Demonstration That Calibration of the Instrument Response to Polarizations Parallel and Perpendicular to the Object Space Projected Slit of an Imaging Spectrometer Enable Measurement of the Atmospheric Absorption Spectrum in Region of the Weak CO2 Band for the Case of Arbitrary Polarization: Implication for the Geocarb Mission

Abstract:

The Tropospheric Infrared Mapping Spectrometer (TIMS) unit rebuilt to operate in a narrow spectral region, approximately 1603 to 1615 nm, of the weak CO₂ band as described by Kumer et al. (2013, Proc. SPIE 8867, doi:10.1117/12.2022668) was used to conduct the demonstration. An integrating sphere (IS), linear polarizers and quarter wave plate were used to confirm that the instrument’s spectral response to unpolarized light, to 45° linearly polarized light and to circular polarized light are identical. In all these cases the intensity components I_p = I_s where I_p is the component parallel to the object space projected slit and I_s is perpendicular to the slit. In the circular polarized case I_p = I_s in the time averaged sense. The polarizer and IS were used to characterize the ratio R_θ of the instrument response to linearly polarized light at the angle θ relative to parallel from the slit, for increments of θ from 0 to 90°, to that of the unpolarized case. Spectra of diffusely reflected sunlight passed through the polarizer in increments of θ, and divided by the respective R_θ showed identical results, within the noise limit, for solar spectrum multiplied by the atmospheric transmission and convolved by the Instrument Line Shape (ILS). These measurements demonstrate that unknown polarization in the diffusely reflected sunlight on this small spectral range affect only the slow change across the narrow band in spectral response relative to that of unpolarized light and NOT the finely structured / high contrast spectral structure of the CO₂ atmospheric absorption that is used to retrieve the atmospheric content of CO₂. The latter is one of the geoCARB mission objectives (Kumer et al, 2013). The situation is similar for the other three narrow geoCARB bands; O₂ A band 757.9 to 768.6 nm; strong CO₂ band 2045.0 to 2085.0 nm; CH₄ and CO region 2300.6 to 2345.6 nm. Polonsky et al have repeated the mission simulation study doi:10.5194/amt-7-959-2014 assuming no use of a geoCARB depolarizer or polarizer. Enabled by measurement of the geoCARB grating efficiencies the simulated intensities I_sm include the slow polarization induced spectral change across the band. These I_sm are input to the retrieval SW that was used in the original study. There is no significant change to the very positive previous results for the mission objective of gas column retrieval.