Atmospheric dristribution of traffic heavy metals at temu-interchange, Epe Lagos, Nigeria

The Atmospheric distribution of traffic heavy metals is a dynamic process that involves the emission of various pollutants released from vehicular traffic. It has affected both human and plants around Temu-interchange hence, the need to characterized the emissions. The objective of the study was determined concentration of Particulate Matter (PM) and Enrichment Factor (EF) depending on the traffic emission. The research was conducted using deposition method. The study area and the control experiment were conducted simultaneously for a period of thirty (30) days between December 2022-January 2023 (Dry Season). Samples were prepared and deposition fluxes (DF) were determined. The particulates were taken to the Central Research Laboratory, Tanke in Ilorin for characterization using X-ray Fluorescence (XRF) spectrometer. EF was also determined. Thirteen elements Fe, Ag, Cd, Zn, Sn, Ti, In, Co, Mn, Pd, Rh and Ru were characterized from the samples collected at the study area while Twenty-one Si, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ge, As, Zr, Sr, Pb, W, Po and Ac elements were characterized from the samples collected at the control unit of the experiment. The EF of the detected elements both in the study area and in the control, experiment was determined. Five elements in the study area were moderately enriched, while elements such as S, Cl and Zn were highly enriched in the control unit. An indication of anthropogenic activities particularly traffic emission influence within the area hence, a clean-up of the study area is highly recommended.


Introduction
The presence of heavy metals in the environment beyond the acceptable limit has been a serious concern to environmentalists.Atmospheric heavy metal contamination has been a major environmental problem in Lagos-State, South-Western, Nigeria due to emissions from industries, energy generation, vehicular traffic, combustion of fossil fuel and poor waste management strategies.Heavy metals are the stable metals or metalloids whose density is greater than 4.5 g/cm3, namely Pb, Cu, Ni, Cd, Zn, Hg and Cr, etc. [1].Evidently, some of these metals are essential for life at very low concentration levels but at high levels of concentration they may lead to harmful effects in humans, plants and animals [2].These heavy metals can be found in the atmosphere in varying proportions in different areas or locations (residential areas, industrial areas) and these varying proportions can be determined by using moss plant as the indicator [3,4].
Environmental challenges posed by the proliferation of human activities such as industrial effluent discharge, vehicular emissions, bush burning, wastes incineration and mining operations have become evident in the industrial and high traffic density areas of Epe, Lagos metropolis but there is dearth of baseline data on the levels of heavy metal pollutants in dust over the areas [1].It was also observed that very little interest has been developed on metal contaminations of street dust despite its direct contact with greater part of every population [5,6].Thus, this study sought to provide preliminary information on the levels of some heavy metals in street dust over the industrial and high traffic density areas at Temu interchange Epe metropolis.The study will form a baseline data for future environmental impact assessment of the street dust over the study area.

Study Area
Lagos state is the most populous and fastest growing in Nigeria.It lies on the Latitude 6 o 27' 55.5192" N and Longtitude 3 o 24' 23.2128" E respectively.Epe is a town and a local Government Area (LGA) Fig. 1 in Lagos State, Nigeria located on the north side of Lekki Lagoon with coordinate of 6° 34' 59.99'' N and 3° 58' 59.99'' E.

Sample Procedure
The samples were carried out between December 2022 to January 2023 (Dry Season) at Temu-Interchange shown in Fig. 3.1.Deposition gauges (0.2mm diameter, 0.3m depth by 20mm height) were placed at the study area for 30 days.Meanwhile a deposition gauge was placed for 30 days in the Lagos State University Epe Campus to serve as our control experiment for the study.The planted deposition gauges were harvested after 30 days and taken to the analytical laboratory in the department of Chemical Engineering Lagos State University in order to determine the deposition fluxes.The dry deposition in the deposition gauges were rinsed with distilled water then filtered through a dry preweighed Whatman membrane filter paper (3.0 μm) on a digital weighing balance.The wet filter papers were preserved in desiccators to prevent further settlement of particles until they are completely dried.The dried filter paper and particles were reweighed to determine the deposition fluxes.The particulates were taken to a private own laboratory, the Central Research Laboratory, Tanke in Ilorin for characterization using X-ray Fluorescence (XRF) spectrometer.The Enrichment Factor of the detected heavy metals was determined.
The deposition fluxes were determined by using the equation below

Deposition flux (DF) = (W2 -W1) (g/m2day) At
Where; W1 is the initial weight of the sample (g), W2 is the final weight of the sample (g), A is the Area of a circular particles (m 2 ), T is the time (days to complete the experiment).

Enrichment Factor (EF)
To calculate the contribution of anthropogenic sources, the enrichment factor EF of each element will be determined using the following relationship;

Characterization of the dry deposition samples
The sampling process was carried out for 30 days at Temu-Interchange while the control unit was in the Lagos State University Epe Campus.The Dry deposition fluxes of aerosol were measured for the study area and control unit Table 1 and Table 2.The PM samples obtained at the sampling locations during the dry season were characterized and the following heavy metals were detected Iron (Fe), Silver (Ag), Cadmium (Cd), Zinc (Zn), Tin (Sn), Titanium (Ti), Lead (Pb), Indium (In), Cobalt (Co), Manganese (Mn), Palladium (Pd), Rhodium (Rh), and Ruthenium (Ru).The data of the characterization of each metal found at the study area and control unit are showed in Table 3 and Table 4.

