Open-path Measurements Research Articles

Relative humidity effects on water vapour fluxes measured with closed-path eddy-covariance systems with short sampling lines

It has been formerly recognised that increasing relative humidity in the sampling line of closed-path eddy-covariance systems leads to increasing attenuation of water vapour turbulent fluctuations, resulting in strong latent heat flux losses. This occurrence has been analyzed for very long (50m) and long (7m) sampling lines. To date, only a few analytical or in situ analyses have been proposed to quantify and correct such effects, among which the comprehensive method by Ibrom et al. (2007) was proved effective for the very long sampling line of a forest eddy-covariance setup.Here we analyze data from eddy-covariance systems featuring short (4m) and very short (1m) sampling lines running at the same clover field and show that relative humidity effects persist also for these setups, and should not be neglected. Starting from the work of Ibrom and co-workers, we propose a mixed method, a composite of two existing approaches, for correcting eddy-covariance fluxes. By means of a comparison with parallel open-path measurements, we show that the mixed method leads to an improved estimation of latent heat fluxes, with respect to the method described by Ibrom et al. (2007). The quantification and correction method proposed here is deemed applicable to closed-path systems featuring a broad range of sampling lines, and indeed applicable also to passive gases as a special case. The methods described in this paper are incorporated, as processing options, in the free and open-source eddy-covariance software packages ECO2S and EddyPro.

Areal-averaged trace gas emission rates from long-range open-path measurements in stable boundary layer conditions

Abstract. Measurements of land-surface emission rates of greenhouse and other gases at large spatial scales (10 000 m2) are needed to assess the spatial distribution of emissions. This can be readily done using spatial-integrating micro-meteorological methods like flux-gradient methods which were evaluated for determining land-surface emission rates of trace gases under stable boundary layers. Non-intrusive path-integrating measurements are utilized. Successful application of a flux-gradient method requires confidence in the gradients of trace gas concentration and wind, and in the applicability of boundary-layer turbulence theory; consequently the procedures to qualify measurements that can be used to determine the flux is critical. While there is relatively high confidence in flux measurements made under unstable atmospheres with mean winds greater than 1 m s−1, there is greater uncertainty in flux measurements made under free convective or stable conditions. The study of N2O emissions of flat grassland and NH3 emissions from a cattle lagoon involves quality-assured determinations of fluxes under low wind, stable or night-time atmospheric conditions when the continuous "steady-state" turbulence of the surface boundary layer breaks down and the layer has intermittent turbulence. Results indicate that following the Monin-Obukhov similarity theory (MOST) flux-gradient methods that assume a log-linear profile of the wind speed and concentration gradient incorrectly determine vertical profiles and thus flux in the stable boundary layer. An alternative approach is considered on the basis of turbulent diffusivity, i.e. the measured friction velocity as well as height gradients of horizontal wind speeds and concentrations without MOST correction for stability. It is shown that this is the most accurate of the flux-gradient methods under stable conditions.

Multiwavelength technique for open-path measurements of the concentrations of atmospheric constituents

A multiwavelength technique for open-path spectrophotometric measurements of the concentrations of the principal constituents of the atmosphere is presented. It uses a standard least squares procedure to minimize deviations of the calculated optical thickness of the working path from the measured value. The novel feature of this technique is that the effects of various sources of error on the computational results are interpreted as the difference in spectra of the probe radiation propagating along the working path and a comparison path. This difference yields a systematic error that is independent of the actual concentrations. The advantage of the multiwavelength technique is that an increased number of measurement wavelengths significantly reduces the effect of the various sources of error on the results.

