Tunable Infrared Laser Differential Absorption Research Articles

Rapid and precise measurement of carbonate clumped isotopes using laser spectroscopy.

Carbonate clumped isotope abundance is an important paleothermometer, but measurement is difficult, slow, and subject to cardinal mass (m/z) interferences using isotope ratio mass spectrometry (IRMS). Here, we describe an optical spectroscopic measurement of carbonate clumped isotopes. We have adapted a tunable infrared laser differential absorption spectrometer (TILDAS) system to measure the abundances of four CO2 isotopologues used for clumped isotope thermometry. TILDAS achieves the same precision (0.01‰ SE) as IRMS measurements rapidly (∼50 min per carbonate analysis) and using small samples (<2 mg of calcite), without making assumptions about 17O abundance in the sample. A temperature calibration based on 406 analyses of CO2 produced by digestion of 51 synthetic carbonates equilibrated at 6° to 1100°C is consistent with results for natural carbonates and previous calibrations. Our system results were indistinguishable from IRMS systems after replicating the InterCarb interlaboratory calibration. Measurement by TILDAS could change the landscape for clumped isotope analysis.

Characteristics of HONO and its impact on O3 formation in the Seoul Metropolitan Area during the Korea-US Air Quality study

Photolysis of nitrous acid (HONO) is recognized as an early-morning source of OH radicals in the urban air. During the Korea–US Air Quality (KORUS-AQ) campaign, HONO was measured using quantum cascade - tunable infrared laser differential absorption spectrometer (QC-TILDAS) at Olympic Park in Seoul from 17 May, 2016 to 14 June, 2016. The HONO concentration was in the range of 0.07–3.46 ppbv, with an average of 0.93 ppbv. Moreover, it remained high from 00:00–05:00 LST. During this time, the mean concentration was higher during the high-O3 episodes (1.82 ppbv) than the non-episodes (1.20 ppbv). In the morning, the OH radicals that were produced from HONO photolysis were 50% higher (0.95 pptv) during the high-O3 episodes than the non-episodes. Diurnal variations in HOx and O3 concentrations were simulated by the F0AM model, which revealed a difference of ~20 ppbv in the daily maximum O3 concentrations between the high-O3 episodes and non-episodes. Furthermore, the HONO concentration increased with an increase in relative humidity (RH) up to 80%; the highest HONO was associated with the top 10% NO2 in each RH group, confirming that NO2 is one of the main precursors of HONO. At night, the conversion ratio of NO2 to HONO was estimated to be 0.88×10−2 h−1; this ratio was found to increase with an increase in RH. The Aitken mode particles (30–120 nm), which act as catalyst surfaces, exhibited a similar tendency with a conversion ratio that increased along with RH, indicating the coupling of surfaces with HONO conversion. Using an artificial neural network (ANN) model, HONO concentrations were successfully simulated with measured variables (r2 = 0.66 as an average of five models). Among these variables, NOx, aerosol surface area, and RH were found to be the main factors affecting the ambient HONO concentrations. The results reveal that RH facilitates the conversion of NO2 to HONO by constraining the availability of aerosol surfaces. This study demonstrates the coupling of HONO with the HOx-O3 cycle in the Seoul Metropolitan Area (SMA) and provides practical evidence of the heterogeneous formation of HONO by employing the ANN model.

Rapid and Precise Analysis of Carbon Dioxide Clumped Isotopic Composition by Tunable Infrared Laser Differential Spectroscopy.

The high precision measurement of doubly substituted ("clumped") isotopologues in CO2 is a topic of significant interest in isotope geochemistry. Here we describe the performance of a new isotope ratio laser spectrometer using tunable infrared laser differential absorption spectroscopy (TILDAS). The TILDAS instrument has two continuous-wave lasers to simultaneously measure the four isotopologues involved in the 12C16O2 + 13C16O18O ⇆ 13C16O2 + 12C16O18O exchange reaction. CO2 samples are trapped in a low volume (∼250 mL) optical multipass cell with a path length of 36 m. Each sample is compared to a reference gas, and clumped isotopologue precision of 0.01‰ (SE) is achieved within 20 min for 15 μmol samples. Similar precision is also achieved for bulk isotopic composition. The degree of rare isotope clumping in excess of strictly random distribution (Δ16O13C18O) measured by this TILDAS instrument varies linearly with theoretically calculated values and shows a very weak dependence on bulk isotopic composition.

