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Quantifying sector-resolved methane fluxes in complex emissions environments is challenging yet necessary for inventory validations. We separate energy and agriculture sector methane using a dynamic linear model of methane, ethane, and ammonia mixing ratios measured at a Northern Colorado site from November 2021 to January 2022. Combining observations with spatially resolved inventories and inverse methods, energy and agriculture methane fluxes are constrained across a ~850 km area. Optimized energy sector fluxes were 22% lower than the inventory despite a ~360% increase in regional energy production since the inventory was constructed, suggesting a regional decline in emissions factors. In contrast, optimized agriculture fluxes were 3× larger than the inventory; we demonstrate this discrepancy is consistent with the spatial distribution of agricultural sources. These results highlight the utility of sector-apportioned methane observations for multi-sector inventory optimization in complex environments, which may prove valuable for national and global quantification of sector-resolved methane fluxes.

We present an open-path mid-infrared dual-comb spectrometer (DCS) capable of precise measurement of the stable water isotopologues H216O and HD16O. This system ran in a remote configuration at a rural test site for 3.75 months with 60 % uptime and achieved a precision of 216O and HD16O (δD) in 1000 seconds. Here, we compare the δD values from the DCS to those from the National Ecological Observatory Network (NEON) isotopologue point sensor network. Over the multi-month campaign, the mean difference between the DCS δD values and the NEON δD values from a similar ecosystem is

We demonstrate the application of mode-locked mid-infrared dual frequency comb spectroscopy for combustion analysis. With two settings of the same dual-comb system, the measurement spans 1500 cm−1 from 2.8 to 5 µm with 0.0067 cm−1 (200 MHz) point spacing, or almost a quarter-million discrete comb modes. Using this broadband spectrometer, we quantify the pyrolysis and smoldering combustion of wood samples. Specifically, we measure 20-second time-resolved mole fractions of CH4, H2O, two isotopologues of CO2 (12C16O2, 13C16O2), two isotopologues of CO (12C16O, 13C16O), ethane, formaldehyde, methanol, formic acid, as well as gas temperature directly above radiatively heated wood samples. The combination of the fine spectral resolution and the broad bandwidth of the dual-comb spectrometer allows for precise separation of absorption signatures of individual molecules from the congested spectra.

Spectrally resolved photoacoustic imaging is promising for label-free imaging in optically scattering materials. However, this technique often requires acquisition of a separate image at each wavelength of interest. This reduces imaging speeds and causes errors if the sample changes in time between images acquired at different wavelengths. We demonstrate a solution to this problem by using dual-comb spectroscopy for photoacoustic measurements. This approach enables a photoacoustic measurement at thousands of wavelengths simultaneously. In this technique, two optical-frequency combs are interfered on a sample and the resulting pressure wave is measured with an ultrasound transducer. This acoustic signal is processed in the frequency-domain to obtain an optical absorption spectrum. For a proof-of-concept demonstration, we measure photoacoustic signals from polymer films. The absorption spectra obtained from these measurements agree with those measured using a spectrophotometer. Improving the signal-to-noise ratio of the dual-comb photoacoustic spectrometer could enable high-speed spectrally resolved photoacoustic imaging., Spectrally resolved photoacoustic images often require the acquisition of data for each wavelength separately. Here, the authors use dual frequency-comb spectroscopy for photoacoustic measurements, enabling spectrally resolved measurements without the need to scan the illumination wavelength.

Using an open-path mid-infrared dual-comb spectrometer, we measure the δD of atmospheric water vapor at a remote site with

Multi-month methane and ethane measurements in the Northern Colorado Front Range using open-path dual-comb spectroscopy are reported. Conditional probability densities of measurements stratified by composition ratio distinguishes between wellhead and non-wellhead emissions.

