We have developed a turnkey, fast response two-channel NO_x monitor (NO_x =NO + NO₂) which provides  simultaneous measurements of both NO_x and NO₂ and thus NO by subtraction.  The NO_x channel employs a carefully controlled concentration of photolytically-produced ozone (O₃) to convert NO into NO₂.  The monitor can measure NO₂ and NO_x with an accuracy of better than 5% and precision of < 0.2 ppb (1s, 1s) with <1 second physical time constant in each measurement channel.  The monitor is based on Aerodyne Research’s patented CAPS (Cavity Attenuated Phase Shift) technology previously employed to measure NO₂concentrations, aerosol optical extinction and aerosol single scattering albedo. 

A CAPS-based NO₂ monitor utilizes a light-emitting diode (LED) as a light source (450 nm for the NO₂ channel and 405 nm for the total NO_x channel) and a sample cell incorporating two high reflectivity mirrors; a vacuum photodiode is used to detect the light emitted from the cell. The square wave modulated light from the LED passes through the absorption cell and is detected as a distorted waveform which is characterized by a phase shift with respect to the initial modulation.   The amount of that phase shift is a function of fixed instrument properties - cell length, mirror reflectivity, and modulation frequency– and of the presence of variable concentrations of nitrogen dioxide.  The mixing ratio is calculated from the value of the cotangent of the phase shift, the speed of light, LED modulation frequency and the absorption coefficient of NO₂.  The use of 405 nm detection in the total NOx channel greatly reduces the possible interference caused by the presence of several ppm of O3 in the sample flow.

We present data from an intercomparison of the NOx monitor with Aerodyne TILDAS monitors measuring both NO and NO2.  The TILDAS monitors utilize infrared diode lasers to probe individual ro-vibrational absorption lines and are considered the “gold standard” for measuring concentrations of small molecules.  The monitors were deployed on the Aerodyne Mobile Laboratory during the MOOSE (Michigan Ontario Ozone Source Experiment) campaign in late 2021.  The figure presented below compares the measurement of both NO₂ and the sum of NO+NO₂ measured by the TILDAS monitors compared with that obtained by the CAPS NO monitor at 1 second resolution.  Note the excellent agreement with respect to both magnitude and time resolution.  Long term comparisons indicate that correlation coefficients at 1 second  resolution approach 1. 

How to cite: Freedman, A., Roscioli, J., Moul, B., and Onasch, T.: Two-Channel, Fast Time-Response CAPS-Based NOx Monitor, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3685, https://doi.org/10.5194/egusphere-egu23-3685, 2023.

The hydroperoxyl radical (HO₂) plays a key role in atmospheric chemistry. It reacts with NO to generate hydroxyl radical (OH) and nitrogen dioxide (NO₂), resulting in the HO_x (= OH + HO₂) cycle that governs the atmospheric oxidation capacity and the formation of air pollution, and a net production of ozone that determines the troposphere ozone budget. Quantitative measurement of its absolute concentration is very important. However, due to its short lifetime and low concentration (typically about 10⁸ to 10⁹ molecule/cm³ under atmospheric condition), most of the currently used methods are indirect methods that require chemical conversion; direct measurement remains very challenging.

The cavity ring-down spectroscopy (CRDS) technique uses high reflectivity mirrors to increase the effective absorption pathlength to tens of kilometers, enabling very high detection sensitivity; the absolute concentration of the target absorbers can be quantitatively determined by the Beer-Lambert law, providing a powerful tool for direct measurement of free radicals. In this work, we report the development of a portable cavity ring-down spectrometer for direct and absolute measurement of HO₂ radical concentration using a distributed feedback (DFB) diode laser operating at 1506 nm. At a pressure of 100 mbar, a detection limit of ~ 7.3×10⁷ molecule/cm³ (1σ, 10s) was achieved with a ring-down time (τ₀) of 136 μs. The corresponding detection sensitivity was 1.5×10^-11 cm^-1, which was close to the state-of-the-art performance.

