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nvironmental Monitoring of NOX, Total Oxidants and the Implications for Photochemistry of Air Pollution over Ilorin Shed, Nigeria

The assessment of oxides of nitrogen (NOX) and total oxidants as ozone (O3) over the air shed of a typical average tropical equatorial city monitored with the aid of high volume sampler for three years is presented using three urban site classes and a control. The mean concentrations for all sites during wet season (May October) were 3.31 ± 0.40 ppb for NOX, 21.86 ± 2.47 for O3 while the dry season (November April) mean concentration values were 2.03 ± 0.23 ppb for NOx and 32.44 ± 5.13 for O3. Mean value at the control site was an order of magnitude lower than the urban sites during the two seasons except concentration of O3 higher during the dry season. Traffic density and trace gas concentrations showed good correlation of 0.98 (P<0.05). NOx was confirmed to be a strong precursor for O3 formation. The influence of Harmattan dust in impeding the photochemistry of O3 formation from November to January (NDJ) months was observed. The overall concentration levels of NOx and O3 for the air shed are 2.67 ± 0.32 and 27.15 ± 3.8 respectively, within the limit of United States Environmental Protection Agency (USEPA) guidelines for air quality standards.


Introduction
Atmospheric pollution is one of the most dominant problems associated with urbanization. Urban areas are associated with high density of industries and transport networks, among other infrastructure, which generate a wide range of air pollutants. The concentrations of oxides of nitrogen (NOX = NO + NO2) in the urban atmosphere are mainly associated with industrial boilers, and non-industrial fuel combustions by both stationary and mobile sources. In addition, some of these primary pollutants are involved in photochemical transformations in the atmosphere under favourable conditions to produce secondary pollutants. Pollutants, which are photo-chemically generated in the atmosphere, are commonly referred to as photochemical oxidants, in view of their formation pathways. Ozone is one of the most recognized parts of photochemical oxidants. Others include peroxyacetylnitrate (PAN), hydrogen peroxide (H2O2), and so on, usually in a web of multi-path photochemical transformation processes. Ozone and PAN are known to be associated mainly with photochemical oxidation of NOX and a range of non-methyl volatile organic compounds (NMVOC) in the atmosphere under favourable photo-dissociation conditions (Thomas et al., 1974) In most developed countries which have urban areas ranging from small to mega-cities, the formulation of polices that could bring about reduction of these pollutants concentrations levels are supported by routine monitoring and modelling, which have now remained essential programmes supported by government and private sector agencies. Experience from these countries indicate that a wide range of methods exist which are routinely deployed for either on-the-spot sampling, or for systematic observations which yield trends in atmospheric mixing values over time.
The most precise of these techniques are those relying on infrared (IR), ultraviolet (UV) and other relevant atomic/molecular spectrometry techniques. These techniques have also been known to be very cost intensive, as well as demanding in both the level of human capacity support and maintenance requirements.
In the less developed countries, especially those in Africa, air quality monitoring and modelling are yet to become routine scientific activities, leading to data which are highly sparse or sometimes not at all in existence. The challenge therefore on scientists is to rise to the demand of providing the needed information to support policies, which could improve the management of urban and rural air quality. These challenges are in most cases, countered by the high cost of atomic and molecular spectroscopy-based analysers. The ability of these countries to deploy these precision techniques is known to be low. As proper management of total environment in these countries requires the acquisition and archiving of trends in atmospheric mixing values of air toxics in both rural and urban areas routinely, the development and application of less expensive but reliable and reproducible techniques have been advocated widely. The colorimetric method (Saltzman, 1954;Saltzman and Gilbert, 1959) is among the techniques, which have been established as adequate for the monitoring of NOX and O3. This technique form the basis of the measurements presented in this study.
The main objective of this study is to determine the concentrations of NOX and O3 and the mixing values of these gases, in the atmosphere of different urban locations in the city of Ilorin, Nigeria. These mixing values will then be compared with similar urban centre measurements elsewhere. The influence of air-shed photochemistry on the concentration of NOx and ozone during the seasonal transitions between Harmattan to Rain and Rain to Harmattan will be evaluated. Additionally, the concentrations of these pollutants will be compared with World Health Organization (WHO) or United States Environmental Protection Agency (EPA) maximum permissible limits and the implications to public health will then be discussed.

