Cloud condensation nuclei in the Amazon Basin: "Marine" conditions over a continent?

Cloud condensation nuclei (CCN) are linked to radiative forcing, precipitat ion, and cloud structure; yet, their role in tropical climates remains larg ely unknown. CCN concentrations (N-CCN) measured during the wet season in t he Amazon Basin were surprisingly low (mean N-CCN at 1% supersaturation: 26 7 +/- 132 cm(-3)) and resembled concentrations more typical of marine locat ions than most continental sites. At low background CCN concentrations, clo ud properties are more sensitive to an increase in N-CCN. Therefore: enhanc ed aerosol emissions due to human activity in the Amazon Basin may have a s tronger impact on climate than emissions in other continental regions. In s pite of the large organic fraction in the Amazonian aerosol, a detailed ana lysis of number distributions and size-dependent chemical composition indic ates that sulfate plays an important role in CCN activity.