On bromine, nitrogen oxides and ozone depletion in the tropospheric plume of Erebus volcano (Antarctica)

Since the discovery of bromine oxide (BrO) in volcanic emissions, there has been speculation concerning its role in chemical evolution and notably ozone depletion in volcanic plumes. We report the first measurements using Differential Optical Absorption Spectroscopy (DOAS) of BrO in the tropospheric plume of the persistently degassing Erebus volcano (Antarctica). These are the first observations pertaining to emissions from an alkaline phonolitic magma. The observed BrO/SO 2 ratio of 2.5 × 10 −4 is similar to that measured at andesitic arc volcanoes. The high abundance of BrO is consistent with high abundances of F and Cl relative to sulfur in the Erebus plume. Our estimations of HBr flux and BrO production rate suggest that reactive bromine chemistry can explain a 35% loss of tropospheric O 3 observed in the Erebus plume at approximately 30 km from source ( Oppenheimer et al., 2010). Erebus also has a permanent lava lake, which could result in generation of NO x by thermal fixation of atmospheric N 2 at the hot lava surface. Any NO x emission could play a potent role in reactive bromine chemistry. However, the presence of NO 2 could not be detected in the plume, about 400 m above the lake, in our DOAS observations of 2005. Nor could we reproduce spectroscopic retrievals that reportedly identified NO 2 in DOAS observations from 2003 made of the Erebus plume ( Oppenheimer et al., 2005). Based on the NO 2 detection limit of our analysis, we can state an upper limit of the NO 2/SO 2 ratio of ≤0.012, an order of magnitude lower than previously reported. Our new result supports a rapid oxidation of NO x in the young plume and is more consistent with measurements of NO y species measured using an instrumented aircraft flying in the plume. Model simulations, tuned for Erebus, were performed to reproduce the BrO/SO 2 observed in the young plume and to investigate the impact of NO x emissions at source on the subsequent formation of BrO in the plume. They support our hypothesis of rapid conversion of NO x to NO y in the vicinity of the lava lake. This study thus places new constraints on the interaction between reactive nitrogen and bromine species in volcanic plumes, and its effects on ozone. ► 1st observation of BrO simultaneously to O 3 depletion in a volcanic plume. ► 1st detection of BrO in the plume of Erebus which impacts the Antarctic troposphere. ► 1st detection of BrO for a phonolitic volcano. ► Reactive Br chemistry can cause the ozone depletion observed