Trace Gas Monitoring Research Articles

CO<sub>2</sub>, CO, and CH<sub>4</sub> measurements from tall towers in the NOAA Earth System Research Laboratory's Global Greenhouse Gas Reference Network: instrumentation, uncertainty analysis, and recommendations for future high-accuracy greenhouse gas monitoring efforts

Abstract. A reliable and precise in situ CO2 and CO analysis system has been developed and deployed at eight sites in the NOAA Earth System Research Laboratory's (ESRL) Global Greenhouse Gas Reference Network. The network uses very tall (> 300 m) television and radio transmitter towers that provide a convenient platform for mid-boundary-layer trace-gas sampling. Each analyzer has three sample inlets for profile sampling, and a complete vertical profile is obtained every 15 min. The instrument suite at one site has been augmented with a cavity ring-down spectrometer for measuring CO2 and CH4. The long-term stability of the systems in the field is typically better than 0.1 ppm for CO2, 6 ppb for CO, and 0.5 ppb for CH4, as determined from repeated standard gas measurements. The instrumentation is fully automated and includes sensors for measuring a variety of status parameters, such as temperatures, pressures, and flow rates, that are inputs for automated alerts and quality control algorithms. Detailed and time-dependent uncertainty estimates have been constructed for all of the gases, and the uncertainty framework could be readily adapted to other species or analysis systems. The design emphasizes use of off-the-shelf parts and modularity to facilitate network operations and ease of maintenance. The systems report high-quality data with > 93% uptime. Recurrent problems and limitations of the current system are discussed along with general recommendations for high-accuracy trace-gas monitoring. The network is a key component of the North American Carbon Program and a useful model for future research-grade operational greenhouse gas monitoring efforts.

Diode lasers: From laboratory to industry

The invention of first laser in 1960 triggered the discovery of several new families of lasers. A rich interplay of different lasing materials resulted in a far better understanding of the phenomena particularly linked with atomic and molecular spectroscopy. Diode lasers have gone through tremendous developments on the forefront of applied physics that have shown novel ways to the researchers. Some interesting attributes of the diode lasers like cost effectiveness, miniature size, high reliability and relative simplicity of use make them good candidates for utilization in various practical applications. Diode lasers are being used by a variety of professionals and in several spectroscopic techniques covering many areas of pure and applied sciences. Diode lasers have revolutionized many fields like optical communication industry, medical science, trace gas monitoring, studies related to biology, analytical chemistry including elemental analysis, war fare studies etc. In this paper the diode laser based technologies and measurement techniques ranging from laboratory research to automated field and industry have been reviewed. The application specific developments of diode lasers and various methods of their utilization particularly during the last decade are discussed comprehensively. A detailed snapshot of the current state of the art diode laser applications is given along with a detailed discussion on the upcoming challenges.

Trace gas detection of benzene, toluene, p-, m- and o-xylene with a compact measurement system using cantilever enhanced photoacoustic spectroscopy and optical parametric oscillator

A compact measurement system based on a novel combination of cantilever enhanced photoacoustic spectroscopy (CEPAS) and optical parametric oscillator (OPO) was applied to the gas phase measurement of benzene, toluene, and o-, m- and p-xylene (BTX) traces. The OPO had a band width (FWHM) of 1.3nm, was tuned from 3237 to 3296nm in steps of 0.1nm and so spectra of BTX at different concentrations were recorded. The power emitted by the OPO increased from 88mW at 3237nm to 103mW at 3296nm. The univariate detection limits (3σ, 0.951s) for benzene, toluene, p-, m- and o-xylene at 3288nm were 12.0, 9.8, 13.2, 10.1 and 16.0ppb, respectively. Multivariate data analysis using science-based calibration was used to resolve the interference of the analytes. The multivariate detection limits (3σ, 3237–3296nm, 591 spectral points each 0.951s) for benzene, toluene, p-, m- and o-xylene in the multi-compound sample, where all other analytes and water interfere were 4.3, 7.4, 11.0, 12.5 and 6.2ppb, respectively. Without interferents, the multivariate detection limits varied between 0.5 and 0.6ppb. The sum of the cross-selectivities (3237–3296nm, 591 spectral points, each 0.951s) per analyte were below 0.05ppb/ppb, with an average of 0.038ppb/ppb. The cross-selectivity of water to the analytes was on average 1.22×10−4ppb/ppb. The OPO is small in size (L×W×H 125×70×45mm), commercially available, and easy to operate and integrate to setups. The combination with sensitive CEPAS enables compact measurement systems for industrial as well as environmental trace gas monitoring.

