SEASONAL VARIABILITY OF CARBON MONOXIDE (CO) IN THE AMBIENT ENVIRONMENT OF IMO STATE, NIGERIA

. The air qualities of four areas in Imo State with a total of twenty two locations were monitored for a period six months with respect to CO level using Gasman air monitor (Crowcon Instruments Ltd, England. The air quality monitoring was conducted in the morning, afternoon and evening hours within dry and wet seasons in order to elucidate the variability of this atmospheric pollutant in the study locations. The results obtained showed significant variation of the air pollutant in the morning, afternoon and evening including within the twenty four weeks, and between dry and wet seasons. The mean values of CO in the dry season are in the order Egbema > Okigwe > Owerri > Orlu, while the order for the mean CO level in wet season is Okigwe > Orlu > Egbema > Owerri. Elevated concentration of CO was observed in the study areas above 10 ppm, the Nigerian National ambient air quality standard. This observation calls for health and environmental concern. Hence, anthropogenic activities and fugitive emission sources of atmospheric pollutants such as increased vehicular traffic, biomass combustion, use of electric power generating sets, leakages from incinerators and flare stacks should be seriously monitored.


Introduction
Pollution of the ambient environment is a serious environmental challenge that has posed significant threat to clean air, a basic and fundamental requirement of human existence and environmental health [1,2,3].Human exposure to air pollutants is unavoidable in today's perspective especially in the urban areas of most developing countries [4], which has been associated with increased morbidity and mortality [5,6].Though, air pollution could be due to natural sources [7].A major anthropogenic source of atmospheric pollution is due to man's quest for a better standard of living, utilizing the natural resources for rapid industrialization and urbanization, consequently causing excessive air pollution [8].This therefore could influence the tendency for the release of toxins to the environment [9], which increases the overall toxic burden of the environment [10].Atmospheric pollution has continued to attract a lot of attention worldwide because of its negative effects on human health and well-being [11,12,13].
Carbon monoxide (CO) is an important trace gas which plays a significant role in the chemistry of most tropospheric reactions [14,15].CO is poisonous and it affects the oxygencarrying capacity of the hemoglobin, could reduce mental health, and may eventually lead to death [16].It contributes to greenhouse gas that warm the atmosphere [17,18].Natural sources of CO emissions include oxidation of natural hydrocarbons and methane, emissions from ocean and vegetation.While the anthropogenic sources include combustion of fossil fuel and biomass, industrial activities and motor vehicles [19,20].
CO which has a lifetime of about four months in the ambient environment [17,21] is believed to be removed from the atmosphere by its reaction with hydroxyl radical, HO • , as in Eq. 1 -2 [17,21], the atomic hydrogen, H, produced in Eq. 1 combines with oxygen molecule in the presence of a third body (M) like N 2 to form hydroproxyl radical.The photochemical reactions are also linked to the formation of ozone as shown in Eq. 3 -5 [22,23].

CO + HO
The result of this is more than two million premature deaths each year, attributable to the effects of urban outdoor and indoor air pollution as reported by World Health Organization, and these effects are more prominent in developing countries [24], and Imo State, a part of Nigeria is not an exception.CO has been associated with mortality due to suffocation and poisoning [25].It was reported that in the months of May and June, 2008 Nigeria recorded about 22 deaths resulting from CO poisoning [26].Seasonal variation of atmospheric pollutants especially CO is an important factor in determining the ambient concentration of these pollutants which may vary from one location to the other due to pollution composition [27,28], and ability of the atmosphere to either absorb or disperse these pollutants [29].In Nigeria, there are two distinct seasons, dry and wet seasons which could vary the atmospheric pollutants significantly.For instance, during dry season when the relative humidity is moderately low with higher wind velocity, pollutants have higher tendency of being swiftly dispersed than during the wet season [21].Fluctuations in time of the seasons and difference in the ambient temperature, relative humidity and wind speed including wind direction could also vary the concentration of atmospheric pollutants over the seasons [30].Abdul-Wahab and Bouhamra reported maximum concentrations of non-methane hydrocarbon, CO, and NOx in the winter period and minimum concentrations in the summer period in Kuwait [31].Also higher values of SO 2 and CO concentrations in winter have been reported [32,33].On the other hand increase in the concentration of atmospheric pollutant in late summer have also been reported, which was attributed to low wind speed, high temperature and subsidence inversion [34] .
The need for this study is necessitated by the fact that the population of Imo State stood at 3,934,899 according to 2006 census [35], which must have increased significantly after ten years.The state have oil wells with enormous natural gas associated with them, which has a rough estimated reserve of about 1422 billion cubic meters, part of which has been continuously flared [36,37].The use of power generators, high volume of vehicular traffic and presence of two stroke engine automobiles like motorcycles and tricycle used for transportation in most parts of the State known for incomplete combustion which could lead to increase emission of carbon monoxide [29].Also the concentration of this pollutant may have increased significantly due to the large influx of old and fairly used vehicles imported into the country [38], which is compounded by poor vehicle maintenance culture.

