First investigation and absolute calibration of clumped isotopes in N2 O by mid-infrared laser spectroscopy.

RATIONALE: Unravelling the biogeochemical cycle of the potent greenhouse gas nitrous oxide (N2 O) is an underdetermined problem in environmental sciences due to the multiple source and sink processes involved, which complicate mitigation of its emissions. Measuring the doubly isotopically substituted molecules (isotopocules) of N2 O can add new opportunities to fingerprint and constrain its cycle. METHODS: We present a laser spectroscopic technique to selectively and simultaneously measure the eight most abundant isotopocules of N2 O, including three doubly substituted species - so called "clumped isotopes". For the absolute quantification of individual isotopocule abundances, we propose a new calibration scheme that combines thermal equilibration of a working standard gas with a direct mole fraction-based approach. RESULTS: The method is validated for a large range of isotopic composition values by comparison with other established methods (laser spectroscopy using conventional isotopic scale and isotope ratio mass spectrometry). Direct intercomparison with recently developed ultrahigh-resolution mass spectrometry shows clearly the advantages of the new laser technique, especially with respect to site specificity of isotopic substitution in the N2 O molecule. CONCLUSIONS: Our study represents a new methodological basis for the measurements of both singly substituted and clumped N2 O isotopes. It has a high potential to stimulate future research in the N2 O community by establishing a new class of reservoir-insensitive tracers and molecular-scale insights.

Seasonal fluxes of carbonyl sulfide in a midlatitude forest.

Carbonyl sulfide (OCS), the most abundant sulfur gas in the atmosphere, has a summer minimum associated with uptake by vegetation and soils, closely correlated with CO2. We report the first direct measurements to our knowledge of the ecosystem flux of OCS throughout an annual cycle, at a mixed temperate forest. The forest took up OCS during most of the growing season with an overall uptake of 1.36 ± 0.01 mol OCS per ha (43.5 ± 0.5 g S per ha, 95% confidence intervals) for the year. Daytime fluxes accounted for 72% of total uptake. Both soils and incompletely closed stomata in the canopy contributed to nighttime fluxes. Unexpected net OCS emission occurred during the warmest weeks in summer. Many requirements necessary to use fluxes of OCS as a simple estimate of photosynthesis were not met because OCS fluxes did not have a constant relationship with photosynthesis throughout an entire day or over the entire year. However, OCS fluxes provide a direct measure of ecosystem-scale stomatal conductance and mesophyll function, without relying on measures of soil evaporation or leaf temperature, and reveal previously unseen heterogeneity of forest canopy processes. Observations of OCS flux provide powerful, independent means to test and refine land surface and carbon cycle models at the ecosystem scale.

Methane emissions from natural gas infrastructure and use in the urban region of Boston, Massachusetts.

Methane emissions from natural gas delivery and end use must be quantified to evaluate the environmental impacts of natural gas and to develop and assess the efficacy of emission reduction strategies. We report natural gas emission rates for 1 y in the urban region of Boston, using a comprehensive atmospheric measurement and modeling framework. Continuous methane observations from four stations are combined with a high-resolution transport model to quantify the regional average emission flux, 18.5 ± 3.7 (95% confidence interval) g CH4 ⋅ m(-2) ⋅ y(-1). Simultaneous observations of atmospheric ethane, compared with the ethane-to-methane ratio in the pipeline gas delivered to the region, demonstrate that natural gas accounted for ∼ 60-100% of methane emissions, depending on season. Using government statistics and geospatial data on natural gas use, we find the average fractional loss rate to the atmosphere from all downstream components of the natural gas system, including transmission, distribution, and end use, was 2.7 ± 0.6% in the Boston urban region, with little seasonal variability. This fraction is notably higher than the 1.1% implied by the most closely comparable emission inventory.

Design and performance of a dual-laser instrument for multiple isotopologues of carbon dioxide and water.

A design and results for an instrument with a quantum cascade laser and an antimonide diode laser to measure simultaneously and with high precision seven isotopologues of carbon dioxide and water vapor. Methods and results for determining the effects that limit absorption noise at the level of 5x10(-6) are presented and discussed.

Mobile Laboratory Observations of Methane Emissions in the Barnett Shale Region.