Dynamics of ammonia volatilisation measured by eddy covariance during slurry spreading in north Italy

Ammonia (NH3) emissions have been quantified during slurry spreading in two experimental trials in two intensively managed agricultural fields in northern Italy, during spring 2009 and 2011. NH3 fluxes have been measured by Eddy covariance (EC) method from the slurry application to the soil incorporation until the end of the emission phenomenon.The EC system was equipped with a fast sensor for NH3 concentration measurements based on Tunable Infrared Laser Differential Absorption Spectrometry (QC-TILDAS). NH3 volatilisation has been monitored in continuous for both experimental trials, confirming the rapidity of the NH3 losses when slurry is spread to the field. Within 24h from the application the volatilisation suddenly decreases, stopping after soil incorporation occurred 24 and 30h from the spreading for the two experimental trials. The maximum NH3 emission levels were 138.3 and 243.5μgm−2s−1 and the total losses of NH4-N were 19.4% and 28.5%, determined 7 days after the spreading for the first and the second trial, respectively. EC measurements have been compared to the emissions estimated by a backward Lagrangian stochastic model, resulting consistent for dynamic and quantitative emitted. To explain the differences between the losses in the two experiments, the relationship between emission and meteorological conditions has been investigated. In particular, rain during the 2009 trial caused a significant reduction in emissions, whereas high air temperatures enhanced the emission phenomenon in the 2011 trial. The results shown that for improving nitrogen efficiency, slurry incorporation has to be performed in times closer from spreading than 24h, under weather conditions which limit NH3 emissions (such as cloudy with low solar radiation and temperature).

Effective line strengths of trans-nitrous acid near 1275 cm−1 and cis-nitrous acid at 1660 cm−1

We determined the effective line strengths of the trans conformer of nitrous acid (HONO) near 1275cm−1 (R-branch of ν3 mode, NOH bend) and of the cis conformer at 1660cm−1 (R-branch of ν2 mode, NO stretch), both at a spectral resolution of 0.001cm−1 by tunable infrared laser differential absorption spectroscopy (TILDAS) utilizing continuous-wave quantum cascade (cw-QC) lasers. Absorbance of one conformer was measured while simultaneously quantifying the mixing ratio of total HONO by catalytic conversion to nitric oxide (NO) followed by calibrated absorption spectroscopy. Line strengths obtained here are consistent with previously reported band strengths for the trans conformer but are lower by a factor of approximately 2.4 for the cis conformer.

Direct measurement of volatile organic compound emissions from industrial flares using real-time online techniques: Proton Transfer Reaction Mass Spectrometry and Tunable Infrared Laser Differential Absorption Spectroscopy

During the 2010 Comprehensive Flare Study a suite of analytical instrumentation was employed to monitor and quantify in real-time the volatile organic compound (VOC) emissions emanating from an industrial chemical process flare burning either propene/natural gas or propane/natural gas. To our knowledge this represents the first time the VOC composition has been directly measured as a function of flare efficiency on an operational full-scale flare. This compositional information was obtained using a suite of proton-transfer-reaction mass spectrometers (PTR-MS) and quantum cascade laser tunable infrared differential absorption spectrometers (QCL-TILDAS) to measure the unburned fuel and associated combustion byproducts. Methane, ethyne, ethene, and formaldehyde were measured using the QC-TILDAS. Propene, acetaldehyde, methanol, benzene, acrolein, and the sum of the C3H6O isomers were measured with the PTR-MS. A second PTR-MS equipped with a gas chromatograph (GC) was operated in parallel and was used to veri...

Eddy covariance measurement of ammonia fluxes: Comparison of high frequency correction methodologies

Several methods are available for evaluating and correcting the underestimation of trace gas fluxes measured by eddy covariance (EC) method and due to the design and the setup of the employed equipment. Different frequency correction factors (CF) to apply to the measured EC ammonia (NH3) fluxes have been applied using the following approaches: (i) the spectral theoretical transfer function (CFTheor) with and without phase shift; (ii) the in situ ogive method (CFO); and (iii) the inductance correction method (CFL). The NH3 fluxes were measured in an experimental field located in south Italy, above a sorghum crop submitted to Mediterranean semi-arid climate and fertilized with urea. A fast analyser based on Tunable Infrared Laser Differential Absorption Spectroscopy (TILDAS) coupled with a quantum cascade (QC) laser was used to measure NH3 concentration at high frequency. The results showed that correction of the values of NH3 EC fluxes over natural surfaces are necessary for taking into account the dumping of high frequency contribution due to the coupling of ammonia fast analyser by QC-TILDAS and sonic anemometer. In particular, the calculated flux losses ranged between −23% (inductance method, with the CF threshold selected to 2.5) and −43% (experimental-ogive method); while the two theoretical transfer function approaches gave comparable loss estimates, i.e. −30 and −31% for the methods with time lag and phase shift, respectively.

