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δ17O and Δ’17O are emerging tracers increasingly used in isotope hydrology, climatology, and biochemistry. Differentiating small relative abundance changes in the rare 17O isotope from the strong covariance with 18O imposes ultra-high precision requirements for this isotope analysis. Measurements of δ17O by Cavity Ringdown Spectroscopy (CRDS) are attractive due to the ease of sample preparation, automated throughput, and avoidance of chemical conversions needed for isotope-ratio mass spectrometry. However, the CRDS approach requires trade-offs in measurement precision and uncertainty. In this protocol document, we present the following: • New analytical procedures and a software tool for conducting δ17O and Δ’17O measurements by CRDS. • Outline a robust uncertainty framework for Δ’17O determinations. • Description of a CRDS performance framework for optimizing throughput, instrumental stability, and Δ’17O measurement precision and accuracy.

Background: Lung cancer is one of the most common malignant tumors worldwide. Currently, effective screening methods for early lung cancer are still scarce. Breath analysis provides a promising method for the pre-screening or early screening of lung cancer. Isoprene is a potential and important breath biomarker of lung cancer. Material and Methods: To investigate the clinical value of isoprene for diagnosing lung cancer patients, a cavity ringdown spectroscopy (CRDS) based near-real time, sensitive analysis method of breath isoprene is developed in our lab. In this paper, 92 breath samples from lung cancer patients, 17 breath samples from patients with benign lesions, and 107 breath samples from healthy people were collected. Results: Research indicates that breath isoprene concentration is significantly higher in healthy individuals ([Formula: see text][Formula: see text]ppbv) than in patients with lung cancer ([Formula: see text][Formula: see text]ppbv) and benign lung lesions ([Formula: see text][Formula: see text]ppbv). The result of Receiver Operating Characteristic (ROC) curve suggests that the concentration of isoprene is meaningful for the diagnosis of lung cancer ([Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text]). Conclusion: This study demonstrates that the CRDS breath isoprene analysis system can effectively analyze a large sample of human breath isoprene, and preliminarily confirms the use of breath isoprene as a biomarker for lung diseases.

To effectively monitor highly heterogeneous urban CO2 emissions using atmospheric observations, there is a need to deploy cost-effective CO2 sensors at multiple locations within the city with sufficient accuracy to capture the concentration gradients in urban environments. These dense measurements could be used as input of an atmospheric inversion system for the quantification of emissions at the sub-city scale or to separate specific sectors. Such quantification would offer valuable insights into the efficacy of local initiatives and could also identify unknown emission hotspots that require attention. Here we present the development and evaluation of a mid-cost CO2 instrument designed for continuous monitoring of atmospheric CO2 concentrations with a target accuracy of 1 ppm for hourly mean measurements. We assess the sensor sensitivity in relation to environmental factors such as humidity, pressure, temperature and CO2 signal, which leads to the development of an effective calibration algorithm. Since July 2020, eight mid-cost instruments have been installed within the city of Paris and its vicinity to provide continuous CO2 measurements, complementing the seven high-precision cavity ring-down spectroscopy (CRDS) stations that have been in operation since 2016. A data processing system, called CO2calqual, has been implemented to automatically handle data quality control, calibration and storage, which enables the management of extensive real-time CO2 measurements from the monitoring network. Colocation assessments with the high-precision instrument show that the accuracies of the eight mid-cost instruments are within the range of 1.0 to 2.4 ppm for hourly afternoon (12:00–17:00 UTC) measurements. The long-term stability issues require manual data checks and instrument maintenance. The analyses show that CO2 measurements can provide evidence for underestimations of CO2 emissions in the Paris region and a lack of several emission point sources in the emission inventory. Our study demonstrates promising prospects for integrating mid-cost measurements along with high-precision data into the subsequent atmospheric inverse modeling to improve the accuracy of quantifying the fine-scale CO2 emissions in the Paris metropolitan area. [ABSTRACT FROM AUTHOR]