Methodology
Study Area; The research was conducted in 22 sites within 4 locations in Imo State which include Owerri, Okigwe, Orlu and Egbema as shown in fig. 1. Imo State is located in the tropical rainforest zone climate which is dominated by plains with elevation ranging from 50-200m above sea level.The annual rainfall is about 2400mm to 4000mm, which is concentrated almost entirely between April and October, with average relative humidity of about 80% and up to 90% occurring during the wet season.The maximum air temperature ranges from 28 to 38 0 C, while the minimum air temperature range from 19 0 C to 24 0 C [39,40,41].

Fig. 1: Map of Imo State showing air quality sampling location
Air Quality Sampling Procedure; Concentration of CO was sampled three times a day (morning, afternoon and evening) with Gasman hand held detector (Crowcon Instruments Ltd, England).Sampling was carried out for six months during dry and wet seasons, between November 2014 and June 2015.The sampling was carried out once a week at each of the 22 air monitoring locations, three times per day, four times a month for a period of six (6) months which was three months in dry season and three months in wet season.
Method of Data Analysis; Data analysis was done using Microsoft excel 2007 and values of all the results from the 22 sampling points in the four locations were recorded as calculated mean values of the air pollutant concentrations in the morning, afternoon and evening hours.The standard deviation (SD) was determined and co-efficient of variation (CV %) was used to determine variation in the concentration of CO within and between the seasons.Variation was categorized as little variation (CV % < 20), moderate variation (CV % = 20 -50) and high variation (CV % > 50) [42].Contour and 3-D surface plots of the air pollutant concentration was modeled using Surfer 12 software to further elucidate the variability of CO in the study locations.

