Meeting the Requirements for High Quality Measurements of the Water Vapor Continuum in the Spectral Region of 700 - 2000 cm-1

Abstract

The water vapor continuum in the spectroscopic window around 1000 cm-1 makes an important contribution to the radiative energy budget of the Earth1. Furthermore, it is important for remote sensing, especially in case of Nadir sounding. Currently, only few spectroscopic reference data for conditions close to the Earth’s atmosphere exist in this spectral region. Also, the understanding of the physical nature of the water vapor continuum is not complete. To make a significant contribution to this field, water vapor continuum measurements in a broad region of 700 - 2000 cm-1 shall be conducted using high resolution FTS and a multireflection cell, which allows an absorption path of up to 160 m. Hereby it is possible to cover the spectral range of the strong v2 water band absorption together with an atmospheric window, and to investigate the foreign and the self-continuum of water. The line parameters of water will also be determined since they are needed to distinguish between continuum and local line contributions. Series with different pressures as well as temperatures down to -40 °C and up to 80 °C are planned. We present requirements for the proposed high quality water vapor measurements and show which efforts have been made to ensure and monitor baseline stability as well as to solve specific problems of the spectral range under investigation: For continuum measurements, an accurate reference measurement is of high importance as the baseline of an actual water vapor measurement cannot be distinguished from the continuum. Therefore, the quality of continuum measurements is particularly limited by the baseline stability and reproducibility. The baseline stability might for example be disturbed by influences on the instrument’s adjustment due to alignment changes in the multi reflection cell, caused by the different conditions between sample and reference measurements. Especially in the spectral region of interest, further matters like the non-linear behavior of the detector or the thermal self-radiation of the instrument arise and need to be handled.