We developed a cavity ringdown spectrometer by utilizing a step-scanning and dithering method for matching laser wavelengths to optical resonances of an optical cavity. Our approach is capable of working with two and more lasers for quasi-simultaneous measurements of multiple gas species. The developed system was tested with two lasers operating around 1654 nm and 1658 nm for spectral detections of 12CH4 and its isotope 13CH4 in air, respectively. The ringdown time of the empty cavity was about 340 µs. The achieved high detection sensitivity of a noise-equivalent absorption coefficient was 2.8 × 10−11 cm−1 Hz−1/2 or 1 × 10−11 cm−1 by averaging for 30 s. The uncertainty of the high precision determination of δ13CH4 in air is about 1.3‰. Such a system will be useful for future applications such as environmental monitoring.

Optical feedback cavity ringdown spectroscopy is presented with a linear Fabry–Pérot cavity and a cost-effective DFB laser. To circumvent the low coupling efficiency caused by the broad laser linewidth, an optical feedback technique is used, and an enhanced coupling efficiency of 31%, mainly limited by impedance mismatch and mode mismatch, is obtained. The trigger of the ringdown event is realized by the shutoff of the laser driving current, and a novel method with the aid of one electronic switch is applied to avoid the ringdown events excited by the unexpected cavity modes during the process of laser current recovery. As a result, the ringdown signal with a signal-to-noise ratio of 2500 is achieved. Through continuous monitoring, the fractional uncertainty of the empty cavity ringdown times is assessed to be 0.04%. An Allan variance analysis indicates a detection sensitivity of 4.3 × 10−10 cm−1 is resulted at an integration time of 120 s, even with a moderate finesse cavity. To further improve the long-term stability, we regularly rectify the empty cavity ringdown time, and an improvement factor of 2.5 is demonstrated.

The present study addresses advanced monitoring techniques for particles and airborne molecular contaminants (AMCs) in cleanroom environments, which are crucial for ensuring the integrity of semiconductor manufacturing processes. We focus on quantifying particle levels and a representative AMC, hydrogen chloride (HCl), having known detrimental effects on equipment longevity, product yield, and human health. We have developed a compact laser sensor based on open-path cavity ring-down spectroscopy (CRDS) using a 1742 nm near-infrared diode laser source. The sensor enables the high-sensitivity detection of HCl through absorption by the 2-0 vibrational band with an Allan deviation of 0.15 parts per billion (ppb) over 15 min. For quantifying particle number concentrations, we examine various detection methods based on statistical analyses of Mie scattering-induced ring-down time fluctuations. We find that the ring-down distributions' 3rd and 4th standard moments allow particle detection at densities as low as ~105 m−3 (diameter > 1 μm). These findings provide a basis for the future development of compact cleanroom monitoring instrumentation for wafer-level monitoring for both AMC and particles, including mobile platforms. [ABSTRACT FROM AUTHOR]

In situ measurements of water vapour isotopic composition in polar regions has provided needed constrains of post-deposition processes involved in the archiving of the climatic signal in ice core records. During polar winter, the temperatures, and thus the specific humidity, are so low that current commercial techniques are not able to measure the vapour isotopic composition with enough precision. Here, we make use of new developments in infrared spectroscopy and combine an optical-feedback frequency-stabilised laser source (OFFS technique) using a V-shaped cavity optical feedback (VCOF) cavity and a high-finesse cavity ring-down spectroscopy (CRDS) cavity to increase the signal-to-noise ratio while measuring absorption transitions of water isotopes. We present a laboratory infrared spectrometer leveraging all these techniques dedicated to measure water vapour isotopic composition at low humidity levels. At 400 ppmv , the instrument demonstrates a precision of 0.01 ‰ and 0.1 ‰ in δ18O and d-excess, respectively, for an integration time of 2 min. This set-up yields an isotopic composition precision below 1 ‰ at water mixing ratios down to 4 ppmv , which suggests an extrapolated precision in δ18O of 1.5 ‰ at 1 ppmv. Indeed, thanks to the stabilisation of the laser by the VCOF, the instrument exhibits extremely low drift and very high signal-to-noise ratio. The instrument is not hindered by a strong isotope–humidity response which at low humidity can create extensive biases on commercial instruments. [ABSTRACT FROM AUTHOR]