Results and Discussion
Summary of results of CO concentration obtained from the 22 air quality monitoring sites in the 4 areas are presented in tables 1 -3 showing, mean, maximum, minimum values, standard deviation and the coefficient of variation (%).Table 1 is the summary of dry season result, table 2 is the summary of wet season result while table 3 is the summary of a combination of dry and wet season result of CO obtained from the air quality monitoring sites in the study areas.In each table the coefficient of variation (CV %) in either morning, afternoon or evening were also presented.Also figure 2 -14 shows the variation of CO in the study locations as presented with column charts, contour and 3-D surface plots.
The result as shown in The dry season result shown in table 1 also indicates that for Owerri elevated values of CO were observed in the evening when compared with morning and afternoon, the order is E>A>M, and the co-efficient of variation is in the order A > E > M. While in Okigwe, the order for the observed mean level of CO is also E > A> M, variation is in the order M > A > E. In Orlu, the mean CO level for morning, afternoon and evening is in the order A > E> M, with variability in the ranking A > M > E. Furthermore, the mean CO values for Egbema is in the order E > M > A, while the co-efficient of variation is also in the order A > M > E. Fig. 2, 3, 4 and 5 shows the variation of CO level in weeks during the air quality monitoring in the dry season.Fig. 2 indicates that CO values for Owerri were highest in week 3, 5, 6 and 8.While in Okigwe, fig. 3 indicates that higher values of CO were observed in week 2, 5, 8 and 12. Also fig. 4 indicates that the level of CO observed in Orlu in week 6, 7 and 8 were higher than other weeks in dry season.Elevated values of CO were observed in the afternoon and evening compared to morning hours as shown in fig. 1, which could be due to higher vehicular and commercial activities in the area [43,44].Also the CO level in Egbema as shown in fig. 5 indicates elevated values from week 1 to week 12.This could be attributed to the presence of gas flare starks in this area [45,46].The results shown in Fig. 6 and 7 are the contour and 3-D surface plots of CO in the study locations in dry season.The results indicates that higher concentrations of CO were observed at Egbema and Okigwe in dry season Table 2 is the summary of wet season result of CO values observed in the study locations.The result indicates that in Owerri (wet season) CO (ppm) values ranged from 24.00 -32.29, 32.71 -39.00 and 36.57-41.57respectively for morning, afternoon and evening.The Owerri mean CO level is in the order E > A > M, and variation is in the ranking M > A >E.While in Okigwe CO values (ppm) ranged from 29.00 -38.50, 38.50 -38.50 and 40.33 -45.83 for morning, afternoon and evening respectively.The mean CO value at Okigwe is in the order E > A > M, while the coefficient of variation is in the order M > A >E. Also the CO values observed in Orlu ranged from 25.20 -39.80, 32.80 -39.80 and 36.40 -43.00 respectively for morning, afternoon and evening, while the mean value is in the ranking E > A > M, and the variation is in the order M > A >E.The CO (ppm) values observed in Egbema ranged from 18.75 -35.00 in the morning, 26.25 -40.50 in the afternoon and 36.50 -44.25 in the evening, The mean CO level is in the order E > A > M, while the order of variability is M > A >E.In all Fig. 4: Variation of CO in Orlu (dry season) higher variability was observed in the morning even though (< 20%) is little variation in the variability ranking when compared with afternoon and evening in the wet season.
Again Fig. 8 indicates that lower values of CO were observed in the morning hours in Owerri in wet season from week 1 -12, and the variation is in the order morning < afternoon < evening.Also fig. 9 indicates that there was no significant difference in the level of CO observed in either morning, afternoon or evening from week 1 -12.While in Orlu as shown in fig. 10, the result indicates that lower value of CO was observed in week 7 in the morning, and elevated values of CO were observed in week 8 -12 in the evening hours.Fig. 11 indicates that in Egbema lower values of CO were observed in the morning from week 1 -2, while elevated concentrations were observed in week 5, 8, 10 and 12.Where M= morning, A= afternoon and E= evening, SD= standard deviation, CV% = co-efficient of variation   Where M= morning, A= afternoon and E= evening, SD= standard deviation, CV% = co-efficient of variation  3 is the summary of combined dry and wet season CO results from the study areas.Some of the results shown in table 3 revealed large standard deviations (SD) which is an indication of the heterogeneous nature of the air samples monitored in the study locations.The results indicates mean value of 32.71, 38.23 and 41.90 ppm in the morning, afternoon and evening respectively in Owerri, while the mean values of CO (ppm) for Okigwe are 37.59, 42.33 and 43.85 respectively for morning, afternoon and evening.It was also observed that the mean CO (ppm) level in Orlu are 36.54,40.20 and 41.73 in the morning, afternoon and evening respectively, and that of Egbema are as follows; 36.67,40.66 and 44.80 respectively for morning, afternoon and evening.
The variability of CO within dry and wet season as shown in table 3 revealed that in the morning co-efficient of variation is in the order; Egbema > Owerri > Orlu > Okigwe.Egbema showed highest variation (30%) in the morning.The order of variation in the afternoon is also Egbema > Owerri > Orlu > Okigwe, while the order of variation in the evening is again Egbema > Owerri > Orlu > Okigwe.In both afternoon and evening, Egbema showed highest level of variation in CO concentration, which is a reflection of the gas flaring activities in this area as earlier noted.
Finally, fig.14 shows the variation of CO in dry and wet seasons.The result as shown in fig.14 revealed some level of variation in the concentration of CO observed in the study areas, as earlier noted, week 1 -12 is dry season while week 13 -24 is wet season.The peaks in fig.14 from week 1 -12 are more elevated than those of week 13 -24.The conspicuous peaks in week 2, 5, 7 and 12 which were all above 50 ppm is an indication of higher variation of CO in dry season than in wet season.The mean level of CO observed in this study was above the Nigerian Ambient Air Quality Standard for CO (10 ppm).Seasonal variation of CO, with highest concentration during the months of August, September and October has been reported [47].Similarly, a significant difference in the mean concentrations of CO during the wet, transition and dry seasons has been reported [48].The higher values of CO observed in the study areas could be attributed to factors such as higher vehicular traffic [49], presence of three stroke engine tricycles known for incomplete combustion which could emit noxious atmospheric pollutant like CO [29], and combustion of biomass [50].

Conclusion
The seasonal variability of CO in the ambient environment of the study areas revealed elevated concentration of the air pollutant in all the study locations above permissible limit which calls for concern.Variation in the level of CO was observed in morning, afternoon and evening, including in different weeks and locations.There was also an observable seasonal variability of the atmospheric pollutant.Variability of atmospheric pollutants could be attributed to changes in meteorological and topographical conditions which could change the atmospheric pollution pattern with different locations and time.Apart from higher vehicular traffic, presence of three stroke engine tricycles known for incomplete combustion and combustion of biomass as being responsible for the observed elevated CO level.Indiscriminate use of power generators especially the very small generating sets due to erratic supply of electricity from the public electricity distribution could have impact significantly to the observed CO level in the study areas.This is supported by the fact that CO once emitted could remain in the ambient environment for about four months as earlier noted.

Fig. 14 :
Fig. 14: Variation of CO in wet and dry season in th air quality monitoring sites

Table 1 :
table 1 for dry season indicates that in Owerri, the mean concentration of CO ranged from 33.71 -40.7, 33.57-48.00.40.57-49.43 ppm for morning, afternoon and evening respectively.The result for Okigwe indicates that CO (ppm) ranged from 30.67 -49.17, 32.50-47.67 and 34.50 -49.83 for morning, afternoon and evening respectively.The CO Summary of dry season CO [ppm] result Orlu ranged from 35.80 -45.40 in the morning, 35.60 -47.00 in the afternoon and 38.80 -49.00 in the evening.While in Egbema the CO (ppm) values ranged from 44.25 -52.00, 39.00 and 39.00 -50.00 for morning, afternoon and evening respectively.