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Despite its crucial role in the Earth's radiative balance, upper-air water vapor (H2O) is still lacking accurate, in situ, and continuous monitoring. Especially in the upper troposphere–lower stratosphere (UTLS), these measurements are notoriously difficult, and significant discrepancies have been reported in the past between different measuring techniques. Here, we present a laboratory assessment of a recently developed mid-IR quantum-cascade laser absorption spectrometer, called ALBATROSS, for balloon-borne measurements of H2O in the UTLS. The validation was performed using SI-traceable reference gas mixtures generated based on the permeation method and dynamic dilution. The accuracy and precision of ALBATROSS were evaluated at a wide range of pressures (30–250 mbar) and H2O amount fractions (2.5–35 ppm), representative of the atmospheric variability in H2O in the UTLS. The best agreement was achieved by implementing a quadratic speed-dependent Voigt profile (qSDVP) line shape model in the spectroscopic retrieval algorithm. The molecular parameters required by this parameterization were determined empirically using a multi-spectrum fitting approach over different pressure conditions. In the laboratory environment, ALBATROSS achieves an accuracy better than ±1.5 % with respect to the SI-traceable reference at all investigated pressures and H2O amount fractions. The measurement precision was found to be better than 30 ppb (i.e., 0.1 % at 35 ppm H2O) at 1 s resolution for all conditions. This performance, unprecedented for a balloon-borne hygrometer, demonstrates the exceptional potential of mid-IR laser absorption spectroscopy as a new reference method for in situ measurements of H2O in the UTLS.

Precision, accuracy, and temporal resolution are key to making full use of atmospheric trace gas records in ice cores. These aspects will become especially crucial for ice cores that aim to extend the ice core record to the last 1.5 Myr, i.e., across the Mid-Pleistocene Transition (as currently drilled within the European project Beyond EPICA – Oldest Ice Core (BE-OIC)). The ice from this period is expected to be close to bedrock and, due to glacier flow, extremely thinned with 15 000 years of climate history contained in only 1 m of ice. Accordingly, for a century-scale resolution, the sample vertical extent must be reduced to a few centimeters containing only about 1–2 mL air STP. We present a novel combined system for the extraction and the simultaneous measurement of CO2, CH4, and N2O concentrations, as well as δ13CO2, which achieves a vertical resolution of 1–2 cm (3.5×3.5 cm cross section) with precisions of 0.4 ppm, 3 ppb, 1 ppb, and 0.04 ‰, respectively, in sublimation tests with standard gas over gas-free ice. This is accomplished by employing a directional and continuous laser-induced sublimation followed by analysis of the sample gas by a quantum cascade laser absorption spectrometer (QCLAS). Besides the low sample volume requirements and the vertical resolution capabilities, the described method holds additional advantages over previous methods, including the immunity of the highly specific QCLAS analysis to drilling fluid contamination as well as the non-destructive nature of the spectroscopic gas analysis. The combined extraction and analysis system was extensively tested by sublimating gas-free ice with introduction of a standard gas to determine the accuracy and characterize potential artifacts. Moreover, Antarctic ice samples were measured to confirm the measurement performance, covering the range of variability expected in Pleistocene ice and highlighting the vertical resolution capabilities critical for its application within BE-OIC.