Study Area
The city of Ilorin is located on latitude 8 20'N and longitude 4 35'E, at an approximate altitude of 306 m above the mean sea level. The city is strategically located in the Guinea Savanna belt of Nigeria, at what may be described as the geographical and cultural confluence of the Northern and Southern parts of the country. As a result of its location, Ilorin experiences two major seasons during an annual cycle, the dry and the wet seasons. The persistence of either season is influenced by the absolute fraction of time during an annual cycle that the line of the Inter-Tropical Convergence Zone (ITCZ) or the Inter-tropical Discontinuity (ITD), as it is called in West Africa, is below or above its line of latitude.
The dry season (under the influence of the north-east trade winds, when the ITD is south of the city), on the average, starts around the middle of October and lasts until the middle of April of the following year, though more pronounced between mid-November and mid-February. Rains initiated by predominance of southwest monsoon winds about the remaining six months of the year in the area, when the ITD lies north of the city. The dry season period is characterized, not only by near absence of rain, but also by transport of dust haze from the Sahara Desert into the region. The winds are generally cold because they form part of the Hadley cell, which is known to move equator-ward from the poles at ground levels, and pole-ward from the equator at upper levels. The Harmattan period, being wintertime in Europe, implies that very cold air, drafted across Europe through the Sahara, arrives in the subregion, carrying the Sahara dusts with it. The season is characterized by high atmospheric turbidity, which inhibits the penetration of a significant fraction of the incident solar radiation to the earth's surface.
In development terms, the city is semi-industrial in the sense that, although a relatively large urban centre geographically by national standard, there seems to be very little presence of large scale industries that could qualify as major point sources. Hence, apart from the few industries, which are sparsely located, the rest of the city is dominated by mobile sources from the expanse of road network across the city, and area sources, which are a mix of residential, commercial and, medium to small scale industries. As for other major cities in the country, no strict demarcation exists between industrial, residential and commercial areas. Most industrial areas have admixtures of commercial and residential premises. The converse holds for commercial and residential areas.

Choice of Sampling Sites
In the absence of clearly demarcated urban industrial, commercial and residential areas, it was not possible to identify sampling sites, which fits exactly into these major categories. In place of such urban classification scheme, the sampling locations were selected across the city to reflect the following categories of interest:  High traffic volume/medium population density (HT-MPD) zones  Medium traffic volume/High population density (MT-HPD) zones  Industrial areas/Low traffic volume (IA-LT) zones, and  Low population density/No traffic (LPD-NT) zone was used as control (CTR).

Preparation of Absorbing and Sampling Solutions
Total oxidants were determined by buffered potassium iodide solution method proposed by (Saltzman and Gilbert, 1959). Determination of oxides of nitrogen concentrations was based on the methods of ICMA (ICMA, 1972), which was an adaptation of the Saltzman (1954) colorimetric, azo-dye forming reagent. The application of these techniques to the monitoring required the preparation of the following range of reagents as sampling and absorbing solutions.

Preparation of 1% Neutral-Buffered Potassium Iodide solution
In order to prepare the neutral-buffered potassium iodide solution, 13.61 g of anhydrous potassium dihydrogen tetra-oxo-phosphate (V) {KH2PO4}, 14.20 g of anhydrous di-sodium hydrogen tetra oxo-phosphate (V) {Na2HPO4}, and 10 g of potassium iodide (KI) were successively dissolved in distilled water in a 1.0 litre volumetric flask. The solution was made up to 1 litre mark on the flask.

Preparation of Saltzman Solution
In order to prepare the colorimetric solution, azo-dye forming reagent, 5.0 g of anhydrous sulphanilic acid {HNC6H4SO3H} and 140 cm 3 of glacial acetic {CH3COOH} acid were added to 250 cm 3 distilled water in 1litre volumetric flask, the solution warmed and allowed to cool to room temperature. Twenty (20) cm 3 of a 0.1 % stock solution of N-(1-naphthyl) ethylene di amine di hydrochloride {2HClNH2 (CH2) 2NHC10H7} and 1 % acetone {CH3COCH3} was added to retain the colour. The volumetric flask was shaken thoroughly and the solution made up to the mark with distilled water.
The chemicals used in the preparation of sampling and absorbing solutions were all of analytical grade. The reagents had been stable prior to their use.

