Open-path FTIR Research Articles

Characterization of forest fuels in a Mass Loss Calorimeter by short open-path FTIR spectroscopy

A FTIR spectroradiometer working in a short open-path configuration has been implemented coupled to a Mass Loss Calorimeter to measure in situ and simultaneously the concentrations of the main gaseous carbon-related by-products from the combustion of forest fuels. A proper methodology to retrieve accurate values of CO and CO 2 concentrations has been developed. This methodology includes the determination of the true optical path of the exhaust gases by using an infrared camera imaging at 4.4 μm. Average factors have been calculated to correct the effect of the temperature in the retrieval procedure. The determination of temporal evolution of CO and CO 2 concentrations has allowed the characterization of the combustion dynamics of the samples. These results open the door to further extensions of the method to the measurement and characterization of forest fuels in a field-scale fire.

Detection of chemical agents in the atmosphere by open-path FT-IR spectroscopy under conditions of background interference: II. Fog and rain

Open-path FT-IR spectra of low-concentration releases of diethyl ether were measured both when a glycol fog was passed into the infrared beam and when large water droplets from a lawn sprinkler were sprayed into the beam. It was shown that the glycol fog, for which the droplet size was much less than the wavelength of the infrared radiation, gave rise to a significant interference such that partial least squares (PLS) regression would only yield reasonable values for the ether concentration if background spectra in which the glycol fog was present were included in the calibration set. On the other hand, target factor analysis (TFA) allowed the presence of the ether to be recognized without precalibration. When large water droplets were present in the beam, any infrared radiation entering the droplet was completely absorbed, so that both PLS and TFA would yield accurate results.

Detection of chemical agents in the atmosphere by open-path FT-IR spectroscopy under conditions of background interference: I. High-frequency flashes

Open-path FT-IR spectra were measured while fireworks were emitting smoke and incandescent particles into the infrared beam. These conditions were designed to simulate the appearance of smoke and explosions in a battlefield. Diethyl ether was used to simulate the vapor-phase spectra of G agents such as sarin. The measured interferograms were corrected by a high-pass filter and were rejected when interfering features were of such high frequency that they could not be removed by application of this filter. The concentration of diethyl ether was calculated correctly by partial least squares regression in the absence of fireworks but significant errors were encountered when the spectra of the oxide particles were not included in the calibration set. Target factor analysis allowed the presence of the analyte to be detected even when the incandescent particles were present in the beam.

Spatial and Temporal Distributions of a Gaseous Pollutant During Simulated Preventive Maintenance and Pipe Leaking Events in a Working Cleanroom

Sulfur hexafluoride (SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> ) of > 99.9% in purity was artificially released to simulate the emission sources in the etching/thin-film area of a working cleanroom in a semiconductor fab at the rate of 492 g/h. The temporal and spatial dispersion patterns of the gas pollutant were studied during the simulated preventive maintenance (PM) and pipe leaking exhaust events experimentally and numerically. Three mobile Fourier transform infrared spectrometers (FTIRs, detection limit: 10 ppb) were used simultaneously to measure the real time SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> concentrations in different locations of the etching/thin-film area of the cleanroom. An additional open-path FTIR with very low detection limit of 0.4 ppb was installed before the recirculation duct in the lithography area to monitor if the pollutant drifted from the etching/thin-film area to the lithography area. The results show that the 3-D numerical model predicts the unsteady gas concentration accurately in both the peak concentration and the time required to reach the peak concentration. Due to high dilution of the pollutant in the cleanroom, the current gas sensors may not be sensitive enough and a better monitoring system and strategy is needed to protect workers from injury and ensure good product yield. The well-mixed model predicts the peak pollutant concentrations within a reasonable range which is 0.34-1.33 times the experimental values except when the monitored distance is very close to the release point. Although it is not able to predict the time required to reach the peak concentration and the time for the concentration to drop below a small level, the simple well-mixed model can be used to obtain an estimation of the peak concentration quickly when the emission rate and the ventilation condition of the cleanroom are different than the current study.