Accurate measurement of water vapor isotopes (δ18O and δ2H) is fundamental for advancing our understanding of the hydrological cycle and improving hydrological model accuracy. This study introduces an innovative calibration methodology using a controlled evaporation mixer (CEM) for determining stable isotopic ratios in atmospheric water vapor via cavity ring-down spectroscopy. The CEM technique reliably produces a stable water vapor stream, crucial for enhancing the precision and accuracy of isotopic measurements. Its rapid adaptation to changes in water vapor concentration and compatibility with different water standards enhance calibration reliability. Demonstrated reproducibility in generating water vapor across a broad concentration range from 900 to over 25,000 ppmv, coupled with a substantial reduction in memory effects, makes this approach highly effective in both laboratory and field settings. This calibration advancement greatly enhances research capabilities for continuous atmospheric water vapor analysis, providing deeper insights into hydrological processes and atmospheric dynamics. [ABSTRACT FROM AUTHOR]

A wide-range, calibration-free tunable diode laser spectrometer is established by combining wavelength modulation and direct absorption spectroscopy (WM-DAS) with continuous wave cavity ringdown spectroscopy (CW-CRDS). This spectrometer combines the benefits of absolute concentration measurements, wide range, and high speed, using WM-DAS with enhanced noise reduction in CW-CRDS. The accurate baseline ringdown time, τ0, is calculated by the absorption peak (measured by WM-DAS) and the ringdown time containing gas absorption information (measured by CW-CRDS at the center wavelength of the spectral line). The gas concentration is obtained without measuring τ0 in real time, thus, greatly improving the measuring speed. A WM-DAS/CW-CRDS spectrometer at 1.57 μm for CO detection was assembled for experimental validation of the multiplexing scheme over a concentration ranging from 4 ppm to 1.09% (0.1 MPa, 298 K). The measured concentration of CO at 6374.406 cm−1 shows that the dynamic range of this tunable diode laser absorption spectrometer is extendable up to five orders of magnitude and the corresponding precision is improved. The measurement speed of this spectrometer can extend up to 10 ms, and the detection limit can reach 35 ppb within 25 s.

The cavity ringdown absorption spectrum of acrolein (propenal, CH2=CH-CH=O) was recorded near 412 nm, under bulk-gas conditions at room temperature and in a free-jet expansion. The measured spectral region includes the 000 band of the T1(n, π*) ← S0 system. We analyzed the 000 rotational contour by using the STROTA computer program [R. H. Judge et al., J. Chem. Phys. 103, 5343 (1995)], which incorporates an asymmetric rotor Hamiltonian for simulating and fitting singlet-triplet spectra. We used the program to fit T1(n, π*) inertial constants to the room-temperature contour. The determined values (cm-1), with 2σ confidence intervals, are A = 1.662 ± 0.003, B = 0.1485 ± 0.0006, C = 0.1363 ± 0.0004. Linewidth analysis of the jet-cooled spectrum yielded a value of 14 ± 2 ps for the lifetime of isolated acrolein molecules in the T1(n, π*), v = 0 state. We discuss the observed lifetime in the context of previous computational work on acrolein photochemistry. The spectroscopically derived inertial constants for the T1(n, π*) state were used to benchmark a variety of computational methods. One focus was on complete active space methods, such as complete active space self-consistent field (CASSCF) and second-order perturbation theory with a CASSCF reference function (CASPT2), which are applicable to excited states. We also examined the equation-of-motion coupled-cluster and time-dependent density function theory excited-state methods, and finally unrestricted ground-state techniques, including unrestricted density functional theory and unrestricted coupled-cluster theory with single and double and perturbative triple excitations. For each of the above methods, we or others [O. S. Bokareva et al., Int. J. Quantum Chem. 108, 2719 (2008)] used a triple zeta-quality basis set to optimize the T1(n, π*) geometry of acrolein. We find that the multiconfigurational methods provide the best agreement with fitted inertial constants, while the economical unrestricted Perdew-Burke-Ernzerhof exchange-correlation hybrid functional (UPBE0) technique performs nearly as well. [ABSTRACT FROM AUTHOR]

