Observations of the downwelling far-infrared atmospheric emission at the Zugspitze observatory

. Measurements of the spectrum of the atmospheric emission in the far-infrared (FIR) range, between 100 and 667 cm − 1 (100–15 µ m) , are scarce because of the detection complexity and of the strong absorption of air at ground level, preventing the sounding of the FIR from low altitude. Consequently, FIR measurements need to be made from high-altitude sites or on board airborne platforms or satellites. This paper describes the dataset of FIR spectral radiances of the atmosphere and snow surface 5 emission , measured in the 100–1000 cm − 1 range by the Far-Infrared Radiation Mobile Observation System (FIRMOS) instrument , during a 2-month campaign carried out from ground at (cid:58)(cid:58)(cid:58)(cid:58) about (cid:58) 3000 m of altitude , on the top of Mount Zugspitze in the German Alps, in 2018-2019. This campaign is part of the preparatory activity of a new space FIR mission, named measurable of the frequency scale, the uncertainty on the metrology on both FIRMOS with shows a frequency scale of about ppm between simulations and observations ( σ real = (1+5 × 10 − 5 ) × σ firmos ), whereas E-AERI has same error but with different σ real − 5 × − 5 ) × ).


Introduction
The Far-InfraRed (FIR)is : , defined here as the longest wavelength region of the infrared spectrum covering the wavenumber 20 range from 667 cm −1 (or equivalently 15 µm wavelength) down to 100 cm −1 (100 µm). The FIR : , contains more than 50% of the energy emitted by the Earth toward the spaceand it : . :::: This ::::::: spectral ::::: region : is modulated by the properties of the most relevant components of the climate system, such as water vapour, carbon dioxide, clouds, and snow surface emissivity (Harries et al., 2008). The characterisation of the FIR radiative properties of these components is therefore essential to improve FIR ::::::: gaseous modelling of spectroscopy (Mlawer et al., 2019), cirrus cloud radiative properties (Cox et al., 2010) and surface emissivity 25 (Chen et al., 2014), which in turn will allow to better understand : a :::::: better :::::::::::: understanding :: of : the Earth radiation budget and to reduce uncertainties in climate models (Huang et al., 2018;Baran et al., 2014;Huang et al., 2007). Nevertheless, due to technical challenges, systematic and global measurements of the FIR from space are still missing, and are scarce from ground and airborne platforms.
To fill this observational gap, a space mission, named Far-infrared-Outgoing-Radiation Understanding and Monitoring (FO-30 RUM) (Palchetti et al., 2020b), is under development by the European Space Agency (ESA) as the ninth Earth Explorer mission to be launched in 2026 (https://www.forum-ee9.eu/). This mission will measure with high accuracy the spectrum of the outgoing infrared radiation from 100 to 1600 cm −1 (100-6.25 µm) covering, for the first time with high spectral resolution, the FIR portion of the spectrum. In preparation to :: for : this mission, an instrument demonstrator, named Far-Infrared Radiation Mobile Observation System (FIRMOS), was developed for field applications from ground-based (and in perspective airborne) 35 platforms to verify with real measurements the sounding capability provided by FIR observations.
At ground level the atmosphere in the FIR spectral region is very opaque because of the high concentration of water vapour.

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In this paper, we describe the measurements acquired during the first field deployment of FIRMOS, between the end of 2018 and the beginning of 2019 at the Alpine observatory of Mt. Zugspitze in the South :::: south : of Germany at about 3000 m altitude, together with all the ancillary measurements needed to provide a complete characterisation of the observed atmospheric and surface states. which is located on the south slope of mountain, 300 m below and 680 m southwest from the Summit (see Fig. 1).

Instrument description and performed measurements
During the campaign, measurements were carried out using instruments deployed at the Zugspitze Summit, at the Schneefernerhaus station, and at the Karlsruhe Institute of Technology (KIT) in Garmisch-Partenkirchen.
The emission spectrum was measured in the FIR with FIRMOS, which was developed at CNR-INO with the support of ESA  (Palchetti et al., 2015). The FTS consists in an interferometric layout with double-input and double-output ports in a Mach-Zehnder configuration, which increases the measurement reliability and the calibration precision (Carli et al., 1999). The instrument is able to cover the FIR radiation using wideband germanium-coated biaxially-oriented 70 polyethylene terephthalate beam-splitters and room-temperature pyroelectric detectors. It was mounted within a plastic box to protect optics and electronics from environmental conditions, such as wind and snowfall, when installed on the site. ::: The :::: full :::::::::: field-of-view :::::: (FOV) :: of :::::::: FIRMOS :: is ::: 22 ::::: mrad.
::::: Figure : 2 shows FIRMOS installation close to the E-AERI instrument, which is permanently installed on the top of the shelter, 85 4 m above FIRMOS, and routinely performs measurements of spectral DLR , when the weather conditions allow the operation.
KIT also operates two lidar systems for aerosols/clouds (Vogelmann and Trickl, 2008), and water vapour and temperature (Klanner et al., 2020) profiling, installed at the Schneefernerhaus station. The advantage of mounting the lidar systems at 90 Schneefernerhaus and the spectrometer at the Summit is that the onset of the measured lidar profiles is typically about 300 m above the laser, which coincides with the location of the spectrometers at the Summit. Cirrus cloud properties are detected with the stratospheric aerosol lidar, which is a pure backscatter lidar operating at the wavelength of 532 nm. This system was operated in a semi-automatic mode, with a sequence of one profile every 4 or 10 minutes and an integration time of 1 min.
Finally, the properties of snow samples were characterised in terms of snow grain type, density (kg m −3 ) and Specific 125 Surface Area (SSA, m 2 kg −1 ), using the DUal Frequency Integrating Sphere for Snow SSA measurement (DUFISSS) sensor, which retrieves the SSA by measuring the reflectance at 1310 nm (Gallet et al., 2009).
The campaign took place from 29 November to 18 December 2018 and from 21 January to 20 February 2019. Measurements were performed when the weather conditions allowed the operations; this occurred for a total of 33 days. The main specifications of the collected measurements, which are available in the provided ::::::: provided :: in ::: the dataset, are summarized :::::::::: summarised 135 in Table 1.

FIRMOS validation
The FIRMOS dataset includes calibrated spectra and noise estimates, calculated starting from the FTS acquired interferograms and following the procedure described in Bianchini and Palchetti (2008). Each spectrum is the average of 4 sky observations and it is calibrated with 4 calibration measurements (2 looking at a hot backbody reference source and 2 looking at a cold source).

Conclusions
In summary, the unique spectral measurements provided by the combination of FIRMOS and E-AERI covering the relevant spectral region of the thermal emission of the atmosphere from 100 to 1800 cm −1 , together with the other supporting ::::::::::: measurements :: (lidars, radiosoundings, atmospheric state, and surface propertiesmeasurements), provide a complete dataset that can be used to constrain radiative properties of water vapour, cirrus ice particles, and snow/ice emissivity over almost all 215 the infrared emission, including the under-explored FIR spectral range.
For the review process, the dataset is also available at https://fts.fi.ino.it/forum/firmos/zugspitze-dataset/ (essd, F20z%sd) LP prepared the manuscript with contributions from all co-authors.
Competing interests. The authors declare that they have no conflict of interest.