Dual-comb spectroscopy measures greenhouse gas concentrations over kilometers of open air with high precision. However, the accuracy of these outdoor spectra is challenging to disentangle from the absorption model and the fluctuating, heterogenous concentrations over these paths. Relative to greenhouse gases, O2 concentrations are well-known and evenly mixed throughout the atmosphere. Assuming a constant O2 background, we can use O2 concentration measurements to evaluate the consistency of open-path dual-comb spectroscopy with laboratory-derived absorption models. To this end, we construct a dual-comb spectrometer spanning 1240 nm to 1700nm, which measures O2 absorption features in addition to CO2 and CH4. O2 concentration measurements across a 560 m round-trip outdoor path reach 0.1% precision in 10 minutes. Over seven days of shifting meteorology and spectrometer conditions, the measured O2 has -0.07% mean bias, and 90% of the measurements are within 0.4% of the expected hemisphere-average concentration. The excursions of up to 0.4% seem to track outdoor temperature and humidity, suggesting that accuracy may be limited by the O2 absorption model or by water interference. This simultaneous O2, CO2, and CH4 spectrometer will be useful for measuring accurate CO2 mole fractions over vertical or many-kilometer open-air paths, where the air density varies.

We combine mid-infrared dual-comb spectroscopy with attenuated total reflectance measurements to provide in-situ monitoring of a chemical reaction. The mid-infrared dual-comb spectrometer measures quantitative absorption cross-sections of mesityl oxide and diacetone alcohol in the region of 3300–2700 cm−1 (3.0–3.7 μm). Based on these spectra, the system is then used to monitor the hydration of mesityl oxide to diacetone alcohol from which we extract reaction kinetics values. This work shows the potential of dual comb systems to measure and monitor reactions in the condensed phase.

A versatile method of dual-comb spectroscopy is presented. Digital control of the comb frequencies enables dynamically adaptable spectrometer resolution to efficiently match a sample’s feature width, optimizing the system for new applications.

Simultaneous retrieval of H216O and H218 concentrations at an agricultural site is demonstrated using near-infrared open-path dual-comb spectroscopy (DCS). Initial retrievals of the isotopologue ratio δ18O have a statistical precision of 5‰ at one hour. Work of the US Government and not subject to copyright.

Dual-comb laser absorption spectroscopy is demonstrated over a 28 km open path over an urban environment. Path averaged concentrations of CO2, H2O and CH4 are retrieved despite significant turbulence induced link loss.

Open-path dual-comb spectroscopy (DCS) is an emerging technique for long open-path measurements across km-scale paths. It provides both broad spectral coverage over hundreds of wavenumbers with high spectral resolution and negligible instrument lineshape. In this chapter, we discuss the principle of open-path DCS; provide an overview of the necessary technology and analysis techniques; review field applications including near-infrared (NIR) measurements of urban CO2 emissions, CH4 emissions from drilling and agricultural operations, measurements to a unmanned aerial vehicle, and recent measurements of volatile organic compounds in the mid-infrared (MIR); and finally provide an outlook especially on how this technique can move to additional spectral regions beyond the NIR and MIR.

Open-path dual-comb spectroscopy measures multispecies livestock emissions, enabling future field-scale ecological studies., Advances in spectroscopy have the potential to improve our understanding of agricultural processes and associated trace gas emissions. We implement field-deployed, open-path dual-comb spectroscopy (DCS) for precise multispecies emissions estimation from livestock. With broad atmospheric dual-comb spectra, we interrogate upwind and downwind paths from pens containing approximately 300 head of cattle, providing time-resolved concentration enhancements and fluxes of CH4, NH3, CO2, and H2O. The methane fluxes determined from DCS data and fluxes obtained with a colocated closed-path cavity ring-down spectroscopy gas analyzer agree to within 6%. The NH3 concentration retrievals have sensitivity of 10 parts per billion and yield corresponding NH3 fluxes with a statistical precision of 8% and low systematic uncertainty. Open-path DCS offers accurate multispecies agricultural gas flux quantification without external calibration and is easily extended to larger agricultural systems where point-sampling-based approaches are insufficient, presenting opportunities for field-scale biogeochemical studies and ecological monitoring.