Online, real-time detection of volatile emissions from plant tissue

Trace gas monitoring plays an important role in many areas of life sciences ranging from agrotechnology, microbiology, molecular biology, physiology, and phytopathology. In plants, many processes can be followed by their low-concentration gas emission, for compounds such as ethylene, nitric oxide, ethanol or other volatile organic compounds (VOCs). For this, numerous gas-sensing devices are currently available based on various methods. Among them are the online trace gas detection methods; these have attracted much interest in recent years. Laser-based infrared spectroscopy and proton transfer reaction mass spectrometry are the two most widely used methods, thanks to their high sensitivity at the single part per billion level and their response time of seconds. This paper starts with a short description of each method and presents performances within a wide variety of biological applications. Using these methods, the dynamics of trace gases for ethylene, nitric oxide and other VOCs released by plants under different conditions are recorded and analysed under natural conditions. In this way many hypotheses can be tested, revealing the role of the key elements in signalling and action mechanisms in plants.

Analysis of ammonia variation in the urban atmosphere

Long-term monitoring of ammonia concentration and other trace gases was made at hourly intervals along with meteorological parameters in Seoul, Korea for a one-year period (1 September 2010–23 August 2011). The mean ammonia concentrations measured at two sites (Gwang-Jin (GJ) and Gang-Seo (GS) districts) were 10.9 ± 4.25 and 12.3 ± 4.23 ppb, respectively. A comparison of the seasonal data shows its relative dominance at both GJ and GS sites during the warm period (summer and spring) over the cool period (winter and fall). Likewise, over the diurnal scale, relative dominance during daytime was also apparent (p < 0.05). A comparison between different studies suggests that the long-term trend of ammonia levels should be tightly affected by an increasing number of vehicles. As such, the results of our statistical analysis also point to the potentially significant role of traffic activities in the elevation of ammonia levels in urban areas.

Ozone production in summer in the megacities of Tianjin and Shanghai, China: a comparative study

Abstract. Rapid economic growth has given rise to a significant increase in ozone precursor emissions in many regions of China, especially in the densely populated North China Plain (NCP) and Yangtze River Delta (YRD). Improved understanding of ozone formation in response to different precursor emissions is imperative to address the highly nonlinear ozone problem and to provide a solid scientific basis for efficient ozone abatement in these regions. A comparative study on ozone photochemical production in summer has thus been carried out in the megacities of Tianjin (NCP) and Shanghai (YRD). Two intensive field campaigns were carried out respectively at an urban and a suburban site of Tianjin, in addition to routine monitoring of trace gases in Shanghai, providing data sets of surface ozone and its precursors including nitrogen oxides (NOx) and various non-methane hydrocarbons (NMHCs). Ozone pollution in summer was found to be more severe in the Tianjin region than in the Shanghai region, based on either the frequency or the duration of high ozone events. Such differences might be attributed to the large amount of highly reactive NMHCs in Tianjin. Industry related species like light alkenes were of particular importance in both urban and suburban Tianjin, while in Shanghai aromatics dominated. In general, the ozone problem in Shanghai is on an urban scale. Stringent control policies on local emissions would help reduce the occurrence of high ozone concentrations. By contrast, ozone pollution in Tianjin is probably a regional problem. Combined efforts to reduce ozone precursor emissions on a regional scale must be undertaken to bring the ozone problem under control.

Strato‐mesospheric ozone measurements using ground‐based millimeter‐wave spectroscopy at Thule, Greenland

On January 2009 a ground‐based millimeter‐wave spectrometer (GBMS) was installed at Thule Air Base (76.5°N, 68.8°W), Greenland, for long‐term winter monitoring of several stratospheric and mesospheric trace gases in the framework of the Network for the Detection of Atmospheric Composition Change. This work is aimed at characterizing the GBMS O3 vertical profiles between 35 and 80 km altitude obtained by applying the optimal estimation method to O3pressure‐broadened spectral line measurements carried out during three winters. In this altitude range, GBMS O3 retrievals are highly sensitive to variations of the atmospheric state, and their accuracy is estimated to be the larger of 11% or 0.2 ppmv. Comparisons of GBMS O3profiles with colocated satellite‐based measurements from Aura Microwave Limb Sounder (MLS) and Thermosphere Ionosphere Mesosphere Energetics and Dynamics Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) show a good agreement below 65 km altitude once the known 10%–20% high bias of SABER O3profiles is considered, with the GBMS displaying an averaged low bias of ∼9% and 17% with respect to MLS and SABER. In the nighttime mesosphere, the GBMS detects the ozone tertiary maximum within 0.1 ppmv (6%) on average with respect to the convolved MLS, SABER, and global 3‐D ROSE model profiles but shifts its position to lower altitudes by 4–5 km compared to the height obtained by the other three data sets. In the 50–80 km altitude range, estimates of mesospheric O3 diurnal variation obtained from the GBMS and the convolved satellite measurements agree well within the ±1 standard deviation (∼0.6 ppmv) of the GBMS mean profile.

