Abstract. Accurate and sustained monitoring of water vapor (H2O) in the upper troposphere-lower stratosphere (UTLS) is essential for quantifying its role in the atmospheric radiative balance, chemistry, and climate variability. However, in situ measurements in this region remain challenging due to the extremely low H2O concentrations, harsh environmental conditions, and strict payload limitations on meteorological balloon platforms. ALBATROSS is a compact (3.6 kg), balloon-borne mid-infrared laser absorption spectrometer developed to provide highly accurate, SI-traceable H2O measurements with high vertical resolution in the UTLS. Following recent comprehensive laboratory characterization, we present the first in-flight validation of ALBATROSS, based on seven balloon flights conducted between 2022 and 2026 from the meteorological observatories of Payerne, Switzerland (six flights) and Lindenberg, Germany (one flight). All flights included simultaneous measurements by a cryogenic frostpoint hygrometer (CFH) and a meteorological radiosonde (Vaisala RS41), with CFH operating either in a tandem-flight configuration or on the same payload as ALBATROSS. In the troposphere (up to 12 km altitude), the ALBATROSS retrieval agrees with both reference instruments within a mean relative difference of –2 ± 7 % with respect to RS41 and –1 ± 8 % with respect to CFH over nearly three orders of magnitude in H2O mixing ratio. Fine‑scale vertical structures and cirrus cloud layers are consistently resolved. In the stratosphere, following the implementation of an in-flight purging system to eliminate contamination from internal moisture sources, ALBATROSS achieves an agreement of 3 ± 10 % relative to CFH between 12 and 28 km altitude. An important advantage of laser absorption spectroscopy is demonstrated through observations of balloon-induced H2O spikes detected simultaneously by ALBATROSS and CFH in the stratosphere. These measurements reveal fast dynamic response of the laser spectrometer of ~1 s, corresponding to an effective vertical resolution of 5 m, compared to 3–4 s (15–20 m) for frostpoint hygrometry. Overall, these results demonstrate the robust performance of ALBATROSS under real atmospheric conditions and establish mid-infrared laser absorption spectroscopy as a reference-quality technique for balloon‑borne observations and long‑term monitoring of UTLS H2O.

The accentuated global market substantially increased worldwide shipping and thus the related greenhouse gas (GHG) emissions. Without effective global measures, emissions from maritime transport will soon undermine any attempts to mitigate climate change. Therefore, the International Maritime Organization committed to new targets for GHG emission reduction. Similarly, the EU commission aims to regulate the emissions from all large ships starting from 2024 [1]. This includes now also CH4 and N2O emissions besides CO2 and SO2. For any market-based measure, however, a robust monitoring, reporting and verification (MRV) system is a prerequisite. Furthermore, air surveillance techniques that are able to assess smoke plumes, are effective approaches to verify compliance and to identify potential violations.Within the framework of the Eurostars 3 project, ZEPHir, we support this effort by creating key enabling technologies for efficient monitoring of ship emissions. Leveraging on our recent advances in compact optical cells [2], laser driving schemes [3], and data-acquisition solutions [4], we target a multi-compound laser absorption spectrometer that can be carried aboard unmanned aerial vehicles (UAVs), such as drones, providing fast, in-situ, and high precision GHG measurements of vessel's exhaust plumes. For this purpose, two custom-made DFB quantum cascade lasers (QCLs) operated in a time-multiplexed regime (intermittent continuous wave operation) are coupled into a segmented circular multipass cell (SC-MPC) with an optical path length of 57 m. As each laser is selected to cover at least two different target species, we are able to assess all the regulated GHG compounds. As shown previously [5], the fastest analytical response is achieved in open-path configuration. However, open-path and thus at atmospheric pressure limits the analytical selectivity. Therefore, we identified a closed-path design at a pressure of 0.3 atm as the best compromise between selectivity and responsivity. Preliminary results indicate a noise equivalent absorbance in the range of 3 to 13 x10-9 cm-1 for all above GHGs within one second of averaging, well suited for measuring typical concentrations found ship plumes.The final aim of this ongoing work is to provide a portable spectrometer to be systematically deployed in maritime ports and harbors for providing quick, flexible and reliable estimates of ship emissions. Referenceshttps://emsa.europa.eu/reducing-emissions/mrv-changes.html M. Graf, L. Emmenegger, and B. Tuzson, "Compact, circular, and optically stable multipass cell for mobile laser absorption spectroscopy", Opt. Lett. 43, 2434-2437 (2018) M. Fischer, B. Tuzson, A. Hugi, R. Brönnimann, A. Kunz, S. Blaser, M. Rochat, O. Landry, A. Müller, and L. Emmenegger, "Intermittent operation of QC-lasers for mid-IR spectroscopy with low heat dissipation: tuning characteristics and driving electronics", Opt. Express 22, 7014-7027 (2014) C. Liu, B. Tuzson, P. Scheidegger, H. Looser, B. Bereiter, M. Graf, M. Hundt, O. Aseev, D. Maas and L. Emmenegger, "Laser driving and data processing concept for mobile trace gas sensing: Design and implementation", Rev. Sci. Instrum. 1 June 2018; 89 (6): 065107 Tuzson, B. and Graf, M. and Ravelid, J. and Scheidegger, P. and Kupferschmid, A. and Looser, H. and Morales, R. P. and Emmenegger, L., "A compact QCL spectrometer for mobile, high-precision methane sensing aboard drones", Atmos. Meas. Tech., 13, 4715–4726