Sampling
The detailed of the air sampling set up used has been presented by (Abdul Raheem et al., 2013). A vacuum pump draws air from the atmosphere into sampling solution in a bubbler tube at a mean flow rate of 1000 cm 3 per minute. The volume of absorbing solution used was 30 cm 3 to ensure that enough sample dissolved in the absorbing solution based on their low concentration in the atmosphere. All joints in the sampling train were made airtight through the application of silicon grease. Sampling was undertaken at sixty-minute intervals, after which, the bubbler was removed carefully and the absorbent solution (sample) transferred into sample bottle for analysis. For NOx a fritted bubbler was used to improve collection efficiency. The method was authenticated by the use of a LaMotte air sampler (LaMotte, 2005) imported from LaMotte and Company U.S.A along with standard reagents for all the pollutants. In order to determine pollutant variability over daytime periods, air sampling was undertaken at each location over seven defined sixty minutes periods for any sampling day. The ten sampling periods were equitably spaced between approximately 6.00 am and 6.00 pm; a minimum of 60 samples were collected at each site per week for the trace gases monitored. The time of sixty minutes was found from trial samplings to be sufficient for the quantitative sampling of these gases without the absorbing solution being saturated. All samplings reported were carried out in triplicates. The sampling exercise lasted for 3 years between mid-Novembers to Mid-Februarys. Since road traffic is the common source cutting across all sites classes, the average traffic densities were determined for all sampling locations. Traffic counts were undertaken at hourly intervals. The minimum and maximum temperatures were between 23 0 C and 34.3 0 C (Abdul Raheem et al., 2008), throughout the sampling time.

Establishment of Atmospheric Concentrations
While the absorbance of the samples and absorbing solutions were measured using Aquamate UV-Visible Spectrophotometer, the absorbance of O3 were measured at 352 nm, while those of NOX measured at 550 nm using the same UV-visible spectrophotometer with 1 cm 3 cuvette. The results of the absorbance were measured for their equivalent concentrations using the standards prepared from iodine (I2) solution for O3. This provides a calibrated iodine solution equivalent to 1.92 μg of ozone per ml (Stanley, 1975, Dara, 2008. As ozone reacts with iodine ion in neutral buffer solution according to the following equation: The tri -iodide ion liberated has an intense yellow colour. The standard solution was always prepared freshly when needed. Sodium di-oxo nitrate III (NaNO2) standard solutions were prepared with the NOX absorbing reagent, which allowed the formation of red azo dye of which the absorbance was picked at 550 nm. The absorbances of the standards against their concentrations were used for calibration curve preparation which was used in deducing the respective concentration of the gas in the absorbing solutions (samples). As the standardization was based on the empirical observation that 0.72 mole of NaNO2 produces the same colour as 1 mole of NO2 (Hesketh, 1972). The concentration of each pollutant in the sampled air was finally established as the ratio of the mass in absorbing solution and total volume of air sampled during the period. Both calibration curves displayed very good linear correlations (R 2 > 0.99) between concentration of the pollutant of interest and the absorbance respectively for O3 and NOX in the measuring range.

Urban Sites Classification and Traffic volume Density
The specific urban locations and site classification by type of activity are presented in Table 1 together with the results of the hourly traffic counts undertaken at the various urban sites where sampling were undertaken. This shows urban site class HT-MPD, represented by the Post Office Roundabout where the highest traffic density of 2,976 and 1917 vehicles per hour were recorded respectively for both dry and wet seasons over the sampling period. The other two sites had 2202, 1240 and 474, 857 respectively while the background site, represented by the University of Ilorin Research Farm recorded few close to no traffic density throughout the sampling period. In order to relate traffic density with the diurnal variations in the monitored

.2 Air-shed Concentrations of Trace Gases and Day-time Trends
The results of the daytime mean values of the assessment of atmospheric mixing values of the trace gases (NOX and O3) in the four urban site types for dry and wet seasons are presented in Table 2 & 3. The results indicate that even though the mean daily concentrations are close in values, significant differences exist among the three site classes. These differences are a reflection of the nature of activities in various areas. For instance, In the case of NOx medium traffic volume and high population density (MT-HPD) zones and industrial area are higher than high traffic and medium population density (HT-MPD) area by an average of about 28 % while for O3 high traffic and medium population density (HT-MPD) and medium traffic and high population density (MT-HPD) are higher than Industrial area by 31 %. The concentration of the trace gases in the control site or background is however, approximately an order of magnitude lower than the other three sites except for O3 where the value is even higher than the industrial area. The high O3 concentration at the CTR site was associated with the time taken for the photochemical transformation of NOX and VOC which are generated at urban sites. During this range of time, the parcels of air carrying the O3 would have experienced advection in the downwind direction towards rural area, thereby producing higher values at downwind distances. This observation where rural areas have higher O3 concentration than urban centres is an established urban-rural dispersion for atmospheric O3 concentration (Wanner et al., 1993;Baumbach and Vogt, 2003). The overall city average for Ilorin from the sites sampled is estimated to be approximately 27.15 ppb for total oxidants and 2.67 ppb for NOX. These are comparable with values of O3 and NOX measured in Lagos in 1993 based on state of the art UV-fluorescence and Chemiluminescence's techniques, and which were respectively in the range 4 to 102 ppb for O3 and 5 to 45 ppb for NOX, including periods of episodic events (Akeredolu et al., 2004).
The daytime trends in concentrations of oxides of nitrogen (NOX = NO + NO2) in the four sites are presented in Figure 1.

