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SEASONAL VARIATION IN ATMOSPHERIC DEPOSITION OF NITRATE, SULPHATE, LEAD, ZINC, AND COPPER IN SOUTHWESTERN NIGERIA

Atmospheric and soil levels of nitrate-nitrogen (NO3-N), sulphate (SO4), lead (Pb), zinc (Zn), and copper (Cu) in southwestern Nigeria were determined. Rainfall and dust samples were collected at 2-week intervals and after a major rainfall and dust storm events at eight locations in a transect from the coast in Badagry to Ilorin, 300 km from the coast. The parameters were evaluated on monthly, seasonal, and annual bases for 2 years. The pH of the deposited materials varied with season and distance from the coast. Higher values were obtained in the dry season and in the hinterland. Soil pH followed a similar trend. The average annual depositions of NO3-N, SO4, Pb, Zn, and Cu ranged from 4.05 to 6.60, 2.58–5.85, 0.60–2.76, 0.96–2.52, and 0.39–2.79 kg ha−1, respectively. Correspondingly the values in soil ranged from 7.45 to 15.65, 3.15–9.50, 1.65–3.35, 1.65–2.80, and 1.75–3.70 kg ha−1. The values also varied with season. Relationships between deposited Pb and Cu and soil values were significant. It was concluded that harmattan dust and/or motorized traffic do contribute significantly to the soil levels of Pb and Cu and less significantly to the levels of pH, NO3-N, SO4, and Zn in the soil.