Ground-based Millimeter-wave Measurements Of Vertical H218O Distribution At Atacama, Chile

We present first results on continuous measurements of vertical distribution of H218O with a ground- based millimeter-wave radiometer operated at Atacama in Chile (23S, 68W, Alt. 4800 m). The aim of the H218O measurements is to understand the detail mechanism of water vapor variations in the stratosphere and mesosphere, because the stratospheric water vapor is a source of HOx and affects various influences on chemical compositions such as a possible delaying of the expected recovery of the ozone layer. For this purpose, under support by SORST Program of JST, we started a project to observe the stratospheric and mesospheric water vapor and its isotopes continuously by using the ground-based millimeter-wave radiometer install at Atacama in 2005, and have been measuring the H218O spectra at 203.407 GHz since 2006. The radiometer is equipped with a superconducting (SIS) mixer receiver that provides an extremely low noise temperature of 150 K (DSB) and with an acousto-optical spectrometer whose bandwidth and effective frequency resolution is 1 GHz and 1 MHz, respectively. The spectrum was obtained every 10 minutes, and the vertical profile of H218O and O3 from 30 to 70 km was retrieved from the 6-h averaged spectrum by using the optimal estimation method. The retrieved vertical profile of H218O is similar to the previous measurements with other methods. In this presentation, we will report on details of the observations, retrieval results and temporal variations of H218O in the stratosphere and mesosphere as well as a discussion on the vertical distribution of the 18O/16O ratio estimated from the retrieved H218O profiles.