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 NO
in the downwind plume despite previous detection of NO2 above the crater. This suggests that near-source NO
was quickly oxidized to HNO3 and HO2NO2, and probably NO3
(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.