Mapping trace gas emission plumes using in situ measurements from unmanned aerial vehicles (UAVs) is an emerging and attractive possibility to quantify emissions from localized sources. Here, we present the results of an extensive controlled-release experiment in Dübendorf, Switzerland, which was conducted to develop an optimal quantification method and to determine the related uncertainties under various environmental and sampling conditions. Atmospheric methane mole fractions were simultaneously measured using a miniaturized fast-response quantum cascade laser absorption spectrometer (QCLAS) and an active AirCore system mounted on a commercial UAV. Emission fluxes were estimated using a mass-balance method by flying the UAV-based system through a vertical cross-section downwind of the point source perpendicular to the main wind direction at multiple altitudes. A refined kriging framework, called cluster-based kriging, was developed to spatially map individual methane measurement points into the whole measurement plane, while taking into account the different spatial scales between background and enhanced methane values in the plume. We found that the new kriging framework resulted in better quantification compared to ordinary kriging. The average bias of the estimated emissions was −1 %, and the average residual of individual errors was 54 %. A Direct comparison of QCLAS and AirCore measurements shows that AirCore measurements are smoothed by 20 s and had an average time lag of 7 s. AirCore measurements also stretch linearly with time at an average rate of 0.06 s for every second of QCLAS measurement. Applying these corrections to the AirCore measurements and successively calculating an emission estimate shows an enhancement of the accuracy by 3 % as compared to its uncorrected counterpart. Optimal plume sampling, including the downwind measurement distance, depends on wind and turbulence conditions, and it is furthermore limited by numerous parameters such as the maximum flight time and the measurement accuracy. Under favourable measurement conditions, emissions could be quantified with an uncertainty of 30 %. Uncertainties increase when wind speeds are below 2.3 m s−1 and directional variability is above 33∘, and when the downwind distance is above 75 m. In addition, the flux estimates were also compared to estimates from the well-established OTM-33A method involving stationary measurements. A good agreement was found, both approaches being close to the true release and uncertainties of both methods usually capturing the true release.

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.

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.

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.

More than 300 non-dispersive infrared (NDIR) CO2 low-cost sensors labelled as LP8 were integrated into sensor units and evaluated for the purpose of long-term operation in the Carbosense CO2 sensor network in Switzerland. Prior to deployment, all sensors were calibrated in a pressure and climate chamber and in ambient conditions co-located with a reference instrument. To investigate their long-term performance and to test different data processing strategies, 18 sensors were deployed at five locations equipped with a reference instrument after calibration. Their accuracy during 19 to 25 months deployment was between 8 and 12 ppm. This level of accuracy requires careful sensor calibration prior to deployment, continuous monitoring of the sensors, efficient data filtering, and a procedure to correct drifts and jumps in the sensor signal during operation. High relative humidity (> ∼85 %) impairs the LP8 measurements, and corresponding data filtering results in a significant loss during humid conditions. The LP8 sensors are not suitable for the detection of small regional gradients and long-term trends. However, with careful data processing, the sensors are able to resolve CO2 changes and differences with a magnitude larger than about 30 ppm. Thereby, the sensor can resolve the site-specific CO2 signal at most locations in Switzerland. A low-power network (LPN) using LoRaWAN allowed for reliable data transmission with low energy consumption and proved to be a key element of the Carbosense low-cost sensor network.

Black carbon (BC) or soot is a constituent of particulate matter (PM) which is relevant for negative human health and climate effects, and despite the lack of direct legal limits, it is recognised as an important atmospheric pollutant to monitor, understand, and control. Aethalometers are instruments which continuously monitor BC by measuring absorption at a number of distinct wavelengths. If collocated elemental carbon (EC) observations are used to transform these values into BC mass, by convention, the result is named equivalent black carbon (EBC). BC emitted by different combustion processes has different optical absorption characteristics, and this can be used to apportion EBC mass into traffic (EBCTR) and woodburning (EBCWB) components with a data processing technique known as the aethalometer model. The aethalometer model was applied to six EBC monitoring sites across Switzerland (using data between 2008 and 2018) and was evaluated by investigating diurnal cycles, model coefficients, and ambient temperature dependence of the two EBC components. For one monitoring site, San Vittore, the aethalometer model failed to produce plausible outputs. The reason for this failure was likely due to a high load of freshly emitted wood smoke during the winter which should be thought of as a third distinct emission source. After model evaluation, the trend analysis indicated that EBCTR concentrations at the remaining five locations significantly decreased between 2008 and 2018. EBCWB also demonstrated significant decreases in most monitoring locations but not at a monitoring site south of the Alps with a high PM load sourced from biomass burning. Ratios of EBC and particulate matter with a diameter of less than 2.5 µm (PM2.5) suggested that EBC contributes 6 %–14 % of the PM2.5 mass in Switzerland. The aethalometer model is a useful data analysis procedure but can fail under certain conditions; thus, careful evaluation is required to ensure the method is robust and suitable in other locations.