Results of mobile ground-based atmospheric measurements conducted during the Barnett Shale Coordinated Campaign in spring and fall of 2013 are presented. Methane and ethane are continuously measured downwind of facilities such as natural gas processing plants, compressor stations, and production well pads. Gaussian dispersion simulations of these methane plumes, using an iterative forward plume dispersion algorithm, are used to estimate both the source location and the emission magnitude. The distribution of emitters is peaked in the 0-5 kg/h range, with a significant tail. The ethane/methane molar enhancement ratio for this same distribution is investigated, showing a peak at ∼1.5% and a broad distribution between ∼4% and ∼17%. The regional distributions of source emissions and ethane/methane enhancement ratios are examined: the largest methane emissions appear between Fort Worth and Dallas, while the highest ethane/methane enhancement ratios occur for plumes observed in the northwestern potion of the region. Individual facilities, focusing on large emitters, are further analyzed by constraining the source location.

Measurement of a doubly substituted methane isotopologue, ¹³CH3D, by tunable infrared laser direct absorption spectroscopy.

Methane is an important energy resource and significant long-lived greenhouse gas. Carbon and hydrogen isotope ratios have been used to better constrain the sources of methane but interpretations based on these two parameters alone can often be inconclusive. The precise measurement of a doubly substituted methane isotopologue, (13)CH3D, is expected to add a critical new dimension to source signatures by providing the apparent temperature at which methane was formed or thermally equilibrated. We have developed a new method to precisely determine the relative abundance of (13)CH3D by using tunable infrared laser direct absorption spectroscopy (TILDAS). The TILDAS instrument houses two continuous wave quantum cascade lasers; one tuned at 8.6 µm to measure (13)CH3D, (12)CH3D, and (12)CH4, and the other at 7.5 µm to measure (13)CH4. With the use of an astigmatic Herriott cell with an effective path length of 76 m, a precision of 0.2‰ (2σ) was achieved for the measurement of (13)CH3D abundance in ca. 10 mL STP (i.e., 0.42 mmol) pure methane samples. Smaller quantity samples (ca. 0.5 mL STP) can be measured at lower precision. The accuracy of the Δ(13)CH3D measurement is 0.7‰ (2σ), evaluated by thermally equilibrating methane with a range of δD values. The precision of ±0.2‰ corresponds to uncertainties of ±7 °C at 25 °C and ±20 °C at 200 °C for estimates of apparent equilibrium temperatures. The TILDAS instrument offers a simple and precise method to determine (13)CH3D in natural methane samples to distinguish geological and biological sources of methane in the atmosphere, hydrosphere, and lithosphere.

Demonstration of an ethane spectrometer for methane source identification.

Methane is an important greenhouse gas and tropospheric ozone precursor. Simultaneous observation of ethane with methane can help identify specific methane source types. Aerodyne Ethane-Mini spectrometers, employing recently available mid-infrared distributed feedback tunable diode lasers (DFB-TDL), provide 1 s ethane measurements with sub-ppb precision. In this work, an Ethane-Mini spectrometer has been integrated into two mobile sampling platforms, a ground vehicle and a small airplane, and used to measure ethane/methane enhancement ratios downwind of methane sources. Methane emissions with precisely known sources are shown to have ethane/methane enhancement ratios that differ greatly depending on the source type. Large differences between biogenic and thermogenic sources are observed. Variation within thermogenic sources are detected and tabulated. Methane emitters are classified by their expected ethane content. Categories include the following: biogenic (<0.2%), dry gas (1-6%), wet gas (>6%), pipeline grade natural gas (<15%), and processed natural gas liquids (>30%). Regional scale observations in the Dallas/Fort Worth area of Texas show two distinct ethane/methane enhancement ratios bridged by a transitional region. These results demonstrate the usefulness of continuous and fast ethane measurements in experimental studies of methane emissions, particularly in the oil and natural gas sector.

Contribution of nitrated phenols to wood burning brown carbon light absorption in Detling, United Kingdom during winter time.