Long-Term Degradation Behavior of 2.3-$\mu\hbox{m}$ Wavelength Highly Strained InAs/InP MQW-DFB Lasers With a p-/n-InP Buried Heterostructure

We investigate the long-term degradation of 2.3-μm wavelength InAs/InP highly strained multiquantum-well distributed-feedback lasers with a p- and n-type InP buried heterostructure during constant power aging. Stable operation beyond 20 000 h is confirmed at an ambient temperature of 45 °C and with a constant output power of 3 mW, and we successfully fabricate a practical optical source for trace gas monitoring applications. A conventional diffusion process dominates the main degradation mechanism as found with conventional telecommunication lasers. Furthermore, we show that some defects are located in the regrown p-type InP layer above the active layer and that downward diffused defects degrade the guide, upper separate confinement heterostructure (SCH), and active layers.

Diode Laser Induced Fluorescence for Gas-Phase Diagnostics

Abstract We highlight the capabilities and potential of diode laser induced fluorescence for measurements in gas-phase reacting flows. Many applications of diode lasers in practical sensing are based on absorption spectroscopy. Fluorescence-based diagnostics possess similar advantages in terms of practicality and implementation-cost but additionally are capable of achieving excellent spatial resolution. Diode laser fluorescence instruments have been employed for high-sensitivity trace gas monitoring in applications ranging from plasma physics to atmospheric chemistry. This article begins by describing the UV-visible diode laser technology used to perform fluorescence. The principles of diode laser induced fluorescence are then reviewed and a comparison is made with absorption spectroscopy. Examples are given of concentration measurements of both atomic and molecular trace gases. Recent work on using diode laser induced atomic fluorescence for precision measurements of flame temperature is also reviewed. We conclude by a discussion of future opportunities for diode laser fluorescence spectroscopy drawing attention to interesting potential target species as well as novel application areas, such as monitoring of synthesis processes for nanomaterials.

Inter-Pulse Spectroscopy Based on Room-Temperature Pulsed Quantum-Cascade Laser for N&lt;sub&gt;2&lt;/sub&gt;O Detection

Mid-infrared lasers are very suitable for high-sensitive trace-gases detection for their wavelengths cover the fundamental absorption lines of most gases. Quantum-cascade (QC) lasers have been demonstrated to be ideal light sources with its special power, tuning and capability of operating in room-temperature. All these merits make it appropriate for the high resolution spectrum analysis. The absorption spectrum monitoring technology based on the QC laser pulsed operating in the room temperature, combining with the strong absorption of the gas molecule in the basic frequency, has become an effective way to monitor the trace gas with the characteristic of high sensitivity, good selectivity and fast response. In this paper, the inter-pulse spectroscopy based on a room-temperature distributed-feedback pulsed QC laser was introduced. Our approach to trace gas monitoring with QC lasers relies on short current pulses which are designed to produce even shorter light pulses. Each pulse corresponds to a single point in a spectrum. The N2O absorption spectrum centered at 2178.2cm-1was also obtained.

Measurements of NO and CO in Shanghai urban atmosphere by using quantum cascade lasers

Nitric oxide (NO) and carbon monoxide (CO) in Shanghai urban atmosphere have been measured during the EXPO 2010 by an optical trace gas monitoring system based on room-temperature pulsed quantum cascade lasers (QCL). The results showed obvious diurnal variation in their concentrations. A great correlation was found by analyzing the diurnal variation, indicating common emission sources for both NO and CO. However analysis of the data from street canyon measurements showed that on a vehicle-by-vehicle basis NO emissions correlated only weakly with CO emissions. A possible explanation to this issue has been given.

