Review of Particulate Matter and Elemental Composition of Aerosols at Selected Locations in Nigeria from 1985-2015

(NAAQS) daily permissible for fine particles of less than 2.5 micrometers in diameter (PM 2.5 ) and coarse dust particles with a diameter of 10 micrometers (PM 10 ) in In addition, the sources of data for the average concentrations for PM 2.5 , PM 10 and related metallic elements during 1985-2015 were assessed. Attempts were also made to compare varied particulate matter loads of atmospheric micro-environments in Nigeria with comparable micro-environments in selected cities around the /PM 10 ratios for the selected studies fall below the WHO guideline (0.5-0.8), suggesting that Nigerian aerosols are mainly made up of coarse, rather than fine particles. In addition, the order of the average highest concentrations of metallic elements for PM 2.5 were magnesium (Mg) > strontium (Sr) > potassium (K) > zinc (Zn) > iron (Fe) > sodium (Na) > aluminium (Al) > chlorine (Cl) > lead (Pb) > silicon (Si), while those of PM 10 were Sr > Zn > Fe > Mg > calcium (Ca) >Na > Pb > manganese (Mn) > K > Al. Conclusions. Seasonal variation of particulate matter loads revealed higher concentrations during the dry season than during the rainy season. In addition, particulate matter loads in rural areas were generally lower than in urban areas. Wind-blown dust from the Sahara Desert is the major contributor to particulate matter loads in northern zones of the country, while sea spray and crustal matter are the highest contributors to particulate matter loads in southern zones. Competing Interests. The authors declare no competing financial interests. 1–18 (2016)

Many epidemiological studies have revealed consistent associations between ambient concentrations of particulate matter with an average diameter of 10 micrometers (PM 10 ) and fine particles of less than less than 2.5 micrometers in diameter (PM 2.5 ) with adverse health effects such as cough, asthma, renal failure, and infertility. 5, 6 Detailed understanding of the concentration levels of important air pollutants like PM 2.5 and PM 10 , polychlorinated biphenyls, dibenzo-pdioxins/dibenzofurans and polycyclic aromatic hydrocarbons in urban and rural areas of the country have remained largely unknown. 7 Although there have been some pioneer studies on PM 2.5 and PM 10 concentrations and source contributions of different urban sources to the ambient air pollution at selected receptor sites in Nigeria, polycyclic aromatic hydrocarbons, dibenzo-p-dioxins/dibenzofurans and polychlorinated biphenyls have yet to be extensively studied. Between 1988 and, segregated studies involving documented contributions of pollutant sources in Nigerian sectorial and national emission inventories were surveyed by a research team. 3,8 In the area of particulate matter levels (PM 2.5 , PM 10 ), segmented studies involving selected receptor sites in Lagos and Abuja have been carried out over the years. 7,10- 16 In addition, a more extensive study involving six Nigerian megacities was undertaken. 17 Furthermore, a sampling campaign of particulate matter load (PM 2.5 , PM 10 ) in ten selected south-eastern Nigerian cities during dry and wet season periods (December 2008to September 2009) was also undertaken. 18 In addition, a monitoring study of ambient particulate matter (PM 10 ) spatial distribution in 17 selected cities in Nigeria over a six year (2001)(2002)(2003)(2004)(2005)(2006) period and the resultant health  Review implications was conducted by Efe (2008). 19 Results from the study indicated that the urban corridors of over 70% of Nigerian cities are sites with a high rate of daily mean/annual mean ambient PM 10 of over 120 µg/m 3 , while < 30% of Nigerian urban centres had a mean annual ambient PM 10 value of 119.2 µg/m 3 . Similarly, it was noted that significant differences existed in PM 10 concentrations across different land-use types, between the built-up areas and those of the surrounding rural areas. Multi-elemental characterization of aerosol samples in Nigeria have been carried out using an array of analytical procedures such as X-ray fluorescence, total reflection X-ray fluorescence, energy dispersive X-ray fluorescence (EDXRF), polarized EDXRF, wavedispersive X-ray fluorescence, atomic absorption spectroscopy, inductively coupled plasma mass spectrometry, inductively coupled plasma optical emission spectroscopy, instrumental neutron activation analysis (INAA), proton-induced X-ray fluorescence and proton-induced gamma ray emission. In a related study, a PM 10 level of 121.8 µg/m 3 was recorded in Enugu as part of a mega-city particulate matter monitoring campaign covering 17 cities in Nigeria over a 6-year period. 19 These recorded values showed marked variations when compared with the WHO and NAAQS daily air quality guideline criteria.

Variations in Nigerian Particulate Matter Micro-Environments: Effects of Location and Climate
Variations in PM 10 and PM 2.5 concentrations within different geopolitical zones can be attributed to regional changes in meteorological factors (temperature, relative humidity, wind speed and visibility) prevalent in such zones. 34 Nigeria has two major seasons (wet and dry seasons) and selected studies in the past have investigated the spatial and seasonal variation of aerosol loadings with climatic parameters. For instance, seasonal variation of meteorological data with aerosol mass concentration has been investigated in the South West and South South zones. 2,35 In a related study, meteorological data depicting seasonal variation in horizontal visibility of four climatic zones (Sahel, North Central, Southern and Coastal zones) in Nigeria was analyzed over a 30 year period (1984-2013) and the results generally showed that good visibility was obtained in the wet season due to the influence of moisture-laden south-easterly wind blowing in from the Atlantic Ocean resulting in rainfall that impinges on air-borne particulates leading to reduced aerosol loadings. 34 On the other hand, poor visibility was a result of dust-laden north-easterly trade wind blowing in from the Sahara Desert, which transports large quantities of dust and anthropogenic emissions, resulting in increased aerosol loadings. 34 A common trend in these studies reveals a weak correlation between climatic parameters and particulate matter concentrations, indicating that subject to seasonal variation, air temperatures tend to be directly proportional to aerosol loadings, while precipitation, relative humidity and wind speeds tend to have the opposite effect on aerosol loadings. In general, the northern geo-political zones tend to have higher aerosol loads than the southern zones due to factors such as higher temperatures, higher wind speeds, lower humidity/precipitation and reduced visibility, especially during atmospheric hazes. On the other hand, cities in the southern zones experience shorter dry season periods (October-February) and longer wet seasons (March-September) due to comparatively lower temperatures, lower wind speeds and higher relative humidity/precipitation

