Ozone Depletion in Tropospheric Volcanic Plumes: From Halogen-Poor to Halogen-Rich Emissions

Tjarda Roberts

doi:10.3390/geosciences8020068

Tjarda Roberts

Open Access

https://doi.org/10.3390/geosciences8020068

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Abstract

Volcanic halogen emissions to the troposphere undergo a rapid plume chemistry that destroys ozone. Quantifying the impact of volcanic halogens on tropospheric ozone is challenging, only a few observations exist. This study presents measurements of ozone in volcanic plumes from Kīlauea (HI, USA), a low halogen emitter. The results are combined with published data from high halogen emitters (Mt Etna, Italy; Mt Redoubt, AK, USA) to identify controls on plume processes. Ozone was measured during periods of relatively sustained Kīlauea plume exposure, using an Aeroqual instrument deployed alongside Multi-Gas SO2 and H2S sensors. Interferences were accounted for in data post-processing. The volcanic H2S/SO2 molar ratio was quantified as 0.03. At Halema‘uma‘u crater-rim, ozone was close to ambient in the emission plume (at 10 ppmv SO2). Measurements in grounding plume (at 5 ppmv SO2) about 10 km downwind of Pu‘u ‘Ō‘ō showed just slight ozone depletion. These Kīlauea observations contrast with substantial ozone depletion reported at Mt Etna and Mt Redoubt. Analysis of the combined data from these three volcanoes identifies the emitted Br/S as a strong but non-linear control on the rate of ozone depletion. Model simulations of the volcanic plume chemistry highlight that the proportion of HBr converted into reactive bromine is a key control on the efficiency of ozone depletion. This underlines the importance of chemistry in the very near-source plume on the fate and atmospheric impacts of volcanic emissions to the troposphere.

Highlights

Volcanoes release large quantities of gases and aerosols to the atmosphere
The Aeroqual measurements provide a useful constraint on the magnitude of ozone depletion in the Kīlauea volcanic plumes at short distances from the source
The Aeroqual measurements provide a useful constraint on the magnitude of ozone depletion in the Kılauea volcanic plumes at short distances from the source

Summary

Introduction

Volcanoes release large quantities of gases and aerosols to the atmosphere. Very large explosive eruptions inject gases directly to the stratosphere, but a significant number of smaller eruptions and continuously passive degassing volcanoes release their emissions to the troposphere; the total globalSO2 flux from passive degassing over 2004–2016 was recently estimated as 23 Tg/yr on average, exceeding volcanic eruptive emissions by about one order of magnitude [1]. The volcanic release contains a number of other gases and particles, including notably the emission of volcanic halogens such as HBr and HCl (e.g., [6]). These were initially assumed to be washed out of the plume in the troposphere and deposited. Volcanic halogens that reach high altitudes may cause reductions in stratospheric ozone levels This has been both observed [11] and simulated by numerical models (e.g., [12,13]) using atmospheric chemistry schemes that were originally developed to study impacts from anthropogenic sources of halogens (chlorofluorocarbons, CFCs) on stratospheric ozone

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