Development of an open-path incoherent broadband cavity-enhanced spectroscopy based instrument for simultaneous measurement of HONO and NO2 in ambient air

We report on the development of an optical instrument based on incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) for simultaneous open-path measurements of nitrous acid (HONO) and nitrogen dioxide (NO2) in ambient air using a UV light emitting diode operating at ∼366 nm. Detection limits of ∼430 pptv for HONO and ∼1 ppbv for NO2 were achieved with an optimum acquisition time of 90 s, determined by an Allan variance analysis. Based on a 1.85 m long high optical finesse open-path cavity, the effective optical path length of 2.8 km was realized in aerosol-free samples or in an urban environment at modest aerosol levels. Such a kilometer long optical absorption is comparable to that achieved in the well established differential optical absorption spectroscopy (DOAS) technology while keeping the instrument very compact. Open-path detection configuration allows one to avoid absorption cell wall losses and sampling induced artifacts. The demonstrated sensitivity and specificity shows high potential of this cost-effective and compact infrastructure for future field applications with high spatial resolution.

Ammonia Emissions from a U.S. Broiler House—Comparison of Concurrent Measurements Using Three Different Technologies

There is a need for robust and accurate techniques for the measurement of ammonia (NH3) and other atmospheric pollutant emissions from poultry production facilities. Reasonable estimates of NH3 emission rate (ER) from poultry facilities are needed to guide discussions about the industry’s impact on local and regional air quality. The design of these facilities features numerous emission points and results in emission characteristics of relatively low concentrations and exhaust flow rates that vary diurnally, seasonally, and with bird age over a considerable range. These factors combine to render conventional emissions monitoring approaches difficult to apply. Access to these facilities is also often restricted for biosecurity reasons. The three objectives of this study were (1) to compare three methods for measuring exhaust NH3 concentrations and thus ERs, (2) to compare ventilation rates using in situ measured fan characteristics versus using manufacturer sourced fan curves, and (3) to examine limitations of the alternative measurement technologies. In this study, two open-path monitoring systems operating outside of the buildings were compared with a portable monitoring system sampling upstream of a primary exhaust fan. The position of the open-path systems relative to the exhaust fans, measurement strategy adopted, and weather conditions significantly influenced the quality of data collected when compared with the internally located, portable monitoring system. Calculation of exhaust airflow from the facility had a large effect on calculated emissions and assuming that the installed fans performed as per published performance characteristics potentially overestimated emissions by 13.6–26.8%. The open-path measurement systems showed promise for being able to obtain ER measurements with minimal access to the house, although the availability of individual fan characteristics markedly improved the calculated ER accuracy. However, substantial operator skill and experience and favorable weather conditions were required to obtain good quality results.

Multiwavelength procedure for determining the concentration of near-ground ozone by an optical open-path measurement system

We present a theoretical basis and develop a convenient mathematical formalism for carrying out a multiwavelength procedure for measuring the concentration of near-ground ozone by the differential optical absorption spectroscopy (DOAS) open-path method. We have analyzed the effect on the measurement results from the difference between the spectra of the probing radiation traveling on the measurement path and the reference path.

Technical note: Evaluation of standard ultraviolet absorption ozone monitors in a polluted urban environment

Abstract. The performance of the EPA Federal Equivalent Method (FEM) technique for monitoring ambient concentrations of O3 via ultraviolet absorption (UV) has been evaluated using data from the Mexico City Metropolitan Area field campaign (MCMA-2003). Comparisons of UV O3 monitors with open path Differential Optical Absorption Spectroscopy (DOAS) and open path Fourier Transform Infrared (FTIR) spectroscopy instruments in two locations revealed average discrepancies in the measured concentrations between +13% to −18%. Good agreement of two separate open path DOAS measurements at one location indicated that spatial and temporal inhomogeneities were not substantially influencing comparisons of the point sampling and open path instruments. The poor agreement between the UV O3 monitors and the open path instruments was attributed to incorrect calibration factors for the UV monitors, although interferences could not be completely ruled out. Applying a linear correction to these calibration factors results in excellent agreement of the UV O3 monitors with the co-located open path measurements; regression slopes of 0.94 to 1.04 and associated R2 values of >0.89. A third UV O3 monitor suffered from large spurious interferences, which were attributed to extinction of UV radiation within the monitor by fine particles (<0.2 µm). The overall performance of this particular monitor was poor owing to a combination of interferences from a contaminated particle filter and/or ozone scrubber. Suggestions for improved operation practices of these UV O3 monitors and recommendations for future testing are made.

