Abstract
Abstract. The TCCON (Total Carbon Column Observing Network) and most NDACC (Network for Detection of Atmospheric Composition Change) sites assume an ideal ILS (instrumental line shape) for analysis of the spectra. In order to adapt the radiant energy received by the detector, an attenuator or different sizes of field stop can be inserted in the light path. These processes may alter the alignment of a high-resolution FTIR (Fourier transform infrared) spectrometer, and may result in bias due to ILS drift. In this paper, we first investigated the sensitivity of the ILS monitoring with respect to application of different kinds of attenuators for ground-based high-resolution FTIR spectrometers within the TCCON and NDACC networks. Both lamp and sun cell measurements were conducted after the insertion of five different attenuators in front of and behind the interferometer. The ILS characteristics derived from lamp and sun spectra are in good agreement. ILSs deduced from all lamp cell measurements were compared. As a result, the disturbances to the ILS of a high-resolution FTIR spectrometer with respect to the insertion of different attenuators at different positions were quantified. A potential strategy to adapt the incident intensity of a detector was finally deduced.
Highlights
The instrumental line shape (ILS) variation caused by inserting attenuators no. 1– 4 is much less than attenuator no. 5. Both TCCON and NDACC ILSs derived from inserting attenuators no. 1– 4 are close to the ILS derived from the default optical scenario, with a modulation efficiency (ME) amplitude change of < 3 % within OPDmax = 45 cm and < 6 % within OPDmax = 180 cm, respectively
Selecting a smaller entrance aperture to decrease incoming intensities is not optimal since the mechanical errors of different apertures may be non-negligible and inconsistent. This may be different from one instrument to the other; the mechanical consistency of each field stop is recommended to be checked further before being used
We investigated the sensitivity of ILS monitoring for groundbased high-resolution FTIR spectrometers with respect to various typical optical attenuators and positions
Summary
In order to achieve consistent results between different FTIR (Fourier transform infrared) sites, the TCCON (Total Carbon Column Observing Network, http://www.tccon.caltech. edu/) and NDACC (Network for Detection of Atmospheric Composition Change; http://www.ndacc.org/) have developed strict data acquisition and retrieval methods for minimizing the site to site differences (Kurylo, 1991; Davis et al, 2001; Washenfelder, 2006; Schneider et al, 2008; Hannigan and Coffey, 2009; Messerschmidt et al, 2010; Wunch et al, 2010, 2011; Kohlhepp et al, 2011; Hase et al, 2012; Wang et al, 2016). Biases between sites may arise due to the behavior of individual spectrometers, if not properly characterized Some of these differences result from a misalignment of an interferometer, which can change abruptly as a consequence of operator intervention or drift slowly due to mechanical degradation over time The NDACC changes the entrance field stop size if incident radiation changes These processes may alter the alignment of a high-resolution FTIR spectrometer and subsequently result in biases due to ILS drift. We designed experiments to investigate the sensitivity of ILS monitoring for ground-based high-resolution FTIR spectrometers with respect to different optical attenuators
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