On the primary factor causing volcanic H₂S and CO₂ accumulation in the lower atmosphere

Abstract

The volcanic gas hazards due to accumulation of volcanic hydrogen sulfide and / or carbon dioxide were often attributed to the molecular weights of the volcanic gases higher than air. This interpretation, however, is inconsistent with the general knowledge in the field of atmosphere science that eddy diffusion is much faster than molecular diffusion in troposphere. To clarify the process controlling the gas diffusion is essential for preventing the volcanic gas hazards. Therefore, in this study, we measured the vertical distribution of the volcanic gases showing molecular weights significantly different from each other, such as molecular hydrogen, carbon dioxide, and hydrogen sulfide simultaneously in a fumarolic field to clarify whether the chemical fractionation due to molecular diffusion is significant or not. In addition, we also evaluated the influence of cooling the volcanic gases on the vertical distribution. Our observation on a fumarolic field of Hakone Kamiyuba area in June 2025 revealed that the molecular diffusion had little influence on the vertical distribution, supporting that the eddy diffusion was the primary factor controlling the diffusion of volcanic gases. Furthermore, we found that cooling promoted the accumulation of volcanic gases in the lower atmosphere close to the ground.