Figure 3b -Source apportionment of PM 2.5-10 in Lagos using chemical mass balance and principal component analysis (Oluyemi and Asubiojo, 2001 10 )
Review (2,  In the case of PM 10 fraction of airborne particulates, least detected elements arranged in ascending order of increasing mass concentrations (ng/ Review it helps to identify the dominant size fraction in the aerosol samples and the emission sources of the particulate matter. For instance, a high PM 2.5 / PM 10 ratio signifies domination by fine particles (PM 2.5 ), while a low ratio signifies domination by coarse particles (PM 10 ). 34 Furthermore, high PM 2.5 / PM 10 ratios are attributed to particulate air pollution from vehicle emissions and secondary particles formed in the atmosphere from gases. On the other hand, lower ratios are related to strong dust emissions and resuspension due to high traffic volume. Several factors can lead to changes in PM 2.5 /PM 10 ratios and these include vehicular emission, high temperature processes, photochemical reactions and a low rate of soil dust resuspension. 36,37 Available PM 2.5 /PM 10 ratios in published air pollution-related studies in Nigeria ( Table 1) ranged from 0.05 to 0.68, with most of the most of the ratios falling below the 0.5 mark, implying that particulate matter in Nigeria is dominated by coarse particles (PM 10 ). 2,12 Comparatively low % PM 2.5 /PM 10 ratios in selected cities in northern Nigeria such as Maiduguri (7%), Kano (19%), Abuja (37%) suggest that the dominant size fraction (PM 10 ) may have as its major contributor the wind-blown dust coming from the Sahara Desert, whereas relatively higher % PM 2.5 /PM 10 ratios in selected cities in southern Nigeria such as Ile-Ife (68%) and Lagos (66%) suggests that the dominant size fraction (PM 2.5 ) may have as its major contributor marine sources such as the Lagos Lagoon and the Atlantic Ocean.  Review mass balance and positive matrix factorization (PMF)] to accurately pinpoint sources of particulate matter loads is often lacking in air pollution related studies carried out in Nigeria. This is a research gap.
Notable studies that are considered pioneer works in the area of source apportionment and receptor modelling of particulate matter pollution in Nigeria include the use of chemical mass balance and principal component analysis to identify contributory sources of coarse (PM 2.5-10 ) and fine (PM 2.5 ) particulates at a receptor site in Lagos. Thus, according to a previous study, prominent sources identified as contributors to the particulate matter load at the study site were soil (35-54%), marine (26-34%), automobile exhausts (0.3-3.5%), refuse incineration (2-3%), and regional sulphate (2-12%). 10 Furthermore, PCFA has been used for source identification of particulate matter loads at four selected receptor sites in Lagos where traffic emission, sea salt, biomass burning, coal combustion and industrial emissions were noted as contributory sources. 29 In a related study, PCFA was also used to apportion five [soil dust (31%), sea spray (23%), heavy oil combustion (14%), industrial (13%) and physical construction ( ) and nitrate (NO 3 ) have been utilized as tracers to apportion stationary and mobile source emissions, respectively, in 3 selected Middle Eastern countries. In the same study, organic carbon (OC)/elemental carbon (EC) was also used as a tracer to estimate secondary OC sources in Palestine, Jordan and Israel. 46 Furthermore, source apportionment of data generated from the United States Supersites Program has revealed that several inorganic marker species can be used to identify major PM 2.5 sources. 48 Elements such as Al, Si, K, Ca, Fe were used as tracers for road Offor, Adie, Ana apportioning source contributors to trace elements found in both fine and coarse fractions depending on the factor loadings of the elements when apportioned to identified factors. 29

Particulate Matter and Elemental Composition of Aerosols in Nigeria
More details about the source contributions to particulate matter loads in selected cities in Nigeria are given in Figures 3(a) to 3(g).  (Figure 4). 80

Appraisal of Air Quality Data from Other Regions Around the Globe
Based on the findings of the WHO study cited earlier, the great majority of cities worldwide exceed the recommended WHO's Air Quality Guideline for maximum annual PM 10 mean levels (20 µg/m 3 ). Globally, relatively few of the monitored cities currently meet the WHO guideline values. These tend to be clustered in high-income countries. Typical concentration ranges for annual PM 10 mean values of selected cities in each of the 6 WHO classified regions are depicted in Figure 5.  guidelines and only around 12% of the total urban population live in cities where the air quality complies with such levels ( Figure 6). 80

Conclusion
This review attempted to portray the recent trends in air pollutionrelated research in Nigeria over the last three decades

Review
Metrological Agency. To remedy this situation, we recommend the development of a national emission standard for suspended particulate matter, establishment of national air quality monitoring networks in major urban/rural areas in the country, fixing emission caps and taxes for industries, road pricing for automobiles during rush hour periods to discourage traffic emissions and stoppage of gas flaring. There is also a need for further research in the area of source apportionment and secondary aerosols to complement current efforts at quantifying particulate matter loads and their elemental concentrations.