Characterization and Sources of Air Particulate Matter at Kwabenya, Near Accra, Ghana

Abstract

Gravimetric, reflectometric and elemental analyses have been carried out on airborne particulate matter sampled in a semi-rural area of Kwabenya, near Accra-Ghana. The PM 10 aerosols were sampled using a Gent sampler, size segregating the aerosol into coarse (PM10.2.5) and fine (PM2.5) fractions. The data and derived information were generated from 216 days of sampling spanning a period of about 14 months, 28th December 2005 to 12th February 2007. The particulate matter (PM) at Kwabenya was dominated by the coarse particulates and showed low levels during the Rainy season and high levels during the Harmattan period. The levels measured during the 2006/07 Harmattan were very high. The mass concentration for the measuring period were in the following ranges; coarse (PM10-2 .5 ) fraction (0.16 - 1794.01 ^ig/m3); PM2 .s(fme) fraction (0.50 - 430.23 ng/m3) and PM10 (0.87 ng/m3 to 2064.89 |ig/m3). Additional information about the ambient air was obtained through the subsequent determination of elemental concentration using energy dispersive x-ray fluorescence (EDXRF) analygia^and black carbon (BC) concentration through the “black smoke method”. The/^lpfnents iafe^ti|led and quantified with the Quantitative X-ray Analysis System (QXAS^ package software were: Al, Si, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Br, Rb, Sr an^ i^ itL ^ i^ co a rse fraction. The following elements were identified and quantified in the fine fraction: Al, Si, S, K, Ca, Ti, Mn, Fe, Cu, Zn, Br, Rb, Sr and Pb. Validation of the quantitative methods with the standard reference filter SRM2783 gave very good agreement (within ±15%) for most elements analysed except for Ni (±43%)which was very close to the detection limit. The elemental concentrations in the two fractions vary from season to season. Using simple correlation analysis some elements correlate, the elemental correlations also vary from season to season, for example during the Harmattan S, Cl, V, Br and Sr correlated very well but during the Rainy season S did not correlate with V and Br. This could serve as possible source indicators. The BC concentration in the fine fraction (ranging from 0.01 to 5.97 figm"3) was generally higher than in the coarse fraction and comparable to data from some developed countries. A receptor model using principal component and regression analysis was used to identify sources contributing to the air particulate matter at Kwabenya. The species used in the model were mass, BC and elemental concentrations. The following major sources were identified in the coarse aerosol: Soil/Dust, Biomass/LDT and Sea aerosol. In the fine aerosol the following sources were identified: Soil/Dust, Biomass/LDT and some industrial sources. The contribution of the sources to the PM load varied from season to season, There was very good agreement between the experimental and model data (mass, BC and elemental concentrations). Comparing the data with WHO limit (50 ^grrf3 for 24-hour mean) and Ghana EPA guideline limit (70 Hgm' 3 for 24 hours) for PMio, a total of 185 and 130 days respectively out of 216 days had values above these limits. For PM2 5 a total of 60 days had values exceeding the WHO limit (25 (agm' 3 for 24-hour mean). The levels of S, Ni and Pb were also comparable to industralised countries. There is the need for some mitigation measures to curb the emission of these elements and fine BC.