Compositional and Air-mass Trajectory Analysis of a Heavy Dust Episode (HDE) Aerosols in Ile-Ife, Nigeria

Abstract

The prevalent problem of air pollution in Nigeria is attributable to the country’s huge population, put in excess 160 million and the trend of industrial growth. In 2010, the country witnessed a rare occurrence of 9-day period of heavy dust episode (HDE). The dust reduced visibility to < 1 km, causing cancellation of several flights. The aim of this study is to assess the level of aerosol mass loadings and, nature and sources of the HDE aerosols. Two fractions of particulate matter (PM) were collected for about two months including the period of the HDE. Samples were collected on Whatman polycarbonate filters using low-volume GENT sampler equipped with a stacked filter unit (SFU) to hold two 47 mm filters. 7-day back trajectory analysis was performed using UGAMP trajectory model driven by ECMWF wind analyses data. Morphological analysis of the PM was done using Scanning Electron Microscopy (SEM), chemical compositions determined with Energy Dispersive X-ray (EDX) and particle number per unit area of filter (particle density) was estimated using optical microscopy. The range of mass concentration of PM2.5, PM2.5-10 and PM10 are 1.24 – 58.7, 5.1 – 354.9 and 8.33 – 379.2 µg m-3, respectively. EDX detected twelve (12) elements: Fe, Na, Mg, Al, Si, S, K, P, Cl, Ca, Mo and O. Elements of crustal origin (Si, Al, Fe, Ca and Mg) account for a high percentage of the elemental composition of the PM. Four distinct classes of particles - mineral dust, NaCl containing agglomerate, Calcium-rich dust and alumina-silicate - were identified from the morphological and compositional analysis of the PM. From the backward trajectory analyses, most of the crustal components of the HDE dust can be attributed to long-range dust transport from North Africa and the Sahel region, while maritime aerosols are attributable to the marine environment in the Atlantic, down South.