VOC analysis by mid-IR laser spectroscopy strongly profits from spectral fine-structures for selective and sensitive detection. This is shown for a wealth of organic molecules (C1-C6) that have unique fingerprints at reduced gas pressure.

Volatile organic compounds (VOCs) exhibit typically broad and mutually overlapping ro-vibrational absorption finger-prints. This complexity has so far limited the applicabi l i t y of laser-based spectroscopy for VOC measurements in complex gas matrices. Here, we exploit a Vernier-type quantum-cascade laser (QCL) as an electrically tunable multiwavelength source for selective and sensitive VOC analysis. This emerging class of lasers provides access to several spectral windows by discrete Vernier tuning ("switching") and continuous coverage wit h i n these windows ("scanning"). We present a versatile driving technique that efficiently combines the two tuning mechanisms. Applied to our Vernier QCL, it enables the rapid acquisition (within 360 ms) of high-resolution spectra from six individual spectral windows, distributed over a wide range from 1063 to 1102 cm-1. Gaining access to the broad absorption envelopes of VOCs at multiple frequencies, along with their superimposed fine structure, which are especially pronounced at a reduced sample pressure, offers completely new opportunities in VOC analysis. The potential of this approach is assessed in a direct-laser-absorption setup with acetaldehyde, ethanol, and methanol as benchmark compounds with significant spectral overlaps. A measurement precision of 1-10 ppb is obtained after integration for 10 s at amount fractions below 10 ppm, and excellent linear i t y is found over at least 3 orders of magnitude. Combined with our dedicated spectral fitting algorit h m , we demonstrate highly selective multicompound analyses with less than 3.5% relative expanded uncertainty, even in the presence of a 40x excess of an interfering compound with complete spectral overlap.

Precise and accurate measurements of ambient HNO3 are crucial for understanding various atmospheric processes, but its ultra-low trace amounts and the high polarity of HNO3 have strongly hindered routine, widespread, direct measurements of HNO3 and restricted field studies to mostly short-term, localized measurement campaigns. Here, we present a custom field-deployable direct absorption laser spectrometer and demonstrate its analytical capabilities for in situ atmospheric HNO3 measurements. Detailed laboratory characterizations with a particular focus on the instrument response under representative conditions for tropospheric measurements, i.e., the humidity, spectral interference, changing HNO3 amount fractions, and air-sampling-related artifacts, revealed the key aspects of our method: (i) a good linear response (R2 > 0.98) between 0 and 25 nmol·mol−1 in both dry and humid conditions with a limit of detection of 95 pmol·mol−1; (ii) a discrepancy of 20% between the spectroscopically derived amount fractions and indirect measurements using liquid trapping and ion chromatography; (iii) a systematic spectral bias due to water vapor. The spectrometer was deployed in a three-week field measurement campaign to continuously monitor the HNO3 amount fraction in ambient air. The measured values varied between 0.1 ppb and 0.8 ppb and correlated well with the daily total nitrates measured using a filter trapping method.