Thermal and geochemical signature of Poás Volcano, Costa Rica (MARCH 2009)

We report results from a multidisciplinary campaign conducted at Poás volcano (Costa Rica) in March 2009. Thermal imagery of the fumaroles sited on the north side of the pyroclastic cone revealed mean apparent temperatures ranging between 25 and 40°C with a maximum apparent temperature of 80°C. The crater lake surface was characterised by mean apparent temperatures varying between 30 and 35°C and a maximum recorded value of 48°C. Thermal profiles across the lake surface revealed steady temperatures and thus thorough convective mixing. The overall mean SO2 flux emitted from the crater was 76 Mg d-1, with approximately equal contributions from both the pyroclastic cone and the lake. Analysis of gas composition using active, open-path FTIR spectroscopy indicated molar ratios of H2O/SO2 = 151, CO2/SO2 = 1.56, SO2/HCl > 40, and SO2/HF > 200 for the lake emission. The plume was also sampled using filter packs. Ion chromatographic analysis revealed the presence of abundant K+ and SO4 2-, with small amounts of Ca2+, Cl-, and Mg2+. This provides a detailed picture of lake surface temperature characteristics and of gas flux and composition of the plume emitted from Poás. The results are consistent with the typical non-eruptive state of this volcano.

Application of Open-Path FT-IR Spectroscopy to Determine Green House Gas Flux from Farmland-1. Closed-Chamber Method-

Flux determination of various gases from farmlands is of great importance. We have applied open-path FT-IR (OPFTIR) spectroscopy combined with a closed-chamber method for measuring greenhouse gases from paddy fields. By applying this technique we were able to measure gases that absorb infrared light on site and simultaneously. To get more quantitative information on the amount of methane from paddy fields, measuring conditions and analyses techniques of observed spectra have been improved. We now get reliable data on the densities of greenhouse gases which are highly space-representative and cannot be got by using conventional techniques. We believe that by proposing appropriate cultivation methods with little environmental load on the basis of the data, we can slow down global warming.

Application of Open-Path FT-IR Spectroscopy to Determine Green House Gas Flux from Farmland-2. Gradient Method-

The gradient method is a way to measure gas fluxes from fields. In the method, a gas flux is calculated from the observed altitude dependencies of the wind speed and of the concentration of the target gas. We constructed a new set of equipments for the gradient method consisting of an open-path FT-IR (OP-FTIR) spectrophotometer, a retroreflector (a corner cube mirror array), and four cup anemometers. We expect to get more space representative values of fluxes with the new equipment than with the conventional methods. The equipment enables us to simultaneously determine the fluxes of many atmospheric gases that absorb infrared light. We simultaneously determined fluxes of H2O, CO2, and CH4 gases from a rice field at Honmachi Rice Field of Tokyo University of Agriculture and Technology. Although we found that some improvements were needed to the equipment and analysis techniques to make the observations more reliable, we obtained reasonable values for the fluxes.

Measurement of Atmospheric Ammonia, Methane, and Nitrous Oxide at a Concentrated Dairy Production Facility in Southern Idaho Using Open-Path FTIR Spectrometry

The number of dairy cows in Idaho has increased by approximately 80% in the last decade, with the majority of these facilities located in southern Idaho, causing air quality concerns in this region. To determine the potential air quality impacts of these facilities, we measured ammonia (NH3), methane (CH4), and nitrous oxide (N2O) concentrations over the pens, wastewater storage pond, and composting area on a 700-cow open-lot dairy using open-path Fourier transform infrared spectrometry (OP/FTIR). Concentrations were measured for one or two days at each location during January, March, June, and September. Median NH3 concentrations over the pens, storage pond, and composting area ranged from 0.14 to 0.39 ppmv, 0.04 to 0.17 ppmv, and 0.06 to 0.22 ppmv, respectively, with concentrations tending to be lower in January. Average CH4 concentrations over the pens, storage pond, and composting area ranged from 2.07 to 2.80 ppmv, 1.87 to 2.15 ppmv, and 1.71 to 1.76 ppmv, respectively. Average N2O concentrations ranged from 0.31 to 0.33 ppmv for all areas, which was similar to global background N2O concentrations. Combined ammonia emissions for the pen and storage pond areas, calculated with a backward Lagrangian stochastic inverse-dispersion technique, were 0.04, 0.25, 0.19, and 0.15 kg NH3 cow-1 d-1 for January, March, June, and September, respectively, and methane emissions were 0.34, 0.55, 0.21, and 0.20 kg CH4 cow-1 d-1 for the same months. Assuming this limited monitoring was representative of the entire year, annual emissions from the pens and storage pond were 57 kg NH3 cow-1 and 120 kg CH4 cow-1. These emission rates were similar to the limited number of comparable studies that have been published. However, more extensive monitoring is needed to better quantify variations in emissions throughout the year and among locations.

