Igor V. Ptashnik

Water vapor self‐continuum absorption in near‐infrared windows derived from laboratory measurements

In most near-infrared atmospheric windows, absorption of solar radiation is dominated by the water vapor self-continuum and yet there is a paucity of measurements in these windows. We report new laboratory measurements of the self-continuum absorption at temperatures between 293 and 472 K and pressures from 0.015 to 5 atm in four near-infrared windows between 1 and 4 m (10000-2500 cm-1); the measurements are made over a wider range of wavenumber, temperatures and pressures than any previous measurements. They show that the self-continuum in these windows is typically one order of magnitude stronger than given in representations of the continuum widely used in climate and weather prediction models. These results are also not consistent with current theories attributing the self continuum within windows to the far-wings of strong spectral lines in the nearby water vapor absorption bands; we suggest that they are more consistent with water dimers being the major contributor to the continuum. The calculated global-average clear-sky atmospheric absorption of solar radiation is increased by 0.75 W/m2 (which is about 1% of the total clear-sky absorption) by using these new measurements as compared to calculations with the MT_CKD-2.5 self-continuum model.

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 H2O−H2O pair interaction. Absorption due to the foreign continuum (i.e. caused mostly by H2O−N2 bimolecular absorption in the Earths 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.

Absorption by water vapour in the 1 to region

Paper associated with the CWVC (Clouds, Water Vapour and Climate) dataset held on the CEDA archive.

Water vapor continuum absorption in near-IR atmospheric windows

The near-infrared water vapor absorption is measured in the 2000–8000 cm−1 spectral region. Spectra were recorded using an IFS 125 HR Fourier spectrometer at a temperature of 287 K and a spectral resolution of 0.03 cm−1. The water vapor continuum absorption spectrum is retrieved using the known absorption in the 2500 cm−1 region as a reference point. It is shown that the continuum absorptions in four windows differ by no more than 20% under investigation conditions. This contradicts the MT_CKD continuum model, which predicts a much stronger variability of the continuum in these windows.

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.

Modeling of helicopter-borne tunable TEA CO2 DIAL system employment for detection of methane and ammonia leakages

Abstract The characteristics of a helicopter-borne lidar based on tunable TEA CO2 laser and its third harmonic designed for remote detection of methane and ammonia leakages from pipelines are analyzed numerically. The spectral range near 3 μm was shown to be most promising for remote sensing of methane emissions. Parameters of radiation of the tunable pulse-periodic mini-TEA CO2 laser and generators of harmonics to be utilized in the helicopter-borne differential absorption lidar are estimated. Emissions of different gas intensities are analyzed for possible detectability at a distance of up to 1 km. The use of the third harmonic of the TEA CO2 laser allows methane emissions from a pipeline to be detected and measured with mean measurement error from 10% to 15% for methane concentrations varying from the background level to the explosion-hazardous one. The optimal pair and possibilities of the ammonia remote sensing on the base of the first harmonic of TEA CO2 laser were determined as well.

Absolute high spectral resolution measurements of surface solar radiation for detection of water vapour continuum absorption

Solar-pointing Fourier transform infrared (FTIR) spectroscopy offers the capability to measure both the fine scale and broadband spectral structure of atmospheric transmission simultaneously across wide spectral regions. It is therefore suited to the study of both water vapour monomer and continuum absorption behaviours. However, in order to properly address this issue, it is necessary to radiatively calibrate the FTIR instrument response. A solar-pointing high-resolution FTIR spectrometer was deployed as part of the ‘Continuum Absorption by Visible and Infrared radiation and its Atmospheric Relevance’ (CAVIAR) consortium project. This paper describes the radiative calibration process using an ultra-high-temperature blackbody and the consideration of the related influence factors. The result is a radiatively calibrated measurement of the solar irradiation at the ground across the IR region from 2000 to 10 000 cm−1 with an uncertainty of between 3.3 and 5.9 per cent. This measurement is shown to be in good general agreement with a radiative-transfer model. The results from the CAVIAR field measurements are being used in ongoing studies of atmospheric absorbers, in particular the water vapour continuum.

Parametrization of transmittance for application in atmospheric optics

Abstract Different ways of improving the line-by-line method are described. Peculiarities of the application of the k-distribution method to parametrization of transmittance of overlapping bands for atmospheric gases are discussed. An expansion of the transmittance in exponential series with parameters determined on the basis of absorption coefficients is proposed.

Measurement of the continuum absorption coefficient of water vapor near 14400 cm−1 (0.694 μm)

The continuum absorption coefficient (CAC) of water vapor (kcont) in the visible region is determined for the first time from the data of laboratory measurements. For this purpose, the absorption spectra of water vapor in the region 14395–14402 cm−1 are recorded with the aid of a high-sensitivity photoacoustic spectrometer with a frequency-tunable single-pulse ruby laser, and the absorption measured in this transparency microwindow is compared with that calculated based on the HITRAN 2004 data bank. In the spectral region under study, kcont = (0.53 ± 0.18) × 10−9 cm−1 mbar−1 at a total pressure of a water vapor-nitrogen mixture of 1000 mbar and a temperature of 295 K. This value of the CAC is roughly 23% higher than the CAC value in the IO-CKD model of the continuum.

Approximation of the width of the line profile narrowed by collisions

For models of hard and soft (in velocity) collisions, algebraic approximations of the line half-width are determined that depend on parameters of collisional broadening and narrowing and on the Doppler half-width. The average difference between the approximated and the exact values of the half-width do not exceed 0.1%. A simple criterion for determining the correspondence of models of hard and soft collisions to the experimental dependence of the half-width on the buffer gas pressure is proposed.