Size distribution of biogenic aerosol particles from the amazon basin

Abstract

The size distribution and elemental composition of natural background aerosol particles from the tropical rain forest of the Amazon Basin were measured. A 45 m high tower installed in an undisturbed forest near Manaus was used and aerosol was sampled in three levels of the tower. Single orifice Batelle-type cascade impactors with 5 stages and backup filter were used. A total of 50 complete cascade impactor sets was sampled in April and May 1987, during the wet season, when no forest burning occurs. Particle-induced X-ray emission (PIXE) was used to measure elemental concentrations of 20 elements (Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Br, Rb, Sr, Zr and Pb). Absolute factor analysis was used to interpret the large data set of the trace element concentrations and to obtain quantitative elemental source profiles. The identified components in all size ranges comprise biogenic aerosol naturally released by the forest, mineral dust aerosol particles and sea salt. The elements associated with the biogenic component were potassium, phosphorus, sulphur, zinc, strontium and others in smaller proportions. The mineral dust component comprises most of the concentration of aluminium, silicon, calcium, titanium, manganese and iron. Chlorine was found to be associated with the aeolean dust component because of the long-range transport of Sahara dust over the Atlantic ocean, bringing Cl together with mineral dust particles. The size distribution of the elements associated with the windblown dust (Al, Si, Ca, Ti, Mn, Fe and Cl) shows in the coarse mode, with an average aerodynamic diameter of 4 μm. Two different biogenic aerosol components were observed. A fine mode component ( d 50 ≈ 0.5 μm), characterised by the the elements S, Zn and Sr and the mode biogenic component ( d 50 ≈ 3.0 μm) characterised by the presence of P, K, Cl and Sr. The coarse biogenic (P, K, Cl, Sr) component is predominant at ground level under the forest canopy. The fine mode biogenic component (S, Zn, Sr) is higher at the top level of the tower.