Remote sensing by open-path FTIR spectroscopy. Comparison of different analysis techniques applied to ozone and carbon monoxide detection

The analysis of FTIR spectra is usually carried out by commercial programs that use methods based on classical least-squares (CLS) procedures These procedures are normally appropriate to analyse O-P FTIR spectra, but some occasional discrepancies with standard extractive methods have been observed. In this work, a line-by-line method (SFIT) is additionally used. Our purpose is to explain the sporadic discrepancies observed between CLS and standard extractive method results and to study the capability of both methods (CLS and SFIT) to analyse open-path measurements. This capability has been studied for ozone and carbon monoxide. It has been demonstrated that in CLS-based methods discrepancies are induced by the experimental background reference spectrum. In these cases, SFIT results are in very good agreement with the standard extractive methods.

Open path atmospheric spectroscopy using room temperature operated pulsed quantum cascade laser

We report the application of a distributed feedback quantum cascade laser for 5.8 km long open path spectroscopic monitoring of ozone, water vapor and CO 2. The thermal chirp during a 140 or 200 ns long excitation pulse is used for fast wavelength scanning. The fast wavelength scanning has the advantage of the measured spectra not being affected by atmospheric turbulence, which is essential for long open path measurements. An almost linear tuning of about 0.6 and 1.2 cm −1 is achieved, respectively. Lines from the ν 3 vibrational band of the ozone spectra centered at 1031 and 1049 cm −1 is used for ozone detection by differential absorption. The lowest column densities (LCD) for ozone of the order of 0.3 ppm m retrieved from the absorption spectra for averaging times less than 20 s are better then the LCD value of 2 ppm m measured with UV DOAS systems. The intrinsic haze immunity of mid-IR laser sources is an additional important advantage of mid-IR open path spectroscopy, compared with standard UV–vis DOAS. The third major advantage of the method is the possibility to measure more inorganic and organic atmospheric species compared to the UV–vis DOAS.

Open-path ozone detection by quantum-cascade laser

Open-path ozone measurements performed by mid-IR differential absorption spectroscopy are reported. Ozone spectrum was taken by fast repetitive sweeping of a quantum-cascade laser wavelength over a spectral feature from the ν3 absorption band of ozone, centered at 1031.2 cm-1. Short (100 ns) sweeping times were essential to prevent line-distortions caused by atmospheric turbulence. For fast wavelength sweeping, a technique that employed the thermal chirp during 140 ns excitation pulses was used. The lowest detection limit of 0.3 ppm.m was estimated from the minimum detectable differential absorption. We present the results from cell and open-path measurements over 440 and 5800 m, together with experimental data regarding the tuning range, the tuning rate and the tuning linearity of the QCL while operated with 140 ns excitation pulses.

FTIR Spectroscopy of the Atmosphere Part 2. Applications

The basic principles and methods of FTIR spectroscopy of the atmosphere were summarized in our previous paper 1. Thanks to the continuous technical development of FTIR spectroscopy (increasing throughput, dynamic alignment, more sensitive detectors, brighter sources, increasing scanning speed, development of focal plane array detectors, flexible spectral manipulations and data handling, etc.) in the last decade, this method has offered a great number of unique applications. In this review article, attempt to summarize the results of the most significant and frequent applications of FTIR spectroscopy to the study of the atmosphere. The possibilities of techniques applied in this field, the extractive and open path measurement methods, and the in situ IR absorption measurements such as remote sensing using the sun, the sky, or natural hot objects as IR sources of radiation are discussed. We have made a special focus to FTIR emission spectroscopy, the so‐called passive technique, since there are a number of originally hot gaseous samples such as volcanic plumes, automobile gases, stack gas plumes, or flames. Most of the subjects discussed in this article can be closely related to environmental analysis of the atmosphere. There is a wide range of atmospheric environmental applications of FTIR spectroscopy; therefore, we have focused our attention in the second part of the article on applications of FTIR spectroscopy in the atmosphere (troposphere) and stratosphere. We have summarized the basic literature in the field of special environmental applications of FTIR spectroscopy, such as power plants, petrochemical and natural gas plants, waste disposals, agricultural, and industrial sites, and the detection of gases produced in flames, in biomass burning, and in flares.