We demonstrate a fast and flexible driving scheme that fully exploits the potential of extended-tuning quantum-cascade lasers (QC-XT). Integrated in a spectroscopic setting, it enables the sensitive and selective detection of volatile organic molecules.

Nitrogen dioxide (NO2) is a major tropospheric air pollutant. Its concentration in the atmosphere is most frequently monitored indirectly by chemiluminescence detection or using direct light absorption in the visible range. Both techniques are subject to known biases from other trace gases (including water vapor), making accurate measurements at low concentration very challenging. Selective measurements of NO2 in the mid-infrared have been proposed as a promising alternative, but field deployments and comparisons with established techniques remain sparse. Here, we describe the development and validation of a quantum cascade laser-based spectrometer (QCLAS). It relies on a custom-made astigmatic multipass absorption cell and a recently developed low heat dissipation laser driving and a FPGA based data acquisition approach. We demonstrate a sub-pptv precision (1 σ) for NO2 after 150 s integration time. The instrument performance in terms of long-term stability, linearity and field operation capability was assessed in the laboratory and during a two-week inter-comparison campaign at a suburban air pollution monitoring station. Four NO2 instruments corresponding to three different detection techniques (chemiluminescence detection (CLD), cavity-attenuated phase shift (CAPS) spectroscopy and QCLAS) were deployed after calibrating them with three different referencing methods: gas-phase titration of NO, dynamic high-concentration cylinder dilution and permeation. These measurements show that QCLAS is an attractive alternative for high-precision NO2 monitoring. Used in dual-laser configuration, its capabilities can be extended to NO, thus allowing for unambiguous quantification of nitrogen oxides (NOx), which are of key importance in air quality assessments.

Mid-infrared laser spectroscopy is a proven technology for reliable and high-precision gas sensing. With the advent of state-of-the-art laser sources, a large variety of analytical techniques has been developed, significantly extending the area of applications beyond the laboratory environment. Yet, full portability and downscaling remained a significant challenge, especially when selectivity, sensitivity and precision were to be preserved.

We describe the development, characterization, and first field deployments of a quantum cascade laser direct absorption spectrometer (QCLAS) for water vapor measurements in the upper troposphere and lower stratosphere (UTLS). The instrument is sufficiently small (30×23×11 cm3) and lightweight (3.9 kg) to be carried by meteorological balloons and used for frequent soundings in the UTLS. The spectrometer is a fully independent system, operating autonomously for the duration of a balloon flight. To achieve the required robustness, while satisfying stringent mass limitations, the concepts for optics and electronics have been fundamentally reconsidered compared to laboratory-based spectrometers. A significant enhancement of the mechanical and optical stability is achieved by integrating a newly designed segmented circular multipass cell which allows for 6 m optical path length in a very compact fashion. The H2O volume mixing ratio is retrieved by calibration-free evaluation of the spectral data, i.e., only relying on SI-traceable measurements and absorption line parameters. The open-path design reduces the risk of contamination and allows for fast response and thus high vertical resolution. Laboratory-based characterization experiments show an agreement within 2 % of reference measurements and a precision of 0.1 % under conditions comparable to the UTLS. The instrument successfully performed two balloon-borne test flights up to 28 km altitude. In the troposphere, the retrieved spectroscopic data show an excellent agreement with the accompanying measurements by a frost point hygrometer (CFH). At higher altitude, the quality of the spectral data remained unchanged, but outgassed water vapor within the instrument enclosure was hindering an accurate measurement of the atmospheric water vapor. Despite this limitation, these test flights demonstrated the operation of a compact laser spectrometer in the UTLS aboard a low-volume meteorological balloon, opening the perspective for future highly resolved, accurate, and cost-efficient soundings.