Simultaneous measurements of atmospheric HONO and NO2 via absorption spectroscopy using tunable mid-infrared continuous-wave quantum cascade lasers

Nitrous acid (HONO) is important as a significant source of hydroxyl radical (OH) in the troposphere and as a potent indoor air pollutant. It is thought to be generated in both environments via heterogeneous reactions involving nitrogen dioxide (NO2). In order to enable fast-response HONO detection suitable for eddy-covariance flux measurements and to provide a direct method that avoids interferences associated with derivatization, we have developed a 2-channel tunable infrared laser differential absorption spectrometer (TILDAS) capable of simultaneous high-frequency measurements of HONO and NO2. Beams from two mid-infrared continuous-wave mode quantum cascade lasers (cw-QCLs) traverse separate 210 m paths through a multi-pass astigmatic sampling cell at reduced pressure for the direct detection of HONO (1660 cm−1) and NO2 (1604 cm−1). The resulting one-second detection limits (S/N=3) are 300 and 30 ppt (pmol/mol) for HONO and NO2, respectively. Our HONO quantification is based on revised line-strengths and peak positions for cis-HONO in the 6-micron spectral region that were derived from laboratory measurements. An essential component of ambient HONO measurements is the inlet system and we demonstrate that heated surfaces and reduced pressure minimize sampling artifacts.

Characterizing a Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS) for measurements of atmospheric ammonia

Abstract. A compact, fast-response Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS) for measurements of ammonia (NH3) has been evaluated under both laboratory and field conditions. Absorption of radiation from a pulsed, thermoelectrically cooled QC laser occurs at reduced pressure in a 0.5 L multiple pass absorption cell with an effective path length of 76 m. Detection is achieved using a thermoelectrically-cooled Mercury Cadmium Telluride (HgCdTe) infrared detector. A novel sampling inlet was used, consisting of a short, heated, quartz tube with a hydrophobic coating to minimize the adsorption of NH3 to surfaces. The inlet contains a critical orifice that reduces the pressure, a virtual impactor for separation of particles, and additional ports for delivering NH3-free background air and calibration gas standards. The level of noise in this instrument has been found to be 0.23 ppb at 1 Hz. The sampling technique has been compared to the results of a conventional lead salt Tunable Diode Laser Absorption Spectrometer (TDLAS) during a laboratory intercomparison. The effect of humidity and heat on the surface interaction of NH3 with sample tubing was investigated at mixing ratios ranging from 30–1000 ppb. Humidity was seen to worsen the NH3 time response and considerable improvement was observed when using a heated sampling line. A field intercomparison of the QC-TILDAS with a modified Thermo 42CTL chemiluminescence-based analyzer was also performed at Environment Canada's Centre for Atmospheric Research Experiments (CARE) in the rural town of Egbert, ON between May–July 2008. Background tests and calibrations using two different permeation tube sources and an NH3 gas cylinder were regularly carried out throughout the study. Results indicate a very good correlation at 1 min time resolution (R2 = 0.93) between the two instruments at the beginning of the study, when regular background subtraction was applied to the QC-TILDAS. An overall good correlation of R2 = 0.85 was obtained over the entire two month data set, where the majority of the spread can be attributed to differences in inlet design and background subtraction methods.

Evaluation of nitrogen dioxide chemiluminescence monitors in a polluted urban environment

Abstract. Data from a recent field campaign in Mexico City are used to evaluate the performance of the EPA Federal Reference Method for monitoring the ambient concentrations of NO2. Measurements of NO2 from standard chemiluminescence monitors equipped with molybdenum oxide converters are compared with those from Tunable Infrared Laser Differential Absorption Spectroscopy (TILDAS) and Differential Optical Absorption Spectroscopy (DOAS) instruments. A significant interference in the chemiluminescence measurement is shown to account for up to 50% of ambient NO2 concentration during afternoon hours. As expected, this interference correlates well with non-NOx reactive nitrogen species (NOz) as well as with ambient O3 concentrations, indicating a photochemical source for the interfering species. A combination of ambient gas phase nitric acid and alkyl and multifunctional alkyl nitrates is deduced to be the primary cause of the interference. Observations at four locations at varying proximities to emission sources indicate that the percentage contribution of HNO3 to the interference decreases with time as the air parcel ages. Alkyl and multifunctional alkyl nitrate concentrations are calculated to reach concentrations as high as several ppb inside the city, on par with the highest values previously observed in other urban locations. Averaged over the MCMA-2003 field campaign, the chemiluminescence monitor interference resulted in an average measured NO2 concentration up to 22% greater than that from co-located spectroscopic measurements. Thus, this interference has the potential to initiate regulatory action in areas that are close to non-attainment and may mislead atmospheric photochemical models used to assess control strategies for photochemical oxidants.

