Composition and sources of aerosol in a central African rain forest duringthe dry season

During the Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO- 96), size-fractionated aerosol samples were collected in November and Decem ber 1996 at a ground site in the tropical rain forest at the N'doki Nationa l Park (NNP) in the Republic of Congo. The samples were analyzed for up to 26 elements using particle-induced X-ray emission. Elements related to mine ral dust and pyrogenic aerosol exhibited greater concentrations during the daytime, while aerosol produced by the rain forest exhibited higher concent rations at night. Samples were also collected at two levels on the tower, a bove and below the canopy, to characterize vegetation sources. Absolute pri ncipal component analysis (APCA) identified three major aerosol source type s in each size fraction, which explained more than 90% of the data variance . The fine-size fraction contained mineral dust (Al, Si, Ca, Ti, and Fe), p yrogenic (black carbon, K, and Zn), and marine/anthropogenic sulfur compone nts. The coarse-size fraction included a mineral dust (Al, Si, Ca, Ti, Mn, and Fe) and two primary biogenic components consisting of K, P, Zn, and S. Absolute principal component scores were calculated for the components of A PCA, and temporal trends were compared to 7 day isopycnic backward trajecto ries. Consistent relationships between the temporal trends of the fine frac tion aerosol components and meteorological patterns were observed. Trade wi nd air masses transported biomass burning and mineral dust aerosol to NNP d uring the first half of the experiment. The fine fraction sulfur component correlated well with the pyrogenic activity before the change in meteorolog ical patterns halfway through the experiment. The fine and coarse sulfur co ncentrations nearly doubled in the latter part of the experiment as a monso on circulation brought sulfur-enriched aerosol from the Atlantic Ocean. Var ious industrial activities on the coast of Cameroon and Gabon probably cont ributed to the high sulfur concentrations as well.