Precise measurement of CO_2 from space using Fabry-Perot based optical setup - current status and development

摘要

The FPICC (Fabry-Perot Interferometer for Column CO_2) is a new instrument developed under the Instrument Incubator Program that uses a novel technique for measuring the absorption of CO_2 sunlight reflected from the Earth. The optical setup consists of three channels. The first channel is built to measure carbon dioxide by using a solid Fabry-Perot etalon to restrict the measurement to light in CO_2 absorption bands. The second and third channels focus on the O_2 A band (759-771 nm) composed of about 300 absorption lines, which vary in strength and width according to pressure and temperature. We performed measurements using solid Fabry-Perot etalons with different FSR and different pre-filters. We demonstrated the instrument's significant capability to detect CO_2 and O_2 in laboratory, as well as in ground based and airborne experiments. The initial tests indicate that when the instrument is used with a sun tracker the sensitivity for CO_2 detection is 2.1 ppm in one second average, and the sensitivity to the oxygen column pressure changes is as low as 0.88 mbar. The reduced sensitivity for the airborne experiments arises because the atmospheric scattering processes make the path length more variable and uncertain. One solution to this problem is to use the glint reflection from water surfaces. For this purpose we design and perform a theoretical study to build a different version of the FPICC instrument to be used on a satellite orbiting the Earth and working in a glint mode. This Fabry-Perot based technique is applicable to other species as well. For example one could use the FPICC instrument for fractionations measurements of the stable carbon isotope (~(13)C/~(12)C). The instrument can be used to study the atmosphere of Mars, which consists primarily of CO_2. A theoretical study and design of a version of the instrument for Mars for CO_2 and CH_4 measurements will be presented. We report results on the recent calibration of the instrument, recent data from ground tests at Goddard, design versions, and theoretical models for the Earth and Mars instruments.