Water-soluble ions in airborne particles over three South American urban areas - Vasconcellos, Pérola de Castro; Souza, Davi Zacarias de; Araújo, Maria Perpétua Batista Monteiro de; Cordova, Ana Maria Leal Cordova; Beherentz, Eduardo; Ccoyllo, Odon Sanchez (South American Emissions, Megacities and Climate Workshop, 2008)