In cavity ring down spectroscopy, the coupling efficiency of the laser beam into the cavity depends on the laser frequency tuning speed and the ratio of the laser linewidth to the cavity mode width. For the DFB laser system, the laser linewidth (~ 2 MHz) was about thousands of times larger than that of the cavity mode (~ 1.2 kHz), which results in the conversion of laser phase noise into amplitude fluctuation, making the cavity injection noisy and limiting the improvement of detection sensitivity. Here, by replacing the DFB diode laser with a narrow linewidth erbium-doped fiber (EDF) laser, the amplitude fluctuation caused by the laser phase noise was reduced and the cavity mode injection efficiency was improved. The sensitivity was improved to 3.9×10^-12 cm^-1 with a short data-acquisition time of 0.2 s. The one order of magnitude improvement makes further ambient applications look promising.

How to cite: Zhao, W., Wang, C., Fang, B., Yang, N., Cheng, F., Hu, X., Chen, Y., Zhang, W., Fittschen, C., and Chen, W.: High-sensitivity detection of HO2 radical by cavity ring-down spectroscopy: prospects for future applications of narrow linewidth lasers, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1963, https://doi.org/10.5194/egusphere-egu23-1963, 2023.

The balloon-borne and airborne instrument SPECIES (SPECtromètre Infrarouge à lasErs in Situ) recently built in our laboratory will be described. This is a mid-infrared absorption spectrometer, including four channels by coupling Interband or Quantum Cascade Lasers (ICLs or QCLs) to Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OF-CEAS). Using cavities of 50 cm length, this leads to very high resolution (< 0.005 cm^-1) spectra and very long optical paths (> 5 km) and thus, low detection limits for the trace gases to be measured. It can contribute to the detailed description and understanding of the functioning of the free troposphere and stratosphere in terms of composition, chemical reactivity and circulation of air masses by carrying out fast (< 2 s) in-situ measurements of reactive trace gases and greenhouse gases among CO, NO_x, CH₂O, ¹²CO₂, ¹³CO₂, CH₄ and N₂O, at very high spatial resolution, i.e. a few meters vertically or hundred meters horizontally. Mini-SPECIES is the lightened version of SPECIES, comprising two lasers coupled to two cavities and reduced electrical power, which allows its integration in aircraft or its operation for long-duration stratospheric balloon flights (> 4 days). High accuracies are obtained when calibration in flight, or at ground before and after the flight, is performed against standards. In addition to providing reference measurements for calibration/validation of space missions, these performances can lead to in-depth characterization of particular atmospheric processes.

How to cite: Catoire, V., Jacquet, P., Chartier, M., Romanini, D., Krysztofiak, G., Ventrillard, I., and Robert, C.: SPECIES: a balloon-borne and airborne instrument coupling infrared lasers with Optical Feedback Cavity Enhanced Absorption Spectroscopy technique for atmospheric in-situ trace-gas measurements, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12978, https://doi.org/10.5194/egusphere-egu23-12978, 2023.

A newly constructed thermal dissociation cavity ring-down spectrometer (TD-CRDS) for simultaneous measuring NO₂, total peroxy nitrates (ΣPNs) and total alkyl nitrates (ΣANs) was presented. NO₂ is detected directly at around 405.46 nm, ΣPNs and ΣANs are detected as NO₂ after thermal decomposition at 180℃ and 360℃. The influences of the recombination reaction of RO₂ radicals in two different types of heated inlets were discussed and compared, and the thermal decomposition efficiency of PNs was found to be higher with the value of 96% at the heated inlet filled with glass beads than the other (72%). Possible interferences, mainly O₃ (including reactions of O₃ via NO and O₃ via NO₂) and NOx (such as the recombination reactions of NOx and peroxy radicals at different thermal temperatures), were quantitatively characterised. The effects were found to be much weaker in the heated inlet filled with glass beads. Thus, a calibration method for measuring ΣPNs and ΣANs was established, especially to solve the accurate measurement of ΣPNs and ΣANs under high amounts of ambient NOx and O₃ in China. At the time resolution of 20 s, the detection limits of the TD-CRDS instrument for NO₂, ΣPNs and ΣANs are 6 pptv (1σ), 15 pptv (1σ) and 15 pptv (1σ), respectively. Finally, we applied the instrument to the Hefei field campaign, obtaining the concentration distribution and variation characteristics of ΣPNs and ΣANs.

How to cite: Lin, C., Hu, R., Xie, P., Zhang, G., Tong, J., and Liu, W.: A three-channel thermal dissociation cavity ring-down spectrometer for the continuous measurement of ambient NO2, total peroxy nitrates and total alkyl nitrates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10765, https://doi.org/10.5194/egusphere-egu23-10765, 2023.

