Atmospheric chemistry of an Antarctic volcanic plume

We report measurements of the atmospheric plume emitted by Erebus volcano, Antarctica, renowned for its persistent lava lake. The observations were made in December 2005 both at source, with an infrared spectrometer sited on the crater rim, and up to 56 km downwind, using a Twin Otter aircraft; with the two different measurement platforms, plume ages were sampled ranging from <1 min to as long as 9 h. Three species (CO, carbonyl sulfide (OCS), and SO₂) were measured from both air and ground. While CO and OCS were conserved in the plume, consistent with their long atmospheric lifetimes, the downwind measurements indicate a SO₂/CO ratio about 20% of that observed at the crater rim, suggesting rapid chemical conversion of SO₂. The aircraft measurements also identify volcanogenic H₂SO₄, HNO₃ and, recognized for the first time in a volcanic plume, HO₂NO₂. We did not find NOx in the downwind plume despite previous detection of NO₂ above the crater. This suggests that near-source NOx was quickly oxidized to HNO₃ and HO₂NO₂, and probably NO32 (aq), possibly in tandem with the conversion of SO₂ to sulfate. These fast processes may have been facilitated by "cloud processing" in the dense plume immediately downwind from the crater. A further striking observation was O₃ depletion of up to ~35% in parts of the downwind plume. This is likely to be due to the presence of reactive halogens (BrO and ClO) formed through heterogeneous processes in the young plume. Our analysis adds to the growing evidence for the tropospheric reactivity of volcanic plumes and shows that Erebus volcano has a significant impact on Antarctic atmospheric chemistry, at least locally in the Southern Ross Sea area.