The tropospheric processing of acidic gases and hydrogen sulphide in volcanic gas plumes as inferred from field and model investigations

摘要

Improving the constraints on the atmospheric fate and depletion rates ofacidic compounds persistently emitted by non-erupting (quiescent) volcanoesis important for quantitatively predicting the environmental impact ofvolcanic gas plumes. Here, we present new experimental data coupled withmodelling studies to investigate the chemical processing of acidicvolcanogenic species during tropospheric dispersion. Diffusive tube samplerswere deployed at Mount Etna, a very active open-conduit basaltic volcano ineastern Sicily, and Vulcano Island, a closed-conduit quiescent volcano inthe Aeolian Islands (northern Sicily). Sulphur dioxide (SO₂), hydrogensulphide (H₂S), hydrogen chloride (HCl) and hydrogen fluoride (HF)concentrations in the volcanic plumes (typically several minutes to a fewhours old) were repeatedly determined at distances from the summit ventsranging from 0.1 to ~10 km, and under different environmentalconditions. At both volcanoes, acidic gas concentrations were found todecrease exponentially with distance from the summit vents (e.g., SO₂decreases from ~10 000 μg/m3at 0.1 km from Etna's ventsdown to ~7 μg/m3 at ~10 km distance),reflecting the atmospheric dilution of the plume within the acid gas-freebackground troposphere. Conversely, SO₂/HCl, SO₂/HF, andSO₂/H₂S ratios in the plume showed no systematic changes withplume aging, and fit source compositions within analytical error. Assumingthat SO₂ losses by reaction are small during short-range atmospherictransport within quiescent (ash-free) volcanic plumes, our observationssuggest that, for these short transport distances, atmospheric reactions forH₂S and halogens are also negligible. The one-dimensional model MISTRAwas used to simulate quantitatively the evolution of halogen and sulphurcompounds in the plume of Mt. Etna. Model predictions support the hypothesisof minor HCl chemical processing during plume transport, at least incloud-free conditions. Larger variations in the modelled SO₂/HCl ratioswere predicted under cloudy conditions, due to heterogeneous chlorinecycling in the aerosol phase. The modelled evolution of theSO₂/H₂S ratios is found to be substantially dependent on whetheror not the interactions of H₂S with halogens are included in the model.In the former case, H₂S is assumed to be oxidized in the atmospheremainly by OH, which results in minor chemical loss for H₂S during plumeaging and produces a fair match between modelled and measuredSO₂/H₂S ratios. In the latter case, fast oxidation of H₂S byCl leads to H₂S chemical lifetimes in the early plume of a few seconds,and thus SO₂ to H₂S ratios that increase sharply during plumetransport. This disagreement between modelled and observed plumecompositions suggests that more in-detail kinetic investigations arerequired for a proper evaluation of H₂S chemical processing in volcanicplumes.