Long-range transport and transformation of volcanic sulfur emissions: A modeling study of the 2020 Nishinoshima event.

Volcanic emissions can undergo substantial chemical transformations during long-range atmospheric transport, potentially influencing air quality in downwind regions. This study quantitatively analyzes a sulfate (SO42-) pollution event in Jeju, South Korea, following the 2020 Nishinoshima volcanic eruption. The transport and transformation of volcanic emissions were simulated using the Community Multiscale Air Quality (CMAQ) model, which revealed three distinct stages in the transport of the volcanic plume: an initial northwestward transport away from the volcano, chemical transformation under stagnant meteorological conditions, and a subsequent stage in which the previously stagnant plume and newly emitted gases were jointly transported toward Jeju. The Integrated Process Rate analysis showed that SO42- formation in both the volcanic source and stagnant areas was primarily governed by aerosol processes, and the Sulfur Tracking Model analysis revealed that these aerosol processes mainly resulted from the gas-phase oxidation of SO2 by hydroxyl (OH) radicals. However, the SO42- production rates attributed to this OH-initiated oxidation differed markedly between the two regions-0.31 μg/m3/h near the volcano and 22.66 μg/m3/h in the stagnant zone-suggesting that rapid SO2 dispersion near the volcano limited chemical transformation, whereas stagnant conditions promoted SO2 accumulation and enhanced oxidation efficiency. As a result, the SO42- formed under these stagnant conditions subsequently recirculated toward the Nishinoshima region and was transported with newly emitted volcanic gases toward Jeju, leading to elevated SO42- levels. Furthermore, altitude-resolved simulations revealed that the relative contributions of emission layers to SO42- concentrations in Jeju evolved over time: lower-layer emissions dominated the early stage, upper-layer emissions became more prominent during the mid-transport phase, and lower-layer contributions regained dominance as the plume approached Jeju. These results demonstrate that the elevated SO42- concentrations observed in Jeju were driven by the combined effects of the vertical structure of volcanic emissions and meteorological conditions. Insights from this case study provide a valuable foundation for future modeling and forecasting of volcanic air quality impacts, emphasizing the importance of incorporating emission altitude and atmospheric dynamics into pollutant transport simulations.