David A. Gryvnak

Absorption of infrared radiant energy by CO_2 and H_2O IV.Shapes of collision-broadened CO_2 lines.

The shapes of the extreme wings of self-broadened CO2 lines have been investigated in three spectral regions near 7000, 3800, and 2400 cm−1. Absorption measurements have been made on the high-wavenumber sides of band heads where much of the absorption by samples at a few atm is due to the extreme wings of strong lines whose centers occur below the band heads. New information has been obtained about the shapes of self-broadened CO2 lines as well as CO2 lines broadened by N2, O2, Ar, He, and H2. Beyond a few cm−1 from the line centers, all of the lines absorb less than Lorentz-shaped lines having the same half-widths. The deviation from the Lorentz shape decreases with increasing wavenumber, from one of the three spectral regions to the next. The absorption by the wings of H2- and He-broadened lines is particularly low, and the absorption decreases with increasing temperature at a rate faster than predicted by existing theories.

Strengths, Widths, and Shapes of the Oxygen Lines near 13,100 cm −1 (7620 Å)

The absorption by the oxygen A band near 13,100 cm(-1) (7620 A) has been investigated. Spectral curves with resolution between 0.6 cm(-1) and 1.2 cm(-1) have been obtained for several samples of O(2) and O(2) + N(2) with path lengths from 8 m to 1185 m and pressures up to 13.6 atm. The strength of the entire band is 4.09 +/- 0.25 g(-1) cm(2) cm(-1), and the relationship between the band strength and the strengths of the individual lines has been determined. Half-widths of self-broadened lines at 1 atm pressure vary from approximately 0.074 cm(-l) at J = 2 to 0.043 cm(-1) for J = 25. The lines are approximately 5% wider for air at the same pressure since broadening by N(2) is more efficient than self-broadening. The wings of the lines absorb less than Lorentz-shaped lines beyond approximately 10 cm(-1) from the centers.

Absorption of Infrared Radiant Energy by CO 2 and H 2 O, V. Absorption by CO 2 between 1100 and 1835 cm −1 (9.1–5.5 μm)*

Samples of CO2 at pressures up to 21.3 atm and with paths as long as 1067 m have been employed to measure CO2 absorption between 1100 and 1835 cm−1. Most of the absorption is due to the ν1 and 2ν2 bands. These bands are forbidden for the symmetric 16O12C16O and 16O13C16O molecules and, therefore, produce only pressure-induced absorption. The asymmetric 16O12C18O and 16O12C17O molecules produce bands that contain components of both intrinsic and pressure-induced absorption. The integrated-absorption coefficient for the intrinsic absorption is 1.08×10−22 (±10%) molecules −1 cm2 cm−1. The integrated pressure-induced absorption coefficient is 1.68×10−22 (±8%) molecules −1 cm2 atm−1 cm−1.

Infrared gas-filter correlation instrument for in situ measurement of gaseous pollutant concentrations.

An ir analyzer employing gas-filter correlation techniques has been designed and constructed to measure the concentrations of CO, NO, SO(2), HCl, and HF in the stacks or ducts of stationary pollutant sources. Use of a retroreflector allows the stack to be double passed, and no sample is extracted. For each gas, small interchangeable fixed-position grating polychromators are used as narrow (~1.5-cm(-1)) multiband spectral filters with the bands corresponding to locations of selected absorption lines. The approximate useful ranges (in parts per million-meters) over which this analyzer operates are 10-4000 for NO, 10-1500 for CO, 50-40,000 for SO(2), 10-2000 for HC1, and 5-200 for HF. The discrimination against other gases and particulates is excellent. The analyzer has been tested in the laboratory and on a variety of pollutant sources.