Completely automated open-path FT-IR spectrometry

Atmospheric analysis by open-path Fourier-transform infrared (OP/FT-IR) spectrometry has been possible for over two decades but has not been widely used because of the limitations of the software of commercial instruments. In this paper, we describe the current state-of-the-art of the hardware and software that constitutes a contemporary OP/FT-IR spectrometer. We then describe advances that have been made in our laboratory that have enabled many of the limitations of this type of instrument to be overcome. These include not having to acquire a single-beam background spectrum that compensates for absorption features in the spectra of atmospheric water vapor and carbon dioxide. Instead, an easily measured "short path-length" background spectrum is used for calculation of each absorbance spectrum that is measured over a long path-length. To accomplish this goal, the algorithm used to calculate the concentrations of trace atmospheric molecules was changed from classical least-squares regression (CLS) to partial least-squares regression (PLS). For calibration, OP/FT-IR spectra are measured in pristine air over a wide variety of path-lengths, temperatures, and humidities, ratioed against a short-path background, and converted to absorbance; the reference spectrum of each analyte is then multiplied by randomly selected coefficients and added to these background spectra. Automatic baseline correction for small molecules with resolved rotational fine structure, such as ammonia and methane, is effected using wavelet transforms. A novel method of correcting for the effect of the nonlinear response of mercury cadmium telluride detectors is also incorporated. Finally, target factor analysis may be used to detect the onset of a given pollutant when its concentration exceeds a certain threshold. In this way, the concentration of atmospheric species has been obtained from OP/FT-IR spectra measured at intervals of 1 min over a period of many hours with no operator intervention.

Probing the magma plumbing of Erebus volcano, Antarctica, by open-path FTIR spectroscopy of gas emissions

Gas emissions from Erebus volcano, Antarctica, were measured by open-path Fourier transform infrared spectroscopy to understand degassing of its magmatic system. Two degassing phonolite lava lakes were present in the summit crater during observation in December 2004. We report analyses of H 2O, CO 2, CO, SO 2, HF, HCl and OCS, (in order of molar abundance) in the plumes. Variations in the proportions of these species strongly reflect the dynamics of degassing, and sourcing of gas from different depths in the magmatic network. The highest observed ratios of CO 2 and H 2O are consistent with gas extracted from the melt at a depth of up to ∼ 2 km below the lava lakes. Magma degassing above this depth contributes to a higher H 2O/CO 2 proportion in the airborne plume. The ratio therefore reflects the balance of deeper vs. shallower contributions of volatiles and, possibly, a combination of closed- and open-system degassing. We observe a strong contrast in HF content in emissions from the two lava lakes, which we attribute to differing levels of magma ascent and/or cooling and crystallization of the magma supply. Fluxes of all gas species were determined using independent SO 2 flux determinations and measured gas ratios. In the case of CO 2 and water, ∼ 1 and ∼ 0.4 m 3 s − 1 , respectively, of parental basanite magma are required to sustain the calculated output. The discrepancy between the two figures is readily explained by sequestration of part of the magma supply at depth such that it only partially degasses its complement of water.

Combination of Neural Network and SBFM Algorithm for Monitoring VOCs Distribution by Open Path FTIR Spectrometry

In this research, the combination of artificial neural network (ANN) modeling and smooth basis function minimization (SBFM) algorithm were applied to Open Path Fourier transform infrared spectroscopy (OP‐FTIR) for monitoring volatile organic compounds' concentration distribution in the air. ANN was utilized to analyze the measured mixture spectra containing chloroform, methanol, and methylene chloride; Then, SBFM was used to reconstruct each component's concentration distribution. The peak concentration locations and maximum concentration for three components are reconstructed accurately. The methodology presented in this paper has significant importance in detecting leaking source spot and monitoring airborne VOCs transport in chemical industrial workplaces.

