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). Interferograms are acquired with similar instruments operated with common detectors, acquisition electronics, and/or optical filters. These interferograms are first converted to spectra and later to retrieved products using dedicated processing algorithms, i.e., GFIT, PROFFIT, or SFIT (Hase et al., 2006; Hannigan and Coffey, 2009; Wunch et al., 2010, 2015). However, 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 (Olsen et al., 2004; Miller et al., 2007; Duchatelet et al., 2010; Hase et al., 2013; Feist et al., 2016). These misalignment effects can be diagnosed via the monitoring of instrumental line shape (ILS) (Hase et al., 1999, 2013). It has become a part of FTIR network practice to regularly use a low-pressure calibration gas cell (HBr or HCl) to diagnose a misalignment of the spectrometer and to realign the instrument when indicated (Hase et al., 1999, 2013; Wunch et al., 2010, 2015). A successful alignment scheme for high-resolution spectrometers was proposed about a decade ago and has become the standard alignment procedure for both TCCON and NDACC (Hase et al., 2013). As a result, the individual systematic errors and site to site biases caused by misalignment due to mechanical degradation are already minimized. However, the ILS measurements are commonly performed using an internal lamp. For the gas measurements, the ILS of the whole system (i.e., including the solar tracking system and the entrance optics) is also of utmost importance (F. Hase and T. Blumenstock, personal communication, 2011).

The TCCON and many NDACC assume an ideal ILS in spectra retrieval. In the TCCON network a maximal variation of the ILS is prescribed, on which the maximal variation for modulation efficiency (ME) amplitude is 5 % (Wunch et al., 2011, 2015). This assumption still holds within the required accuracy of the results. The TCCON prescribes a constant entrance field stop. In order to adapt the intensity of the incident radiation, an attenuator is inserted in the light path. 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. An alternative way that does not alter the alignment of a spectrometer is done via the selection of a suitable amplifier gain, depending on incoming intensities. However, this method has limited contribution and may be plagued with deteriorating SNR (signal-to-noise ratio) of the spectrum. For measurement performed within the TCCON and NDACC networks, the degree of ILS changes caused by the above processes is not fully quantified. In this paper, we designed experiments to investigate the sensitivity of ILS monitoring for ground-based high-resolution FTIR spectrometers with respect to different optical attenuators.

In Fig. 3, the typical interfering gases and the solar Fraunhofer lines within the HCl and HBr fitting regions are shown. In the TCCON case, H2O and CH4 have non-negligible absorptions in the same region as HCl. The NDACC case is more complicated; both N2O and SO2 show strong interferences in HBr region. Furthermore, non-negligible solar Fraunhofer lines within both HCl and HBr regions are shown. Therefore, optical background removal is rather important, especially for the sun cell measurement. Since the interfering items in the solar spectrum are not easy to quantify, each target spectrum (with a cell inserted in the optical path) is divided by a reference spectrum (without a cell inserted into the optical path) to remove the optical background.

The default scenarios were used to examine the consistency between the lamp and sun ILS retrieval. Typical lamp and sun spectra used for TCCON and NDACC ILS retrievals are shown in Fig. 4. The sun spectra in both HCl and HBr regions exhibited more interference than the lamp spectra. The lamp spectra are nearly free of interference except the non-constant transmission due to the glass body of the gas cells, whereas the atmospheric structures are obviously shown in sun spectra.

Figure 5 shows the fitted cases for TCCON and NDACC ILS retrievals after removing the optical background. LINEFIT achieved good ILS fittings for both TCCON and NDACC regardless of lamp or sun spectrum. The ILS modulation efficiencies and phase errors deduced from Fig. 5 are shown in Fig. 6. It concludes that the lamp and solar spectrum can achieve consistent ILS retrievals for both TCCON and NDACC, though the solar spectrum is much more structured than the lamp spectrum. The three groups of experiments deduced the same conclusion.

We investigated the sensitivity of ILS monitoring for ground-based high-resolution FTIR spectrometers with respect to various typical optical attenuators and positions. We confirmed that the ILS measurement using sun and internal lamp returned consistent results for the Bremen instrument. ILSs deduced from all scenarios of lamp cell measurements are compared. We observed that the ILS retrievals are very sensitive to various attenuators at different positions. The ILS disturbances with respect to the insertion of various attenuators in front of and behind the interferometer were quantified.

The insertion of any of the attenuators no. 1–4 or their derivatives in front of or behind the interferometer did not change the ILS much, and could potentially be taken to adapt the incident intensity. Selecting a smaller (bigger) entrance field stop (i.e., attenuator no. 5) to decrease (increase) incoming intensities is not optimal since the mechanical errors of different field stops may be non-negligible and inconsistent. This may be different from one instrument to the other; hence, the mechanical consistency of each field stop is recommended to be checked further before being used.