Measurement of Volcanic SO2 Concentration in Miyakejima Using Differential Optical Absorption Spectroscopy (DOAS)

摘要

Since the volcanic eruption in 2000, continuous monitoring of sulfur dioxide (SO2) gas has been conducted with in-situ samplers located along the seashore road in Miyakejima, a volcano island around 180 kmsouth of Tokyo. The purpose of these sampling measurements has been to issue warning on the hazardous air pollution to the local residents. Therefore, the resulting data do not provide direct information on pollution levels inside the restricted areas where high concentration of SO2 still takes place frequently. From the ecological point of view, it is desirable to have pollution data covering wider regions of the island. In this paper we report on our differential optical absorption spectroscopy (DOAS) measurements carried out inside the highly-polluted, restricted areas in Miyakejima in December 2009 and September 2010. The system is based on continuous light emitted from a xenon light sources, while detector setups consisting of a telescope and a compact spectrometer detect the light after passing a nearly horizontal optical path of460 m-1300 m. By virtue of the portability of the DOAS observation systems, we achieved the measurement of the concentrations inside the restricted districts in the eastern and southwestern parts of the island. The DOAS results in both of these districts revealed the occurrence of pollution of volcanic gas even when no pollution was observed at nearby sampling stations. In addition, simultaneous measurements with two nearly orthogonal DOAS paths were conducted for examining the spatial distribution of the volcanic gas over the spatial range of several hundred meters. The result of this two paths measurement has indicated the importance of orography, in addition to the wind speed and wind direction, in determining the spatial concentration of SO2 emitted from the volcano crater.