干旱区生态环境知识资源中心

Two episodes of long-range aerosol transport (4000 km) from southern Africa into the central tropical South Atlantic are documented. Stable nitrogen isotope analysis, multielemental analysis, and meteorological observations on local and regional scales are used to describe the observed surface aerosol chemistry during these transport episodes. The chemical, kinematic, and thermodynamic analyses suggest that for the central tropical South Atlantic, west Africa between 0 degrees and 10 degrees S is the primary air mass source region (over 50%) during austral spring. Over 70% of all air arriving in the lower and middle troposphere in the central tropical South Atlantic comes from a broad latitudinal band extending from 20 degrees S to 10 degrees N. Air coming from the east subsides and is trapped below the midlevel and trade wind inversion layers. Air from the west originates at higher levels (500 hPa) and contributes less than 30% of the air masses arriving in the central tropical South Atlantic. The source types of aerosols and precursor trace gases extend over a broad range of biomes from desert and savanna to the rain forest. During austral spring, over this broad region, processes include production from vegetation, soils, and biomass burning. The aerosol composition of air masses over and the atmospheric chemistry of the central South Atlantic is a function of the supply of biogenic, biomass burning, and aeolian emissions from tropical Africa. Rainfall is a common controlling factor for all three sources. Rain, in turn, is governed by the large-scale circulations which show pronounced interannual variability. The field measurements were taken in an extremely dry year and reflect the circulation and transport fields typical of these conditions.