Carbon monoxide (CO) and nitrous oxide (N2O) are two key parameters in the observation of the atmosphere, relevant to air quality and climate change, respectively. For CO, various analytical techniques have been in use over the last few decades. In contrast, N2O was mainly measured using gas chromatography (GC) with an electron capture detector (ECD). In recent years, new spectroscopic methods have become available which are suitable for both CO and N2O. These include infrared (IR) spectroscopic techniques such as cavity ring-down spectroscopy (CRDS), off-axis integrated cavity output spectroscopy (OA-ICOS) and Fourier transform infrared spectroscopy (FTIR). Corresponding instruments became recently commercially available and are increasingly used at atmospheric monitoring stations. We analysed results obtained through performance audits conducted within the framework of the Global Atmosphere Watch (GAW) quality management system of the World Meteorology Organization (WMO). These results reveal that current spectroscopic measurement techniques have clear advantages with respect to data quality objectives compared to more traditional methods for measuring CO and N2O. Further, they allow for a smooth continuation of historic CO and N2O time series. However, special care is required concerning potential water vapour interference on the CO amount fraction reported by near-IR CRDS instruments. This is reflected in the results of parallel measurement campaigns, which clearly indicate that drying the sample air leads to an improved accuracy of CO measurements with such near-IR CRDS instruments.

Detailed knowledge about the urban NO2 concentration field is a key element for obtaining accurate pollution maps and individual exposure estimates. These are required for improving the understanding of the impact of ambient NO2 on human health and for related air quality measures. However, city-scale NO2 concentration maps with high spatio-temporal resolution are still lacking, mainly due to the difficulty of accurate measurement of NO2 at the required sub-ppb level precision. We contribute to close this gap through the development of a compact instrument based on mid-infrared laser absorption spectroscopy. Leveraging recent advances in infrared laser and detection technology and a novel circular absorption cell, we demonstrate the feasibility and robustness of this technique for demanding mobile applications. A fully autonomous quantum cascade laser absorption spectrometer (QCLAS) has been successfully deployed on a tram, performing long-term and real-time concentration measurements of NO2 in the city of Zurich (Switzerland). For ambient NO2 concentrations, the instrument demonstrated a precision of 0.23 ppb at one second time resolution and of 0.03 ppb after 200 s averaging. Whilst the combined uncertainty estimated for the retrieved spectroscopic values was less than 5 %, laboratory intercomparison measurements with standard CLD instruments revealed a systematic NO2 wall loss of about 10 % within the laser spectrometer. For the field campaign, the QCLAS has been referenced to a CLD using urban atmospheric air, despite the potential cross sensitivity of CLD to other nitrogen containing compounds. However, this approach allowed a direct comparison and continuous validation of the spectroscopic data to measurements at regulatory air quality monitoring (AQM) stations along the tram-line. The analysis of the recorded high-resolution time series allowed us to gain more detailed insights into the spatio-temporal concentration distribution of NO2 in an urban environment. Furthermore, our results demonstrate that for reliable city-scale concentration maps a larger data set and better spatial coverage is needed, e.g., by deploying more mobile and stationary instruments to account for mainly two shortcomings of the current approach: (i) limited residence time close to sources with large short-term NO2 variations, and (ii) insufficient representativeness of the tram tracks for the complex urban environment.

Until recently, atmospheric carbon dioxide (CO2) and methane (CH4) measurements were made almost exclusively using nondispersive infrared (NDIR) absorption and gas chromatography with flame ionisation detection (GC/FID) techniques, respectively. Recently, commercially available instruments based on spectroscopic techniques such as cavity ring-down spectroscopy (CRDS), off-axis integrated cavity output spectroscopy (OA-ICOS) and Fourier transform infrared (FTIR) spectroscopy have become more widely available and affordable. This resulted in a widespread use of these techniques at many measurement stations. This paper is focused on the comparison between a CRDS "travelling instrument" that has been used during performance audits within the Global Atmosphere Watch (GAW) programme of the World Meteorological Organization (WMO) with instruments incorporating other, more traditional techniques for measuring CO2 and CH4 (NDIR and GC/FID). We demonstrate that CRDS instruments and likely other spectroscopic techniques are suitable for WMO/GAW stations and allow a smooth continuation of historic CO2 and CH4 time series. Moreover, the analysis of the audit results indicates that the spectroscopic techniques have a number of advantages over the traditional methods which will lead to the improved accuracy of atmospheric CO2 and CH4 measurements.