Sensing HDO/H2O in the Ural’s atmosphere using ground-based measurements of IR solar radiation with a high spectral resolution

The Ural Atmospheric Fourier Station (UAFS) based on a Bruker IFS-125M Fourier spectrometer conjugated with an A547N automated solar tracker is described. The UAFS is located in the forest area (57.038°N; 59.545°E) and is intended for trace gas monitoring in the background atmosphere. The examples of solar-radiation near-IR atmospheric transmittance spectra measured with a high spectral resolution are presented, as well as the first results of retrieval of the heavy water fraction in the Ural atmosphere.

External cavity tunable quantum cascade lasers and their applications to trace gas monitoring

Since the first quantum cascade laser (QCL) was demonstrated approximately 16 years ago, we have witnessed an explosion of interesting developments in QCL technology and QCL-based trace gas sensors. QCLs operate in the mid-IR region (3-24 μm) and can directly access the rotational vibrational bands of most molecular species and, therefore, are ideally suited for trace gas detection with high specificity and sensitivity. These sensors have applications in a wide range of fields, including environmental monitoring, atmospheric chemistry, medical diagnostics, homeland security, detection of explosive compounds, and industrial process control, to name a few. Tunable external cavity (EC)-QCLs in particular offer narrow linewidths, wide ranges of tunability, and stable power outputs, which open up new possibilities for sensor development. These features allow for the simultaneous detection of multiple species and the study of large molecules, free radicals, ions, and reaction kinetics. In this article, we review the current status of EC-QCLs and sensor developments based on them and speculate on possible future developments.

On-line, real time monitoring of exhaled trace gases by SIFT-MS in the perioperative setting: a feasibility study

A study is described of the first on line, real time analyses of the exhaled breath of five anaesthetized patients during the complete perioperative periods of laparoscopic surgery. These breath analyses were achieved using a selected ion flow tube, SIFT-MS, instrument, located in the operating theatre at an acceptable distance from the operating table, and coupled to the endotracheal tube in the ventilation circuit via a 5 metre long capillary tube. Thus, inhalation/exhalation breathing cycles, set to be at a frequency of 10 per minute, were sampled continuously for water vapour, the metabolites acetone and isoprene and the propofol used to induce anaesthesia for each operating period that ranged from 20 min (shortest) to 80 min (longest). Whilst there was some loss of water vapour along the long sampling line, the concentrations of the other trace compounds were not diminished. The breath acetone was essentially at a constant level for each patient, but increased somewhat over the longest operating period due to the onset of lipolysis. Most interesting is the clear increase of breath isoprene following abdomen inflation with carbon dioxide. The vapour of the intravenously injected propofol was detected in the exhaled breath and remained essentially constant during the perioperative period. These analyses were performed totally non-invasively and the data were immediately and constantly available to the anaesthetist and surgeon. Exploitation of this development could influence decision making and potentially improve patient safety within the perioperative setting.

Application of a super-luminescent diode coupled with an interference filter for mixed gas sensing

Laser-based spectroscopic techniques are very attractive methods for trace gas, environmental and industrial monitoring. By employing narrow spectral-width tunable coherent light sources, a single absorption line of the substance to be analyzed can be probed with a laser, avoiding interference signals from other species. Here we report a difference frequency generation (DFG) source in mid-infrared region using a tunable laser (825 – 870nm) and a fixed wavelength diode laser (1060nm) in combination with a PPLN having single period. Application of the approach for carbon dioxide (CO2) monitoring in air was studied. Many gas sensing applications do not require extreme sensitivities up to ppb order and in this case, super-luminescent diodes (SLD) are also of great interest. They are commonly produced in the spectral range of optical fibers telecommunications (1.3 – 1.7 m), where overtone vibration bands of many gas molecules of interest. As a typical example, measurements of ammonia (NH3) trace gas were studied for combustion measurement applications. A novel approach of using fiber-coupled SLD along with an interference filter can also be considered for such application to avoid influence of water vapor (H2O). The interest of this system consists in the possibility to detect several species with a single instrument.

In-situ monitoring of trace gases in a non–urban environment

A set of commercial instruments measuring carbon monoxide (CO), ozone (O3), sulfur dioxide (SO2), and nitrogen oxides [nitric oxide (NO), nitrogen dioxide (NO2), and odd nitrogens (NOX)] was integrated and deployed in a non– urban environment. The deployment occurred between July 2, 2007 and August 7, 2007 in Richland, WA. The mixing ratios of all species were lower than in most rural–suburban environments, and strong diurnal patterns were observed. NO2 was depleted by photochemically formed ozone during the day and replenished at night as ozone was destroyed. The highest ozone concentration during these episodes was 45 ppb. The overall average was 15 ppb with readings approaching near zero at times. This observation is low compared to average daytime summer readings of 60–80 ppb in highly populated and industrialized urban areas in the Pacific Northwest region. Back-trajectory analysis and prevailing weather conditions both indicated that much of the ozone was transported locally or was produced in– situ. Analysis of SO2 as a tracer for O3 advection further indicated lack of long–range regional transport of pollutants to Richland. We also present results of analysis of high ozone episodes and comparisons relative to other areas in the Pacific Northwest region. These results provide a useful sample data set to study the historical record of air quality in rural Eastern Washington.

