Measurements Of Water Vapor Content Research Articles

Reduction of false alarms in forest fire surveillance using water vapour concentration measurements

In this work a theoretical model to evaluate the capabilities of our lidar system in forest fire detection is reported. In particular, a new idea of minimization of false alarm is shown. In a forest fire, in fact, a lot of ashes and in the first stage a large amount of water vapour are emitted. Measurements of water vapour increase with respect to standard humidity in the atmosphere due to a forest fire event, by means of Raman analysis, permit to minimize the false alarm. A simulation of one case of study permits to estimate the maximum range of detection and minimum sensibility of our lidar system. In this paper the theoretical results are shown.

Simultaneous water vapor concentration and temperature measurements in unsteady hydrogen flames

Techniques applicable to visualization and scalar measurements in large-scale hydrogen fires are highly desired, but lacking. In this research, planar thermal images of a buoyancy-driven unsteady laminar hydrogen/air flame and line images of a thin filament stretched across this flame were obtained using an infrared (IR) camera at a sampling frequency of 348 Hz. A pulsing frequency of 11 Hz was measured. Transient line measurements of temperature ( T) and water vapor mole fraction ( X H 2 O ) have been achieved using inverse radiation calculations. Instantaneous X H 2 O and T distributions during a flame–vortex interaction cycle were obtained with temporal and spatial resolutions of 3 ms and 1.7 mm, respectively. The instantaneous distributions of X H 2 O and T were effected by preferential diffusion and the altered velocity field of the flame. Vortices caused more scalar fluctuations outside the flame surface than inside. The present X H 2 O and T measurements are consistent with detailed chemistry calculations of the flame and laser diagnostic measurements of similar flames. With some modifications, the thermal imaging based technique can be extended to large-scale hydrogen fire applications.

On the correlation structure of water vapor and carbon dioxide in the atmospheric surface layer: A basis for flux partitioning

High‐frequency measurements of water vapor (q) and carbon dioxide (c) concentrations were collected over the course of a transition from dry to wet surface conditions in an agricultural setting on the eastern shore of Virginia. Daytime correlation coefficients between q and c were close to −1 during the dry conditions but became degraded during the wet conditions. An application of wavelet analysis to the high‐frequency time series showed that the degraded q‐c correlations on the wet day were mainly caused by the influence of large‐scale eddies, which introduced positively correlated q‐c components to the half‐hour time series. Consistent differences in q‐c correlation were also observed for smaller eddy scales, which are more indicative of the surface‐atmosphere exchange. Correlations between q and c for this range of eddy scales were likewise closer to −1 for dry conditions, when transfer efficiencies of both scalars exhibited greater similarity. These correlations are influenced by the nonidentical source‐sink distributions of the water vapor and carbon dioxide fluxes and the relative magnitude of their constituent fluxes. A new method is introduced to estimate the components of the water vapor flux (transpiration and direct evaporation) and carbon dioxide flux (photosynthesis and respiration) by applying flux variance similarity assumptions separately to the stomatal and the nonstomatal exchange and by considering q‐c correlation. Water use efficiency for the vegetation, and how it varies with respect to vapor pressure deficit, is the only input needed for this approach that uses standard eddy covariance measurements. Reasonable estimates yielded by this technique when applied to the contrasting wet and dry days demonstrate its potential for flux partitioning.

In situ combustion measurements of H2O and temperature near 2.5 µm using tunable diode laser absorption

In situ combustion measurements of water vapor concentration and gas temperature were carried out with a new tunable diode laser sensor near 2.5 µm. Recent availability of room-temperature semiconductor diode lasers operating at longer wavelengths provides access to fundamental vibrational bands (ν1 and ν3) of H2O. These bands have stronger absorption line strength compared to the overtone (2ν1, 2ν3) and combination (ν1 + ν3) vibrational bands in the near-infrared region probed previously with telecommunication diode lasers. The absorption transitions of H2O vapor in the 2.5–3.0 µm region are systematically analyzed via spectral simulation, and optimal spectral line pairs are selected for combustion measurements in the temperature range of 1000–2500 K. Fundamental spectroscopic parameters (line strength, line position and line-broadening coefficients) of the selected transitions are determined via laboratory measurements in a heated cell. Absorption measurements of H2O concentration and temperature are then made in a laboratory flat-flame burner to illustrate the potential of this sensor for sensitive and accurate measurements in combustion gases with short optical path lengths.

Automated soil respiration measurements: new information, opportunities and challenges

Automated soil respiration measurements: new information, opportunities, and challenges Automated Soil Respiration Workshop – a Terrestrial Ecosystem Response to Atmospheric and Climate Change (TERACC) sponsored workshop, Durham, New Hampshire, USA, September 2007 Soils are the largest carbon pool in terrestrial ecosystems, and soil respiration is the major pathway of carbon transfer from the soils to the atmosphere. Measuring and predicting soil respiration has been challenging because the CO 2 efflux from soil integrates numerous complexities belowground (Zhou & Luo, 2006). Current models of soil respiration lack a theoretical underpinning with which to predict how fluxes reflect different plant and microbial CO 2 sources and mechanisms. Recent technological advances in automated soil respiration systems are generating unprecedented numbers of high temporal-resolution observations (Savage and Davidson, 2003). Automated soil respiration (ASR) provide valuable information often missed with less frequent manual measurements, and they present the opportunity to move beyond empirical (gap filling) models towards a predictive understanding of the key mechanisms that determine soil respiration fluxes. However, these continuous measurements present new challenges in that they require the additional management of complex equipment and large datasets as well as novel analytical approaches. In September 2007, researchers met in Durham, New Hampshire, USA for a workshop on Automated Soil Respiration Measurements (http://www.umaine.edu/teracc). The overall goal of the workshop was to initiate communication within the automated soil respiration measurement

In-situ Measurement of Water Vapor and Oxygen Concentration in a PEMFC Channel by Tunable Diode Laser Absorption Spectroscopy Techniques

n this study in-situ measurements of water vapor and oxygen concentration in gas flow channels in an operating polymer electrolyte membrane fuel cell (PEMFC) are obtained using Tunable Diode Laser Absorption Spectroscopy (TDLAS). Wavelengths of the diode laser are 1392nm for measurement of water vapor concentration and 760nm for measurement of oxygen concentration, respectively. It was successfully demonstrated that the optical remote sensing based on TDLAS techniques can detect local variations of oxygen and water vapor concentration in the channel of a PEMFCs under load.

In-situ Measurement of Water Vapor and Oxygen Concentration in a PEMFC Channel by Tunable Diode Laser Absorption Spectroscopy Techniques

not Available.

Strong low-pass filtering effects on water vapour flux measurements with closed-path eddy correlation systems

Turbulent water vapour fluxes measured with closed-path eddy correlation (EC) systems are unintentionally low-pass filtered by the system in a manner that varies with environmental conditions. Why and how is described here. So is the practical method that systematically corrects long-term flux datasets for this substantial measurement error. In contrast to earlier studies, a large number of spectra and raw data have been used in the analysis to define the low-pass filtering characteristic of the EC system. This revealed that the cut-off frequency of the closed-path EC system for water vapour concentration measurements decreases exponentially with increasing relative humidity. After correction for this unintended filtering, the fluxes are consistent with CO 2 and H 2O fluxes that were measured with an open-path sensor at the same time. The correction of water vapour flux measurements over a Beech forest in Sorø, Zealand, Denmark, amounted on average to 42% of the measured flux, while it was only 4% for the CO 2 flux, which was measured with the same EC system. We recommend using the described method to correct water vapour fluxes measured in any closed-path EC system for unintended low-pass filtering effects. Other than for CO 2 is the magnitude of the correction for water vapour flux measurements unsatisfactorily high, i.e. the EC system needs to be technically improved. Our results suggest that such high correction can be avoided by keeping relative humidity in the entire gas transport system of the EC system lower than 30%, e.g. by heating intake filters and tubes.

Deep Air Sparging—15 to 46 m beneath the Water Table

AbstractAir sparging can be successfully applied to deep aquifers to remediate significant hydrocarbon mass in order to ultimately reach a point at which monitored natural attenuation can manage dissolved phase plumes. Air sparging is typically applied in shallow ground water conditions, less than 15 m beneath the water table (bwt). Deep air sparging, 15 to 46 m bwt, is more challenging but a viable alternative for treatment of residual petroleum hydrocarbons and dissolved phase constituents within a thick smear zone. An application to remediate an aquifer impacted by gasoline‐range petroleum hydrocarbons at depths greater than 76 m below ground surface (bgs) and up to 40 m thick (vertically) is reported here. Deep air sparging at 15, 31, and 46 m bwt was assessed using diagnostic tools involving flow rate, pressure, tracer gas, water table elevation, vapor concentration, and ground water chemistry measurements. The diagnostic tools identified preferential flow directions and a maximum depth of effective air injection. The full‐scale deep air sparging/soil vapor extraction (SVE) system removed more than 150,000 kg of hydrocarbon mass at the study site over a 3‐year period and ultimately greater than 100,000 kg/year with system expansion.

Technical note: Water vapour concentration and flux measurements with PTR-MS

Abstract. The most direct approach for measuring the exchange of biogenic volatile organic compounds between terrestrial ecosystems and the atmosphere is the eddy covariance technique. It has been applied several times in the last few years using fast response proton-transfer-reaction mass spectrometry (PTR-MS). We present an independent validation of this technique by applying it to measure the water vapour flux in comparison to a common reference system comprising an infra-red gas analyser (IRGA). Water vapour was detected in the PTR-MS at mass 37 (atomic mass units) corresponding to the cluster ion H3O+·H2O. During a five-week field campaign at a grassland site, we obtained a non-linear but stable calibration function between the mass 37 signal and the reference water vapour concentration. With a correction of the high-frequency damping loss based on empirical ogive analysis, the eddy covariance water vapour flux obtained with the PTR-MS showed a very good agreement with the flux of the reference system. The application of the empirical ogive method for high-frequency correction led to significantly better results than using a correction based on theoretical spectral transfer functions. This finding is attributed to adsorption effects on the tube walls that are presently not included in the theoretical correction approach. The proposed high-frequency correction method can also be used for other trace gases with different adsorption characteristics.

Rapid measurements of temperature and H 2O concentration in IC engines with a spark plug-mounted diode laser sensor

Crank angle-resolved measurements of gas temperature and water vapor concentration are made during the compression stroke of an internal combustion (IC) engine with a novel diode laser absorption sensor. The temperature is determined from the ratio of optical absorption for two overtone transitions of water vapor in the intake gas mixture, and the H 2O concentration is determined from this inferred temperature and the absorption for one of the transitions. The measurements sample a short-path region (6 mm) of the in-cylinder gases near the spark plug, which has been modified to provide optical access. Hence, the sensor can be installed in nearly any engine via the spark plug port. Wavelength modulation spectroscopy is employed with second harmonic detection (WMS-2 f) to enable the short-path measurements over a range of temperatures and pressures from 500 to 1050 K and 1 to 50 atm at a bandwidth of 7.5 kHz. The accuracy of the sensor is validated in a static cell, giving RMS errors of less than 3% in temperature and less than 3.6% in H 2O concentration over a wide range of conditions. Crank angle-resolved measurements are performed in unfired and fired engine cylinders, illustrating the potential of this sensor for investigating a range of difficult-to-model trends in current and proposed IC engine combustion schemes.

Frequency-stabilized cavity ring-down spectrometer for high-sensitivity measurements of water vapor concentration

We present a portable spectrometer that uses the frequency-stabilized cavity ring-down spectroscopy technique capable of high-precision measurements of trace water vapor concentration. Measuring one of the strongest rovibrational transitions in the ν1+ν3 water vapor combination band near ˜ν=7181.156 cm-1, we compare spectroscopic and thermodynamic determinations of trace water vapor in N2, and find systematic differences attributable to water vapor background effects and/or uncertainties in line intensities. We also compare the frequency-stabilized ring-down method with other cavity ring-down approaches that are based on unstabilized probe lasers and unstabilized ring-down cavities. We show that for the determination of water vapor concentration, the frequency-stabilized cavity ring-down method has the minimum measurement uncertainty of these techniques. The minimum noise-equivalent absorption coefficient of the spectrometer was 1.2×10-10 cm-1 Hz-1/2, which further corresponds to a minimum detectable water vapor mole fraction equal to 0.7×10-9 for an absorption spectrum of 10 minutes duration.