We show for the first time quantitative online measurements of five nitrated phenol (NP) compounds in ambient air (nitrophenol C6H5NO3, methylnitrophenol C7H7NO3, nitrocatechol C6H5NO4, methylnitrocatechol C7H7NO4, and dinitrophenol C6H4N2O5) measured with a micro-orifice volatilization impactor (MOVI) high-resolution chemical ionization mass spectrometer in Detling, United Kingdom during January-February, 2012. NPs absorb radiation in the near-ultraviolet (UV) range of the electromagnetic spectrum and thus are potential components of poorly characterized light-absorbing organic matter ("brown carbon") which can affect the climate and air quality. Total NP concentrations varied between less than 1 and 98 ng m(-3), with a mean value of 20 ng m(-3). We conclude that NPs measured in Detling have a significant contribution from biomass burning with an estimated emission factor of 0.2 ng (ppb CO)(-1). Particle light absorption measurements by a seven-wavelength aethalometer in the near-UV (370 nm) and literature values of molecular absorption cross sections are used to estimate the contribution of NP to wood burning brown carbon UV light absorption. We show that these five NPs are potentially important contributors to absorption at 370 nm measured by an aethalometer and account for 4 ± 2% of UV light absorption by brown carbon. They can thus affect atmospheric radiative transfer and photochemistry and with that climate and air quality.

Aircraft emissions of methane and nitrous oxide during the alternative aviation fuel experiment.

Given the predicted growth of aviation and the recent developments of alternative aviation fuels, quantifying methane (CH(4)) and nitrous oxide (N(2)O) emission ratios for various aircraft engines and fuels can help constrain projected impacts of aviation on the Earth's radiative balance. Fuel-based emission indices for CH(4) and N(2)O were quantified from CFM56-2C1 engines aboard the NASA DC-8 aircraft during the first Alternative Aviation Fuel Experiment (AAFEX-I) in 2009. The measurements of JP-8 fuel combustion products indicate that at low thrust engine states (idle and taxi, or 4% and 7% maximum rated thrusts, respectively) the engines emit both CH(4) and N(2)O at a mean ± 1σ rate of 170 ± 160 mg CH(4) (kg Fuel)(-1) and 110 ± 50 mg N(2)O (kg Fuel)(-1), respectively. At higher thrust levels corresponding to greater fuel flow and higher engine temperatures, CH(4) concentrations in engine exhaust were lower than ambient concentrations. Average emission indices for JP-8 fuel combusted at engine thrusts between 30% and 100% of maximum rating were -54 ± 33 mg CH(4) (kg Fuel)(-1) and 32 ± 18 mg N(2)O (kg Fuel)(-1), where the negative sign indicates consumption of atmospheric CH(4) in the engine. Emission factors for the synthetic Fischer-Tropsch fuels were statistically indistinguishable from those for JP-8.

Measurements of nitrous acid in commercial aircraft exhaust at the Alternative Aviation Fuel Experiment.

The Alternative Aviation Fuel Experiment (AAFEX), conducted in January of 2009 in Palmdale, California, quantified aerosol and gaseous emissions from a DC-8 aircraft equipped with CFM56-2C1 engines using both traditional and synthetic fuels. This study examines the emissions of nitrous acid (HONO) and nitrogen oxides (NO(x) = NO + NO(2)) measured 145 m behind the grounded aircraft. The fuel-based emission index (EI) for HONO increases approximately 6-fold from idle to takeoff conditions but plateaus between 65 and 100% of maximum rated engine thrust, while the EI for NO(x) increases continuously. At high engine power, NO(x) EI is greater when combusting traditional (JP-8) rather than Fischer-Tropsch fuels, while HONO exhibits the opposite trend. Additionally, hydrogen peroxide (H(2)O(2)) was identified in exhaust plumes emitted only during engine idle. Chemical reactions responsible for emissions and comparison to previous measurement studies are discussed.

Monomer, clusters, liquid: an integrated spectroscopic study of methanol condensation.

We have combined static pressure, spectroscopic temperature, Fourier transform infrared spectroscopy (FTIR), and small angle X-ray scattering (SAXS) measurements to develop a detailed picture of methanol condensing from a dilute vapor-carrier gas mixture under the highly supersaturated conditions present in a supersonic nozzle. In our experiments, methanol condensation can be divided into three stages as the gas mixture expands in the nozzle. In the first stage, as the temperature decreases rapidly, small methanol n-mers (clusters) form, increase in concentration, and evolve in size. In the second stage, the temperature decreases more slowly, and the n-mer concentrations continue to rise. Thermodynamic and FTIR experiments cannot, however, definitively establish if the average cluster size is constant or if it continues to increase. Finally, when the vapor becomes supersaturated enough, liquid droplets form via nucleation and growth, consuming more monomer and reducing the concentration of clusters. At the point where liquid first appears, cluster formation has already consumed up to 30% of the monomer. This is significantly more than is predicted by a model that describes the vapor phase as an equilibrium mixture of methanol monomer, dimer, and tetramer. An energy balance suggests that a significant fraction of the cluster population is larger than the tetramer, while preliminary SAXS measurements suggest that these clusters contain, on average, 6 monomers.

