HR: 14:55h
AN: V12E-06    [PDF]
TI: Mixing Ratios of Br, BrO, Cl, and ClO in High-Temperature Volcanic Gases
AU: * Gerlach, T M
EM: tgerlach@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory 1300 SE Cardinal Ct, #100, Vancouver, WA 98683-9589 United States
AB: Bobrowski et al. recently reported 1-ppb levels of BrO gas in plume 4-7 km downwind of the summit of Soufriere Hills volcano, Montserrat (Nature, 15 May 2003, v. 423, p. 273-276). This first detection of BrO in volcanic plume is potentially important evidence of halogen-catalyzed tropospheric ozone destruction. Bobrowski et al. conclude that volcanoes either directly emit BrO or emit bromine species that are rapidly converted to reactive bromine in volcanic plume. Thermodynamic speciation models show that HBr, HCl, and HF are by far the dominant halogen species emitted at $900-1200\deg$C by degassing magma of arc, divergent plate, and hot spot volcanoes; they have molar mixing ratios exceeding those of BrO, ClO, and FO by factors $>$10$^{8}$; the predicted mixing ratios of BrO, ClO, and FO are generally $<$10$^{-13}$. Direct magma degassing is therefore not the source of the observed 1-ppb levels of BrO in the Soufriere Hills plume, nor is it likely to be a significant source of ClO. Furthermore, attempts to enhance BrO by high-temperature mixing of volcanic gas and air, as can occur in close proximity to erupting magma and by advection of air through hot volcanic domes, failed to boost BrO levels above 1 ppb in the speciation models. Thus it does not appear that the volcano is in any way the direct source of BrO observed in the Soufriere Hills plume, and alternatives for its origin need to be considered. For example, volcanoes may emit bromine and chlorine species that can be converted rapidly to reactive Br and Cl by gas phase and/or heterogeneous reactions in the volcanic plume. The conversion rates of HBr and HCl, which clearly are emitted by degassing magma, to reactive Br and Cl may be enhanced by relatively high mixing ratios for OH in volcanic plumes; speciation models give OH mixing ratios for degassing magma in the range 10$^{-8}$ to 10$^{-5}$, compared to typical background atmospheric values of 10$^{-14}$. Speciation models also indicate that direct magma degassing of reactive Br and Cl as atomic Br and Cl at mixing ratios of 10 to 300 ppb is plausible; atomic Br and Cl mixing ratios increase dramatically to levels of 2500 to 6500 ppb in high-temperature volcanic gas/air mixtures containing $\sim$10% air; the models predict no significant reactive F species. Direct volcano degassing of atomic Br and Cl could give rise to BrO and ClO downwind by reaction with O$_{3}$; this may account for the observed 1-ppm levels of BrO in the Soufriere Hills plume. Although the mixing ratios of reactive Br and Cl are several orders of magnitude lower than those of HBr and HCl in magma degassing, they can be greatly increased by oxidation from mixing with air, and unlike HBr and HCl, they are not susceptible to scrubbing. Their role in the destruction of tropospheric and stratospheric ozone during volcanism may therefore merit further consideration.
DE: 8409 Atmospheric effects (0370)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting