Development and recent evaluation of the MT_CKD model of continuum absorption

Water vapour continuum absorption is an important contributor to the Earth's radiative cooling and energy balance. Here, we describe the development and status of the MT_CKD (MlawerTobinCloughKneizysDavies) water vapour continuum absorption model. The perspective adopted in developing the MT_CKD model has been to constrain the model so that it is consistent with quality analyses of spectral atmospheric and laboratory measurements of the foreign and self continuum. For field measurements, only cases for which the characterization of the atmospheric state has been highly scrutinized have been used. Continuum coefficients in spectral regions that have not been subject to compelling analyses are determined by a mathematical formulation of the spectral shape associated with each water vapour monomer line. This formulation, which is based on continuum values in spectral regions in which the coefficients are well constrained by measurements, is applied consistently to all water vapour monomer lines from the microwave to the visible. The results are summed-up (separately for the foreign and self) to obtain continuum coefficients from 0?to 20?000?cm(-1). For each water vapour line, the MT_CKD line shape formulation consists of two components: exponentially decaying far wings of the line plus a contribution from a water vapour molecule undergoing a weak interaction with a second molecule. In the MT_CKD model, the first component is the primary agent for the continuum between water vapour bands, while the second component is responsible for the majority of the continuum within water vapour bands. The MT_CKD model should be regarded as a semi-empirical model with strong constraints provided by the known physics. Keeping the MT_CKD continuum consistent with current observational studies necessitates periodic updates to the water vapour continuum coefficients. In addition to providing details on the MT_CKD line shape formulation, we describe the most recent update to the model, MT_CKD_2.5, which is based on an analysis of satellite- and ground-based observations from 2385 to 2600?cm(-1) (approx. 4?μm).     

Recent advances in measurement of the water vapour continuum in the far-infrared spectral region

We present a new derivation of the foreign-broadened water vapour continuum in the far-infrared (far-IR) pure rotation band between 24?μm and 120?μm (85-420?cm(-1)) from field data collected in flight campaigns of the Continuum Absorption by Visible and IR radiation and Atmospheric Relevance (CAVIAR) project with Imperial College's Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) far-IR spectro-radiometer instrument onboard the Facility for Airborne Atmospheric Measurement (FAAM) BAe-146 research aircraft; and compare this new derivation with those recently published in the literature in this spectral band. This new dataset validates the current Mlawer-Tobin-Clough-Kneizys-Davies (MT-CKD) 2.5 model parametrization above 300?cm(-1), but indicates the need to strengthen the parametrization below 300?cm(-1), by up to 50 per cent at 100?cm(-1). Data recorded at a number of flight altitudes have allowed measurements within a wide range of column water vapour environments, greatly increasing the sensitivity of this analysis to the continuum strength.     

The water vapour self- and water-nitrogen continuum absorption in the 1000 and 2500 cm(-1) atmospheric windows

The pure water vapour and water-nitrogen continuum absorption in the 1000 and 2500?cm(-1) atmospheric windows has been studied using a 2?m base-length White-type multi-pass cell coupled with a BOMEM DA3-002 Fourier transform infrared spectrometer. The measurements were carried out at the National Institute of Standards and Technology (NIST, Gaithersburg, MD) over the course of several years (2004, 2006-2007, 2009). New data on the H(2)O:N(2) continuum in the 1000?cm(-1) window are presented and summarized along with the other experimental results and the continuum model. The experimental data reported on the water vapour continuum in these atmospheric windows basically agree with the most reliable laboratory data from the other sources. The MT_CKD (Mlawer-Tobin-Clough-Kneizys-Davies) continuum model significantly departs from the experimental data in both windows. The deviation observed includes the continuum magnitude, spectral behaviour and temperature dependence. In the 2500?cm(-1) region, the model does not allow for the nitrogen fundamental collision-induced absorption (CIA) band intensity enhancement caused by H(2)O:N(2) collisions and underestimates the actual absorption by over two orders of magnitude. The water vapour continuum interpretation as a typical CIA spectrum is reviewed and discussed.     

Airborne and satellite remote sensing of the mid-infrared water vapour continuum

Remote sensing of the atmosphere from space plays an increasingly important role in weather forecasting. Exploiting observations from the latest generation of weather satellites relies on an accurate knowledge of fundamental spectroscopy, including the water vapour continuum absorption. Field campaigns involving the Facility for Airborne Atmospheric Measurements research aircraft have collected a comprehensive dataset, comprising remotely sensed infrared radiance observations collocated with accurate measurements of the temperature and humidity structure of the atmosphere. These field measurements have been used to validate the strength of the infrared water vapour continuum in comparison with the latest laboratory measurements. The recent substantial changes to self-continuum coefficients in the widely used MT_CKD (Mlawer-Tobin-Clough-Kneizys-Davies) model between 2400 and 3200?cm(-1) are shown to be appropriate and in agreement with field measurements. Results for the foreign continuum in the 1300-2000?cm(-1) band suggest a weak temperature dependence that is not currently included in atmospheric models. A one-dimensional variational retrieval experiment is performed that shows a small positive benefit from using new laboratory-derived continuum coefficients for humidity retrievals.     

Water vapour foreign-continuum absorption in near-infrared windows from laboratory measurements

For a long time, it has been believed that atmospheric absorption of radiation within wavelength regions of relatively high infrared transmittance (so-called 'windows') was dominated by the water vapour self-continuum, that is, spectrally smooth absorption caused by H(2)O--H(2)O pair interaction. Absorption due to the foreign continuum (i.e. caused mostly by H(2)O--N(2) bimolecular absorption in the Earth's atmosphere) was considered to be negligible in the windows. We report new retrievals of the water vapour foreign continuum from high-resolution laboratory measurements at temperatures between 350 and 430?K in four near-infrared windows between 1.1 and 5?μm (9000-2000?cm(-1)). Our results indicate that the foreign continuum in these windows has a very weak temperature dependence and is typically between one and two orders of magnitude stronger than that given in representations of the continuum currently used in many climate and weather prediction models. This indicates that absorption owing to the foreign continuum may be comparable to the self-continuum under atmospheric conditions in the investigated windows. The calculated global-average clear-sky atmospheric absorption of solar radiation is increased by approximately 0.46?W?m(-2) (or 0.6% of the total clear-sky absorption) by using these new measurements when compared with calculations applying the widely used MTCKD (Mlawer-Tobin-Clough-Kneizys-Davies) foreign-continuum model.     

Polarimetric measurements of long-wave infrared spectral radiance from water

Polarimetric measurements of the thermal infrared spectral radiance from water are reported and are compared with calculations from a recently published model over the spectral range of 600-1600 cm(-1) (6.25-16.67-mum wavelength). In this spectral range, warm water viewed under a dry, clear atmosphere appears vertically polarized by 6-12%. The measured spectral degree of polarization agrees with calculations within the measurement uncertainty (~0.5% polarization in spectral regions with high atmospheric transmittance and 1.5% polarization in spectral regions with low atmospheric transmittance). Uncertainty also arises from temporal changes in water and air temperatures between measurements at orthogonal polarization states, indicating the desirability of simultaneous measurements for both polarization states.     

Distribution of tritium in water vapour and precipitation around Wolsung nuclear power plant

The distribution of tritium in water vapour and precipitation with discharge of tritiated water vapour and meteorological factors was studied around the Wolsung nuclear power plant (NPP) site during the period 2004-2008. The tritium concentrations in atmospheric water vapour and precipitation had a temporal variation with relatively high values in the early summer. Spatial distribution of tritium concentrations was affected by various factors such as distance from the NPP site, wind direction, tritium discharge into the atmosphere and atmospheric dispersion factor. The annual mean concentrations of atmospheric HTO and precipitation were correlated with the amount of gaseous tritium released from the Wolsung NPP. The tritium concentrations in precipitation decrease exponentially with an increase of the distance from the Wolsung NPP site.     

Remote sensing of sea surface temperature through infrared radiometry — a review

A review on the estimation of sea surface temperature (SST) from space using infrared radiometry is presented. The principle of the remote sensing technique, the infrared radiometers and the theory of SST measurement have been explained briefly. The absorption of infrared radiation from earth by the atmospheric constituents has been a major problem in the retrieval of SST from space-borne sensors. The effect of clouds and other atmospheric constituents, especially moisture content/water vapour, has been discussed and the various techniques used for evaluating the atmospheric correction and their limitations are summarised. Development of separate algorithms for each oceanic area coupled with validation by realistic sea truth measurements has been suggested for improving the accuracy of SST measurements from space. The first author was supported by a CSIR fellowship.     

Simple Monte Carlo methods to estimate the spectra evaluation error in differential-optical-absorption spectroscopy

Differential-optical-absorption spectroscopy (DOAS) permits the sensitive measurement of concentrations of trace gases in the atmosphere. DOAS is a technique of well-defined accuracy; however, the calculation of a statistically sound measurement precision is still an unsolved problem. Usually one evaluates DOAS spectra by performing least-squares fits of reference absorption spectra to the measured atmospheric absorption spectra. Inasmuch as the absorbance from atmospheric trace gases is usually very weak, with optical densities in the range from 10(-5) to 10(-3), interference caused by the occurrence of nonreproducible spectral artifacts often determines the detection limit and the measurement precision. These spectral artifacts bias the least-squares fitting result in two respects. First, spectral artifacts to some extent are falsely interpreted as real absorption, and second, spectral artifacts add nonstatistical noise to spectral residuals, which results in a significant misestimation of the least-squares fitting error. We introduce two new approaches to investigate the evaluation errors of DOAS spectra accurately. The first method, residual inspection by cyclic displacement, estimates the effect of false interpretation of the artifact structures. The second method applies a statistical bootstrap algorithm to estimate properly the error of fitting, even in cases when the condition of random and independent scatter of the residual signal is not fulfilled. Evaluation of simulated atmospheric measurement spectra shows that a combination of the results of both methods yields a good estimate of the spectra evaluation error to within an uncertainty of ~10%.     

An aerosol-process for the synthesis of nanostructured molybdenum oxide catalysts by integrated chemical vapour synthesis/chemical vapour deposition at atmospheric pressure

We report the synthesis of composite nanoparticles by an integrated CVS/CVD process at atmospheric pressure. Iron oxide and silica support particles were generated by chemical vapour synthesis (CVS), using Fe(CO)5 and Si(OC2H5)4 and were directly coated in the aerosol state with molybdenum oxide by chemical vapour deposition of Mo(CO)6. Depending on the CVS temperature hematite (600 degrees C) or maghemite (1500 degrees C) iron oxide phases were determined by XRD and FTIR. Core-shell structures with a coating thickness in the lower nm range were obtained for CVD temperatures below 150 degrees C. Complete encapsulation of the core particles and uniform elemental distribution is shown by TEM and EELS measurements. Higher CVD temperatures lead to unwanted homogenous decomposition of the molybdenum precursor. Additional aerosol temperature treatment was used to reach further oxidation and the formation of a mixed oxide shell, indicated by FTIR measurements. The results show the potential of the process for the synthesis of structured core-shell nanoparticles.     

Measurements of the foreign-broadened continuum of water vapor in the 6.3 microm band at -30 degrees C

The foreign-broadened continuum of water vapor in the nu2 band (5-7.7 microm, 1300-2000 cm(-1)) is important for satellite-based retrievals of water vapor in the upper troposphere, where temperatures are below -25 degrees C. Continuum coefficients have previously been measured mostly at or above +23 degrees C. We present continuum coefficients in the nu(2) band retrieved from measurements made in Antarctica at temperatures near -30 degrees C: atmospheric transmission at South Pole Station and atmospheric emission at Dome C. The continuum coefficients derived from these measurements are generally in agreement with the widely used Mlawer, Tobin-Clough, Kneizys, Davies continuum. Differences are at most 30%, corresponding to a 6% relative error in retrieved upper-tropospheric humidity.     

Modelling of fatigue-corrosion in Ti6246 alloy at 500°c

This paper deals with a study of the influence of the atmospheric environment on the resistance against fatigue crack propagation of a Ti6246 titanium alloy at 500°C. Tests were performed in the near‐threshold and mid‐rate ranges under controlled gaseous environments, including vacuum and argon, with controlled partial pressures of water vapour and oxygen. A predominant detrimental influence of water vapour is demonstrated even under very low partial pressure, while oxygen gas is shown to moderate the influence of water vapour. A crack‐propagation model relating the growth rate to the effective stress intensity factor range is proposed to account for this competitive influence of water vapour and oxygen with respect to the test frequency.     