Dual quantum cascade laser trace gas instrument with astigmatic Herriott cell at high pass number.

We have developed and demonstrated a high-sensitivity trace gas instrument employing two mid-infrared quantum cascade lasers and an astigmatic Herriott sample cell with up to a 240 m path length. Several aspects of astigmatic Herriott cell optics have been addressed to enable operation at a high pass number (up to 554), including aberrations and pattern selection to minimize interference fringes. The new instrument design, based on the 200 m cell, can measure various atmospheric trace gases, depending on the installed lasers, with multiple trace gases measured simultaneously. Demonstrated concentration noise levels (1 s average) are 40 parts per trillion [(ppt) 10(-12)] for formaldehyde, 10 ppt for carbonyl sulfide, 110 ppt for hydrogen peroxide (H2O2), and 180 ppt for nitrous acid (HONO). High-precision measurements of nitrous oxide and methane have been recorded at the same time as high-sensitivity measurements of HONO and H2O2.

Multicomponent Breath Analysis With Infrared Absorption Using Room-Temperature Quantum Cascade Lasers.

Breath analysis is a powerful noninvasive technique for the diagnosis and monitoring of respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD). Nitric oxide (NO) and carbon monoxide (CO) are markers of airway inflammation and can indicate the extent of respiratory diseases. We have developed a compact fast response laser system for analysis of multiple gases by infrared absorption. The instrument uses room temperature quantum cascade lasers to simultaneously measure NO, CO, carbon dioxide (CO(2)) and nitrous oxide (N(2)O) in exhaled breath. Four breath flow rates are employed to explore their exchange dynamics in the lungs and airways. We obtain 1-s detection precisions of 0.5-0.8 parts-per-billion (ppb) for NO, CO, and N(2)O with an instrument response time of less than 1 s. The breath analysis system has been demonstrated in a preliminary study of volunteers. It is currently deployed in a trial clinical study.

Laboratory evaluation of an aldehyde scrubber system specifically for the detection of acrolein.

We demonstrate the use of an aldehyde scrubber system to resolve isobaric aldehyde/alkene interferences in a proton transfer reaction mass spectrometer (PTR-MS) by selectively removing the aldehydes from the gas mixture without loss of quantitative information for the alkene components. The aldehyde scrubber system uses a bisulfite solution, which scrubs carbonyl compounds from the gas stream by forming water-soluble carbonyl bisulfite addition products, and has been evaluated using a synthetic mixture of acrolein and isoprene. Trapping efficiencies of acrolein exceeded 97%, whereas the transmission efficiency of isoprene was better than 92%. Quantification of the PTR-MS response to acrolein was validated through an intercomparison study that included two derivatization methods, dinitrophenylhydrazine (DNPH) and O-(4-cyano-2-ethoxybenzyl)hydroxylamine (CNET), and a spectroscopic method using a quantum cascade laser infrared absorption spectroscopy (QCL) instrument. Finally, using cigarette smoke as a complex matrix, the acrolein content was assessed using the scrubber and compared with direct QCL-based detection.

What are the instrumentation requirements for measuring the isotopic composition of net ecosystem exchange of CO2 using eddy covariance methods?