Trends in Remote Spectroscopic Sensing and Imaging - Experimental Techniques and Chemometric Concepts

For studying environmental pollution locally performed analyses are non-representative as the pollutants are distributed heterogeneously. Thus, comprehensive pollution information can only be obtained by means of spatially probing the entire area of interest. Optical spectroscopy is a powerful method because active and passive sensing is feasible over large distances. Recent experimental techniques like light detection and ranging (lidar), differential optical absorption spectroscopy (DOAS), open-path FTIR spectroscopy and chemical imaging are discussed. However, sophisticated experimental techniques alone are not sufficient to establish a successful analytical method - innovative statistical data evaluation (chemometric) techniques are also needed for reliable sensor calibration and data analyses. All spectroscopic open-path techniques are inherently hampered by unpredictable and changing measurement conditions as well as by incomplete chemical knowledge during calibration. Thus, new methods are presented, which calibration models and prevent artifacts from impacting quantification of pollutants. Novel chemical imaging techniques perform spatially resolved spectroscopy and enable multicomponent analyses at unprecedented spatial and temporal resolution. However, spectroscopic imaging produces humongous data sets and thus imposes new challenges as data storage and evaluation is a computational burden and extremely time consuming. Recent data compression algorithms utilize multidimensional wavelet transformations for shrinking data sets considerably prior to chemometric evaluation and archiving. Keywords: Remote pollution sensing, Optical spectroscopy, Chemical imaging, Optimized chemometrics, Robust quantification

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.

Reconstruction of Air Contaminant Concentration Distribution in a Two-dimensional Plane by Computed Tomography and Remote Sensing FTIR Spectroscopy

This research combined open path FTIR (OP-FTIR) technique and computed tomography (CT) to reconstruct air contaminant concentration distribution in a two-dimensional plane. Remote sensing FTIR instrument was used to scan radial beam geometry and obtain path integrated concentration (PIC) data of acetone gas in the measuring plane. Smooth basis function minimization (SBFM) algorithm was adopted to reconstruct gaseous concentration distribution. For the purpose of finding out the preferable number of Gaussians used in SBFM algorithm, single-Gaussian, double-Gaussian, and three-Gaussian models were used respectively. Experimental results showed that the reconstruction result of acetone concentration distribution by SBFM algorithm with double-Gaussian model agreed with real distribution more qualitatively and quantitatively than single-Gaussian and three-Gaussian. Also, it has been proved that simulated annealing algorithm used in the optimization process of SBFM reconstruction was feasible and effective. Although computed tomography and remote sensing FTIR technique (CT-RS-FTIR) is still at the laboratory study stage, with further improvement of SBFM algorithm and beam geometry, it promises to be used in air pollution monitoring widely.

Applying Open-Path FTIR with Computed Tomography to Evaluate Personal Exposures. Part 1: Simulation Studies

This paper presents the theoretical background and the numerical evaluation results obtained using computed tomography coupled with open-path Fourier transform infrared (CT-FTIR) measurements to estimate personal exposures. In this simulation study, we first tested the one-dimensional scenario with a five-beam segment geometry. A series of Gaussian plumes and the corresponding path-integrated concentrations (PICs) were simulated. The personal exposures were estimated as the average of the point estimates calculated from the workers' locations and the concentration profiles reconstructed from the Smooth Basis Function Minimization algorithm. It was found that the running-average PIC updating strategy has similar performance as the spline PIC updating strategy. However, the latter strategy gives delayed estimates of the workers' exposures since it requires additional measurements before and after the time period of interest. In the two-dimensional scenario, we simulated a series of single-mode bivariate Gaussian plumes with a nine-beam radial geometry. The average of the estimated exposures from the CT-FTIR approach was close to the average of the true exposures. The concordance correlation factors between the true and estimated exposures were reasonably good (between 0.50 and 0.58). This study demonstrated that the CT-FTIR approach is feasible for industrial hygiene monitoring.

Applying Open-Path FTIR with Computed Tomography to Evaluate Personal Exposures. Part 2: Experimental Studies

This paper presents the experimental evaluation results of using computed tomography coupled with OP-FTIR (CT-FTIR) measurement to estimate personal exposures. Experimental data were collected inside a ventilation chamber with a remote controlled robot as a surrogate for a real human. While the robot moved inside the chamber, a tracer gas (carbon monoxide) was released from a line source. A personal sampling device measured the true exposure on the robot. The estimated personal exposures were calculated from both the area sampling array data and the CT-FTIR measurements along with the information about the robot's locations in real time. The location information was obtained by applying image analysis on recorded digital videotapes. The average slopes of the regression lines between the true and estimated exposures was 0.76 with 1 included in the 95% confidence interval. The concordance correlation factor (CCF) between the true and the CT-FTIR estimated exposures was 0.52, which was similar to the findings from previous simulation studies. Kriging the area sampling array data with an exponential algorithm instead of a liner algorithm improved the CCF value from 0.60 to 0.75. This suggests that using a different basis function for the SBFM algorithm might improve the performance of our estimation approach. Based on the sensitivity and specificity analysis of the experimental data, we demonstrated that this approach is suitable as a warning system.