Background: Lung cancer is one of the most common malignant tumors worldwide. Currently, effective screening methods for early lung cancer are still scarce. Breath analysis provides a promising method for the pre-screening or early screening of lung cancer. Isoprene is a potential and important breath biomarker of lung cancer. Material and Methods: To investigate the clinical value of isoprene for diagnosing lung cancer patients, a cavity ringdown spectroscopy (CRDS) based near-real time, sensitive analysis method of breath isoprene is developed in our lab. In this paper, 92 breath samples from lung cancer patients, 17 breath samples from patients with benign lesions, and 107 breath samples from healthy people were collected. Results: Research indicates that breath isoprene concentration is significantly higher in healthy individuals (2 2 1. 3 ± 1 2 2. 2 ppbv) than in patients with lung cancer (1 1 2. 0 ± 3 6. 6 ppbv) and benign lung lesions (1 2 7. 9 ± 4 1. 2 ppbv). The result of Receiver Operating Characteristic (ROC) curve suggests that the concentration of isoprene is meaningful for the diagnosis of lung cancer (AUC = 0. 8 2 2 , sensitivity = 6 3. 6 % , specificity = 9 0. 2 % , P < 0. 0 1). Conclusion: This study demonstrates that the CRDS breath isoprene analysis system can effectively analyze a large sample of human breath isoprene, and preliminarily confirms the use of breath isoprene as a biomarker for lung diseases. [ABSTRACT FROM AUTHOR]

δ 17 O and Δ' 17 O are emerging tracers increasingly used in isotope hydrology, climatology, and biochemistry. Differentiating small relative abundance changes in the rare 17 O isotope from the strong covariance with 18 O by Cavity Ringdown Spectroscopy (CRDS) are attractive due to the ease of sample preparation, automated throughput, and avoidance of chemical conversions needed for isotope-ratio mass spectrometry. However, the CRDS approach requires trade-offs in measurement precision and uncertainty. In this protocol document, we present the following:•New analytical procedures and a software tool for conducting δ 17 O by Cavity Ringdown Spectroscopy (CRDS) are attractive due to the ease of sample preparation, automated throughput, and avoidance of chemical conversions needed for isotope-ratio mass spectrometry. However, the CRDS approach requires trade-offs in measurement precision and uncertainty. In this protocol document, we present the following:•New analytical procedures and a software tool for conducting δ 17 O and Δ' 17 O measurements by CRDS.•Outline a robust uncertainty framework for Δ' 17 O determinations.•Description of a CRDS performance framework for optimizing throughput, instrumental stability, and Δ' 17 O measurement precision and accuracy., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2023 International Atomic Energy Agency. Published by Elsevier B.V.)

Current formaldehyde (HCHO) measurement networks rely on the TO-11A offline chemical derivatization technique, which can be resource intensive and limited in temporal resolution. In this work, we evaluate the field performance of three new commercial instruments for continuous in situ formaldehyde monitoring: the Picarro cavity ring-down spectroscopy G2307 gas concentration analyzer and Aeris Technologies' mid-infrared absorption Pico and Ultra gas analyzers. All instruments require regular drift correction, which is accomplished through instrument zeroing using dinitrophenylhydrazine (DNPH)-coated cartridges, Drierite, or molecular sieves, while heated Hopcalite failed to remove all incoming HCHO. We show that a modified precision estimate accounting for regular instrument zeroing results in values of 0.09, 0.20, and 0.22 ppb at a 20 min integration time for the G2307, Ultra, and Pico, respectively. After applying standard addition and dynamic dilution calibrations, all instruments agreed within 13 % and were well correlated with each other (all r ≥ 0.90). TO-11A HCHO observations resulted in a normalized mean bias of -58 % compared to co-located Picarro G2307 measurements (r=0.62 , slope = 0.38, int = 0.07 ppb HCHO). Using a 6-month deployment period in the Atlanta metropolitan area, we determined that the Picarro G2307 and Aeris units have sufficient accuracy and precision to capture the Atlanta spatial HCHO gradient. We find that midday HCHO concentrations have decreased by 22.3 % since 1999 in the city's urban core, and DNPH measurements at a nearby Photochemical Assessment Monitoring Station (PAMS) site show a greater decrease of 53 %. [ABSTRACT FROM AUTHOR]