Combination of a photoacoustic detector with a diffusion sampler for the measurement of water vapor concentration in ethylene glycols for the natural gas industry

A diffusion sampling based photoacoustic (PA) system for measuring the concentration of water as an impurity in different glycol samples (mono-, di- and triethylene glycol) is presented. Water from the glycol samples diffuses through a sampling membrane into the gaseous phase, where its concentration is determined by a PA system based on a telecommunication type diode laser operating at room temperature. The system was calibrated by measuring the PA signal as a function of the water content of the glycol samples. The sensitivity of the system was found to be dependent on the different glycol types, but in all cases the minimum detectable water concentration was in the low mg/l range. The selectivity of the method was proven by the fact that within the wavelength tuning range of the laser diode, other components, which may diffuse through the membrane, do not generate measurable PA signals. The sensitivity of the system was found to increase exponentially with the temperature of the glycol sample. Altogether, the system has a good potential for industrial applications.

Diode laser absorption spectroscopy of water vapor in a scramjet combustor

A sensor based on tunable diode laser absorption spectroscopy was constructed for time-resolved temperature and water vapor concentration measurements in a scramjet combustor. The sensor probed two absorption lines near 1390 nm with two time-multiplexed lasers used to measure temperature and water vapor concentration at up to 20 kHz. A demonstration experiment was performed in the supersonic, expanding exhaust region of the combustor, showing the measurement to be repeatable, able to resolve temporal trends during tunnel operation, and sensitive to changes in combustor operating conditions.

Estimation of spatially distributed latent heat flux over complex terrain from a Raman lidar

A method is presented in which estimates of evaporation may be made over an area approaching three quarters of a square kilometer, with relatively fine (25 m) spatial resolution, using three-dimensional measurements of water vapor concentration from a scanning Raman lidar. The method is based upon Monin–Obukhov similarity theory applied to spatially and temporally averaged data. Data from the lidar is used to sense the location and orientation of the surface and the location of the water vapor measurements with respect to that surface. Maps of the spatial distribution of evaporation have been produced showing the evaporation rates at regular intervals throughout the day. The method was applied to the SALSA experimental site during the 1997 summer field campaign. The estimates of evaporation rates made during the campaign compare favorably with estimates made using sap flux methods with RMS differences of 18 W/m 2. While the method has certain limitations, the three-dimensional character of the data allows for the detection of anomalous situations so that analysts may alter the analysis technique or reject the estimates from the affected regions. This information can be used in a wide variety of ways to study the spatial variations in evaporation caused by changes in soil type and moisture content, canopy type and topography.

Long-range persistence in climatological and hydrological time series: analysis, modeling and application to drought hazard assessment

We present power spectra of time-series data for tree ring width chronologies, atmospheric temperatures, river discharges and precipitation averaged over hundreds of stations worldwide. The average power spectrum S for each of these phenomena is found to have a power-law dependence on frequency with exponent − 1 2 : S(f) ∝f − 1 2 . An advection-diffusion model of the vertical transport of heat and water vapor in the atmosphere is presented as a first-order model of climatic and hydrological variability. The model generates variability with the observed spectrum. The model is validated with a correlation analysis of temperature and water vapor concentration measurements from the TIROS operational vertical sounder (TOVS). Drought frequency analyses based on synthetic lognormal streamflows with the above power spectrum are presented. We show that the presence of long memory as implied by the power-law power spectrum has a significant effect on the likelihood of extended droughts compared with the drought hazard implied from standard autoregressive models with short memory.