Variation of atmospheric pollutant concentration
Fe, Ag, Cd, Zn, Sn, Ti, Pb, In, Co and few other elements were both detected in the samples from the sampling site location as well as from control unit; this could be linked to the high movements of vehicles and motorist at the study area.High concentrations of these particulate matters automatically cause pollution in the area.The high Iron (Fe) concentration detected in dry deposition indicates the ubiquity of this toxic metal in the environment and shows that the area is enriched with Fe.According to [8,9], the recommended daily averaging for Fe is 0.003 μg/m 3 and 0.02 μg/m 3 respectively and these values are far below the values 0.1958 (μg/m 3 )10 6 from sampling site and 661.6800 (μg/m 3 )10 6 from the control experiments respectively [10,11,12] as shown in the Table 3 and Table 4.The Iron (Fe) concentrations were high in the samples collected from the control unit.The analysis revealed that the mean concentration for the control unit was (661.6800mg/m 3 ) while the mean concentration for the study area was (0.1958 mg/m 3 ).This trend shows that the Lagos State University Epe Campus was more polluted with Iron (Fe).These average concentration values are higher than the value stipulated by the NAAQS [8,1].
The result revealed that higher mean level of Zinc (Zn) was observed in the control unit (474.9139mg/m 3 ) compared to the mean level revealed in the study area at Temu-Interchange (0.0011mg/m 3 ) this shows that the control unit was more polluted in Zinc.The results also revealed that higher mean level of Titanium (Ti), Lead (Pb) and Manganese (Mg) were observed in the control unit compared to the study area as shown in Fig. 2 and Fig.
3 Elements such as Silver (Ag), Cadmium (Cd), Tin (Sn), Indium (In), Cobalt (Co), Palladium (Pd), Rhodium (Rh) and Ruthenium (Ru) were found at the study area but not found at the control unit after the characterizations of element from the samples collected.This shows that different elements can be found at different locations.

Enrichment Factor Analysis
Calculation of EF values help to determine whether a certain element has additional or anthropogenic sources other than its major crustal sources.Sources of metals in PM include both natural and anthropogenic processes [13,14,1].Elements such as Al, Si, Ti, and Fe are commonly used as a point of reference for the calculation of factors, because they are very abundant in crustal material and are not significantly affected by pollution.The Enrichment factor of the analysis for each metal Samples were calculated as shown in the Table 5 and Table 6, using the Iron (Fe) as reference element in this study, based on the average elemental concentration data of the upper continental crust.The crustal element data were taken from [15].

Enrichment Factor analysis for dry deposition
The enrichment factor for most elements in the dry season was within 2 -5 which shows that all elements were moderately enriched.Except for these element Sulfur (S), Chlorine (Cl), and Zinc (Zn) which were higher than 40 as shown in Table 5 and Table 6.The high enrichment of these elements (S, Cl, Zn) in the control experiment suggests that the dominant sources for these elements are non-crustal, this means that a variety of pollution emissions, such as vehicles activities and mechanical erosion of metallic surfaces [12,18].The comparison of the enrichment factors for both the sampling site and the control experiment unit is as shown in Fig.4

Conclusion
The present study has pointed out that the dry deposition of the characterized samples at Temu-Interchange were moderately enriched and the atmosphere of the environment is safe for mankind and animals, while the dry deposition of the characterized samples at the control unit which is the Lagos State University Epe Campus had some elements highly enriched.This shows that the environment needs an urgent clean up since elements like S, Cl, Zn have their enrichment factor >10.The observed elevated concentrations at the control experiment are an indication that the sources of these pollutants are due to increased industrial and commercial activities during working hours.The concentration of these atmospheric pollutants must have increased significantly due to the emission from bad vehicles and motorist at the control experiment.
This calls for best environmental management practices to alleviate and reduce the associated risks.This study discovered that most of the toxic heavy metals were, Silicon (Si), Zinc (Zn), Chlorine (Cl) and Iron (Fe), are ubiquitous in the ambient environment.These atmospheric pollutants pose the greatest negative effects on the environment.

Disclosure of conflict of interest
No conflict of interest to be disclosed.

Figure 1
Figure 1 Geographical map of Temu-Interchange Epe, Lagos State This study was conducted at Temu-Interchange Fig.1 Epe metropolis.(6°35'40.54"Nand 3°58'39.5"E)areas of Lagos state, Nigeria.The sites were carefully chosen based on the following criteria: accessibility to the Moss plant, availabilityof open spaces and of course areas with high traffic volume (transportation), as well as industrial activities.These activities will contribute immensely to the dust and element generation in the study area[3,] EF = (Cx/Cref) sample / (Cx/Cref) crust Where: EFx = Enrichment Factor of element x Cx = concentration of element x Cref = concentration of the reference element (Cx/Cref) sample = proportion of concentration in the particulate matter (Cx/Cref) crust = proportion of concentration in the crust

Figure 2 Figure 3
Figure 2 Mean and standard deviation of the samples in the dry season at study area

Table 1
Dry Deposition Flux of Temu-Interchange

Table 2
Dry Deposition Flux Control Experiment LASU Epe Campus

Table 3
Characterized Dry Samples for Temu-Interchange

Table 4
Characterized Dry Samples for Control Experiments

Table 5
Enrichment Factor for dry deposition at Temu-Interchange

Table 6
Enrichment Factor for dry deposition at the control Experiment Figure 4 Plot for enrichment dry against the element