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

Cloud condensation nuclei (CCN) are linked to radiative forcing, precipitation, and cloud structure; yet, their role in tropical climates remains largely unknown. CCN concentrations (N-CCN) measured during the wet season in the Amazon Basin were surprisingly low (mean N-CCN at 1% supersaturation: 267 +/- 132 cm(-3)) and resembled concentrations more typical of marine locations than most continental sites. At low background CCN concentrations, cloud properties are more sensitive to an increase in N-CCN. Therefore: enhanced aerosol emissions due to human activity in the Amazon Basin may have a stronger impact on climate than emissions in other continental regions. In spite of the large organic fraction in the Amazonian aerosol, a detailed analysis of number distributions and size-dependent chemical composition indicates that sulfate plays an important role in CCN activity.