Simultaneous observations of stratospheric and mesospheric water vapor isotopomers in a tropical part of Chile

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The aim of this project is to observe temporal and spatial variation of water vapor isotopic ratio in the middle atmosphere in order to understand the formation and transportation mechanism of water vapor. We developed a new radiometer system to obtain mm-wave H_2O (183GHz) and H_2^<18>O (203GHz) spectra simultaneously. The new system was installed at Atacama highland at an altitude of 4800m, since the 183GHz H_2O spectrum is hardly observable in low altitude places. The results through a two-year period of the project are summarized bellow(1)Development of the dual-frequency mm-wave radiometer We designed new radiometer system equipped with two superconductive receivers to observe different water vapor isotopomers simultaneously. The two receivers are set in a common 4-Kelvin cryostat, while the optical paths for the two receivers are completely separated. In the laboratory, we confirmed that the new system was functioning well.(2)H_2^<18>O observation at Las Campanas precedent to Atacama : From September 2003 to February 2004,we carried out continuous observations of H_2^<18>O at Las Campanas Observatory (alt.2,500m) in Chile. Since the spectral intensity of H_2^<18>O is very weak, Ta〜50mK, no such continuous ground-based observation has been ever done. We found that the seasonal variation of H_2^<18>O mixing ratio is similar to that of normal H_2O.(3)183GHz H_2O observation in Atacama : The new radiometer system was installed in Atacama in September 2004,yet the instrumental setup was delayed because of bad working condition and unexpected hardware troubles due to low pressure environment at the site. We did not perform the dual-frequency operation during the project period, whereas we succeeded in detecting 183GHz H_2O spectrum in the single-frequency observation mode. We exhibited the feasibility of water vapor isotopic ratio measurements in Atacama and established the foundations for future observation through this project.