The reaction of HO 2 with NO is one of the most important steps in radical cycling throughout the stratosphere and troposphere. Previous literature experimental work revealed a small yield of nitric acid (HONO 2 ) directly from HO 2 + NO. Atmospheric models previously treated HO 2 + NO as radical recycling, but inclusion of this terminating step had large effects on atmospheric oxidative capacity and the concentrations of HONO 2 and ozone (O 3 ), among others. Here, the yield of HONO 2 , φ HONO 2 , from the reaction of HO 2 + NO was investigated in a flow tube reactor using mid-IR pulsed-cavity ringdown spectroscopy. HO 2 , produced by pulsed laser photolysis of Cl 2 in the presence of methanol, reacted with NO in a buffer gas mixture of N 2 and CO between 300 and 700 Torr at 278 and 300 K. HONO 2 and its weakly bound isomer HOONO were directly detected by their v 1 absorption bands in the mid-IR region. CO was used to suppress HONO 2 produced from OH + NO 2 and exploit a chemical amplification scheme, converting OH back to HO 2 . Under the experimental conditions described here, no evidence for the formation of either HONO 2 or HOONO was observed from HO 2 + NO. Using a comprehensive chemical model, constrained by observed secondary reaction products, all HONO 2 detected in the system could be accounted for by OH + NO 2 . At 700 ± 14 Torr and 300 ± 3 K, φ HONO 2 = 0.00 ± 0.11% (2σ) with an upper limit of 0.11%. If all of the observed HONO 2 was attributed to the HO 2 + NO reaction, φ HONO 2 = 0.13 ± 0.07% with an upper limit of 0.20%. At 278 ± 2 K and 718 ± 14 Torr, we determine an upper limit, φ HONO 2 ≤ 0.37%. Our measurements are significantly lower than those previously reported, lying outside of the uncertainty of the current experimental and recommended literature values.

To effectively monitor the highly heterogeneous urban CO2 emissions using atmospheric observations, there is a need to deploy cost-effective CO2 sensors at multiple locations within the city with sufficient accuracy to capture the concentration gradients in urban environments. Its measurements could be used as input of an atmospheric inversion system for the quantification of emissions at the sub-city scale or separate specific sectors. Such quantification would offer valuable insights into the efficacy of local initiatives and could also identify unknown emission hotspots that require attention. Here we present the development and evaluation of a mid-cost CO2 instrument designed for continuous monitoring of atmospheric CO2 concentrations with a target accuracy of 1 ppm on hourly mean measurement. We assess the sensor sensitivity in relation to environmental factors such as humidity, pressure, temperature and CO2 signal, which leads to the development of an effective calibration algorithm. Since July 2020, eight mid-cost instruments have been installed within the city of Paris and its vicinity to provide continuous CO2 measurements, complementing the seven high-precision Cavity Ring-Down Spectroscopy (CRDS) stations that have been in operation since 2016. A data processing system, called CO2 calqual, has been implemented to automatically handle data quality control, calibration and storage, which enables the management of extensive real-time CO2 measurements from the monitoring network. Colocation assessments with the high-precision instrument show that the accuracies of the eight mid-cost instruments are within the range of 1.0 to 2.4 ppm for hourly afternoon (12–17 UTC) measurements. The long-term stability issues require manual data checks and instrument maintenance. The analyses show that CO2 measurements can provide evidence for underestimations of CO2 emissions in the Paris region and a lack of several emission point sources in the emission inventory. Our study demonstrates promising prospects in integrating mid-cost measurements along with high precision data into the subsequent atmospheric inverse modeling to improve the accuracy of quantifying the fine-scale CO2 emissions in the Paris metropolitan area. [ABSTRACT FROM AUTHOR]