Quantifying sources and sinks, as well as chemical activity of trace gases in the lower troposphere, requires accurate measurements of the concentrations of the species of interest. While there exist in-situ measurement techniques, which are highly accurate, point-like measurements are only sufficiently representative in the spatial domain for a small area. This holds true in particular in high-gradient environments, e.g., urban settings. Hence, measuring those concentrations averaged on the length scale of a few kilometers is desirable. Furthermore, quantifying emissions requires combining concentration measurements with regional transport models, which cover a comparable spatial resolution.

We present a long open-path setup that measures average concentrations on the kilometer-scale in the urban boundary layer. Our setup, which is operational since March 2022, is based on a Bruker IFS 125 HR Fourier transform spectrometer, a commercially available spectrometer, which offers high resolution and throughput. The instrument choice provides flexible and explorative experimental setups such as variable and high spectral resolution and the extension of spectral coverage from the shortwave-infrared to the near ultra-violet spectral range. Here, we present the results on CO₂ and CH₄ from our 2022 measurement campaign as well as analysis of the diurnal variability in comparison to the prediction of local and regional transport models.

How to cite: Schmitt, T., Kuhn, J., Pilz, L., Maiwald, R., May, M., Kleinschek, R., Edinger, P., Schmitt, S., Hase, F., Griffith, D. W. T., and Butz, A.: Long open-path measurements of CO2 and CH4 with an 125HR FTS in an urban environment., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4403, https://doi.org/10.5194/egusphere-egu23-4403, 2023.

Laser spectrometers have shown good capability in measuring mixing ratios for atmospheric greenhouse gases (GHGs). Given the fact that spectral bands of CO₂, CH₄, and N₂O covering from near to mid-infrared (NIR to MIR) wavelengths, and the limitations in the wavelength coverage of the laser and photodetector, it is very challenging to analyze CO₂, CH₄, and N₂O simultaneously for most cavity-enhanced analyzers using NIR lasers. Alternative solutions, such as combining multiple quantum cascade laser (QCL) beams and analyzing all GHGs in the MIR wavelengths, would significantly increase the instrumental cost.

Here, we present a recently developed analyzer utilizing the advantages of detecting CO₂/H₂O in the NIR spectral region and N₂O/CH₄ in the MIR region, respectively. Through a unique optical design, two independent optical paths are formed in a single Herriott gas cell so that low temperature-related drift and mechanical robustness are achieved. The mixing ratios of CO₂ and H₂O are analyzed by a NIR laser and a photodetector at ~4995cm^-1, while CH₄ and N₂O are analyzed by a QCL and an MCT photodetector at ~1275cm^-1. The analyzer facilitates high-sensitivity, field-deployable measurements of CO₂, CH₄, N₂O and H₂O altogether in a compact, portable instrumental design. In addition, this analyzer can be completely powered by rechargeable battery, facilitating all-day in situ measurements without grid power supply.

In the laboratory, side-to-side comparisons were performed between our newly developed analyzer and another commercial gas analyzer based on cavity ring down spectroscopy (CRDS). The results showed high consistency in GHG mixing-ratio measurements with the two spectrometers. Attached to soil chambers, we also found comparable performance of two analyzers in determining GHG fluxes. In particular, we found that the presented analyzer could precisely capture transient changes in gas mixing ratios from the soil chamber. Recently, field deployment in different soil conditions, including upland forest soils and riparian soils, was carried out for simultaneous N₂O, CH₄, CO₂ soil flux measurements. The overall results suggest that our analyzer is suitable for continuous GHG flux monitoring under variable field conditions, and shows potentials in simultaneous measurements of multiple GHG fluxes from natural ecosystems.

How to cite: Yu, L., Wang, Y., Niu, Z., Su, Z., Zheng, L., and Lin, T.-J.: A portable, high-precision optical analyzer based on hybrid laser absorption cell for simultaneous measurements of N2O, CH4, and CO2 fluxes from soils, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5232, https://doi.org/10.5194/egusphere-egu23-5232, 2023.

