XYLENE POWER LTD.

INFRARED ABSORPTION

By Charles Rhodes, P. Eng., Ph.D.

OVERVIEW OF INFRARED ABSORPTION:
For an overview of absorption of radiation by gaseous molecules please refer
to:CRISP

THE ROLE OF INFRARED ABSORPTION:
Recall from the Radiation Physics section that the emission temperature of an element of surface area of the earth is given by:Ta = Te [(1 - Fr) / Ft]^.25
where the emissivity Ft is measured in the infrared spectral range and the albedo Fr is measured in the solar spectral range. Recall also that:Ft = Integral from Z = 0 to Z = infinity of:
{Ftw(W) [15/Pi^4] Z^3 dZ / [exp(Z) - 1]}
where Z = (h F) / (K Ta)

In order to understand Ftw(W) it is necessary to understand the infrared absorption characteristics of the greenhouse gases and how these gases affect the infrared spectrum of the earth. The gases of most interest are water vapor, carbon dioxide, ozone, methane and nitrous oxide.

VIEW FROM OUTER SPACE:
When the earth is viewed from outer space with an infrared thermal emission spectrometer, the recorded spectrum consists of radiation from species that will readily interact with electromagnetic radiation. Oxygen and nitrogen do not interact with photons in the thermal emission spectral range and hence are not seen. Water vapor, which interacts with photons almost right across the thermal emission spectrum is seen, as are carbon dioxide and ozone. High resolution thermal emission spectroscopy equipment for satelite use is being developed.

The following Tropical Clear Sky thermal emission spectrum was recorded by an IRIS instrument (Infra Red Interferometer Spectrometer) on board a Nimbus-3 or Nimbus-4 satelite over the tropical central Pacific Ocean on an exceptionally clear day when there was almost no cloud in its 100 km wide field of view. This spectrum is undated but the satelite mission records indicate that this spectrum was recorded either in the time interval April 14, 1969 to July 22, 1969 or in the time interval April 18, 1970 to January 30, 1971.

This spectrum also shows the width (575 cm^-1 to 775 cm^-1) of the main carbon dioxide absorption band over the equitorial Pacific Ocean in 1970 as seen by the overhead satelite with minimum interference by water vapor.

For wavenumbers less than 400 cm^-1 and greater than 1300 cm^-1 even a small amount of water vapor obscures everything else. The above plot also shows a dark green 255 K black body line corresponding to the temperature of high altitude water vapor.

In the wavenumber band 996 cm^-1 to 1068 cm^-1 ozone in the upper atmosphere obscures the water vapor. At wavenumbers 1029 cm^-1 and 1054 cm^-1 an overhead satelite sees radiation of an intensity corresponding to the temperature of the ozone layer.

At a wavenumber of 669 cm^-1 carbon dioxide has a much stronger absorption than water vapor. Hence, when the earth is viewed from a satelite through 669 cm^-1 bandpass filter the carbon dioxide in the upper atmosphere obscures the water vapor from the satelite's view. Since the upper atmosphere is cold (215 K), that is the effective temperature seen by the satelite at 669 cm^-1.

In the wavenumber ranges 850 cm^-1 to 980 cm^-1 and 1080 cm^-1 to 1200 cm^-1 a dense cloud of water vapor will prevent infrared radiation originating below the cloud from reaching a spacecraft above the atmosphere. Hence the spacecraft sees the temperature at the top of the cloud.

On a clear day when there is almost no cloud the remaining water vapor is almost transparent in the wavenumber range 475 cm^-1 to 1200 cm^-1. Under these circumstances within this wavenumber range the satelite can see almost down to ground level, except within the carbon dioxide and ozone absorption bands. The satelite sees condensing water vapor above the surface of the ocean. Near the equator the equivalent black body temperature seen by the satelite is in the range 265 degrees K to 285 degrees K, depending on the season. This spectral band is known as the 'water vapor window". This band allows infrared emission to space from warm water vapor regions near the warm ocean surface. This relatively high temperature thermal emission carries away most of the Earth's thermal radiant energy, thus keeping the Earth cool. Increased obstruction of the "water window" by carbon dioxide absorption leads to an increase in the Earth's surface temperature.

Careful examination of the thermal emission spectrum recorded by the IRIS instrument reveals carbon dioxide side band absorption lines at 515 cm^-1, 548 cm^-1, 800 cm^-1 and 850 cm^-1, outside the main carbon dioxide absorption band but between 475 cm^-1 and 875 cm^-1. These side bands likely arise due to rotation of the CO2 molecule. Amplitude modulation theory indicates that there will be an infinite series of such side bands with diminishing amplitude away from the center frequency. The side band absorption lines become much more important at larger carbon dioxide concentrations.

The IRIS data shows that within the wavenumber range 400 cm^-1 to 1600 cm^-1, at the present atmospheric carbon dioxide concentration, there is no significant carbon dioxide gas absorption outside the 475 cm^-1 to 875 cm^-1 band.

Laboratory measurements of infrared absorption by carbon dioxide show that the carbon dioxide absorption band consists of a central strong absorption line at 669 cm^-1 (wavelength = 15 um) and a series of side bands of diminishing amplitude. Increasing the concentration of carbon dioxide in the measurement apparatus increases the strength of the absorption sidebands and hence effectively increases the width of the carbon dioxide absorption band.