Multipass open-path Fourier-transform infrared measurements for nonintrusive monitoring of gas turbine exhaust composition

The detection limits for NO and NO2 in turbine exhausts by nonintrusive monitoring have to be improved. Multipass mode Fourier-transform infrared (FTIR) absorption spectrometry and use of a White mirror system were found from a sensitivity study with spectra simulations in the mid-infrared to be essential for the retrieval of NO2 abundances. A new White mirror system with a parallel infrared beam was developed and tested successfully with a commercial FTIR spectrometer in different turbine test beds. The minimum detection limits for a typical turbine plume of 50 cm in diameter are approximately 6 parts per million (ppm) for NO and 9 ppm for NO2 (as well 100 ppm for CO2 and 4 ppm for CO).

An application of tomographic reconstruction of atmospheric CO/sub 2/ over a volcanic site based on open-path ir laser measurements

The southern area of Italy is characterized by the presence of many active volcanic areas. In Pozzuoli (Naples, Italy), an urban area characterized by high volcanic risk, a gas emitting site is present. A high percentage of CO/sub 2/ is emitted, whose atmospheric concentration measurement is an important task in many environmental and scientific applications. In this paper, we describe the utilization over that area of a mobile infrared (IR) laser system, able to measure the CO/sub 2/ concentration along rectilinear atmospheric paths up to 1-km length, and the result of tomographic processing applied to retrieve a two-dimensional CO/sub 2/ concentration field. The laser system computes the link averaged concentration by processing the received IR laser radiation propagated along an open-air rectilinear link connecting the transmitter/receiver laser unit and a passive retroreflector device. A one-day measurement campaign has been made and 15 different atmospheric propagation links were considered moving the transmitter/receiver unit and some retroreflectors.

Comparison of Open-Path and Point Measurements of a Gaseous Pollutant in the Vicinity of a Model Building

Field experiments were carried out in the vicinity of anisolated building using open-path and point monitoring airpollution detectors to measure concentrations of a tracer gasreleased from a source located either on the building or at thedownwind face of the building. Experiments were conducted underneutral or slightly unstable weather conditions. Either threeor five fast-response point monitors were located along thebeam path of an open-path instrument, in the wake of thebuilding. Experiments were performed with different sourcelocations, detector locations and building orientations to thewind. Visual comparison of data time series showed that datafrom the two types of instruments followed similar trends. Thebest degree of correlation occurred for an averaging timesimilar to the response time of the open-path monitor. Meanconcentrations detected by the open-path instrument weregenerally in very good agreement with the path averaged meanconcentrations from the point monitors. The point monitors werealso capable of providing time-resolved data needed to describedetailed concentration fluctuation statistics.