CONTINUUM ABSORPTION BY H 2 O VAPOR IN THE INFRARED AND MILLIMETER REGIONS

The continuum absorption by laboratory samples of pure H 2 O and of H 2 0 + N 2 throughout the infrared and millimeter regions has been studied. The samples cover a wide range of pressures and temperatures from 296 K to 428 K. Measurements have been made in the well-known 4 μm and 8 to 12 μm windows as well as in many very narrow “windows” between rotation and vibration-rotation lines. An empirical continuum has been derived for each of several spectral regions to account for the “excess” absorption observed for pure H 2 O over that calculated on the basis of known line strengths and widths and simple line shapes. The results indicate that much of the continuum absorption previously attributed to dimers, or clusters of H 2 O molecules, is due to the extreme wings of self-broadened H 2 O lines. If this is true, the lines must have three important characteristics that are not predicted by simple line-shape theories : (1) self-broadened lines absorb much more in the extreme wings than do N 2 -broadened lines of the same intensity and width; (2) the absorption in the extreme wings decreases with increasing temperature faster than is predicted by simple theory; (3) characteristic (2) is more pronounced for self-broadened lines than for N 2 broadened lines.

Strengths, Widths, and Shapes of the Lines of the 3ν CO Band

Spectral curves of several CO samples have been used to investigate the 3ν band whose center is near 6350 cm−1. The strength of the band for the common 12C16O isotope has been found to be 0.0130±0.0005 atm−1·cm−1STP·cm−1. An empirical equation, Fm=1+0.011m, has been derived to account for the influence of vibration—rotation on line strengths. The half‐widths of the self‐broadened lines at 1 atm pressure vary from approximately 0.090 cm−1 at | m | = 1 to 0.062 cm−1 at | m | = 20. The widths of self‐broadened lines are 1.08±0.005 times as great as N2‐broadened lines at the same pressure. The Lorentz line shape appears to be appropriate for the collision‐broadened lines within a few cm−1 of their centers; but the extreme wings of the lines are sub‐Lorentzian.

Absorption of Infrared Radiation by CO 2 and H 2 O. II. Absorption by CO 2 between 8000 and 10 000 cm −1 (1–1.25 Microns)*

Transmission spectra of several samples of pure CO2 have been obtained in the 8000–10 000 cm−1 (1–1.25 μ) region. Sample path lengths were varied from 16.5 to 933 m, and pressures from approximately 1 to 14.6 atm. Each of the important absorption bands has been identified and its strength determined. The average half-width, at one atmosphere pressure, of lines P8–P32 of the 0203 band has been found to be approximately 0.092 cm−1, with some evidence of a slight decrease with increasing J. Several replotted spectral curves are included, as are tables of the integrated absorptance.

Laboratory Measurements Of The Infrared Absorption By H2O And CO2 In Regions Of Weak Absorption

Laboratory measurements of the infrared absorption by CO2 and H2O in regions of weak absorption has provided new information on the shapes of the extreme wings of the absorption lines. CO2 lines absorb much less in the wings than Lorentz-shaped lines with the same intensities and half-widths; N2-broadened lines absorb even less than self-broadened lines. Both self-broadened and N2-broadened H2O lines appear to absorb more over a large portion of the wings than Lorentz-shaped lines. Like CO2 lines, the wings of self-broadened H2O lines absorb much more than the N2-broadened H2O lines. Absorption by the wings of all of the lines decreases unpredictably fast with increasing temperature. Much of the H2O absorption frequently attributed to dimers may be due to the extreme wings of self-broadened H2O lines.

Atmospheric Attenuation In The Infrared Windows

None of the so-called atmospheric windows in the infrared are completely transparent, even in an atmosphere free of haze, fog, dust, or other particulate matter. Absorption by the atmospheric gases occurs because of many, very weak absorption lines in the windows and the extreme wings of very strong lines whose centers are located outside the windows. Most of the absorption lines between the visible and approximately 18 pm result from simultaneous changes in the vibrational and rotational energy levels of the absorbing molecules. At longer wavelengths most of the lines are pure rotational. A few pure rotational H2O lines appear at wavelengths as short as approximately 8 μm. (Ref. 1)