Improved representation of volcanic sulfur dioxide depletion in Lagrangian transport simulations: a case study with MPTRAC v2.4

The lifetime of sulfur dioxide (SO2) in the Earth's atmosphere varies from orders of hours to weeks, mainly depending on whether cloud water is present or not. The volcanic eruption on Ambae Island, Vanuatu, in July 2018 injected a large amount of SO2 into the upper troposphere and lower stratosphere (UT/LS) region with abundant cloud cover. In-cloud removal is therefore expected to play an important role during long-range transport and dispersion of SO2. In order to better represent the rapid decay processes of SO2 observed by the Atmospheric Infrared Sounder (AIRS) and the TROPOspheric Monitoring Instrument (TROPOMI) in Lagrangian transport simulations, we simulate the SO2 decay in a more realistic manner compared to our earlier work, considering gas-phase hydroxyl (OH) chemistry, aqueous-phase hydrogen peroxide (H2O2) chemistry, wet deposition, and convection. The either newly developed or improved chemical and physical modules are implemented in the Lagrangian transport model Massive-Parallel Trajectory Calculations (MPTRAC) and tested in a case study for the July 2018 Ambae eruption. To access the dependencies of the SO2 lifetime on the complex atmospheric conditions, sensitivity tests are conducted by tuning the control parameters, e.g., by changing the release height, the predefined OH climatology data, the cloud pH value, the cloud cover, and other variables. Wet deposition and aqueous-phase H2O2 oxidation remarkably increased the decay rate of the SO2 total mass, which leads to a rapid and more realistic depletion of the Ambae plume. The improved representation of chemical and physical SO2 loss processes described here is expected to lead to more realistic Lagrangian transport simulations of volcanic eruption events with MPTRAC in future work