Diurnal variability of size-differentiated inorganic aerosols and their gas-phase precursors during January and February of 2003 near downtown Mexico City

Size-differentiated atmospheric aerosol particles along with gas-phase precursors were measured at a site near downtown Mexico City (MER site) during January and February 2003 to provide information regarding the diurnal variability and partitioning of semi-volatile inorganic compounds between the gas phase and different-size particles. The aerosols were sampled with cascade impactors (MOUDI’s) in the following periods: 1st (06:00–09:00 h, LST), 2nd (09:00–12:00 h, LST), 3rd (12:00–15:00 h, LST) and 4th (15:00–18:00 h, LST). The gas-phase measurements were continuously recorded with an open-path FTIR spectrometer. Overall, inorganic aerosol size/composition measurements observed a bimodal distribution: one mode was present in the accumulation size range (0.18–0.32mm, aerodynamic diameter) and the other in the coarse mode (over 1mm, aerodynamic diameter). During the morning sampling periods, the highest concentrations occurred mainly over the accumulation mode while during the afternoon sampling periods, concentration peaks were observed over both accumulation and coarse modes. More than halfofthe ammonium was found in the accumulation mode. The rest of the ions (sodium, chloride, sulfate, nitrate, calcium and potassium) were prominent in both modes. The significant presence ofsodium and crustals (calcium and potassium) are explained in terms ofthe potential influence ofthe dry salt-lake ofTexcoco and resuspended dust/soil, respectively. Based on the analysis ofthe time-resolved PM and gas-phase composition, the significant presence ofgas-phase ammonia (>35 ppb) during the morning sampling periods was ofimportance in neutralizing the aerosol particles. r 2004 Elsevier Ltd. All rights reserved.

Application of the open path FTIR with COL1SB to measurements of ozone and VOCs in the urban area

Abstracts The CO nstrained L1 -norm minimization with the S ynthetic B ackground generation method (COL1SB) is newly developed open-path FTIR analysis software, designed to efficiently handle chemical interferences and to systematically generate background spectra. The performance of COL1SB was evaluated mainly against artificially generated field spectra. The present paper aims at application of COL1SB to ambient monitoring. In order to satisfy low detection limit requirements of ambient monitoring, COL1SB was further improved to accurately determine path averaged water vapor pressures, to dynamically select appropriate spectral bands, and to use site specific background spectra. The modified COL1SB was applied to measure ozone, ammonia, methane, carbon monoxide, and 26 VOC species in the park located at the urban area from 12:00 a.m., 25 May to 12:00 a.m., 26 May 2000. The open-path FTIR measurements for ozone, water vapor, alkane concentrations compared very well with those by sampling methods both in magnitudes and in diurnal variations. Furthermore, the detection rates of VOCs were considerably enhanced such that the detection rates based on the correlation coefficients appeared to be above 50% for 12 species out of 16 representative chemical species. Especially ethylene and isoprene, chemically reactive olefins, had high detection rates exceeding 80% despite of their low concentrations. These detection rates by COL1SB are 2–5 times higher than those derived by the conventional CLS method in the previous work (SPIE Conference on Environmental Monitoring and Remediation Technologies SPIE (1998) 3534, 140).

試料の位置・大きさがオープンパスＦＴ‐ＩＲの信号強度に与える影響

Effects of (1) sizes and positions of target gas distributions, (2) diameter and divergence of IR beam used for measurements, and (3) the types of reflection mirror upon signal intensities in the open-path FT-IR measurements were examined in detail experimentally and with theoretical simulations. These factors were found to affect only a little when the target gas distribution is much larger than the IR beam diameter. On the other hand, when the target gas distribution is similar to or smaller than the IR beam size, signal intensity was found to be dependent on these factors in complicated manners. In such cases, it is highly difficult to get quantitative information of the target gas.

Applying Open-Path FTIR with a Bi-Beam Strategy to Evaluate Personal Exposure in Indoor Environments: Experimental Results of a Validation Study

Applying Open-Path FTIR with a Bi-Beam Strategy to Evaluate Personal Exposure in Indoor Environments: Experimental Results of a Validation Study