Particles and gases in the emissions from the 1980–1981 volcanic eruptions of Mt. St. Helens

Abstract

Detailed airborne measurements of particles and gases were made in the volcanic effluents from Mt. St. Helens from March 1980 through August 1981. The results of these measurements are summarized and are used to deduce emission fluxes of particles and gases from the volcano. Measurements on the periphery of the plume of effluents produced by the violent paroxysmal eruption of May 18, 1980, showed number concentrations of particles <2 μm and >2 μm in diameter up to 1500 and 20,000 times greater, respectively, than in the ambient air. Mass concentrations of particles in the periphery of the plume on May 18 were ∼7000 μg m−3 with particles >2 μm making the dominant contribution. However, the percentage of the total mass in particles <2 μm increased with time after May 18 and was ∼100% of the emissions after June 18, 1980. The fluxes of particles from Mt. St. Helens ranged from 1020 s−1 during the May 18 eruption to 2×1015 s−1 in phreatic emissions. The particulate mass fluxes ranged from 6×104 to 10−2 kg s−1. The total input to the atmosphere of particulate mass from Mt. St. Helens from March 28, 1980, to March 28, 1981, is estimated to be ∼2 Tg. A sometimes substantial, but highly variable, percentage (0–91) of the mass of material in the plume of effluents consisted of soluble halides and sulfates. Fluxes of cloud condensation nuclei (active at 1% supersaturation) in the plume from the volcano ranged from 1016 to 2×1017 s−1. Gases in the effluents included H2O, SO2, H2S, CO2, N2, NO, CO, CH3, Cl, COS, and CS2. An unusually high percentage (35–100) of the sulfur was H2S, particularly during the paroxysmal and intraeruptive periods. Ozone was depressed below ambient values in the plume. SO2‐to‐SO4 = and NO2‐to‐NO3− conversions were observed in the plume at rates of ∼0.1–1% h−1. About 1% of the total material in the tropopause plume from Mt. St. Helens on May 18, 1980, was estimated to have reached the stratosphere. This yields estimates for the stratospheric injections on May 18, 1980, of 3×109 (and 1.5×108) kg of H20, 1.3×105 (and 6×104) kg of particles <2 μm, 1×1018 (and 1×1018) Aitken nuclei, 6×104 (and 3×104) kg of SO2, 3×104 (and 3×104) kg of H2S, 4×109 (and 1×109) kg of CO2 and 3×107 (and 3×107) kg of CO where, in each case, the first number is the total injection (i.e., volcanic plus entrained tropospheric material) and the number in parentheses is the material that derived from the volcano itself.