We measure speed-dependent Voigt lineshape parameters with temperature-dependence exponents for several hundred spectroscopic features of pure water spanning 6801–7188 cm(−1). The parameters are extracted from broad bandwidth, high-resolution dual frequency comb absorption spectra with multispectrum fitting techniques. The data encompass 25 spectra ranging from 296 K to 1305 K and 1 to 17 Torr of pure water vapor. We present the extracted parameters, compare them to published data, and present speed-dependence, self-shift, and self-broadening temperature-dependent parameters for the first time. Lineshape data is extracted using a quadratic speed-dependent Voigt profile and a single self-broadening power law temperature-dependence exponent over the entire temperature range. The results represent an important step toward a new high-temperature database using advanced lineshape profiles.

We report argon-broadened water vapor transition parameters and their temperature dependence based on measured spectra spanning 6801–7188 cm(−1) from a broad-bandwidth, high-resolution dual frequency comb spectrometer. The 25 collected spectra of 2% water vapor in argon ranged from 296 K to 1305 K with total pressure spanning 100 Torr to 600 Torr. A multispectrum fitting routine was used in conjunction with a quadratic speed-dependent Voigt profile to extract broadening and shift parameters, and a power-law temperature-dependence exponent for both. The measurements represent the first broad bandwidth, argon-broadened water vapor absorption study, and are an important step toward a foreign-gas-perturbed, high-temperature database developed using advanced lineshape profiles.

The output of a laser frequency comb is composed of 100,000+ perfectly spaced, discrete wavelength elements or comb teeth, that act as a massively parallel set of single frequency (CW) lasers with highly stable, well-known frequencies. In dual-comb spectroscopy, two such frequency combs are interfered on a single detector yielding absorption information for each individual comb tooth. This approach combines the strengths of both cw laser spectroscopy and broadband spectroscopy providing high spectral resolution and broad optical bandwidths, all with a single-mode, high-brightness laser beam and a simple, single photodetector, detection scheme. Here I will touch on the application of this system for open-path measurements of atmospheric trace gases (CH4, CO2, CO, NH3, water, ethane, and N2O) and volatile organic compounds (acetone, isopropanol, propane) with field applications targeting industrial oil and gas monitoring and agriculture.

This manuscript describes the design of a robust, mid-infrared dual-comb spectrometer operating in the 3.1-µm to 4-µm spectral window for future field applications. The design represents an improvement in system size, power consumption, and robustness relative to previous work while also providing a high spectral signal-to-noise ratio. We demonstrate a system quality factor of 2×106 and 30 hours of continuous operation over a 120-meter outdoor air path.

Open-path measurements of atmospheric gas species over km-scale path lengths are well suited to quantify emissions from sources like oil and gas, forest fires, and industry. is a relatively new technique that combines high-resolution and broad spectral coverage with no instrument lineshape and near perfect frequency calibration. These features have enabled open-path DCS to provide accurate measurements of multiple trace gas species simultaneously in the near-infrared across path lengths ranging from 100 m to several km. However, in order to reach the sensitivity necessary to detect many atmospheric trace constituents, including volatile organic compounds (VOCs), operation in the mid-infrared (or UV/Vis) is required.Here, we show a mid-infrared open-path dual comb spectrometer operating in the 3-4 and 4.5-5 μm spectral regions. We have used this spectrometer to measure methane, ethane, and propane (arising primarily from oil and gas activity) across a 1-km-long path in Boulder, CO for 1 week with an ethane sensitivity of ∼0.1 ppb for a 2-minute time resolution. In addition, we show quantitative measurements of intentionally released acetone and isopropanol with a 1-σ sensitivity of 5.7 ppm·m and 2.4 ppm·m, respectively. In the 4.5-5 μm region, we have used this system to detect N2O, CO, and O3. Finally, we have developed a second-generation instrument in the 3-4 μm region that is more compact and has improved stability. This system was recently deployed in a van at an active oil and gas drilling operation. We present preliminary measurements of methane, ethane, and higher hydrocarbons from this deployment as well as initial efforts at emissions quantification.