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 ∼0.16 s 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 ∼1.0 ppbV Hz −1/2, which allows a detection limit of ∼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.

Real-Time Measurements of SO2, H2CO, and CH4 Emissions from In-Use Curbside Passenger Buses in New York City Using a Chase Vehicle

The Aerodyne Mobile Laboratory "chased" in-use curbside passenger buses operated by various operators in New York City. With the cooperation of New York State's Metropolitan Transit Authority, the relationships between the emissions of the several gas-phase species and particulate loadings were investigated across several bus technologies, bus types, and fuels (diesel, ultralow sulfur diesel, and compressed natural gas, CNG). The CNG buses followed did not employ an oxidation catalyst. The buses characterized were not prescreened in any fashion and were measured while deployed on their normal in-service routes. This paper focuses on the fuel-based mass emissions of SO2, H2CO, and CH4, measured using tunable infrared laser differential absorption spectroscopy. Sulfur dioxide emissions from buses known to be burning ultralow sulfur diesel (<30 ppm(m) S) were 16 times lower than those from buses burning normal commercial diesel fuel, nominally less than 300 ppm(m) sulfur. Emissions of formaldehyde and methane from in-use CNG buses were approximately 15 times greater than those from diesel powered buses.

Real-Time Measurements of Nitrogen Oxide Emissions from In-Use New York City Transit Buses Using a Chase Vehicle

New diesel engine technologies and alternative fuel engines are being introduced into fleets of mass transit buses to try to meet stricter emission regulations of nitrogen oxides and particulates: Real-time instruments including an Aerodyne Research tunable infrared laser differential absorption spectrometer (TILDAS) were deployed in a mobile laboratory to assess the impact of the implementation of the new technologies on nitrogen oxide emissions in real world driving conditions. Using a "chase" vehicle sampling strategy, the mobile laboratory followed target vehicles, repeatedly sampling their exhaust. Nitrogen oxides from approximately 170 in-use New York City mass transit buses were sampled during the field campaigns. Emissions from conventional diesel buses, diesel buses with continuously regenerating technology (CRT), diesel hybrid electric buses, and compressed natural gas (CNG) buses were compared. The chase vehicle sampling method yields real world emissions that can be included in more realistic emission inventories. The NO, emissions from the diesel and CNG buses were comparable. The hybrid electric buses had approximately one-half the NOx emissions. In CRT diesels, NO2 accounts for about one-third of the NOx emitted in the exhaust, while for non-CRT buses the NO2 fraction is less than 10%.

NO and NO2Emission Ratios Measured from In-Use Commercial Aircraft during Taxi and Takeoff

In August 2001, the Aerodyne Mobile Laboratory simultaneously measured NO, NO2, and CO2 within 350 m of a taxiway and 550 m of a runway at John F. Kennedy Airport. The meteorological conditions were such that taxi and takeoff plumes from individual aircraft were clearly resolved against background levels. NO and NO2 concentrations were measured with 1 s time resolution using a dual tunable infrared laser differential absorption spectroscopy instrument, utilizing an astigmatic multipass Herriott cell. The CO2 measurements were also obtained at 1 s time resolution using a commercial non-dispersive infrared absorption instrument. Plumes were measured from over 30 individual planes, ranging from turbo props to jumbo jets. NOx emission indices were determined by examining the correlation between NOx (NO + NO2) and CO2 during the plume measurements. Several aircraft tail numbers were unambiguously identified, allowing those specific airframe/engine combinations to be determined. The resulting NOx emission indices from positively identified in-service operating airplanes are compared with the published International Civil Aviation Organization engine certification test database collected on new engines in certification test cells.

