In situ measurements of tropospheric volcanic plumes in Ecuador and Colombia during TC 4

A NASA DC‐8 research aircraft penetrated tropospheric gas and aerosol plumes sourced from active volcanoes in Ecuador and Colombia during the Tropical Composition, Cloud and Climate Coupling (TC 4 ) mission in July–August 2007. The likely source volcanoes were Tungurahua (Ecuador) and Nevado del Huila (Colombia). The TC 4 data provide rare insight into the chemistry of volcanic plumes in the tropical troposphere and permit a comparison of SO 2 column amounts measured by the Ozone Monitoring Instrument (OMI) on the Aura satellite with in situ SO 2 measurements. Elevated concentrations of SO 2 , sulfate aerosol, and particles were measured by DC‐8 instrumentation in volcanic outflow at altitudes of 3–6 km. Estimated plume ages range from ∼2 h at Huila to ∼22–48 h downwind of Ecuador. The plumes contained sulfate‐rich accumulation mode particles that were variably neutralized and often highly acidic. A significant fraction of supermicron volcanic ash was evident in one plume. In‐plume O 3 concentrations were ∼70%–80% of ambient levels downwind of Ecuador, but data are insufficient to ascribe this to O 3 depletion via reactive halogen chemistry. The TC 4 data record rapid cloud processing of the Huila volcanic plume involving aqueous‐phase oxidation of SO 2 by H 2 O 2 , but overall the data suggest average in‐plume SO 2 to sulfate conversion rates of ∼1%–2% h −1 . SO 2 column amounts measured in the Tungurahua plume (∼0.1–0.2 Dobson units) are commensurate with average SO 2 columns retrieved from OMI measurements in the volcanic outflow region in July 2007. The TC 4 data set provides further evidence of the impact of volcanic emissions on tropospheric acidity and oxidizing capacity.