs emitted into the environment, CO2 emission accounted for 98.96 % of the amount.The cost of carbon tax for the total amount of GHGs was $109.52 billion (N33.40trillion) while that for the emissions in the year 2014 was $161.84 million (N49.36 billion).According to a report on national GHG inventory under UNFCC for the year 2000, Nigeria contributed about 2.14 x 10 8 t CO2e of GHG to the atmosphere (National Communication, 2014).The energy sector (fuel combustion and fugitive emissions) was reported to contribute the largest proportion (70 %) to direct GHG emissions in Nigeria (National Communication, 2014).Of this amount, 1.33 x 10 8 tCO2e were released into the atmosphere due to fuel combustion which consisted of 1.15 x 10 8 tons (114,724 Gg) of CO2, 6.79 x 10 8 tons (679 Gg) of CH4 and 9.0 x 10 3 tons (9 Gg) of N2O.From this present study, it was estimated that 1.70 x 10 7 tons of CO2, 3.26 x 10 3 tons of CH4 and 484 tons of N2O (1.73 x 10 7 t CO2e) were emitted through the use of both gasoline and diesel in the year 2000.From the values aforementioned, it was observed that this study's estimate of GHGs is 13.1 % of that reported for the energy sector in the national GHG inventory.This consists of 14.5 % (CO2), 0.5 % (CH4) and 5.4 % (N2O) of the corresponding gas reported in the energy sector.The significant difference between the value of GHGs obtained in this study and that reported for the sector in the national inventory is largely due to the encompassing inventory of the emissions in the energy sector (energy industries, manufacturing and construction, transport, commercial, residential, agriculture, forestry and fishing activities, gas flaring, petroleum refining and fugitive process) of the country as against the GHG estimation of emissions from gasoline and diesel consumption.Also, 40.3 % (5.66 x 10 7 t CO2e) of the GHGs from the energy sector were reported to be from gas flaring activities for the year under consideration (National communication, 2014).In addition, the transport subsector of the energy sector was reported to have emitted 2.57 x 10 7 t CO2e of GHGs into the atmosphere, which is an amount fairly higher than the value (1.73 x 10 7 t CO2e) obtained in this study.It is worth mentioning that the GHG inventory for the transport subsector entailed emissions from road, rail, aviation and marine sections.

E. Comparison of results
To our best knowledge, previous studies on GHGs emission inventory for gasoline and diesel fuels in Nigeria are very scarce in the literature.Thus, we have compared our results with data provided in this regard by government agencies and institutions such as United States Energy Information (USEIA), World Bank, United States Department of Energy (USDOE), and Emission Database for Global Atmospheric Research (EDGAR).Emissions of CO2, CH4 and N2O as obtained in our study were compared with those provided by USEIA, World Bank and USDOE (for CO2) and EDGAR (for both CH4 and N2O emissions).
Figure 5 illustrates the amounts of CO2 released via fossil fuel combustion in Nigeria from 1980 to 2014 as obtained from USEIA, USDOE and World Bank, and this present study.It is pertinent to know that CO2 emission data for both USEIA and World Bank were only updated to the year 2013 as at the time of reporting this work while those of USDOE were given to the year 2014.From Figure 5, it can be noticed that the values of CO2 emissions evaluated in this study were slightly lower than those of USEIA, USDOE and World Bank.This can be linked to the fact that this work only considered CO2 emission inventory for gasoline and diesel of all the fossil fuels (kerosene, natural gas etc.) used in the country.Thus, the difference in emission values observed in Figure 5 can be due to the other fossil fuels not accounted for in our study but evaluated by USEIA and USDOE.Analysis of variance (ANOVA) carried out on all the CO2 data from USDOE, USEIA, World Bank and this study showed that the values were significant and statistically different (Fobserved (78.26) > Fcritical (3.99)) from one another with P-value <<0.05 at 95 % confidence level.Beside USDOE and World Bank having correlation coefficient of unity (1) -showing excellent relationship between the CO2 data sources -other CO2 data correlations revealed weak and positive correlation coefficients (USEIA and World Bank (0.3117), our data and USEIA (0.2519), USDOE and USEIA (0.3116), our data and World Bank (0.2118) and, USDOE and our data (0.3116)).Comparison between the estimated CH4 emissions in this work and those of EDGAR dataset of CH4 emissions in the country is presented in Figure 6.Relatively similar trends were noticed between the CH4 emission data from 1980 to 1993.Thereafter, a considerably sharp increase in the amounts of CH4 emissions due to fossil fuels burning was observed for the EDGAR CH4 data from 1993 to 2008.This sudden and progressive increase in CH4 emissions from 1993 upward as reflected in EDGAR data for CH4 may be attributed to the monumental use of natural gas in the industrial sector of the country at that point in time as the EDGAR database provides emission values for fossil fuels combustion, of which only gasoline and diesel were considered and evaluated in this present study.The two CH4 emission data were found to be statistically not the same (Fobserved (23.32) > Fcritical (4.01)), though significant with Pvalue <<0.05 at 95 % confidence level.Also, a moderate and positive relationship was noticed between the two emission data with a correlation coefficient of 0.6281.In comparing the estimated N2O emissions obtained in this work with those provided by EDGAR database for N2O emissions, similar pattern to that of CH4 emissions comparison (see Figure 6) was noticed in Figure 7.The only exception is the fact that the quantities of N2O evaluated in this study were slightly higher than those reported in EDGAR database.Similarly, both data sets were statistically not equal (Fobserved (15.13) > Fcritical (4.0)) with P-value of 0.00027 at 95 % confidence level and correlation coefficient of 0.3814.These implied the significance of both data and the existence of a weak-positive relationship between them.