Over 900 event-based precipitation samples were collected in 2019–2021 in the Tien Shan and its foothills and analysed using cavity ring-down spectroscopy. δ D and δ18 O values were highest in summer and lowest in winter, and annual cycles of deuterium excess (d-excess) varied between sites, reflecting local conditions. The δ18 O and δ D values increased from north to south in all seasons except autumn, and latitude was a statistically significant predictor of δ18 O and δ D in the overall data set, along with elevation in winter and elevation and longitude in autumn. Elevation was a significant predictor of d-excess in all seasons, and local air temperature was a more important control over δ18 O and δ D than precipitation depth. Local meteoric water lines were derived using seven regression methods applied to non-weighted and weighted precipitation. Non-weighted ordinary least squares regression and reduced major axis regression methods are recommended overall, except for summer when the precipitation-weighted least squares regression should be used, particularly in the south. Atmospheric back-trajectory and mixing-model analyses were applied in combination to identify air mass source regions and their relative contribution to precipitation. Recycled moisture from irrigated land in the Amu Darya and Syr Darya basins and from the study catchments accounted for 29 %–71 % of precipitation, depending on the site and season. In the Chon Kyzyl-Suu catchment, local re-evaporation from Issyk-Kul accounted for up to 85 % of precipitation. These findings highlight the importance of moisture from terrestrial sources, especially irrigated land, for the formation of precipitation in the Tien Shan. [ABSTRACT FROM AUTHOR]

Fluid inclusions are a window into deep geological fluids, providing unique access to their nature and composition. The isotopic composition of CO2 and CH4 hosted in fluid inclusions is a powerful proxy to assess the origin and transformation of deep geological fluids, giving insights into carbon sources, fluxes, and degassing in a wide variety of geodynamic settings. Over the last 5 decades, techniques have been developed to extract fluid inclusions from their host minerals and measure their bulk composition. These techniques are often challenged by analytical artifacts including high blank levels of CO2 and CH4, fluid re-speciation, gas adsorption, and diffusion. Since these processes may alter the pristine composition of gases liberated from fluid inclusions, rigorous protocols are needed in order to evaluate the isotopic integrity of the extracted volatile species. In this study, we introduce new protocols for bulk off-line fluid inclusion extraction for the analysis of δ13C-CH4 and δ13C-CO2 using a Cavity Ring-Down Spectroscopy (CRDS) analyser (Picarro G2201-i). Two mechanical fluid extraction techniques are compared: ball milling in ZrO2 jars and sample crushing in a stainless steel sealed tube under a hydraulic press. Blanks and isotopically labelled tests with the ball milling technique suggest that rotation speed, grinding stock filling degree and filling type alter the CH4 and CO2 concentrations and isotopic compositions measured by the CRDS analyser. In contrast, the crushing technique does not generate measurable quantities of blank CH4 and CO2. The protocols presented in this study allow to extract, detect, and analyse δ13C of CH4 and CO2 for concentrations above 10 and 1,000 ppm respectively. Interlaboratory experiments allowed to replicate previously measured δ13C-CH4 values in natural fluid inclusions within 1‰ with both extraction techniques. This study highlights the potential of combining simple bulk off-line fluid inclusion extraction techniques with a CRDS analyser for δ13C analysis of CO2 and CH4 without gas separation being required. [ABSTRACT FROM AUTHOR]

Accurate estimates of greenhouse gas emissions and sinks are critical for understanding the carbon cycle and identifying key drivers of anthropogenic climate change. In this study, we investigate the variability in CO and CO2 concentrations and their ratio over the Mexico City metropolitan area (MCMA) using long-term, time-resolved columnar measurements at three stations, employing solar-absorption Fourier transform infrared spectroscopy (FTIR). Using a simple model and the mixed-layer height derived from a ceilometer, we determined the CO and CO2 concentrations in the mixed layer from the total column measurements and found good agreement with surface cavity ring-down spectroscopy measurements. In addition, we used the diurnal pattern of CO columnar measurements at specific time intervals to estimate an average growth rate that, when combined with the space-based Tropospheric Monitoring Instrument (TROPOMI) CO measurements, allowed for the derivation of annual CO and CO2 MCMA emissions from 2016 to 2021. A CO emission decrease of more than 50 % was found during the COVID-19 lockdown period with respect to the year 2018. These results demonstrate the feasibility of using long-term EM27/SUN column measurements to monitor the annual variability in the anthropogenic CO2 and CO emissions in Mexico City without recourse to complex transport models. This simple methodology could be adapted to other urban areas if the orography of the regions favours low ventilation for several hours per day and the column growth rate is dominated by the emission flux. [ABSTRACT FROM AUTHOR]

