The cycle of biogenic sulfur compounds over the Amazon Basin. I. Dry season

Abstract

We determined the concentrations of gaseous and particulate sulfur species over the Amazon Basin during July-August 1985. The concentration of dimethylsulfide (DMS) in the mixed layer was near 10 parts per trillion (ppt); methylmercaptan (MeSH) levels were about 1 order of magnitude lower. Above 1000 m, DMS levels decreased rapidly with altitude to about 1-2 ppt at 5 km. At ground level within the forest canopy, concentrations up to 40 ppt were observed. The distribution and diurnal behavior of DMS, MeSH, and hydrogen sulfide in the rain forest canopy and measurements by a flux chamber suggest that both the soil and the tree canopy contribute to the emissions of these gases. The SO2 levels measured over central Amazonia were relatively low: averages were 27 and 18 ppt in the boundary layer and the free troposphere, respectively. Together with the concentrations of aerosol sulfate and methanesulfonate (boundary layer averages: 129 and 6 ppt, respectively), these data suggest that the flux of biogenic sulfur from tropical forests (at least in the Amazon Basin) is lower than has been estimated previously. This is further supported by relatively low concentrations of sulfate in rain collected near Manaus. The sulfur flux from the Amazon rain forests is of the order of 3-8 nmol m-2 min-1; per unit area, it appears to be comparable to the flux from the ocean surface (about 6 nmol m-2 min-1). However, comparison of atmospheric and riverine fluxes of sulfur in the Amazon region suggests that biogenic sulfur cycling through the atmosphere may have considerable geochemical and ecological significance.