The measurement of singly substituted, stable isotopologues, such as ¹³CO₂, by mid-IR spectroscopy is well established. In addition, there is a great interest to exploit the information carried by more exotic isotopologues, i.e. of low abundance, multiply substituted (clumped) isotopic species or site-specific isotopomers. This information on isotopic composition can be used as proxy to constrain formation pathways, source attribution, temperature histories or dating (radioactive isotopes) of the respective molecules. The established method to perform such isotopic analysis is isotope ratio mass spectrometry (IRMS). This approach, however, in particular for rare isotopologues, typically requires very demanding instruments, several hours of analysis time and extensive sample preparation to separate isobaric interferences.

Here, we demonstrate an alternative analytical method based on optical interrogation of the molecules by directly probing their ro-vibrational frequencies. This makes the method inherently suitable to distinguish between isotopomers (structural isomers). Furthermore, we propose a low temperature approach that substantially reduces the spectral interferences due to hot-band transitions of more abundant isotopologues. The effectiveness and versatility of this strategy are highlighted by three different applications: i) high-precision mid-IR measurements of clumped ¹²C¹⁸O₂, ii) the detection of ¹⁴CO₂ in enriched CO₂ samples, and iii) a new scheme for determination of the intramolecular distribution (terminal and central positions) of ¹³C in propane.

We developed a quantum cascade laser (QCL) spectrometer using a Stirling-cooled circular multipass absorption cell. The distributed feedback (DFB) QCL is driven in intermittent continuous wave (iCW) mode [1] with a repetition rate of 6.5 kHz. Its beam passes through a compact segmented circular multipass cell (SC-MPC) [2] with an optical path length of 6 m. The SC-MPC is placed in a vacuum chamber that is maintained at 5ּ 10^-5 mbar and cooled down to 150 K.

The precision in the ratios [¹²C¹⁸O₂]/[¹²C¹⁶O₂] and [¹²C¹⁶O¹⁸O]/[¹²C¹⁶O₂] is 0.05 %₀ with 25 s integration time. Its accuracy is confirmed by agreement with literature values of the equilibrium constant, K, of the exchange reaction for CO₂ samples equilibrated at 300 K and 1273 K [3].

As proof of concept, we adapted the system to allow the detection of the radiocarbon ¹⁴C in enriched CO₂ samples. Due to its ultra-low abundance (10^-12), the absorption signatures of this isotopic species is completely hidden by the spectral contributions of the other, more abundant, CO₂ isotopologues. Therefore, it is the perfect candidate for low-temperature spectroscopy. We present first results on ¹⁴CO₂ with a precision of 50 ppt.

And finally, we demonstrate the first high-resolution spectra of propane and its site-specific isotopomers (1-¹³C and 2-¹³C). We distinguish their individual contributions to the overall absorption spectrum and show a precision better than 0.1 ‰ for both isotopomer ratios (2-¹³C)/¹²C and (1-¹³C)/¹²C.

[1] M. Fischer et al., Opt. Express, 22(6), 7014–7027 (2014), doi: 10.1364/OE.22.007014.

[2] M. Graf, L. Emmenegger, and B. Tuzson, Opt. Lett., 43(11), 2434-2437, (2018), doi: 10.1364/OL.43.002434.

[3] A. Nataraj et al. Opt. Express 30, 4631-4641 (2022): doi.org/10.1364/OE.447172.

How to cite: Emmenegger, L., Nataraj, A., Gianella, M., Faist, J., and Tuzson, B.: Low-temperature mid-IR absorption spectroscopy for isotopomer-specific measurements, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16250, https://doi.org/10.5194/egusphere-egu23-16250, 2023.

How to cite: Safi, E., Rennick, C., Chung, E., and Arnold, T.: A novel Laser-based technique for in-situ automated measurements of atmospheric CH4 isotopologues and implications for current and future emission scenarios , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12554, https://doi.org/10.5194/egusphere-egu23-12554, 2023.