In this study, we report highly precise and accurate measurements of the P(5), P(6), and O(3) transitions of the 2-0 band of HD occurring at approximately 1.5 µm. HD spectra were acquired in the pressure range of 100-900 Torr using a cavity ring-down spectrometer linked to an optical frequency comb. The line-shape analysis was performed using the Hartmann-Tran profile in the so-called β-corrected version. For the P(5) transition, we improved the accuracy of previous line center frequency determinations by more than two orders of magnitude. Moreover, for the P(5) and P(6) line centers, the total standard uncertainty is below 3 MHz, similar to those provided by quantum electrodynamics predictions, making them useful for direct comparisons that might foster future improvements of the theoretical model. [ABSTRACT FROM AUTHOR]

Cavity ringdown spectroscopy is employed to measure the absolute concentration of the ground state OH(X) radical at the ignition region in the microwave plasma-assisted combustion (PAC) of a premixed methane/air mixture. A 2.45-GHz solid-state microwave source was used to generate the plasma used in this article. The PAC platform consisted of a triple-layered coaxial cylindrical quartz combustor with the argon plasma conducted in the innermost cylinder and the premixed methane/air as the coflow. This configuration allowed for the coupling of the plasma and reactants outside the combustor making the plasma-assisted ignition region accessible to the cavity ringdown beam. Optical emission spectroscopy and visual imaging were used to obtain information about the excited state species along with plasma and flame geometries, respectively. Increasing the plasma power at a constant fuel equivalence ratio resulted in a blue inverted cone-shaped flame anchored to the tip of the plasma. A single peak was observed in the excited state OH(A) emission profile in the hybrid zone, whereas no OH(X) peak was observed in the hybrid zone. The OH(X) number density was measured at $0.12\times 10^{15}$ molecules/cm3 at ${z}\,\,= 2$ mm before increasing to a peak of $1.85\times 10^{15}$ molecules/cm3 at ${z}\,\,= 8$ mm, then subsequently dropping off. The representation of both OH(A) and OH(X) provides a complete picture of the role played by the OH radical, an important intermediate specie in the microwave generated argon PAC of a methane/air mixture. The ability to track the evolution of both states of the OH radical in the ignition zone further sheds light on the role played by this important species in influencing the plasma enhancements of a premixed methane/air combustion. The results obtained further confirmed the hypothesis that OH(X) is more involved in the stabilization reactions, whereas OH (A) is more prevalent in the ignition process. [ABSTRACT FROM AUTHOR]

Near-infrared cavity ringdown spectra were recorded following the photolysis of dihalomethanes in O2/N2 mixtures. In particular, photolysis of CH2I2 under conditions previously reported to produce the simplest Criegee intermediate, CH2O2, gave a complex, structured spectrum between 6800 and 9000 cm-1, where the lowest triplet-singlet transition (ã-X̃) of CH2O2 might be expected. To help identify the carrier of the spectrum, extensive electronic structure calculations were performed on the ã and X̃ states of CH2O2 and the lowest two doublet states of the iodomethylperoxy radical, CH2IO2, which also could be produced by the chemistry and whose Ã-X̃ transition likely lies in this spectral region. The conclusion of these calculations is that the ã-X̃ transition of CH2O2 clearly falls outside the observed spectral range and would be extremely weak both because it is spin-forbidden and because of a large geometric change between the ã and X̃ states. Moreover, only a shallow well (with a barrier to dissociation of less than 1900 cm-1) is predicted on the ã state, which likely precludes the existence of long-lived states. Calculations for the Ã-X̃ transition of CH2IO2 are generally consistent with the observed spectrum in terms of both the electronic origin and vibrational frequencies in the à state. To confirm the carrier assignment to CH2IO2, calculations beyond the Franck-Condon approximation were carried out to explain the hot band structure of the large-amplitude, low-frequency O-O-C-I torsion mode, ν12. Photolysis of other dihalomethanes produced similar spectra which were analyzed and assigned to CH2ClO2 and CH2BrO2. Experimental values for the electronic energies and frequencies for several à state vibrations and the ν12 vibration of the X̃ state of each are reported. In addition, the observed spectra were used to follow the self-reaction of the CH2IO2 species and its reaction with SO2. The rates of these reactions are dramatically faster than those of unsubstituted alkyl peroxy radicals and approach those of the Criegee intermediate. [ABSTRACT FROM AUTHOR]