F. Quantitative estimates of uncertainty i. Uncertainty analysis for total emission
The quality of emission inventories for the GHGs depends largely on the accuracy of fuel consumption statistics.In this study, the GHGs emissions inventory have been carried out base on standard and best practices subject to the data available to us through NNPC.However, we strongly encourage better and extensive collection of these data taking into account the volume of gasoline and diesel consumed by the end users.By this, we meant reliable data that has taken care of possible smuggling these products, tanker accidents, proper accounting of fuels at their final destinations across the country etc.The data on volumes of gasoline and diesel obtained from NNPC were considered to be significantly sufficient for use in study bearing in mind possible but insignificant inappropriateness and bias in data collection due to both systematic and human errors.Tier 2 approach was chosen for this study because the coefficient of variation for the input variables was more than 0.3 (0.57) and that the input variables did not fit into normal distribution.Goodness-of-fit tests (Chi-Squared test, Kolmogorov-Smirnov test, and Anderson-Darling test) carried out on the input parameters using EasyFit® assigned lognormal and triangle distributions to VG and VD, respectively.Based on these distributions, the range of the mean of CO2, CH4, N2O and total GHGs at 95 % confidence was obtained by running the simulation on Analytica®.
Table 3 gives the simulated mean, relative uncertainties of the mean, lower and upper confidence levels of the mean for the quantity of emissions (CO2, CH4, N2O and total GHGs) released into the environment from petroleum products consumed.The estimated mean of CO2, CH4, N2O and total GHGs obtained prior to quantifying the estimate of uncertainties associated with them was found to be relatively higher than those obtained for the simulated mean as presented in Table 3.These discrepancies in the mean may be ascribed to the nature and statistical distribution of the input data, the collection and mode of collection of the data by the national agency.For this study, the running of the simulation model was carried out using 250, 500, 1000, 2000 and 3000 iterations, respectively.The simulation with 3000 iterations was considered the best for the output model since the standard deviation remained constant at this number of iteration.

ii. Estimating Uncertainty in the Model Output
As observed in Table 3, the range of the total amount of GHGs emitted is between 1.36 × 10 7 tCO2 e and 1.27 × 10 8 tCO2 e with the corresponding relative uncertainties of -80.93 % and 78.36 %.This is clearly illustrated in the cumulative probability distribution of the output model as presented in Figure 8. Also, the ranges of the total CO2, CH4 and N2O are presented in Table 3.The simulated mean values are 5.96 × 10 6 tons, 2.93 × 10 5 tons, 1.91 × 10 5 tons and 7.14 × 10 7 tCO2 e for CO2, CH4, N2O and total GHGs, respectively.The relative uncertainties associated with CO2, CH4 and N2O are also presented in Table 3.
The range of relative uncertainty obtained for each emission parameter (CO2, CH4, N2O and total GHGs) is strongly connected to the mode of collection and nature of the data used in evaluating the uncertainties.The values of the ranges of relative uncertainty as presented in column 5 of Table 3, seems similar due to the characteristics of the data as the same data were employed in the estimation of the relative uncertainties.Columns 2, 3 and 4 of Table 3, show the minimum, mean and maximum values, respectively, of the simulated quantities of the emission parameters of CO2, CH4, N2O and total GHGs (at 95 % confidence level), obtained using Analytica® (4.5) to estimate uncertainties associated with the parameters.The columns present statistical values of emissions connected to the cumulative probability distributions generated for all the emission parameters.From Figure 8, it can be observed that the minimum value of total GHGs is more than 10 M and less than 20 M (around 14 M) while the maximum value is above 120 M and less than 140 M (around 130 M).These values are close to those (minimum and maximum values) reported for total GHGs in Table 3. iii.Sensitivity analysis on emission It was observed that out of the two input parameters (VG and VD) that contribute to the estimation of the total GHGs, VD is more sensitive to the uncertainty of the emission estimates (see Figure 6).The result of the sensitivity analysis implies that the best way to reduce uncertainty in the total amount of GHGs is to reduce uncertainty in the data of VD with more accurate data collection and the use of state-of-theart instruments.

