Elemental concentrations and source identification of air particulate matter at a Nigerian site: A preliminary study

Sources and levels of particulate matter in North African and Sub-Saharan cities: a literature review

In order to assess the significance of PM in ambient air, it is necessary to evaluate their physical and chemical characteristics as well as identify their major emission sources. On a global scale, particulate matter in the atmosphere arises mainly from the combustion process of motorized vehicles, but natural sources are still considered as the major contributors. In Africa, PM emissions differ from those in developed countries; human activities such as biomass burning in households, poor household waste management, and the high number of diesel-powered vehicles are the predominant anthropogenic sources. Natural contributions are also observed. Saharan dust and savanna fires are the most common atmospheric natural sources of particulate matter. The present literature review gives an overview of the status of air quality in African cities and highlights the various sources of particulate matter emissions and local human activities specific to each African region. This could likely serve as a reference to evaluate the current air quality in this region and will be a useful tool in the future to develop pollution mitigation strategies at the source. Recommendations are proposed in the conclusion in order to reduce emissions from their sources, taking into account the low-income African countries.

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

BACKGROUND

Over the past 30 years, various studies in Nigeria have monitored atmospheric particulate matter loads and elemental composition of ambient air around diverse receptor sites.

OBJECTIVES

A comparative discussion of the different sampling techniques, pre-treatment and analytical methods employed between 1985 and 2015.

METHODS

Air pollution indices from studies covered in this review were compared to relevant guideline standards such as the World Health Organization (WHO) 24-hr guideline and the United States Environmental Protection Agency (USEPA)'s National Ambient Air Quality Standard (NAAQS) daily permissible limits for fine particles of less than 2.5 micrometers in diameter (PM2.5) and coarse dust particles with a diameter of 10 micrometers (PM10) in ambient air. In addition, the sources of data for the average concentrations for PM2.5, PM10 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 world.

DISCUSSION

Results showed that PM2.5 concentration ranged from 5-248 μg/m3, while PM10 concentration ranged from 18-926 μg/m3, revealing that about 50% of the particulate matter loads in Nigeria exceeded both the WHO (25 μg/m3, 50 μg/m3) and NAAQS (35 μg/m3, 150 μg/m3) guideline limits for PM2.5 and PM10 respectively. PM2.5/PM10 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 PM2.5 were magnesium (Mg) > strontium (Sr) > potassium (K) > zinc (Zn) > iron (Fe) > sodium (Na) > aluminium (Al) > chlorine (Cl) > lead (Pb) > silicon (Si), while those of PM10 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.

Spatial and temporal variations of the particulate size distribution and chemical composition over Ibadan, Nigeria

Spatial and temporal variation of suspended particulate matter was measured in Ibadan, Nigeria during the raining and dry months of 2013 and 2014, respectively. Six different locations were considered, reflecting city-spread, population density, lifestyle, and vehicular- and industrial-related activities. Elemental characterisation of the samples were carried out using energy dispersive x-ray fluorescence (ED-XRF) spectroscopy, while the black carbon content was determined using an optical transmissometer. Backward trajectory analysis indicated that most air mass was of maritime origin except on few occasions that suggested the Sahara desert origin. Average PM2.5/PM10 ratio for the entire sampling period was 0.32 ± 0.04. This is slightly lower than the reported values in the coastal city of Lagos, partly because of more adequately paved and better road networks resulting in lower dust re-suspension and/or there are relatively finer fraction particulates from other sources, which might include marine, industrial, and secondary emission-related sources. Black carbon showed predominance in the fine fractions and was, majorly, of traffic origin. Major enriched elements of concern are S, Zn, As, and Pb aside from those of sea origin. Refuse burning was observed to be the principal source of Pb and Zn in all the sites except one, which showed major industrial-related activities source. This study will provide data for future measurement and modelling works as well as providing a benchmark for relevant agencies for policy making on setting emission standards for the country.