SPATIO-TEMPORAL VARIABILITY OF TRACE METALS IN ATMOSPHERIC FINE PARTICULATE MATTER FROM SELECTED INDUSTRIAL SITES IN OGUN STATE, NIGERIA

摘要

The spatial and temporal variations of fine particulate matter bound trace metals were investigated in three industrial estates namely Ota Industrial Estate (OTE), Ewekoro Community (EWC) and Agbara Industrial Estate (AGE) and one background (Covenant University Farm Ota) in Ogun State. The sampling period was six (6) months covering the wet and dry seasons. One hundred and fifty (150) samples were collected according to standard methods using environtech gravimetric sampler, after which fine particulate matter (PM2.5) mass concentration was computed gravimetrically. The total trace metals and chemical sequential extraction of twenty-two (22) trace metals (As, Cr, Pb, Ni, Cd, Mn, Ti, V, Ba, Fe, Al, Mo, Ca, Zn, Cu, Mg, Sr, Se, Ag, K, Na, and Sb) were further analysed by inductively coupled plasma mass spectrometry. Daily mean meteorological parameters for the study period were obtained from Nigerian Meteorological Agency. Cumulative lifetime cancer risk, air quality index and source apportionment of pollutants were also assessed. Spatio-temporal mass distribution of PM2.5 in the investigated sites indicated that the average annual PM2.5 mass concentrations were fifteen to twenty six (15-26) times and twelve to twenty two (12-22) times higher than the current annual World Health Organisation (WHO) and United States Environmental Protection Agency (US EPA) guideline of 10 and 12 μg/m3 respectively. OTE and CUF recorded the highest and lowest PM2.5 levels across seasons respectively while distinct seasonal variations trend showed dry > wet. Comparison of the annual average concentrations of Pb, As, Cd, Cr, Ni, Mn, V trace metals to the appropriate standard limit values reveals that out of these 7 trace metals, only V (0.199 - 0.862 μgm-3) was below the existing WHO limit of one (1 μg/m3).The meteorological parameter results revealed that relative humidity and rainfall significantly decreased the accumulation of fine particles whereas increased wind speed and temperature, showed a corresponding increase in the concentration of PM2.5. Speciation and bioavailability studies identified Cr, V, Cu, Cd and Cr, V Cu, Zn as the most labile metals present in OTE and EWC respectively. The sum of the cumulative lifetime cancer risk of inhalation exposure route was above EPA’s acceptable risk range of 1 × 10-6 to 1× 10-4 for both adults and children with the highest occurrence (9.53 × 10-2) in Ewekoro community. The absence of 0 to 50 gradations representing the good air quality index category was conspicuously highlighted. Scanning electron microscopy images of fine particles indicate that they are clustered into the following groups: soil dust mineral, soot aggregation, alumino silicate and fly ash particles. Hierarchical agglomerative clustering analysis of fine particles indicated major clustered groups while source identification using principle component analysis identified the origin of this source such as industrial processes, vehicle emissions, re-suspended and crustal dust, biomass burning, solid waste combustion and coal oil combustion. To improve on the present status of air quality, Nigerian Environmental Standards and Regulations Enforcement Agency (NESREA) should enforce environmental laws, ensure that all industries adopt standard emission control strategies and encourage the cultivation of green belts.