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 are reported here. These measurements provide a rare constraint on Cl output from an andesitic eruption at high emission rate (> 10⁴ t d^− 1 SO₂). Four S/Cl ratios measured during a period of lava dome growth indicate a depth of last magma equilibration of 2–5 km. The S/Cl ratios in combination with COSPEC SO₂ emission rate measurements indicate HCl emission rates of 1500–3600 t d^− 1 during dome growth. SO₂ and HCl emission rates at Redoubt Volcano correlate with each other and were low prior to the eruption, high during the eruption, and low after the eruption. S/Cl ratios measured by filter pack at andesitic volcanoes have a small range of variance, with no clear trends seen for eruptive versus passive activity. The very few S/Cl ratio measurements by filter pack at andesitic volcanoes are not as predictive of future volcanic activity as has been demonstrated for basaltic volcanoes. This may be because there are so few of these measurements. We have demonstrated it is possible to collect these samples by air between explosions during lava dome-building eruptions. We recommend more filter pack sampling be performed at andesitic volcanoes to determine the technique's utility for volcano monitoring. Filter pack data has been demonstrated to be useful for calculating the depth of magma equilibration at volcanoes including Redoubt Volcano.