Water vapor is the dominant greenhouse gas, and its abundance in the upper tropospheric/lower stratospheric region (UTLS, 8-25 km altitude) is of great importance to the Earth's radiative balance. Reliable predictions of the climate evolution as well as the understanding of cloud-microphysical processes require the accurate and frequent measurement of water vapor concentrations at these altitudes. The only established method for high-accuracy UTLS water vapor measurements aboard of meteorological balloons is cryogenic frost-point hygrometry (CFH). However, the cooling agent required for its operation (CHF3) is to be phased out due to its strong global warming potential. It is, therefore, a major, worldwide challenge to ensure the continuation of the observation of this key Environmental Climate Variable (ECV) of the World Meteorological Organization (WMO). As an alternative method, we present a compact and lightweight instrument based on quantum cascade laser absorption spectroscopy (QCLAS) that reduces systematic errors by contactless and contamination-minimized measurements. Its construction addresses the stringent constraints posed by the harsh environmental conditions found in the UTLS. This is achieved by a fundamental reconsideration of main components of the spectrometer. We developed a highly versatile segmented circular multipass cell (SC-MPC) which supports compact and well-controlled beam folding [1]. The SC-MPC consists of a monolithic aluminum ring with 10.8 cm inner radius, containing 57 quadratic, spherically curved segments, seamlessly shaped into the internal ring surface. The collimated mid-IR beam (λ = 6 µm) from the distributed feedback quantum cascade laser (DFB-QCL) is directly coupled to the MPC without the need for additional beam-shaping optics. This leads to a resilient optical setup suitable for mobile applications and rough environmental conditions. Water vapor amount fractions of <10 ppmv can be measured with a precision better than 1% at 1 Hz. Measuring in open-path mode ensures quick response and minimal interference by water desorbing from surfaces. The instrument weighs less than 4 kg (including battery) and has an average power consumption of 15 W. An elaborate thermal management system that comprises phase change materials and thermoelectric cooling ensures excellent internal temperature stability despite an outside temperature difference of up to 80 K. Specifically developed hard- and software guarantee autonomous operation for the duration of flight [2]. Extensive stability assessments in climate chambers as well as validation experiments using dynamically generated, SI-traceable water vapor mixtures were performed in collaboration with the Swiss Federal Institute of Metrology (METAS). In cooperation with the German Weather Service (DWD) in Lindenberg, the instrument was successfully tested and compared to CFH in two consecutive balloon-ascents in December 2019 up to 28 km altitude, experiencing temperatures and pressures as low as –65°C and 16 hPa, respectively. The drastic reduction in mass and size of a laser absorption-spectrometer and its successful deployment under harshest conditions represents a paradigm change in portable laser spectroscopy and opens the door to previously inaccessible applications. [1] Graf, M.; Emmenegger, L.; Tuzson, B. Opt. Lett. 2018, 43, 2434-2437 [2] Liu, C. et al., L. Rev. Sci. Instrum. 2018, 89 (6), 065107 (9 pp.)

