In-situ aircraft observations of the 2000 Mt. Hekla volcanic cloud: Composition and chemical evolution in the Arctic lower stratosphere

An instrumented NASA aircraft made comprehensive, in-situ measurements of trace gas concentrations and aerosol properties while flying through the eruptive cloud from Mt. Hekla in February and March, 2000. The data reveal novel aspects of the properties and evolution of the volcanic cloud in the lower arctic stratosphere. Thirty-five hours after the initial, sub-Plinian explosion on February 26, 2000, the aircraft intersected the cloud at an altitude of 11.3 km. SO₂ concentrations in the cloud exceeded 1 ppmv, but no H₂S was observed. Large HF concentrations of ∼50 ppbv were nearly equal to the HCl concentration, the same ratio of halogen species adsorbed on fallen ash. Although reactive nitrogen species are rarely detected in volcanic clouds, significant HNO₃ concentrations of 3 ppbv above background were measured. A bimodal aerosol size distribution with total number densities exceeding 8000 particles/cm³ and total aerosol volume of 65 μm³/cm³ was observed. Approximately 1/3 of the fine aerosol particles were non-volatile (volcanic ash) and the remaining 2/3 were volatile (sulfate aerosol and ice). The volcanic cloud was highly structured with clearly delineated boundaries. In the 18-day period following the initial eruption, increases in SO₂, sulfate aerosol, HCl, and HF volume mixing ratios were again detected. Analysis of the partitioning of sulfur between the gas and aerosol phases in these later cloud encounters shows that the rate of SO₂ oxidation to sulfuric acid was broadly consistent with changing OH concentrations at the time of the vernal equinox.