We developed a cavity ringdown spectrometer by utilizing a step-scanning and dithering method for matching laser wavelengths to optical resonances of an optical cavity. Our approach is capable of working with two and more lasers for quasi-simultaneous measurements of multiple gas species. The developed system was tested with two lasers operating around 1654 nm and 1658 nm for spectral detections of 12 CH 4 and its isotope 13 CH 4 in air, respectively. The ringdown time of the empty cavity was about 340 µs. The achieved high detection sensitivity of a noise-equivalent absorption coefficient was 2.8 × 10 -11 cm -1 Hz -1/2 or 1 × 10 -11 cm -1 by averaging for 30 s. The uncertainty of the high precision determination of δ13CH4 in air is about 1.3‰. Such a system will be useful for future applications such as environmental monitoring.

The carbon isotope composition of respired CO2 (δ13CR) and bulk organic matter (δ13CB) of various plant compartments informs about the isotopic fractionation and substrate of respiratory processes, which are crucial to advance the understanding of carbon allocation in plants. Nevertheless, the variation across organs, species and seasons remains poorly understood. Cavity Ring-Down Laser Spectroscopy was applied to measure δ13CR in leafy shoots and woody stems of maple (Acer platanoides L.), oak (Quercus robur L.) and cedar (Thuja occidentalis L.) trees during spring and late summer. Photosynthesis, respiration, growth and non-structural carbohydrates were measured in parallel to evaluate potential drivers for respiratory fractionation. The CO2 respired by maple and oak shoots was 13C-enriched relative to δ13CB during spring, but not late summer or in the stem. In cedar, δ13CR did not vary significantly throughout organs and seasons, with respired CO2 being 13C-depleted relative to δ13CB. Shoot δ13CR was positively related to leaf starch concentration in maple, while stem δ13CR was inversely related to stem growth. These relations were not significant for oak or cedar. The variability in δ13CR suggests (i) different contributions of respiratory pathways between organs and (ii) seasonality in the respiratory substrate and constitutive compounds for wood formation in deciduous species, less apparent in evergreen cedar, whose respiratory metabolism might be less variable. [ABSTRACT FROM AUTHOR]

The weak absorption spectra of isoprene and acetone have been measured in the wavelength range of 261–275 nm using cavity ringdown spectroscopy. The measured absorption cross-sections of isoprene in the wavelength region of 261–266 nm range from 3.65 × 10−21 cm2·molecule−1 at 261 nm to 1.42 × 10−21 cm2·molecule−1 at 266 nm; these numbers are in good agreement with the values reported in the literature. In the longer wavelength range of 270–275 nm, however, where attractive applications using a single wavelength compact diode laser operating at 274 nm is located, isoprene has been reported in the literature to have no absorption (too weak to be detected). Small absorption cross-sections of isoprene in this longer wavelength region are measured using cavity ringdown spectroscopy for the first time in this work, i.e., 6.20 × 10−23 cm2·molecule−1 at 275 nm. With the same experimental system, wavelength-dependent absorption cross-sections of acetone have also been measured. Theoretical detection limits of isoprene and comparisons of absorbance of isoprene, acetone, and healthy breath gas in this wavelength region are also discussed.