Open-path dual-comb spectroscopy is used to quantify CO2, CH4, H2O and NH3 concentrations adjacent to a cattle feedlot. In all cases except NH3, the data is compared against closed-path cavity ring-down spectroscopy.

Dual-comb photoacoustic spectroscopy (DCPAS) is a new spectroscopy technique that measures photoacoustic signals in thousands of wavelength bands simultaneously. We report the first demonstration of this technique and show DCPAS spectra of polymer films. Work of the United States government, not subject to copyright.

Open-path mid-infrared dual-comb spectroscopy is performed over a 2 km- long link with bandwidth spanning the atmospheric transmission window near 4.7 pm. We quantify mixing ratios for CO, CO2, N2O, H2O, and O3. Work of the US Government and not subject to copyright.

We use a broadband mid-IR dual-comb spectrometer operating between 2.8 and 5 microns to measure time-resolved mole fractions of 12 gaseous species and isotopologues in the pyrolysis products above a heated Eucalyptus wood sample.

Open-path dual-comb spectroscopy in the 4.5-4.9 pm region is performed over a 300 m- long link to quantify mixing ratios for CO, CO2, N2O, H2O, and O3.

We deploy a mid-infrared dual-comb spectrometer in a van for field measurements of volatile organic compounds (VOCs). We also use a similar, laboratory system to measure ambient VOCs over a 2km round-trip path. Work of the US Government and not subject to copyright.

We demonstrate a robust, portable mid-infrared dual-comb spectrometer based on modelocked fiber lasers and difference frequency generation, allowing high resolution, open-path spectroscopy in the 3.1-4 m spectral region. Work of the US Government and not subject to copyright.

Near-infrared dual-comb spectroscopy is performed over two open-air paths upwind and downwind from pens containing approximately four hundred cattle. Time- resolved enhancements of CO2, CH4, NH3 and H2O were observed on the downwind path. Work of the US Government and not subject to copyright.

We used a MIR dual frequency comb spectroscopy system to measure emissions at a fracking site. We present preliminary measurements of methane, ethane, and higher hydrocarbons. The methane shows good agreement with a co-located point sensor.

We report argon-broadened water vapor transition parameters and their temperature dependence based on measured spectra spanning 6801–7188 cm−1 from a broad-bandwidth, high-resolution dual frequency comb spectrometer. The 25 collected spectra of 2% water vapor in argon ranged from 296 K to 1305 K with total pressure spanning 100 Torr to 600 Torr. A multispectrum fitting routine was used in conjunction with a quadratic speed-dependent Voigt profile to extract broadening and shift parameters, and a power-law temperature-dependence exponent for both. The measurements represent the first broad bandwidth, argon-broadened water vapor absorption study, and are an important step toward a foreign-gas-perturbed, high-temperature database developed using advanced lineshape profiles.

We measure speed-dependent Voigt lineshape parameters with temperature-dependence exponents for several hundred spectroscopic features of pure water spanning 6801–7188 cm−1. The parameters are extracted from broad bandwidth, high-resolution dual frequency comb absorption spectra with multispectrum fitting techniques. The data encompass 25 spectra ranging from 296 K to 1305 K and 1 to 17 Torr of pure water vapor. We present the extracted parameters, compare them to published data, and present speed-dependence, self-shift, and self-broadening temperature-dependent parameters for the first time. Lineshape data is extracted using a quadratic speed-dependent Voigt profile and a single self-broadening power law temperature-dependence exponent over the entire temperature range. The results represent an important step toward a new high-temperature database using advanced lineshape profiles.