Mobile Laboratory with Rapid Response Instruments for Real-Time Measurements of Urban and Regional Trace Gas and Particulate Distributions and Emission Source Characteristics

Recent technological advances have allowed the development of robust, relatively compact, low power, rapid response (approximately 1 s) instruments with sufficient sensitivity and specificity to quantify many trace gases and aerosol particle components in the ambient atmosphere. Suites of such instruments can be deployed on mobile platforms to study atmospheric processes, map concentration distributions of atmospheric pollutants, and determine the composition and intensities of emission sources. A mobile laboratory containing innovative tunable infrared laser differential absorption spectroscopy (TILDAS) instruments to measure selected trace gas concentrations at sub parts-per-billion levels and an aerosol mass spectrometer (AMS) to measure size resolved distributions of the nonrefractory chemical components of fine airborne particles as well as selected commercial fast response instruments and position/velocity sensors is described. Examples of the range of measurement strategies that can be undertaken using this mobile laboratory are discussed, and samples of measurement data are presented.

Cross road and mobile tunable infrared laser measurements of nitrous oxide emissions from motor vehicles

Context Abstract: Nitrous oxide (NO2) is a potent greenhouse gas whose atmospheric budget is poorly constrained. One known atmospheric source is the formation of N2O on three-way motor vehicle catalytic converters followed by emission with the exhaust. Previous estimates of the magnitude of this N2O source have varied widely. Two methods employing tunable infrared lasers to measure N2O/CO2 ratios from a large number of on-road motor vehicles have been developed. Both methods add support to lower estimates of N2O emissions from the US motor vehicle fleet, although significant uncertainty remains.Main Abstract: Two tunable infrared laser differential absorption spectroscopy (TILDAS) techniques have been used to measure the N2O emission levels of on-road motor vehicle exhausts. Cross road, open path laser measurements were used to assess N2O emissions from 1361 California catalyst equipped vehicles in November, 1996 yielding an emission ratio of (8.8±2.8)×10−5 N2O/CO2. A van mounted TILDAS sampling system making on-road N2O measurements in mixed traffic in June, 1998 in Manchester, New Hampshire yielded a mean N2O/CO2 ratio of (12.8±0.3)×10−5, based on correlated N2O and CO2 concentration peaks attributed to motor vehicle exhaust plumes. The correlation of N2O emissions with vehicle type, model year and NO emissions are presented for the California data set. It is found that the N2O emission distribution is highly skewed, with more than 50% of the emissions being contributed by 10% of the vehicles. Comparison of our results with those from four European tunnel studies reveals a wide range of derived N2O emission indices, with the most recent studies (including this study) finding lower values.

Characterization of On-Road Vehicle NO Emissions by a TILDAS Remote Sensor

ABSTRACT A tunable infrared laser differential absorption spectrometer (TILDAS) was used to remotely sense the nitric oxide (NO) emissions from 1,473 on-road vehicles. The real-world measurement precision of this instrument in the limit of low NO concentration is 5 ppm of the vehicle exhaust, which corresponds to a 3o detection limit of 15 ppm. Our analysis of the distribution of negative concentration measurements produced during this experiment supports this claim, showing that the instrumental noise for this set of measurements was at most 8 ppm in the limit of low NO concentration. The high sensitivity of this instrument allowed us to measure the NO emissions of even the cleanest vehicles. The measured vehicle fleet NO emissions closely fit a gamma distribution with 10% of the fleet contributing about 50% of the total fleet emissions. Newer vehicles had lower NO emissions than older ones, but high NO emitters were found in every vehicle age cohort. On a vehicle-by-vehicle basis, NO emissions correlated very weakly with vehicle velocity, acceleration, power per unit mass, carbon monoxide (CO) emissions, and hydrocarbon (HC) emissions. High NO emitting vehicles could not be identified by remote sensing of CO or HC emissions and vice versa. When we compared the NO emissions for 117 vehicles measured more than one time, about half of the high NO emitters were found to be very consistent, while the other half varied significantly.

Tunable infrared laser detection of pyrolysis products of explosives in soils

A research program involving two applications of tunable infrared laser differential absorption spectroscopy (TILDAS) with multipass, long-path absorption cells to the detection of explosives contamination in soils is reported. In the first application, sensitive, specific real-time species concentration measurements by TILDAS have led to new understanding of the processes involved in explosives detection by the heating of contaminated soils and the quantification of the resulting pyrolysis gases. In the second, we present results of our calculations of the properties of astigmatic off-axis resonator absorption cells, which show that useful TILDAS path lengths can be achieved inside a cone penetrometer probe.