Properties of Transglutaminase Crosslinked Whey Protein Isolate Coatings and Cast Films

Whey protein is an excellent barrier material, potentially providing the low oxygen permeability and water vapour transmission rate required for packaging materials for sensitive foods. A topical issue is how to reduce the permeability of the whey protein‐based layers. One possible strategy is to apply crosslinking agents. In the present study, the enzyme transglutaminase (TG) was used for this purpose, and the following properties have been measured: oxygen permeability, water vapour transmission rate, effective water vapour diffusion, sorption and permeation coefficients, swellability, light transmission and surface energy. The use of TG was found to reduce the oxygen permeability, reduce the water vapour transmission rate and reduce the effective water vapour diffusion coefficient of the whey protein layers. This study also showed that it was possible to use sorption experiments to approximate the water vapour transmission rates. This work is novel because it deals with the packaging‐related properties of whey films crosslinked with TG. Previous studies have focused on the influence of TG on protein formulations or on the measurement of film properties, which is of little relevance for packaging applications. Copyright © 2014 John Wiley & Sons, Ltd.     

Interferometer for ground-based observations of emitted spectral radiance from the troposphere: evaluation and retrieval performance

We evaluate the spectral quality, radiometric noise, and retrieval performance of a Fourier transform infrared spectrometer, which has been developed for recording spectrally resolved observations in a region of the spectrum which is important both for the science of Earth's climate and applications, such as the remote sensing of temperature and atmospheric gas species. This spectral region extends from 100 to 1600 cm(-1) and encompasses the two fundamental, rotation and vibration, absorption bands of water vapor. The instrument is a customized version of a Bomem AERI (Atmospheric Emitted Radiance Interferometer) spectrometer, whose spectral coverage has been extended in the far infrared with the use of uncooled pyroelectric detectors. Retrieval examples for water vapor and temperature profiles are shown, which also allow us to intercompare the retrieval performance of both H(2)O vibration and rotation bands.     

Accurate modeling of spectral fine-structure in Earth radiance spectra measured with the Global Ozone Monitoring Experiment

We present what we believe to be a novel approach to simulating the spectral fine structure (<1 nm) in measurements of spectrometers such as the Global Ozone Monitoring Experiment (GOME). GOME measures the Earth's radiance spectra and daily solar irradiance spectra from which a reflectivity spectrum is commonly extracted. The high-frequency structures contained in such a spectrum are, apart from atmospheric absorption, caused by Raman scattering and by a shift between the solar irradiance and the Earth's radiance spectrum. Normally, an a priori high-resolution solar spectrum is used to simulate these structures. We present an alternative method in which all the required information on the solar spectrum is retrieved from the GOME measurements. We investigate two approaches for the spectral range of 390-400 nm. First, a solar spectrum is reconstructed on a fine spectral grid from the GOME solar measurement. This approach leads to undersampling errors of up to 0.5% in the modeling of the Earth's radiance spectra. Second, a combination of the solar measurement and one of the Earth's radiance measurement is used to retrieve a solar spectrum. This approach effectively removes the undersampling error and results in residuals close to the GOME measurement noise of 0.1%.     

High-Resolution Fourier-Transform Ultraviolet-Visible Spectrometer for the Measurement of Atmospheric Trace Species: Application to OH

A compact, high-resolution Fourier-transform spectrometer for atmospheric near-ultraviolet spectroscopy has been installed at the Jet Propulsion Laboratory's Table Mountain Facility (34.4 degrees N, 117.7 degrees W, elevation 2290 m). This instrument is designed with an unapodized resolving power near 500,000 at 300 nm to provide high-resolution spectra from 290 to 675 nm for the quantification of column abundances of trace atmospheric species. The measurement technique used is spectral analysis of molecular absorptions of solar radiation. The instrument, accompanying systems designs, and results of the atmospheric hydroxyl column observations are described.     