The HITEMP2010 and HITRAN2012 databases are important tools for predicting molecular absorption under various environmental conditions. At room temperature, the databases can be quite accurate, owing to their development using room temperature absorption data. However, at elevated temperatures common to combustion and planetary research, their broadband accuracy is largely unexplored. We utilize a dual frequency comb spectrometer and a high temperature optical cell to assess the capability of these databases to accurately predict water vapor absorption for over 600 transitions at conditions up to 1300 K from 6800 cm−1 to 7200 cm−1. We demonstrate that at 1300 K, HITEMP2010 and HITRAN2012 accurately predict the existence and position (within 0.033 cm−1) of >99% of transitions with an intensity greater than 1E−28 (cm−1/molecule cm−2), which is the approximate minimum detectable intensity for our absorption spectra. Of the slightly less than 1% of transitions that are misreported, all have linestrengths below 1E−23 (cm−1/molecule cm−2). HITEMP more often predicts transitions that are not observed in the absorption spectra, while HITRAN more often fails to predict an observed line. Updated temperature-scaling coefficients for air-broadening and updated line assignments improve the high temperature performance of HITRAN compared to HITEMP for lineshapes and positions. Our analysis also shows that adding simple temperature-scaling relationships for air-pressure shift and self-broadening half width increase the predictive accuracy of simulations using both databases, and should be employed whenever transition-specific information is unavailable.

We present the first quantitative intercomparison between two open-path dual-comb spectroscopy (DCS) instruments which were operated across adjacent 2 km open-air paths over a 2-week period. We used DCS to measure the atmospheric absorption spectrum in the near infrared from 6023 to 6376 cm−1 (1568 to 1660 nm), corresponding to a 355 cm−1 bandwidth, at 0.0067 cm−1 sample spacing. The measured absorption spectra agree with each other to within 5 × 10−4 in absorbance without any external calibration of either instrument. The absorption spectra are fit to retrieve path-integrated concentrations for carbon dioxide (CO2), methane (CH4), water (H2O), and deuterated water (HDO). The retrieved dry mole fractions agree to 0.14 % (0.57 ppm) for CO2, 0.35 % (7 ppb) for CH4, and 0.40 % (36 ppm) for H2O at ∼ 30 s integration time over the 2-week measurement campaign, which included 24 °C outdoor temperature variations and periods of strong atmospheric turbulence. This agreement is at least an order of magnitude better than conventional active-source open-path instrument intercomparisons and is particularly relevant to future regional flux measurements as it allows accurate comparisons of open-path DCS data across locations and time. We additionally compare the open-path DCS retrievals to a World Meteorological Organization (WMO)-calibrated cavity ring-down point sensor located along the path with good agreement. Short-term and long-term differences between the open-path DCS and point sensor are attributed, respectively, to spatial sampling discrepancies and to inaccuracies in the current spectral database used to fit the DCS data. Finally, the 2-week measurement campaign yields diurnal cycles of CO2 and CH4 that are consistent with the presence of local sources of CO2 and absence of local sources of CH4.

Stratospheric aerosols (SAs) are a variable component of the Earth's albedo that may be intentionally enhanced in the future to offset greenhouse gases (geoengineering). The role of tropospheric-sourced sulfur dioxide (SO2) in maintaining background SAs has been debated for decades without in-situ measurements of SO2 at the tropical tropopause to inform this issue. Here we clarify the role of SO2 in maintaining SAs by using new in-situ SO2 measurements to evaluate climate models and satellite retrievals. We then use the observed tropical tropopause SO2 mixing ratios to estimate the global flux of SO2 across the tropical tropopause. These analyses show that the tropopause background SO2 is about 5 times smaller than reported by the average satellite observations that have been used recently to test atmospheric models. This shifts the view of SO2 as a dominant source of SAs to a near-negligible one, possibly revealing a significant gap in the SA budget.

