Modeling of hydroxyl oxidation and wet deposition in volcanic sulfur dioxide plumes: the July 2018 Ambae eruption

Sulfur dioxide (SO₂) is one of the most abundant gases released by volcanic eruptions. It is of concern due to its potential to influence climate on the global scale. Next to advection and diffusion, various chemical and geophysical processes need to be considered to properly represent volcanic SO₂ plumes in Lagrangian transport simulations. The chemical reaction of SO₂ with the hydroxyl radical (OH) and wet deposition are major processes depleting SO₂ from volcanic plumes. In this study, we implemented and revised new modules for OH chemistry and wet deposition in the Massive Parallel Trajectory Calculations (MPTRAC) Lagrangian transport model. In MPTRAC, the OH chemistry module adopts the newest JPL data evaluation to calculate the temperature- and pressure-dependent reaction coefficient of the termolecular reaction of SO₂ and OH. Similar to other Lagrangian chemistry transport models, monthly mean zonal mean OH fields are applied in the MPTRAC simulations. However, here we propose to introduce a correction factor depending on the solar zenith angle in order to represent the diurnal variations of the OH concentrations. The wet deposition module of MPTRAC mainly uses cloud ice and liquid water content retrieved from ECMWF’s meteorological data as well as an effective Henry constant, considering both, the dissociation and dissolution of SO₂ in cloud water. The revised and improved MPTRAC model is evaluated in the case study on the July 2018 eruption of Ambae, Vanuatu, the most voluminous volcano of the New Hebrides archipelago. It is reported that during the eruption, the island of Ambae suffered from acid rain, which means that the volcanic plume encountered clouds and significant wet deposition of SO₂ due to precipitation. A series of sensitivity tests was conducted to assess the two new chemistry modules of MPTRAC. An inspection of the time series of SO₂ total mass revealed that the diurnal variations due to the OH chemistry are now properly represented in the model. The trajectories of the SO₂ parcels have large influence on the wet deposition because they determine whether they pass through cloudy regions or not at a certain time and location. Therefore, it is important to use the most accurate meteorological data and source information. With the revised MPTRAC model, we found that a significant part of the loss of SO₂ total mass from the Ambae eruption is represented in the simulations. However, satellite observations reveal even shorter tropospheric lifetimes of the volcanic SO₂ plume from the Ambae eruption, which indicates that future work needs to focus on including additional chemical and geophysical processes in the simulations.