We will present the preliminary results of the CarbonSurvey project (Towards the Next Generation of Sensors for Surveying the Atmospheric Carbon Cycle). As outlined by the European Commission green report in the framework of the Operational Anthropogenic CO2 Emissions Monitoring and Verification Support (MVS) capacity, the existing ground-based networks currently do not meet all the operational requirements for the Copernicus CO2 MVS capacity due to the lack of in situ measurement data from urban areas and other emission hot spots. The main expected contribution of this project is to address this weakness through the development of a new generation of instruments to enable unprecedented CO2 monitoring capabilities, the biggest GHG contributor to human-caused global warming. The necessary scientific and technical contributions required to reach the main goal of the project involve two complementary developments: (i) firstly, a gas analyzer capable of obtaining the vertical profile (with resolution in altitude) of CO2 concentration. The system, which is based on the Laser Heterodyne Radiometry (LHR) technique, will operate from the Earth's surface analyzing the effect in the spectrum of the received sunlight of the atmospheric components to accurately find the distribution of the concentration of CO2 in the atmospheric column. Thus, this instrument will provide a characterization of the CO2 in the atmospheric volume located above the measurement site. Secondly, (ii) the project aims to develop Photacoustic Spectroscopy (PAS) cost-competitive photonic solution for in situ urban GHG measurements. As a main difference with today’s commercially available instrumentation, the system proposed, based on a compact photoacoustic sensing cell, will combine small-size, high sensitivity, and a straightforward field deployment capacity. This directly enables the possibility of providing an accurate, and potentially gap-free, map of the concentration of gases at ground level. These two sets of instruments provide indeed complementary information for a full reconstruction of the map of CO2 around the areas of interest. It is important to remark that both instrument designs will be equipped as well with an important additional feature: the ability to determine the isotopic fingerprint of CO2 in order to discern between natural and anthropogenic CO2 sources, such as fossil fuel combustion or biogenic respiration.

The CarbonSurvey project is funded by the Spanish State Research Agency, it started December 2022 and last until November 2024. We will present the sensors design and the preliminary laboratory results. By the end of the project, we will have both sensor prototypes fully operational and calibrated at the Izaña Atmospheric center. Moreover, the sensing systems will be specifically designed for a straightforward in situ deployment in different areas of interest, providing full coverage of the most important blind spots existing today. This new generation of sensors could establish the necessary basis to guide decision-making policies in the green transition process ahead.

How to cite: Ruiz-Llata, M., Martín-Mateos, P., Bonilla-Marique, O., Moreno-Oyervides, A., Moreno-Sanoyan, Y., and García, O.: Advances and results of two novel sensors for Surveying the Atmospheric Carbon Cycle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15362, https://doi.org/10.5194/egusphere-egu23-15362, 2023.

George Washington University and Mesa Photonics are developing and deploying a Laser Heterodyne Radiometer (LHR) that simultaneously measures CO₂, CH₄, H₂O, and O₂ mixing ratios throughout the troposphere and lower stratosphere. Spectral fits are constrained by fitting the oxygen spectral line shape – which depends only on pressure and temperature – to improve GHG retrieval precision and provide dry-air corrections. This constraint is achieved by fitting pressure and temperature profiles obtained from the meteorology data measured by radiosondes (vertical resolution ranging from 5-500 m) as part of NOAA’s Integrated Global Radiosonde Archive (IGRA).

Atmospheric spectra are simulated for the column using a spectral simulation package (“LahetraSim”) that uses parameters from the HITRAN spectral database to model spectra through the HITRAN Application Programming Interface (HAPI). Integrated path absorption spectra are calculated using the initial sun angle and estimated radiosonde pressure and temperature profiles. Concentration profiles for CO₂, CH₄, and H₂O can then be iterated on their vertical distributions and refining the pressure and temperature profiles to best fit the oxygen spectrum.

Here we present a comparison of our spectral simulation software to that of an alternative method that has been presented for LHR data processing which makes use of the Planetary Spectrum Generator (PSG) API. Spectral simulations are generated using several atmospheric databases, templates, and transfer models. This tool allows for the extraction of Modern Era Retrospective Analysis, Version 2 (MERRA-2) vertical profiles of pressure and temperature and species abundances from HITRAN. Two limitations of the MERRA-2 database are the latency of approximately 2 months for the latest data availability and the coarse vertical resolution (~1 km) of the available data.  Another benefit of our approach is that approximation of both Planetary Boundary Layer (PBL) and Tropopause Heights can be extracted from the temperature and pressure profiles and these heights can be iterated to refine atmospheric layers; thus increasing their relevance of LHR to GHG modeling.

How to cite: Miller, J. H., Flores, M., Bomse, D., and Gomez, A.: An Analysis of Spectral Simulation Methods for Laser Heterodyne Radiometry for the Vertical Profiling of Greenhouse Gas (GHG) Mixing Ratios, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16612, https://doi.org/10.5194/egusphere-egu23-16612, 2023.