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The Atmospheric Chemistry of Volcanic Plumes


École Normale Supérieure


Séminaires ENS


Tjarda Roberts

LPC2E-CNRS, Orléans Abstract: Volcanoes release gases and particles to the atmosphere, by explosive eruptions and by continuous passive degassing. The most commonly studied gas is SO2, which becomes oxidized into the atmosphere to form sulfate particles that impact climate. Volcanoes also release halogens (e.g. HBr, HCl, HI). These were assumed to simply be washed out of the atmosphere, but are now known to also undergo oxidation in the plume, as evidenced by observations of volcanic halogen radicals (BrO, OClO). To understand these observations, a suite of numerical models of the volcanic plume chemistry has been developed, from the hot crater to the regional scale. The models show how multi-phase chemical reactions in the plume lead to the formation of BrO and OClO. This volcanic plume halogen chemistry acts to destroy tropospheric ozone. It also impacts NOxy HOx and mercury chemistry, and can critically influence sulfur oxidation processes. The model results demonstrate the need to include volcanic halogens for a complete understanding of the formation of sulfate particles and chemistry-climate impacts of volcanic plumes. To better characterize volcanic emissions, we have applied small low-cost sensors for SO2, H2S, HCl, CO and H2 and particles to measure the plume composition in-situ at the volcano summit. Here, the pollution levels are very high (e.g. tens of ppmv SO2) but decline rapidly as the plume disperses and dilutes in the atmosphere. Ongoing work is advancing these methods for long-term measurements in concentrated and dilute plumes, near and far from volcanoes. However, at low pollution levels, it becomes challenging to accurately measure gas abundances using small sensors. In a project in Fairbanks, Alaska, versions of the sensors for CO, NO, NO2, ozone and particles have been deployed to measure urban pollution. By cross-comparing the sensor output to Air-Quality monitors and developing algorithms of the sensor response functions it becomes possible to accurately measure urban pollutants (down to tens ppbv gas). A field-demonstrated and future application of the small sensors is to deploy on moving platforms to characterize and spatially map pollutant plumes.