The record of past greenhouse gas composition from ice cores is crucial for our understanding of global climate change. Future ice core projects will aim to extend both the temporal coverage (extending the timescale to 1.5 Myr) and the temporal resolution of existing records. This implies a strongly limited sample availability, increasing demands on analytical accuracy and precision, and the need to reuse air samples extracted from ice cores for multiple gas analyses. To meet these requirements, we designed and developed a new analytical system that combines direct absorption laser spectroscopy in the mid-infrared (mid-IR) with a quantitative sublimation extraction method. Here, we focus on a high-precision dual-laser spectrometer for the simultaneous measurement of CH4, N2O, and CO2 concentrations, as well as δ13C(CO2). Flow-through experiments at 5 mbar gas pressure demonstrate an analytical precision (1 σ) of 0.006 ppm for CO2, 0.02 ‰ for δ13C(CO2), 0.4 ppb for CH4, and 0.1 ppb for N2O, obtained after an integration time of 100 s. Sample–standard repeatabilities (1 σ) of discrete samples of 1 mL STP (Standard Temperature and Pressure) amount to 0.03 ppm, 2.2 ppb, 1 ppb, and 0.04 ‰ for CO2, CH4, N2O, and δ13C(CO2), respectively. The key elements to achieve this performance are a custom-developed multipass absorption cell, custom-made high-performance data acquisition and laser driving electronics, and a robust calibration approach involving multiple reference gases. The assessment of the spectrometer capabilities in repeated measurement cycles of discrete air samples – mimicking the procedure for external samples such as air samples from ice cores – was found to fully meet our performance criteria for future ice core analysis. Finally, this non-consumptive method allows the reuse of the precious gas samples for further analysis, which creates new opportunities in ice core science.

Abstract. The record of past global background atmospheric greenhouse gas composition is crucial for our understanding of global climate change. The ”Beyond EPICA Oldest Ice Core” project is currently pushing the frontier of this knowledge forward by the retrieval of an ice core reaching back to 1.5 million years ago. The oldest section of this core will have been strongly thinned by glacier flow with about 15 kyr being trapped in as little as 1m thickness of ice. This reduces the available sample volume to only a few mL of air for the targeted century-scale resolution of greenhouse gas records. Under these conditions, the required accuracy for multiple greenhouse gases cannot be achieved with currently available analytical methods. Here, we present a new approach to unlocking such challenging atmospheric archives with a high-precision mid-IR dual-laser direct absorption spectrometer. The instrument is designed to simultaneously measure CH4, N2O, CO2 concentrations as well as δ13C(CO2) using discrete samples of only 1 mL STP, and it achieves a precision of 1.6 ppb, 1.0 ppb, 0.03 ppm and 0.04 ‰, respectively. Repeated measurement cycles of air samples demonstrate an excellent accuracy level, and high reproducibility of the spectroscopic and the gas handling system. In addition, this non-invasive method allows reuse of the precious gas samples for further analysis, which opens new opportunities in ice core science.

A compact and lightweight mid-infrared laser absorption spectrometer has been developed as a mobile sensing platform for high-precision atmospheric methane measurements aboard small unmanned aerial vehicles (UAVs). The instrument leverages two recent innovations: a novel segmented circular multipass cell (SC-MPC) design and a power-efficient, low-noise, intermittent continuous-wave (icw) laser driving approach. A system-on-chip hardware control and data acquisition system enables energy-efficient and fully autonomous operation. The integrated spectrometer weighs 2.1 kg (including battery) and consumes 18 W of electrical power, making it ideally suited for airborne monitoring applications. Under stable laboratory conditions, the device achieves a precision (1σ) of 1.1 ppb within 1 s and 0.1 ppb CH4 at 100 s averaging time. Detailed investigations were performed to identify and quantify the effects of various environmental factors, such as sudden changes in pressure, temperature, and mechanical vibrations, which commonly influence UAV-mounted sensors. The instrument was also deployed in two feasibility field studies: an artificial methane release experiment and a study on vertical profiles in the planetary boundary layer. In both cases, the spectrometer demonstrated its airborne capability of capturing subtle and/or sudden changes in atmospheric CH4 mole fractions and providing real-time data at 1 s time resolution.

Mid-IR spectroscopy using QCLs allows sensitive, selective, and fast detection of trace-gases. Recent developments permit rugged and lightweight instruments that create tantalizing options in environmental sciences and other fields, i.e. medicine or industrial process control.

Widely electrically tuneable QCLs are highly attractive for the sensitive and selective detection of organic molecules. Both high spectral resolution and broad coverage are shown for an XT-QCL providing 6-channels based on the Vernier effect.

A lightweight instrument has been developed to measure water vapor up to the lower stratosphere aboard meteorological balloons. The sensor relies on a segmented circular multipass cell which is especially suited for mobile field applications.