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Volcanic aerosols

Par ECOLA Dernière modification 25/08/2014 17:32

Volcanic Aerosols in the Southwest Pacific


Researchers: J. Lefevre, P. Marchesiello, C. Menkes

The Melanesian Volcanic Arc (MVA) emits about 12 kT(SO2).d-1 from continuous passive (non-explosive) volcanic degassing, which makes 20% of global sulfur dioxide emission from volcanoes. Here, we assess from up-to-date and long-term observations the volcanic SO2 emission of Ambrym, one of the dominant volcanoes in the MVA, and we investigate the role of its continuous release of sulfate precursors on the regional distribution of aerosols, using both satellite observations and WRF-Chem/GOCART simulations.

Schematic drawing of aerosols in SPCZ

(c) Jérome Lefèvre

 Our choice of model parameterizations for convection, vertical mixing and cloud properties provides a reliable chemical weather representation that allows a cross-examination of model solution and observations. This work enables the identification of biases and limitations affecting both the model (missing sources) and satellite sensors and algorithms (for aerosol detection and classification) and leads to the implementation of improved transport and aerosol processes in the modeling system.

On one hand, the model confirms a 50% underestimation of SO2 emissions due to satellite swath sampling of the Ozone Monitoring Instrument (OMI), in agreement with field studies. The OMI irregular sampling also produces a level of noise that impairs its surveillance capacity during short-term volcanic events. On the other hand, the model reveals a large sensitivity on aerosol composition and Aerosol Optical Depth (AOD) due to choices of both the source function in WRF-Chem and size parameters for sea-salt in FlexAOD, the post-processor used to compute offline the simulated AOD 

We then proceeded to diagnose the role of SO2 volcanic emission in the regional aerosol composition. The model shows that transport processes and cloud properties associated with the South Pacific Convergence Zone (SPCZ) have a large influence on the oxidation of SO2 (lifetime ~20 h) and on the transport pathways of volcanic species across the Pacific atmosphere. Turbulent mixing and convective transport drive their venting above the trade inversion, while the large-scale circulation in the free troposphere drives their long-range transport toward the Maritime Continent and the South Pacific. In the tropical cloudy air, the sulfate production in the aqueous phase is very efficient, resulting in the formation of a large cloud of highly scattering sulfate aerosols, also very well captured by MODIS but misclassified by CALIOP. By contrast, drier conditions in regions of large-scale subsidence promote the formation of sulfate aerosols in the gas phase. Model sensitivity experiments indicate that aerosol resuspension due to evaporating droplets is a significant pathway for the supply of volcanic sulfur species in the remote marine boundary layer. By strongly modulating the irreversible loss due to wet scavenging, this aerosol process has a similar influence on the sulfur burden as natural emission from volcanoes or biogenic sources like DMS. The results emphasize the importance of MVA passive degassing and SPCZ dynamics on the atmospheric distribution and environmental impact of volcanic volatiles including reactive halogens at local to regional scales.

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