Comparison of InP- and GaSb-based VCSELs emitting at 2.3 μm suitable for carbon monoxide detection

Carbon monoxide (CO) is a prominent and toxic gas mainly generated by imperfect burning in fires or ovens. For trace gas monitoring and fire detection a compact, long term stable and calibration-free CO sensor is desirable. One very interesting measuring method is Tuneable Diode Laser Absorption Spectroscopy (TDLAS), which utilizes the unique properties of Vertical-Cavity Surface-Emitting Lasers (VCSELs). Two approaches to reach the required wavelength range for the absorption lines of CO at around 2350nm are discussed in this paper. The first approach is an expansion of the emission wavelength range of the now well-established lasers based on InP and a second and new one is fabricating VCSELs based on GaSb. From the epitaxial point of view the gain of the active material and the realization of a tunnel junction as well as optical, thermal and electrical characteristics of the mirror materials are important issues. For a proper choice of the device design the structuring of the used materials also plays a fundamental role— in particular the substrate removal. With simultaneous considerations of all these crucial issues, devices on InP and GaSb substrates have been fabricated. Both types work in continuous-wave mode, generating light in single-mode emission at the desired wavelength of the CO absorption line, which enables CO measurements down to a concentration limit of 2ppm.

Laser beam spatial profile determination by pulsed photoacoustics: exact solution

The pulsed photoacoustic spectroscopy technique is one of the oldest but respectable techniques which offer some unique features that are relevant to trace gas monitoring. Despite obvious advantages there are some drawbacks that have so far alienated this spectroscopy technique from routine air monitoring. One of them is the additional instrumentation necessity to fulfill the requirement for an accurate knowledge of the spatial and temporal profiles of the excitation light source. Here we present one method for a laser beam spatial profile determination and simultaneous vibrational–translational relaxation time calculation. It is based on the temporal shape analysis of the photoacoustic signals. Symmetric laser beam spatial profiles are obtained with a high level of accuracy. Even though this method does not work in real time, it gives a good basis for future beam profile investigations. Experimental signals used in our analysis are obtained after infrared multiphoton absorption in the SF6–Ar mixture.

Optimization and magnetic-field tunability of quantum cascade laser for applications in trace gas detection and monitoring

We explore the possibilities of using advanced tools for global optimization, namely the genetic algorithm, to obtain structural parameters of gain-maximized quantum cascade laser (QCL) emitting at specified wavelengths, suitable for detection of pollutant gasses, such as SO2, HNO3, CH4 and NH3, in the ambient air. Upon completing this initial optimization stage, we introduce a strong external magnetic field perpendicular to the epitaxial layers, to fine tune the laser output properties. This magnetic field alters the electron energy spectrum by splitting the continuous energy subbands into discrete Landau levels whose arrangement influences the magnitude of the optical gain. In addition, strong effects of band nonparabolicity result in subtle changes in the lasing wavelength at magnetic fields which maximize the gain, thus allowing us to explore the prospects of multi-wavelength emission of the given structure, and achieving resonance with additional compounds, absorbing at wavelengths close to the original one. Numerical results are presented for GaAs/AlxGa1−xAs based QCL structures designed for operation in the mid-infrared part of the spectrum.

Reliable 2.3-$\mu$m Wavelength Highly Strained InAs–InP MQW-DFB Lasers With p-/n-InP Buried Heterostructure

We have realized reliable 2.3-mum wavelength InAs-InP multiquantum-well distributed-feedback (DFB) lasers for trace gas monitoring applications. The estimated median lifetime exceeds 1x105 hours during aging at an ambient temperature of 45degC and with a constant output power of 3 mW. Furthermore, the relative increase in operating current is proportional to the square root of time, and it is clarified that the main degradation mechanism in DFB lasers with highly strained InAs quantum wells is dominated by a diffusion process as found with conventional telecommunication lasers.