In situ and simultaneous measurement of the three most abundant isotopologues of methane using mid-infrared laser absorption spectroscopy is demonstrated. A field-deployable, autonomous platform is realized by coupling a compact quantum cascade laser absorption spectrometer (QCLAS) to a preconcentration unit, called trace gas extractor (TREX). This unit enhances CH4 mole fractions by a factor of up to 500 above ambient levels and quantitatively separates interfering trace gases such as N2O and CO2. The analytical precision of the QCLAS isotope measurement on the preconcentrated (750 ppm, parts-per-million, µmole mole−1) methane is 0.1 and 0.5 ‰ for δ13C- and δD-CH4 at 10 min averaging time. Based on repeated measurements of compressed air during a 2-week intercomparison campaign, the repeatability of the TREX–QCLAS was determined to be 0.19 and 1.9 ‰ for δ13C and δD-CH4, respectively. In this intercomparison campaign the new in situ technique is compared to isotope-ratio mass spectrometry (IRMS) based on glass flask and bag sampling and real time CH4 isotope analysis by two commercially available laser spectrometers. Both laser-based analyzers were limited to methane mole fraction and δ13C-CH4 analysis, and only one of them, a cavity ring down spectrometer, was capable to deliver meaningful data for the isotopic composition. After correcting for scale offsets, the average difference between TREX–QCLAS data and bag/flask sampling–IRMS values are within the extended WMO compatibility goals of 0.2 and 5 ‰ for δ13C- and δD-CH4, respectively. This also displays the potential to improve the interlaboratory compatibility based on the analysis of a reference air sample with accurately determined isotopic composition.

We present the continuous data record of atmospheric CO2 isotopes measured by laser absorption spectroscopy for an almost four year period at the High Altitude Research Station Jungfraujoch (3580 m a.s.l.), Switzerland. The mean annual cycles derived from data of December 2008 to September 2012 exhibit peak-to-peak amplitudes of 11.0 μmol mol−1 for CO2, 0.60‰ for δ13C and 0.81‰ for δ18O. The high temporal resolution of the measurements also allow us to capture variations on hourly and diurnal timescales. For CO2 the mean diurnal peak-to-peak amplitude is about 1 μmol mol−1 in spring, autumn and winter and about 2 μmol mol−1 in summer. The mean diurnal variability in the isotope ratios is largest during the summer months too, with an amplitude of about 0.1‰ both in the δ13C and δ18O, and a smaller or no discernible diurnal cycle during the other seasons. The day-to-day variability, however, is much larger and depends on the origin of the air masses arriving at Jungfraujoch. Backward Lagrangian particle dispersion model simulations revealed a close link between air composition and prevailing transport regimes and could be used to explain part of the observed variability in terms of transport history and influence region. A footprint clustering showed significantly different wintertime CO2, δ13C and δ18O values depending on the origin and surface residence times of the air masses. Several major updates on the instrument and the calibration procedures were performed in order to further improve the data quality. We describe the new measurement and calibration setup in detail and demonstrate the enhanced performance of the analyzer. A measurement precision of about 0.02‰ for both isotope ratios has been obtained for an averaging time of 10 min, while the accuracy was estimated to be 0.1‰, including the uncertainty of the calibration gases.