The Mars Water Cycle: Determining the Role of Exchange with the Regolith

The near-surface nighttime atmospheric water vapor concentrations inferred by Ryanet al.(J. A. Ryan, R. D. Sharman, and R. D. Lucich 1982.J. Geophys. Res.87,7279–7284) from Viking Lander air temperature measurements are a factor of 2–3 lower than the same quantities estimated from daytime atmospheric column water vapor abundances observed from the Viking Orbiters. We show that a physical model of the atmospheric boundary layer and regolith can produce a nighttime depletion of this magnitude by diffusion of water into the regolith and adsorption onto regolith grains. Quantitative validation of the model is not possible at present due to the lack of direct measurements of the near-surface atmospheric water vapor concentration and by uncertainties regarding surface regolith and atmospheric boundary layer properties. However, if the diurnal exchange of water vapor with the surface is as large as is suggested by the Viking Lander and Orbiter measurements, then the exchange of water between the atmosphere and regolith also is important in the seasonal cycle of water vapor. Further characterization of these processes can be made using measurements from the various landing site and atmospheric profiling experiments to be conducted by the Mars Pathfinder and Mars Surveyor Lander and Orbiter missions.

Simultaneous water vapor concentration and temperature measurements using 1.31-micron diode lasers

This paper reports the development of a compact, inexpensive sensor for simultaneous water vapor concentration and temperature measurements suitable for aeropropulsion exhaust applications. High sensitivity is achieved with an electronically balanced dual detector strategy that circumvents requirements for custom-fabricated lasers operating at specific wavelengths or high-frequency modulation techniques. Using widely available, broadly tunable InGaAsP diode lasers near 1.31 jxm, simultaneous measurements are demonstrated in a fiber-coupled, wavelength multiplexed configuration with a limiting density sensitivity of 1015 cm~3 in a 50-cm path and an rms standard deviation of 42 K over a range from 300 to 1300 K. Initial results suggest the possibility of extending this temperature range to 1900 K and above using other line pairs. (1) where Iv is the monochromatic laser intensity at frequency v, mea- sured after propagating a pathlength t through a medium with an absorbing species number density N. The strength of the absorp- tion is determined by the temperature-dependent line strength S(T), and the line shape function g(v — v()). The line shape function de- scribes the temperature- and pressure-dependent broadening mech- anism of the fundamental line strength. The temperature dependence of the line strength arises from the Boltzmann population statistics governing the internal energy level population distribution of the absorbing species. The single-mode distributed feedback (DFB) diode lasers used in this work are sufficiently narrow in frequency that they may be con- sidered essentially monochromatic with respect to the absorption line shape. The laser frequency may be tuned over a range that en- compasses the entire line shape function so that the resultant trans- mission can be integrated to remove the pressure and temperature dependence of the line-broadening mechanisms. The recorded ab- sorbance, then, is proportional only to the temperature-dependent line strength and the absorbing species number density. It is usu- ally possible to select an absorbing ground state whose line strength is relatively constant over some target temperature range so that the absorbance is a direct measurement of species number density. Separate temperature measurements may be used to correct for tem- perature variations, if necessary. Alternatively, two absorption transitions may be probed (using one or two lasers, depending on the target transition separation and the laser tuning range). The ratio of the integrated absorbance of each transition is a pure function of temperature,2

A latitudinal survey of mesospheric and upper stratospheric water vapor

As part of the LAtitudinal DIstribution of Middle Atmosphere Structure (LADIMAS) campaign, measurements of mesospheric and upper stratospheric water vapor concentration were made over a latitudinal range from 53 N to 63 S. The 22-GHz emission line of water vapor was observed by a new, portable, cryogenically cooled microwave radiometer that was carried on board the German research vessel Polarstern as it sailed from Bremerhaven, Germany, to the Antarctic during November and December, 1991. Water vapor profiles were obtained at approximately 5 deg latitude intervals for an altitude range of 40 to 80 km.

Measurement of molecular concentrations and line parameters using line-locked second harmonic spectroscopy with an AlGaAs diode laser

The technique of line-locked wavelength modulation with 2f detection is applied to the measurement of water vapor concentration and absorption line parameters by using an 820-nm AlGaAs communications diode laser. Measurements of the 2f signal as a function of the modulation amplitude yield accurate concentrations and line parameters over a pressure range of an order of magnitude and half-widths from 0.02 to 0.15 cm(-1). Usingtwo different spectral lines, we determined concentrations and line parameters with 1% precision, and the absolute accuracy of the line parameters is 3% or better. The results have been used to calculate calibration curves for a diode laser humidity monitor.