PRECIPITATION CHEMISTRY IN INTERTROPICAL AFRICA

Rainwater samples from Kollo (Niger) and Lamto (Ivory Coast) were coll ected during the year 1994. Two phases were analyzed, the dissolved wa s obtained with a 0.2 mu m filtration and the total was obtained after evaporation and an HF:HNO3 digestion. Mg, V, Cr, Mn, Co, Zn, Rb, Sr, Pa, Pb, U, Al, Fe, Cs, REEs and Th were analyzed in both phases and Na , K, Ca and Cl in the dissolved phase. Except for Zn in Lamto with a v alue of 53, enrichment factors EF, calculated with respect to Al norma lization in the total phase, do not show any significant enriched valu es (EF > 10). However, event by event, at low Al amounts, some element s V, Cr, Mn, Co, Zn, Pb and Mg, Rb, Sr and Ba are obviously enriched. Anthropogenic activities for metals and Ba or natural emissions for Mg , Rb, Sr can explain these enriched values. The REE patterns of precip itation particles are almost flat with La/Yb values (16.5-21.5) higher than the upper-crust values(13.6). These values are comparable with t hose measured in suspended sediments from Congo rivers. Dissolved-tota l distributions are: Zn > Sr > Mn > Co > Mg > Ba, Cr > Rb > V > Pb > U > Th > Cs > La > Ce > Al > Fe for Lamto samples and Zn > Sr > Mg > Mn > Rb > Co > Ba > Cr > Pb > V > Cs > Th > U > Al > La > Ce > Fe for Ko llo samples. The percentage of the dissolved fraction is conversely pr oportional to the amount of Al in precipitations. Precipitation partic les in Lamto and Kollo are strongly depleted in Mg, Mn, Rb, Sr and Ba compared to the upper-crust composition. The Rb/La mean ratios of 1.62 in Lamto and 1.70 in Kollo differ From the upper-crust value (3.73) a nd indicate that these particles have already been involved in weather ing processes. All the information obtained in this study shows that t errigeneous particle emission is the main source of trace elements, in this region. Nevertheless, the contribution from other sources (ocean , vegetation, human activities) become evident when the amount of crus tal dust particles in the atmosphere is very low. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.