Better quantification of isotope ratios of atmosphere-ecosystem exchange of CO2 could substantially improve our ability to probe underlying physiological and ecological mechanisms controlling ecosystem carbon exchange, but the ability to make long-term continuous measurements of isotope ratios of exchange fluxes has been limited by measurement difficulties. In particular, direct eddy covariance methods have not yet been used for measuring the isotopic composition of ecosystem fluxes. In this article, we explore the feasibility of such measurements by (a) proposing a general criterion for judging whether a sensor's performance is sufficient for making such measurements (the criterion is met when the contribution of sensor error to the flux measurement error is comparable to or less than the contribution of meteorological noise inherently associated with turbulence flux measurements); (b) using data-based numerical simulations to quantify the level of sensor precision and stability required to meet this criterion for making direct eddy covariance measurements of the 13C/12C ratio of CO2 fluxes above a specific ecosystem (a mid-latitude temperate forest in central Massachusetts, USA); (c) testing whether the performance of a new sensor-a prototype pulsed quantum cascade laser (QCL) based isotope-ratio absorption spectrometer (and plausible improvements thereon)-is sufficient for meeting the criterion in this ecosystem. We found that the error contribution from a prototype sensor (approximately 0.2 per thousand, 1 SD of 10 s integrations) to total isoflux measurement error was comparable to (1.5 to 2x) the irreducible 'meteorological' noise inherently associated with turbulent flux measurements above this ecosystem (daytime measurement error SD of approximately 60% of flux versus meteorological noise of 30-40% for instantaneous half-hour fluxes). Our analysis also shows that plausible instrument improvements (increase of sensor precision to approximately 0.1 per thousand, 1 SD of 10 s integrations, and increased sensor stability during the half-hour needed to integrate eddy covariance measurements) should decrease the contribution of sensor error to the point where it is less than the contribution from meteorological noise. This suggests that new sensors using QCL-based isotope ratio absorption spectroscopy should make continuous long-term observations of the isotopic composition of CO2 fluxes via eddy covariance methods feasible.

Measurement of nitrogen dioxide in cigarette smoke using quantum cascade tunable infrared laser differential absorption spectroscopy (TILDAS).

Although nitrogen dioxide (NO(2)) has been previously reported to be present in cigarette smoke, the concentration estimates were derived from kinetic calculations or from measurements of aged smoke, where NO(2) was formed some time after the puff was taken. The objective of this work was to use tunable infrared laser differential absorption spectroscopy (TILDAS) equipped with a quantum cascade (QC) laser to determine if NO(2) could be detected and quantified in a fresh puff of cigarette smoke. A temporal resolution of approximately 0.16s allowed measurements to be taken directly as the NO(2) was formed during the puff. Sidestream cigarette smoke was sampled to determine if NO(2) could be detected using TILDAS. Experiments were conducted using 2R4F Kentucky Reference cigarettes with and without a Cambridge filter pad. NO(2) was detected only in the lighting puff of whole mainstream smoke (without a Cambridge filter pad), with no NO(2) detected in the subsequent puffs. The measurement precision was approximately 1.0 ppbVHz(-1/2), which allows a detection limit of approximately 0.2 ng in a 35 ml puff volume. More NO(2) was generated in the lighting puff using a match or blue flame lighter (29+/-21 ng) than when using an electric lighter (9+/-3 ng). In the presence of a Cambridge filter pad, NO(2) was observed in the gas phase mainstream smoke for every puff (total of 200+/-30 ng/cigarette) and is most likely due to smoke chemistry taking place on the Cambridge filter pad during the smoke collection process. Nitrogen dioxide was observed continuously in the sidestream smoke starting with the lighting puff.

Infrared laser spectrometer with balanced absorption for measurement of isotopic ratios of carbon gases.

Measurement of the isotopic compositions of carbon dioxide and methane is a powerful tool for quantifying their atmospheric sources and sinks, which is especially important considering the dramatic increase in these greenhouse gases during the industrial era. Laser absorption spectroscopy is a technique which has demonstrated the high sensitivity needed for isotopic measurement. A significant problem in the spectroscopic measurement of isotopic abundances is the large difference in concentrations of the major and minor isotopic constituents. The measurement of two isotopic species using lines of similar strength but very unequal concentrations leads to low precision, with either the minor constituent having too small an absorption depth, or the major constituent having too great an absorption depth. If lines with unequal strength are chosen to compensate for the absorption depth imbalance, then precision tends to suffer due to the greater temperature sensitivity of the weaker line strength. We describe the development of a compact instrument for isotopic analysis CO2 and CH4 using tunable infrared laser absorption spectroscopy which combines novel optical design and signal processing methods to address this problem. The design compensates for the large difference in concentration between major and minor isotopes by measuring them with pathlengths which differ by a factor of 72 within the same multipass cell. We have demonstrated the basic optical design and signal processing by determining delta13C (CO2) isotopic ratios with precision as small as 0.2/1000 using a lead salt diode laser based spectroscopic instrument.