Figure 1: Diurnal Variation of Traffic Density with Trace Gas Concentrations for HT-MPD Sites
NOX exhibits an early morning maxima between 6.30 and 7.10 am. However, a second maximum appears to show up between 9 and 10 am, which gives way a little later and remains low until late in the evening when the value starts to increase.

Figure 2: Diurnal Variation of Traffic Density with Trace Gas Concentrations for MT-HPD Sites
The daily mean values for the three urban sites are equal, while the background station or control site is also, nearly an order of magnitude lower than the urban sites. The observed trends in the NOX values may be affected by a combination of factors. Figures 1 to 3 indicate that some weak correlation appears to exist between NO2 and traffic density for HT-MPD and MT-HPD sites, while being anti-correlated with traffic for the IA-LT sites. Indications are that the early morning peak is most likely dominated by NO emissions, which become photochemically transformed in the late morning and afternoon to NO2. The weak correlation with traffic may be indicative that other urban sources (industrial, diesel and gasoline generators, and for IA-LT sites, residential burning of firewood and charcoal) combined with potential emissions from savannas burning (which are rampant during the Harmattan season) may have higher source strengths than traffic.
Abdul Raheem,Adekola & Obioh 7 The trends in measured atmospheric concentration of total oxidants (presented as O3) at the four sites are presented in Figure 5, unlike the trends for NOX, total oxidants concentrations Figure 3: Diurnal Variation of Traffic Density with Trace Gas Concentrations for IA-LT Sites appear to have low values in the morning but increases gradually during the day reaching a peak in the mid-afternoon before eventual gradual decrease to minimum values in the evening. Figures 1 to 3 indicate that O3 has an inverse-correlation with traffic density during daytime and reduces towards zero values during the hours when the sun is switched off. These are clearly indicative of the strong influence of solar radiation in the photochemical processes leading to O3 formation (Abdul Raheem et al., 2008). Since O3 is not emitted by urban sources, but through photochemical transformation, the observed profile has expected should be strongly correlated with the trends in surface penetration of solar radiation. It is also evident that O3 is anti-correlated with NOX in all urban sites classes (Figures 1 to 3), which demonstrates the precursor-role of NOX in O3 formation. This could be confirmed by overall regression equations 2 and 3 obtained from regression results showing constants with O3 as dependent variable for Ilorin data during wet and dry seasons (Tables 4 and 5 (3)

The trend of total oxidant and titration effect
When NO2 is exposed to UV light (λ < 424nm), oxygen atom and NO molecule are generated through the NO2 photolysis reaction. As a result, O3 is formed as the product of the reaction between O and O2 NO2 + h ‫ט‬ → NO + O (4) O + O2 → O3 + M Except when going through the O3 production cycle, the NO from reaction equation 4 can react with O3 to regenerate NO2 as in equation 6 O3 + NO → NO2 + O2 Equations 4 -6 represent the photochemistry state for O3, NO and NO2. Reaction equation 6 is known as titration reaction. Nevertheless, in the case of a fixed O3 production rate, a decrease in NOX emission will reduce the NO available for the titration reaction and, consequently, affect the O3 -NO -NO2 equilibrium in the photostationary cycle (Chou et al., 2006)

Figure 4: Mean Daily Mixing Ratios of O3, Nox and O3+Nox of Ilorin Town during Rainy Season
Figures 4 & 5 illustrate the analysis of total oxidants as estimated by O3 + NO2 instead of O3. The values are the daily hourly averages of the mixing ratios throughout the period. Contrary to the findings of Chou et al., (2006), O3 + NO2 show the same seasonal variation with ozone, particularly O3 + NO2 during wet season. These enhance the seasonal variation with the maxima during the dry season. However the seasonal cycle of ozone / oxidant with dry season maximum and wet season minimum as shown is in accordance with results by other authors (Pissimanis et al., 2000;Monks, 2000). It was observed that the dry season maximum is due to the long range transport of the polluted continental air masses and the prevalence of photochemical reactions. The wet season minimum is as a result of the low ozone concentration in the air masses transported from the regional trans-boundary.