A quantum cascade laser based absorption spectrometer for continuous and direct measurements of NO and NO2 was employed at the high-altitude monitoring site Jungfraujoch (3580 m a.s.l., Switzerland) during a three-month campaign in spring/summer 2012. The total reactive nitrogen, NOy, was also measured in the form of NO after conversion on a gold catalyst. The aim was to assess the suitability of the instrument for long-term monitoring of the main reactive nitrogen species under predominantly free tropospheric air conditions. A precision (1σ) of 10 and 3 ppt for NO and NO2 was achieved under field conditions after 180 s averaging time. The linear dynamic range of the instrument has been verified for both species from the detection limit up to 45 ppb. The spectrometer shared a common sampling inlet with a chemiluminescence-based analyzer. The comparison of the time series shows excellent agreement between the two techniques and demonstrates the adequacy of the laser spectroscopic approach for this kind of demanding environmental applications.

We describe the first high precision real-time analysis of the N2O site-specific isotopic composition at ambient mixing ratios. Our technique is based on mid-infrared quantum cascade laser absorption spectroscopy (QCLAS) combined with an automated preconcentration unit. The QCLAS allows for simultaneous and specific analysis of the three main stable N2O isotopic species, 14N15N16O, 15N14N16O, 14N14N16O, and the respective site-specific relative isotope ratio differences δ15Nα and δ 15Nβ. Continuous, stand-alone operation is achieved by using liquid nitrogen free N2O preconcentration, a quasi-room-temperature quantum cascade laser (QCL), quantitative sample transfer to the QCLAS and an optimized calibration algorithm. The N2O site-specific isotopic composition (δ15Nα and δ15Nβ) can be analysed with a long-term precision of 0.2‰. The potential of this analytical tool is illustrated by continuous N2O isotopomer measurements above a grassland plot over a three week period, which allowed identification of microbial source and sink processes.

Fast response optical analyzers based on laser absorption spectroscopy are the preferred tools to measure field-scale mixing ratios and fluxes of a range of trace gases. Several state-of-the-art instruments have become commercially available and are gaining in popularity. This paper aims for a critical field evaluation and intercomparison of two compact, cryogen-free and fast response instruments: a quantum cascade laser based absorption spectrometer from Aerodyne Research, Inc., and an off-axis integrated cavity output spectrometer from Los Gatos Research, Inc. In this paper, both analyzers are characterized with respect to precision, accuracy, response time and also their sensitivity to water vapour. The instruments were tested in a field campaign to assess their behaviour under various meteorological conditions. The instrument's suitability for eddy covariance flux measurements was evaluated by applying an artificial flux of CH4 generated above a managed grassland with otherwise very low methane exchange. This allowed an independent verification of the flux measurements accuracy, including the overall eddy covariance setup and data treatment. The retrieved fluxes were in good agreement with the known artificial emission flux, which is more than satisfactory, given that the analyzers were attached to separate sonic anemometers placed on individual eddy towers with different data acquisition systems but similar data treatment that are specific to the best practice used by the involved research teams.

Important information about the biogeochemical cycle of nitrous oxide (N2O) can be obtained by measuring its three main isotopic species, 14N15N16O, 15N14N16O, and 14N14N16O, and the respective site-specific relative isotope ratio differences δ15Nα and δ15Nβ. Absorption laser spectroscopy in the mid-infrared is a direct method for the analysis of the 15N isotopic composition of N2O, yet not sensitive enough for atmospheric N2O mixing ratios (320 ppb). To enable a fully-automated high precision analysis of N2O isotopic species at ambient mixing ratios, we built and optimized a liquid nitrogen-free preconcentration unit to be coupled to a quantum cascade laser (QCL) based spectrometer. During standard operation 10 l of ambient air are preconcentrated on a HayeSep D trap and desorbed in 50 ml of synthetic air. Rigorous tests were conducted, using FTIR, quantum cascade laser absorption spectroscopy (QCLAS), GC-FID and component-specific ozone and oxygen analysers to investigate recovery rates, conservation of isotopic signatures and spectral interferences after preconcentration. We achieve quantitative N2O recovery of >99% with only minor, statistically not significant isotopic fractionation and no relevant spectral interferences from other atmospheric constituents. The developed preconcentration unit also has the potential to be applied to other trace gases and their isotopic composition.