Measuring chemical emissions using open-path Fourier transform infrared (OP-FTIR) spectroscopy and computer-assisted tomography

This paper reports on a field study that was part of a large-scale, multi-seasonal research study with the North Carolina Department of Environment, Health, and Natural Resources, to measure nitrogen emissions from an intensive swine confinement facility. The study measured emission rates using tracer gases and a horizontal network of open-path Fourier transform infrared (FTIR) optical rays placed less than a meter above the surface of an approximately 6 acre intensive swine waste lagoon in Eastern North Carolina. This network of rays simultaneously monitored the ammonia and the tracer gases every 2 min. The open-path measurements were combined with the mathematical mapping techniques of computer-assisted tomography (CAT) to create two-dimensional concentration maps of the gases for the entire lagoon surface. For this study, a ratioing technique was applied to the tomographic concentration maps to estimate the nitrogen emission rates (from ammonia) using known tracer emission rates. The average concentrations of ammonia measured in August, November, and May were 0.81, 0.25, and 0.74 ppm, respectively. In general, ammonia concentrations were lowest at the center of the lagoon and could vary across a lagoon from 2 to 4 times depending upon the time of the day and the meteorological conditions. Emission rates were only calculated for November and May, up until midnight. In November 1997, the average flux was 1910 μg N m −2-min −1 (range 542–4695 μg N m −2-min −1) and in May the average flux was 4775 μg N m −2-min −1 (range 2572–8499 μg N m −2-min −1). This study was important because it not only provided nitrogen emission rate measurements using a new technology which can measure concentrations over large areas in real time, it was the first large-scale outdoor field study using this application.

Experimental evaluation of an environmental CAT scanning system for mapping chemicals in air in real-time.

An innovative method is being developed which creates real-time, two-dimensional maps of chemical concentrations in air for environmental and occupational applications. This method, we call environmental CAT scanning, combines the real-time measuring technique of open-path Fourier transform infrared spectroscopy with the mapping capabilities of computed tomography to produce accurate spatial and temporal information about contaminant concentrations and dispersion patterns. With this system, a network of open-path measurements is obtained over an area; measurements are then processed using a tomographic algorithm to reconstruct the concentrations. This article describes a thorough experimental evaluation of an environmental CAT scanning system using a field-ready prototype system deployed in a room-size exposure chamber; point sample measurements were taken simultaneously in the chamber along with the CAT measurements. Twenty-eight experiments were performed using single or multiple plumes of a tracer gas in the chamber. Tomographic maps were compared with the point sample reference maps to evaluate the CAT scanning system for accuracy of concentration measurement and plume location. Quantitative agreement was very good; concentrations reconstructed with the tomographic maps agreed to within 17 percent of the point sample maps, and plume locations were reconstructed to within six inches of the plumes in the point sample maps. This technique shows real promise as a rapid and accurate method for mapping chemicals over large areas.

Mapping chemicals in air using an environmental CAT scanning system: evaluation of algorithms

A new technique is being developed which creates near real-time maps of chemical concentrations in air for environmental and occupational environmental applications. This technique, we call Environmental CAT Scanning, combines the real-time measuring technique of open-path Fourier transform infrared spectroscopy with the mapping capabilitites of computed tomography to produce two-dimensional concentration maps. With this system, a network of open-path measurements is obtained over an area; measurements are then processed using a tomographic algorithm to reconstruct the concentrations. This research focussed on the process of evaluating and selecting appropriate reconstruction algorithms, for use in the field, by using test concentration data from both computer simultation and laboratory chamber studies. Four algorithms were tested using three types of data: (1) experimental open-path data from studies that used a prototype opne-path Fourier transform/computed tomography system in an exposure chamber; (2) synthetic open-path data generated from maps created by kriging point samples taken in the chamber studies (in 1), and; (3) synthetic open-path data generated using a chemical dispersion model to create time seires maps. The iterative algorithms used to reconstruct the concentration data were: Algebraic Reconstruction Technique without Weights (ART1), Algebraic Reconstruction Technique with Weights (ARTW), Maximum Likelihood with Expectation Maximization (MLEM) and Multiplicative Algebraic Reconstruction Technique (MART). Maps were evaluated quantitatively and qualitatively. In general, MART and MLEM performed best, followed by ARTW and ART1. However, algorithm performance varied under different contaminant scenarios. This study showed the importance of using a variety of maps, particulary those generated using dispersion models. The time series maps provided a more rigorous test of the algorithms and allowed distinctions to be made among the algorithms. A comprehensive evaluation of algorithms, for the environmental application of tomography, requires the use of a battery of test concentration data before field implementation, which models reality and tests the limits of the algorithms.