Implications for Atmospheric Photochemistry of Ilorin Urban
It is well known that NOX and hydrocarbons serve as precursors in presence of solar radiation for formation of total oxidants. An illustration of the diurnal trends in solar radiation in a typical site in South West Nigeria, which shares the same climatic conditions as Ilorin, is presented in Figure 6 [Obioh's personal communication]. This indicates that solar radiation is initiated at approximate 7.00 hours local time, reaches its peak around mid-day and then gradually reduces to zero at approximately 19.00 hours local time, in what may be closely related to a Gaussian distribution.   The diurnal O3 trends presented in Figures 1 to 3, 7 and 8 are illustrative of the strong correlation between solar radiation levels and O3 concentration. The implication of NOX in the photochemistry of O3 formation is presented in Figures 9, 10, 11 and 12. From these figures, it is apparent that NOX values are high in the morning hours in response to the early and mid-morning high traffic, with much less presence of O3. However, in the afternoon hours, even with the late afternoon high traffic, the values of NOX remain low. At the same time, the value of O3 that was low in the morning 11 and 12 is now showing significantly high values. This indicates that much of the NOX emission during the day had been photo-oxidized to O3, thus confirming a widely known urban photochemical process. Thus NOx dominated the polluted air shed in Ilorin in the morning to mid-afternoon, and by O3 in the late afternoon. The atmospheric turbidity during the Harmattan season is generally known to be high, inhibiting optimal surface penetration of solar radiation. As a result, the rates of atmospheric photochemical kinetic processes are expected to be low. This implies that O3 production currently recorded may be lower than the atmospheric concentrations outside the peak Harmattan season. To enable the quantitative evaluation of the potential role played by Harmattan aerosol in the O3 photochemical production process, the typical seasonal variability of solar radiation level during an annual cycle in Ilorin is presented in Figure 13. This indicates that global solar radiation is minima during the DJF and JJA months during periods when atmospheric aerosols and clouds dominate and provide some shield to the surface penetration of solar radiation. As a result of poor surface solar radiation penetration, limited photochemical reactions dominate these two seasons (Harmattan and wet seasons). The transition periods between Harmattan to Rain (March) and Rain to Harmattan (November) appear to be more favourable to total oxidant formations (Abdul Raheem et al., 2013).

Comparative Analysis of Implications for Human Health
From the available data of mean levels of NOX and O3 in Ilorin urban and control sites, the comparative assessment of the level of pollution in the city with some of the known cities of the world has been undertaken as a means for inferring potential health implications of ambient concentrations of NOX, and O3. The overall average level for the city, using all the data at our disposal was estimated to be 27.15 ± 3.8 ppb total oxidants and 2.67 ± 0.32 of NOX. The Ilorin city oxides of nitrogen and ozone concentration levels compared with NAAQS standards as shown in Tables 6. The value obtained for Ilorin may be considered high in view of its limited industrial and transport infrastructure. This is illustrative of the need for early establishment of local emission factors, identification of reduction strategies which would have to be implemented as a long-term programme to reduce the adverse impact of these potential high ambient concentrations on human health as the city strives to improve its industrial and social service infrastructure some of which are directly implicated in the emission of these trace gases.

Conclusion
The study has provided estimates of the mean concentrations of two trace gases, day-time trends and the potential contributions to the atmospheric photochemistry in the city's air-shed. Results show colorimetric / spectrophotometric technique could be used to obtain results needed to support air quality management as well as some level of understanding of the source strengths, air-shed photochemistry and potential impacts. In particular, it has been confirmed that NOx plays a strong precursor role in the formation of surface O3 in the city. It is also established that some weak correlation exists between NOx and traffic density for HT-MPD and MT-HPD sites, while being anti-correlated with traffic for the IA-LT sites. Indications are that the early morning peak is most likely dominated by NO emissions, which become photochemically transformed in the late morning and afternoon to NO2. For O3, it is demonstrated that there is an inverse-correlation between O3 concentrations and traffic density during daytime. The O3 values reduce towards zero values during the hours when the sun is switched off, which clearly indicate the strong influence of solar radiation in the photochemistry of O3 formation (Abdul Raheem et al., 2013). The observed profile as expected should be strongly correlated with the trends in surface penetration of solar radiation. It is observed that solar radiation penetration to the surface is minimum during the Harmattan season when the atmospheric aerosol content reaches maximum values and minimum during seasonal transitions between Harmattan to Rain (March) and Rain to Harmattan (November).