Characterization and Source Identification of Ambient Particulate Matter (PM2.5) in the Greater Accra Metropolitan Area, Ghana

Abstract

A 2-year research was conducted to investigate mass concentration, weather impacts, chemical composition and sources of ambient particulate matter (PM2.5) and their fingerprints in the Greater Accra Metropolitan Area, Ghana. Sampling of the PM2.5 fraction of airborne particulate matter was done 24-hourly every six days on a polytetrafluoroethylene (PTFE) filter of pore size 0.2 µm using an ARA N-FRM sampler. The investigation was carried out at three separate sites within the Greater Accra Metropolitan area: Adabraka (AD), Dansoman (DA) and the University of Ghana (UG) spanning January 2021 to December 2022. Gravimetric analysis and energy-dispersive X-ray fluorescence (EDXRF) spectroscopy were employed to determine the mass concentration and characterization of the airborne particulate matter. The samples were positioned in the XRF equipment and subjected to high-energy X-rays. The X-rays excite the atoms in the sample, which makes them give off secondary (fluorescent) X rays that showed what elements are in the sample. X-rays released were detected and analyzed to ascertain the presence of components in the PM2.5 samples. Every element possesses a distinct X ray energy or wavelength, facilitating accurate quantification. The intensity of the fluorescent X rays correlates with the concentration of each element in the sample. Calibration with standards Iron (Fe) and Molybdenum (Mo) were employed to translate observed intensities into quantitative concentrations. The minimum, maximum, and annual mean mass concentration values for PM2.5 obtained at the three locations during the research periods were AD 60.76 µgm-3 (20.70 μg m−3 - 133.07 μg m−3), DA 49.90 µgm-3 (8.32 μg m−3 - 120.59 μg m−3) and UG 42.49 µgm-3 (8.32 μg m−3 - 120.59 μg m−3) respectively, for the 2021 study period. For the 2022 study period, the minimum, maximum, and annual mean mass concentration values were AD (20.79 μg m−3, 162.18 μg m−3, 60.54 μg m−3), DA (16.63 μg m−3, 138.86 μg m−3, 49.28 μg m−3), and UG (12.48 μg m−3, 153.86 μg m−3, 46.37 μg m−3) respectively. These annual mean mass concentration values exceeded the air quality standards of Ghana, as well as those set by the World Health Organization (WHO), the United States Environmental Protection Agency (USEPA), and the European Union (EU). Analysis of meteorological parameters and their possible impacts on particulate matter levels revealed a positive correlation between temperature and PM2.5 mass concentrations (r = 0.23), and negative correlations with relative humidity (r = -0.22), rainfall (r = -0.18), and wind speed (r = 0.31) during the entire study period. These findings underscore the influence of meteorological conditions on PM2.5 concentrations. Notably, the presence of crustal components was highest during the seasonal Harmattan period, from late December to early February, characterized by the transportation of Saharan dust over West Africa. Using the Enrichment Factor (EF) modeling tool, the extent of anthropogenic influence on species present in the observed elemental composition of PM2.5 sampled was assessed. Source apportionment analysis using the U.S. EPA’s positive matrix factorization (PMF) 5.0 model identified and differentiated six anthropogenic sources. The contributions from various sources to pollution levels are as follows: Biomass burning contributes 20% in AD, 20% and 21% in DA, and 17% and 19% in UG. Solid waste burning accounts for 17% and 14% in AD, 7% and 6% in DA, and 19% in UG. Sea salt spray contributes 9% and 7% in AD, 10% and 8% in DA, and 7% and 6% in UG. Vehicular and industrial emissions are significant, with 25% and 28% in AD, 29% and 35% in DA, and 24% and 22% in UG. Soil dust has contributions of 13% in AD, 16% and 11% in DA, and 14% in UG. Lastly, re-suspended dust accounts for 16% and 18% in AD, 18% and 19% in DA, and 19% and 20% in UG. This emphasizes their substantial impacts on PM2.5 levels at the studied sites in Greater Accra Metropolitan Area. This study provides valuable insights into the dynamics of ambient PM2.5 in Greater Accra Metropolitan Area, contributing to a better understanding of air quality management and mitigation strategies in urban environments in sub-Saharan Africa.