Application of SpectraScan tunable diode laser instruments to fugitive emissions

The advantages of lasers over broad‐band light sources for spectroscopic measurements have long been recognized. Laser techniques offer the ability to measure the concentrations of trace species in a gas stream in “real time” with sensitivity and molecular selectivity not possible with broad band spectroscopic techniques. Until recently, the use of laser‐based instruments has been largely limited to laboratory or one‐of‐a‐kind devices. However, the increased availability of solid‐state tunable diode lasers, recent advancements in fiber optic coupling and advanced signal processing have made it possible to develop instruments that offer customers enabling technology at an affordable price. Spectrum Diagnostix has developed a family of tunable diode laser‐based instruments used in a variety of applications including: fugitive release detection of HF and H2S in the refining and petrochemical industries, stack monitoring, and using extractive sample probe, in situ process control. SpectraScan instruments obtain their high sensitivity and chemical selectivity utilizing a technique known as wavelength modulated spectroscopy. Described simply, a near infra‐red diode laser’s wavelength is scanned rapidly and repeatedly through a molecular absorption line. A photodetector senses the instantaneous fraction of emitted laser power that is transmitted through the chemical bearing gas. Measurement of the relative amplitudes of offline to online transmission yields a precise value of the quantity of chemical along the laser beam’s path. The amplitude modulated signals are then detected using established radio receiver and signal processing techniques. The SpectraScan monitors are designed for permanent installation in harsh industrial and petrochemical environments and are approved for Class I, Division 2 hazardous area use. Describes the SpectraScan instruments, their field applications, and reviews operating data compiled from open path measurements of HF in refineries.

Quantitative open-path FTIR with the use of isotopic standards

Emphasis on air quality has increased the need for a rapid method of measuring air pollutants. Remote Fourier-transform infrared spectroscopy (FTIR) offers rapid, on-site analysis of large areas and requires no sample preparation. Simple quantitative methods that reflect conditions present at the time of the measurement are other advantages of remote FTIR. This is especially important when one is attempting to use ambient background sources where the intensity of the source alters absorption intensities and calibration curves. Quantitative remote FTIR is based on Beer's law and matching the analyte spectrum with reference spectra. The reference spectra are measured separately and do not account for variation in source intensity or temperature, or for unknown interferences. Substituting deuterium for one or more hydrogens in a volatile organic compound (VOC) shifts the vibrational frequency, allowing the deuterated compound to be measured simultaneously with the VOC. The reference spectra for the Beer's law plot can be measured at the same time as the analyte and a slope and y intercept conversion allows determination of the analyte concentration. We have used isotopic references for quantitative open-path FTIR measurements of acetone and methanol and have achieved better than 10% accuracy in the parts-per-million concentration range. © 1999 John Wiley & Sons, Inc. Field Analyt Chem Technol 3: 105–110, 1999

Near — Infrared Diode Laser Air Monitoring

Inexpensive near-infrared diode lasers are now being used to enable high sensitivity, real time monitoring of gases both in open path measurements of urban air quality and in industrial environments for stack gas emission monitoring. Individual species are detected in a highly selective manner via overtone and combination bands of their vibrational spectra. Operating at room temperature and with simple optical components they can be made into portable instruments, ideal for field measurements. Combined with optical fibres they can be used for accessing remote and possibly dangerous locations. This paper presents current progress on the development of such a system illustrating recent results on ammonia monitoring at 1540 nm and acetylene at 780 nm. Sensitivities of the order of parts per million and below have been attained. Advantages and disadvantages of this approach to air pollution monitoring will be described. Pressure broadening results are also given for the 2v1 band of ammonia.