Improved instrumental line shape monitoring for the ground-based, high-resolution FTIR spectrometers of the Network for the Detection of Atmospheric Composition Change

Abstract

Abstract.We propose an improved monitoring scheme for the instrumental line shape (ILS) of high-resolution, ground-based FTIR (Fourier Transform InfraRed) spectrometers used for chemical monitoring of the atmosphere by the Network for Detection of Atmospheric Composition Change (NDACC). Good ILS knowledge is required for the analysis of the recorded mid-infrared spectra. The new method applies a sequence of measurements using different gas cells instead of a single calibration cell. Three cells are used: cell C1 is a refillable cell offering 200 mm path length and equipped with a pressure gauge (filled with 100 Pa N 2 O), cells C2 and C3 are sealed cells offering 75 mm path length. C2 is filled with 5 Pa of pure N 2 O. Cell C3 is filled with 16 Pa N 2 O in 200 hPa technical air, so provides pressurebroadened N 2 O lines. We demonstrate that an ILS retrieval using C1 improves significantly the sensitivity of the ILS retrieval over the current calibration cells used in the network, because this cell provides narrow fully saturated N 2 O lines. The N 2 O columns derived from C2 and C3 allow the performance of a highly valuable closure experiment: adopting the ILS retrieved from C1, the N 2 O columns of C2 and C3 are derived. Because N 2 O is an inert gas, both columns should be constant on long timescales. Apparent changes in the columns would immediately attract attention and indicate either inconsistent ILS results or instrumental problems of other origin. Two different cells are applied for the closure experiment, because the NDACC spectrometers observe both stratospheric and tropospheric gases: C2 mimics signatures of stratospheric gases, whereas C3 mimics signatures of tropospheric gases.

The Network for the Detection of Atmospheric Composition Change (NDACC): History, status and perspectives

“…9, ozone structure in the TTL (tropical tropopause layer or tropopause transition layer) as the UT/LS is referred to in the tropics, is displayed longitudinally using the composite tropical SHADOZ data (Thompson et al, 2003;2012). The eastern Indian Ocean through Pacific region displays a sharp ozone gradient and high tropopause.…”

The influence of instrumental line shape degradation on NDACC gas retrievals: total column and profile

“…For a perfectly aligned spectrometer, it would meet the ideal nominal ILS characteristics if smear and vignetting effects were neglected, and thus have an ME amplitude of unity and a PE of zero along the whole interferogram. However, if a FTIR spectrometer were subject to misalignment, the ME amplitude would deviate from unity and the PE would deviate from zero (Hase et al, 2012). This results in an imperfect ILS.…”

“…For standard NDACC measuring conditions, L ≥180 cm and θ defined by the entrance field stop size in the light path. The LINEFIT software calculates the deviation of the measured ILS from the ideal ILS (Hase et al, 2001(Hase et al, , 2012. It retrieves a complex ME as a function of the OPD, which is represented by a ME amplitude and a phase error (PE) (Hase et al, 1999).…”

“…FTIR spectrometers are highly precise and stable devices, and if carefully aligned, the instrumental line shape (ILS) might not be far from the theoretical limit. However, their alignment can change abruptly as a consequence of operator intervention or drift slowly due to mechanical degradation over time (Olsen and Randerson, 2004;Duchatelet et al, 2010;Hase et al, 2012;Feist et al, 2016). Moreover, the NDACC observation may change the entrance field stop size if incident radiation changes.…”

“…edu/, last access: 11 May 2018) and NDACC (Network for Detection of Atmospheric Composition Change; http: //www.ndacc.org/, last access: 11 May 2018) have developed strict data acquisition and retrieval methods to minimize site to site differences (Hase et al, 2012;Wunch et al, 2010Wunch et al, , 2011Washenfelder, 2006;Messerschmidt et al, Cosine and sine bending of scanner bar: causes a chord increase 0.073 × (sin(π × OPD/360)+ ME amplitude: 0.86 in misalignment before half-maximum OPD and causes cos(π × OPD/360))−0.073 (cm) PE: −0.029 rad. a chord decrease in misalignment after half-maximum OPD j Constant shear plus cosine and sine bending of scanner bar: −0.073 × (sin(π × OPD/360)+ ME amplitude: 1.16 causes a chordal decrease in misalignment before cos(π × OPD/360))+0.103 (cm) PE: −0.056 rad.…”