iv. Solutions to reduce GHGs emission in Nigeria
At this point in time when nations of the world are mitigating against the release of emissions (gaseous or particulate), particularly GHGs into the atmosphere from the combustion of fossil fuels, Nigeria as a country is not on the same page as others despite clear evidences of the effects of global warming and climate change in the country.National emission inventory from various sources are scarce, emission regulations and standards are nothing to write about compared to global best practices and the response of successive governments in the country towards emissionrelated issues call for serious concern.Free-to-theenvironment release of emissions from the burning of fossil fuels in various combustors without an atom of restriction from any quarters is the order of the day in the country.In agreement with international cry as heralded by various agencies and organizations such as United Nations and IPCC that countries should cut down drastically GHGs emissions, Nigeria being a signatory must see the need to urgently strategize to partake in this global drive toward ensuring a safe world.National policy on emissions should be reformulated which would include undertaking national inventory to quantify emissions from time to time, making rules and regulations on emissions from various sources and enforcing them, setting national emission standards subject to review with time, utilization and enforcement of emissions abatement technologies, use of alternative fuel vehicles, use of low or neutral alternative fuels (biofuels, compressed natural gas and liquefied natural gas) and so on.Solving the erratic power supply problem in the country can also help in reducing emissions since companies, institutions, households, offices, business owners etc. will run on electricity instead of burning fuels in electric generator sets as options available for energy supply.
Subject to the GHG footprint as estimated in this study and in consonance with the international outcry for sustainable development in terms of sustainable energy development and sustainable environment, the large quantity of CO2 from consumption of gasoline and diesel needs prompt attention.Similarly, the country being a party to the new birth world order of sustainable development must strategize, plan and implement towards achieving the sustainable development goals, especially goals 7 (ensuring access to affordable, reliable and sustainable energy) and 13 (urgent action to combat climate change and its impacts) in agreement to this work.
IV. CONCLUSION Burning of fossil fuels (gasoline and diesel) has caused poor indoor and outdoor air quality in the country, which has significantly contributed to public health and environment issues.Using the fuel consumption data with the Tier 1 approach recommended for national GHGs estimation and Analytica™ software, the total amount of GHGs emitted into the environment for the period under consideration was 7.30 x 10 8 tCO2 e (gasoline; 5.20 x 10 8 tCO2 e and diesel; 2.10 x 10 8 tCO2 e) from 2.13 x 10 11 and 7.45 x 10 10 litres of gasoline and diesel, respectively.The range of the total amount of GHGs emitted is between 1.36 × 10 7 tCO2 e and 1.27 × 10 8 tCO2 e with the relative uncertainties of -80.93 % and 78.36 %.Measures to abate GHGs emission should be put in place and enforced by the government.Currently, no measure is in place in the country to curb emissions from fuel combustion.
S. O. Giwa * , M. A. Sulaiman, C. N. Nwaokocha Inventory of Greenhouse Gases Emissions from Gasoline and Diesel Consumption in Nigeria Department of Agricultural and Mechanical Engineering, College of Engineering and Environmental Studies, Olabisi Onabanjo University, Ibogun Campus, P.M.B. 5026, Ifo, Ogun State, Nigeria.

Figure 1 :
Figure 1: Volume of gasoline and diesel consumed in Nigeria from 1980 to 2014.

Figure 2 :
Figure 2: Amounts of CO2, CH4 and N2O released from gasoline consumption in Nigeria.

Figure 3 :
Figure 3: Amounts of CO2, CH4 and N2O released from diesel consumption in Nigeria.

Figure 4 :
Figure 4: Amounts of greenhouse gases released from petroleum products consumption.

Figure 5 :
Figure 5: Comparison of present study with other emission data sets (CO2).

*Figure 6 :
Figure 6: Comparison of present study with other emission data sets (CH4).

Figure 7 :
Figure 7: Comparison of present study with other emission data sets (N2O).

Figure 8 :
Figure 8: Uncertainty analysis on total GHGs emitted from gasoline and diesel consumption.

Figure 9 :
Figure 9: Sensitivity of total GHGs emitted to input parameters.

Table 2 : Diesel consumption and the gases emitted.
a Source: NNPC