Analysis of aircraft exhausts with Fourier-transform infrared emission spectroscopy

Because of the worldwide growth in air traffic and its increasing effects on the atmospheric environment, it is necessary to quantify the direct aircraft emissions at all altitudes. In this study Fourier-transform infrared emission spectroscopy as a remote-sensing multi-component-analyzing technique for aircraft exhausts was investigated at ground level with a double pendulum interferometer and a line-by-line computer algorithm that was applied to a multilayer radiative transfer problem. Initial measurements were made to specify the spectral windows for traceable compounds, to test the sensitivity of the system, and to develop calibration and continuum handling procedures. To obtain information about the radial temperature and concentration profiles, we developed an algorithm for the analysis of an axial-symmetric multilayered plume by use of the CO(2) hot band at approximately 2400 cm(-1). Measurements were made with several in-service engines. Effects that were due to engine aging were detected but have to be analyzed systematically in the near future. Validation measurements were carried out with a conventional propane gas burner to compare the results with those obtained with standard measurement equipment. These measurements showed good agreement to within +/-20% for the CO and NO(x) results. The overall accuracy of the system was found to be +/-30%. The detection limits of the system for a typical engine plume (380 degrees C, ? = 50 cm) are below 0.1% for CO(2), ~0.7% for H(2)O, ~20 ppmv (parts per million by volume) for CO, and ~90 ppmv for NO.     

Laser-based diagnostics for the measurement of liquid water film thickness

Three different diagnostic techniques are investigated for measurement of the thickness of liquid water films deposited on a transparent quartz plate. The methods are based on laser-induced fluorescence (LIF) from low concentrations of a dissolved tracer substance and spontaneous Raman scattering of liquid water, respectively, both excited with 266?nm of radiation, and diode laser absorption spectroscopy (DLAS) in the near-infrared spectral region. Signal intensities are calibrated using liquid layers of known thickness between 0 and 1000?μm. When applied to evaporating liquid water films, the thickness values derived from the direct DLAS and Raman scattering measurements correlate well with each other as a function of time after the start of data recording, while the LIF signal derived thickness values decrease faster with time due to selective tracer evaporation from the liquid. The simultaneous application of the LIF with a tracer-free detection technique can serve as an in situ reference for quantitative film thickness measurements.     

Portable Fourier transform infrared spectroradiometer for field measurements of radiance and emissivity

A hand-held, battery-powered Fourier transform infrared spectroradiometer weighing 12.5 kg has been developed for the field measurement of spectral radiance from the Earth's surface and atmosphere in the 3-5-μm and 8-14-μm atmospheric windows, with a 6-cm(-1) spectral resolution. Other versions of this instrument measure spectral radiance between 0.4 and 20 μm, using different optical materials and detectors, with maximum spectral resolutions of 1 cm(-1). The instrument tested here has a measured noise-equivalent delta T of 0.01 °C, and it measures surface emissivities, in the field, with an accuracy of 0.02 or better in the 8-14-μm window (depending on atmospheric conditions), and within 0.04 in accessible regions of the 3-5-μm window. The unique, patented design of the interferometer has permitted operation in weather ranging from 0 to 45 °C and 0 to 100% relative humidity, and in vibration-intensive environments such as moving helicopters. The instrument has made field measurements of radiance and emissivity for 3 yr without loss of optical alignment. We describe the design of the instrument and discuss methods used to calibrate spectral radiance and calculate spectral emissivity from radiance measurements. Examples of emissivity spectra are shown for both the 3-5-μm and 8-14-μm atmospheric windows.     

Nanoporous polymeric transmission gratings for high-speed humidity sensing

Nanoporous polymeric transmission gratings are demonstrated to be an excellent platform for high-speed optical humidity sensing. The grating structures were fabricated with a modified holographic, polymer-dispersed liquid crystal (H-PDLC) system. The sensing mechanism was based on changes in the relative transmission associated with the adsorption and desorption of water vapour by nanopores. The spectral changes due to varying humidity levels were measured by a spectrometer and compared with the calculated results based on the coupled wave theory. When the relative humidity (RH) changed from 40% to 95%, the relative transmission at 475?nm increased from 6.3% to 46.6% and that at 702?nm increased from 4%?to 64%; these results indicate the sensor's high sensitivity. In addition, the sensor demonstrated excellent reversibility and reproducibility over a large RH range (from 20% to 100% RH). Moreover, the response time of the sensor was measured to be less than 350?ms, making it suitable for many high-speed humidity-sensing applications.