High precision measurements of atmospheric nitrous oxide and methane using thermoelectrically cooled mid-infrared quantum cascade lasers and detectors.

A compact, fast response, mid-infrared absorption spectrometer using thermoelectrically (TE) cooled pulsed quantum cascade (QC) lasers and TE detectors has been developed to demonstrate the applicability of QC lasers for high precision measurements of nitrous oxide and methane in the earth's atmosphere. Reduced pressure extractive sampling with a 56 m path length, 0.5 l volume, multiple pass absorption cell allows a time response of <0.1s which is suitable for eddy correlation flux measurements for these gases. Precision of 0.3 ppb (rms, 1s averaging time) or 0.1% of the ambient concentration for N(2)O (4 ppb or 0.2% of ambient for CH(4)), has been demonstrated using QC lasers at 4.5 microm (7.9 microm for CH(4)), corresponding to an absorbance precision of 4 x 10(-5) Hz(-1/2) (8 x 10(-5) Hz(-1/2) for CH(4)). Stabilization of the temperature of the optical bench and the pulse electronics results in a minimum Allan variance corresponding to 0.06 ppb for N(2)O with an averaging time of 100 s (0.7 ppb with an averaging time of 200 s for CH(4)). The instrument is capable of long-term, unattended, continuous operation without cryogenic cooling of either laser or detector.

Hydrazine detection limits in the cigarette smoke matrix using infrared tunable diode laser absorption spectroscopy.

Infrared absorption lines of hydrazine are broad and typically not baseline resolved, with line strengths approximately 100 times weaker than the more widely studied compound ammonia. Hardware and software improvements have been made to a two-color infrared tunable diode laser (IR-TDL) spectrometer in order to improve the limit of detection (LOD) of hydrazine (N2H4) in the cigarette smoke matrix. The detection limit in the smoke matrix was improved from 25 parts-per-million-by-volume (ppmv) to 4.2 ppmv using a 100 m pathlength cell with acquisition of background spectra immediately prior to each sample and 100 ms temporal resolution. This study did not detect hydrazine in cigarette smoke in the 964.4-964.9 cm(-1) spectral region, after mathematically subtracting the spectral contributions of ethylene, ammonia, carbon dioxide, methanol, acrolein, and acetaldehyde. These compounds are found in cigarette smoke and absorb in this spectral region. The LOD is limited by remaining spectral structure from unidentified smoke species. The pseudo random noise (root mean square) in the improved instrument was 2 x 10(-4) absorbance units (base e) which is equivalent to a 0.09 ppmv hydrazine gas sample in the multipass cell. This would correspond to a detection limit of 0.44 ppmv of hydrazine, given the dilution of the smoke by a factor of 5 by the sampling system. This is a factor of 10 less than the 4.2 ppmv detection limit for hydrazine in the smoke matrix, and indicates that the detection limit is primarily a result of the complexity of the matrix rather than the random noise of the TDL instrument.

Quad quantum cascade laser spectrometer with dual gas cells for the simultaneous analysis of mainstream and sidestream cigarette smoke.

A compact, fast response, infrared spectrometer using four pulsed quantum cascade (QC) lasers has been applied to the analysis of gases in mainstream (MS) and sidestream (SS) cigarette smoke. QC lasers have many advantages over the traditional lead-salt tunable diode lasers, including near room temperature operation with thermoelectric cooling and single mode operation with improved long-term stability. The new instrument uses two 36 m, 0.3 l multiple pass absorption gas cells to obtain a time response of 0.1s for the MS smoke system and 0.4s for the SS smoke system. The concentrations of ammonia, ethylene, nitric oxide, and carbon dioxide for three different reference cigarettes were measured simultaneously in MS and SS smoke. A data rate of 20Hz provides sufficient resolution to determine the concentration profiles during each 2s puff in the MS smoke. Concentration profiles before, during and after the puffs also have been observed for these smoke constituents in SS smoke. Also, simultaneous measurements of CO(2) from a non-dispersive infrared (NDIR) analyzer are obtained for both MS and SS smoke. In addition, during this work, nitrous oxide was detected in both the MS and SS smoke for all reference cigarettes studied.