Airborne filter pack measurements of S and Cl in the plume of Redoubt Volcano, Alaska February-May 2009

► Filter pack data from six airborne campaigns at Redoubt volcano, Alaska reported. ► Four S/Cl ratios indicate of magma equilibration depth of 2−5 km. ► S/Cl ratios and COSPEC SO2 rate indicate 1500-3600 T d-1 HCl during dome growth. ► S/Cl ratios measured by filter pack at andesitic volcanoes have a small range of variance. ► Filter pack samples are currently not as useful at andesitic as basaltic volcanoes.