We demonstrate the first frequency comb laser absorption spectroscopy in an industrial environment. Recent advancements in robust frequency comb design enabled installation of the sensor in an operating power plant, where we simultaneously measured temperature, H 2 O and CO 2 concentration in the exhaust of a 16 MW stationary gas turbine. The frequency comb laser spectrometer probed 16,000 individual wavelengths of light spaced by 0.007 cm −1 (0.0014 nm) near 1440 nm, spanning 279 absorption features of H 2 O and 43 features of CO 2 . Fits to the measured absorption spectra yield simultaneous temperature, H 2 O and CO 2 concentrations with between 10 and 60 second time resolution. Measurements over a 5 hour period tracked variations in the exhaust consistent with various changes to the gas turbine operation. Much larger wavelength ranges (200+ nm) and different time resolutions are possible depending on the desired precision by changing various settings on the same spectrometer. Overall, this work demonstrates the potential for frequency comb laser absorption spectroscopy in industrial combustion environments.

Dual-comb spectroscopy has recently attracted significant interest due to its fast acquisition times, absolute frequency accuracy, negligible lineshape, and coherent probe light. We have recently expanded our near-infrared dual-comb spectroscopy efforts to the mid-infrared, which offers significantly improved sensitivity for many trace gas species and access to other species which cannot be measured in the Near-Infrared. Our Mid-Infrared spectrometer is based on two erbium fiber optical frequency combs that generate light spanning from about 3 to 5 microns using a two-branch difference frequency generation (DFG) design with a periodically poled lithium niobate crystal (PPLN). The data product of the spectrometer are interferograms. Once digitized, the interferograms are corrected for residual phase noise of the frequency combs and coadded in real-time on a field-programmable gate array (FPGA). Finally, the optical spectrum is calculated through a Fourier transform of the coadded interferogram. I will present three measurement modalities we implemented with this spectrometer. In a laboratory gas cell measurement, we characterized low-pressure gas phase propane, demonstrating excellent agreement with literature spectra obtained with high-resolution FTIR. In a separate measurement, we performed in-situ monitoring of a chemical reaction using attenuated total reflection spectroscopy. Finally, open-path measurements of atmospheric trace gases (methane, CO2, water, ethane) and volatile organic compounds (acetone, isopropanol) demonstrate the spectrometer's capability to monitor atmospheric trace gases and quantify emissions from sources like oil and gas, forest fires and industry.

Si

We present a compact, mid-infrared (MIR) dual-comb spectrometer using difference- frequency generation. The spectrometer covers 3 μm - 4 μm, within the functional group region.We demonstrate detection of acetone in a controlled-release experiment.

Si$_3$N$_4$ waveguides, pumped at 1550 nm, can provide spectrally smooth, broadband light for gas spectroscopy in the important 2 ${\mathrm{\mu}}$m to 2.5 ${\mathrm{\mu}}$m atmospheric water window, which is only partially accessible with silica-fiber based systems. By combining Er+:fiber frequency combs and supercontinuum generation in tailored Si$_3$N$_4$ waveguides, high signal-to-noise dual-comb spectroscopy (DCS) spanning 2 ${\mathrm{\mu}}$m to 2.5 ${\mathrm{\mu}}$m is demonstrated. Acquired broadband dual-comb spectra of CO and CO$_2$ agree well with database line shape models and have a spectral-signal-to-noise as high as 48$/\sqrt{\mathrm{s}}$, showing that the high coherence between the two combs is retained in the Si$_3$N$_4$ supercontinuum generation. The DCS figure of merit is 6$\times 10^6/\sqrt{\mathrm{s}}$, equivalent to that of all-fiber DCS systems in the 1.6 ${\mathrm{\mu}}$m band. Based on these results, future DCS can combine fiber comb technology with Si$_3$N$_4$ waveguides to access new spectral windows in a robust non-laboratory platform.

In this paper, we give an overview of quantum cascade detector technology for the near- and mid-infrared wavelength range. Thanks to their photovoltaic operating principle, the most advanced quantum cascade detectors offer great opportunities in terms of high detection speed, reliable room temperature operation, and excellent Johnson noise limited detectivity. Besides some important features dealing with their fabrication and their general characteristics, we will also briefly present some possibilities for performance improvement. Elementary theoretical considerations adopted from photoconductive detectors confirm that optimization of such devices always involves various trade-offs.