Remote Measurement of Sulfur Dioxide in a Volcanic Plume using Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS)

摘要

Volcanic plume is a major natural source of atmospheric trace gases, influencing tropospheric and stratospheric trace gas budget. A variety of techniques have been employed to measure trace species in volcanic plumes. Remote sensing techniques have been preferred because they allow the measurement without in-situ sampling near a volcano, which is often impractical or hazardous. Multi-axis differential optical absorption spectroscopy (MAX-DOAS) technique, one of the remote sensing techniques for air quality measurement, uses the scattered sunlight as a light source and measures it at various elevation angles by sequential scanning with a stepper motor. Volcanic plume was observed with a ground-based robust MAX-DOAS instrument from 22 May to 4 June 2004 in Sakurajima, Japan. Measured MAX-DOAS data were analyzed to identify and quantify SO{sub}2 in the volcanic plume utilizing its specific structured absorption features in the ultra violet region (305.5 - 316 nm). Vertical scan through the multiple elevation angles was performed at different azimuth directions perpendicular to the plume dispersion direction to retrieve cross-sectional distribution of SO{sub}2 in the plume based on the measured slant column density (SCD, the integrated concentration along the light path) values. Maximum SCD up to 1.3×10{sup}18 molecules cm{sup}(-2) was observed in the viewing direction toward the center of the plume. Based on the estimated cross section of the volcanic plume total emission strength of SO{sub}2 from the volcano was estimated to be 4.2×10{sup}25 molecules s{sup}(-1). The portable MAX-DOAS instrument can be utilized for remote sensing of vertical profile of air pollutants in a volcanic plume as well as an industrial plume.