Atmospheric evolution of sulfur emissions from Kı̅lauea: real-time measurements of oxidation, dilution, and neutralization within a volcanic plume.

The high atmospheric concentrations of toxic gases, particulate Matter, and acids in the areas' immediately-Surrounding volcanoes can have negative impacts on human and ecological health. To better understand the atmospheric fate of volcanogenic emissions in the near field (in the first few hours after emission), we have carried out realtime measurements of key chemical components of the volcanic plume from IClauea on the Island of Hawan. Measurements were made at two locations, one km northnortheast of the vent and the other 31 km to the southwest, with sampling at each site spanning a range of meteorological conditions and volcanic influence. Ingtrumentation included a sulfur dioxide monitor and an Aerosol Chemical Speciation Monitor, allowing for a measurement of the partitioning between the two major sulfur species (gas-phase SO2 and particulate sulfate) every S min. During trade wind conditions, which sent the plume toward the southwest site, sulfur partitioning exliibited a clear diurnal pattern, indicating photochemical oxidation of SO2 to sulfate; this enabled the quantitative determination of plume age (S h) and instantaneous SO2 oxidation rate (2.4 x 10(-6) at solar noon). Under stagnant conditions near the crater, the extent of SO2 oxidation was Substantially higher, suggesting faster oxidation: The particles within the plume were extremely acidic, with pH values (controlled largely by anibient relative humidity) as low as -0.8 and strong acidity (controlled largely by absolute sulfate levels) up to 2200 nmol/m(3). The high variability of sulfur partitioning and particle composition underscores the chemically dynamic nature of volcanic plumes, which may have important implications for human and ecological health.