In the Earth's atmosphere the central absorption band is saturated, but the side bands are not. Increasing the atmospheric CO2 concentration brings more CO2 absorption side bands into play. On Earth the atmospheric CO2 pressure is about .000387 bar. Experimental evidence from the planet Venus indicates that a its atmospheric CO2 pressure of about 90 bars causes a temperature increase of about 400 degrees C.

Although the above IR spectrum reveals a lot of useful information it cannot be used for global warming calculations because it is representative of nearly ideal cloudless conditions, not average conditions, and the value of Fr is not known for these nearly ideal conditions.

WATER VAPOR:
The physical structure of a water (H2O) molecule causes it to have a very complex families of rotational and vibrational states. The molecule is Vee shaped because the two hydrogen atoms bond to each other as well as bonding to the oxygen atom.

Chemically each of the two hydrogen atoms give up one electron to the oxygen atom, causing the oxygen atom to be negatively charged andt the hydrogen atoms to be positively charged. The mass of the hydrogen atoms is small compared to the mass of the oxygen atom, so the center of mass is located near the oxygen atom and the rotational moment of inertia is small.
The moment of inertia is different in each of the three planes of rotation. Quantization of angular momentum leads to a wide range of rotational states forming absorption lines in the wavenumber range 200 cm^-1 to 600 cm^-1.(50 um to 16.6 um)

There are also vibrational states relating to the hydrogen-oxygen chemical bond. The interacion between the two hydrogen atoms massively complicates these vibrational states. These vibrational states form a broad absorption band in the frequency range start 1300 cm^-1 to 1900 cm^-1 (7.7 um to 5.2 um). This absorption band has an extremely narrow passband at about 1600 cm^-1.

Between the rotational states and the vibrational states Water vapor (H2O) has an infrared passband that extends from 600 cm^-1 to 1300 cm^-1 (20 um to 7.7 um).

The two water vopor absorption bands cut off the high wavenumber and low wavenumber ends of the infrared thermal emission spectrum emitted by the earth. Water vapor also has weak wideband absorption. Experimental data shows that increasing the water vapor concentration in the atmosphere increases the altitude at which the thermal emission occurs, and vice versa. It is possible that this wide band absorption is related to H2CO3 formed by combination of CO2 disolving in H20 cloud droplets. The exact physical mechanism of this wide band absorption remains to be resolved.

At any particular location the water vapor concentration goes through daily oscillations due to the rotation of the earth. The average water vapor concentration seeks a level that minimizes energy related to the daily water vapor concentration oscillations. The water vapor concentration adopts a level where the fluctuations in (1 - Fr) due to water vapor cancel out the fluctuations in Ft due to water vapor. Thus the temperature remains relatively constant and water is a moderating influence.

Recall that greenhouse warming is given by:
(Ta - Te) / Te = {[(1 - Fr) / Ft]^.25 - 1}
The effect of adding an increment of water vapor to the atmosphere is to increase albedo Fr and to decrease Ft. As the concentration of water vapor in the atmosphere increases the albedo increases which in turn reduces the amount of absorbed solar power, which reduces the rate of water evaporation. On average the rate of water evaporation matches the rate of precipitation. The rate of precipitation is set by the rate of infrared radiative cooling by the atmosphere. In the above equation these two effects cancel one another.

The effect of clouds is to moderate daily temperature swings because clouds reduce absorbed solar radiation during the day and trap infrared radiation during the night. When the atmospheric temperature is very low the water vapor concentration in the atmosphere is low, so there are relatively large daily temperature swings.

The concentration of water vapor in the atmosphere cannot be directly controlled by mankind. Water vapor goes through a phase change and tends to establish its own equilibrium.

Shown below is experimental relative infrared absorbance data published by NIST. The left hand column gives the wavenumber in cm^-1. The other columns give the absorbance results of ten successive experimental spectrum scans for the wavenumber range 450 cm^-1 to 2000 cm^-1. The far right column gives the average of the ten experimental runs in the spectral range of immediate interest.

CARBON DIOXIDE:
From a global warming perspective the most important gas is carbon dioxide. Its laboratory measured relative transmittance versus:
Wavenumber= F / C
for five experimental runs is tabulated below using data from NIST. The claimed resolution is 4 cm^-1.

Note that the absorbance of carbon dioxide contains multiple absorption lines (side bands) of diminishing strength around the main translational absorption wave number= 669 cm^-1. At higher carbon dioxide concentrations these sidebands play a major role in global warming. The main absorptin wavenumber 669 cm^-1 appears to be amplitude modulated by 9 cm^-1 and 51 cm^-1 signals, probably due to molecular rotation states.

The infrared absorption spectrum of carbon dioxide (CO2) recorded by the IRIS instrument in 1970 has a strong absorption band centered at 669 cm^-1 (15 um). The main absorption band clearly extends from 575 cm^-1 to 775 cm^-1 (17.39 um to 12.9 um). The IRIS instrument shows other smaller absorption bands on both sides of the main absorption band. The origin of these sidebands cannot be determined from the IRIS data alone. However, comparison of the IRIS data with the NIST data shows that the absorptions visible at 515 cm^-1, 548 cm^-1 and 800 cm^-1 on the IRIS data are all due to carbon dioxide.

At the central portion of the 15 um carbon dioxide absorption band only the temperature of the cold upper atmosphere is visible from outer space. At this wavelength the carbon dioxide in the lower atmosphere is opaque. Increasing the carbon dioxide concentration increases the absorption by side bands, making these sideband wavenumbers opaque as well. Thus, increasing the carbon dioxide concentration increases the apparent width of the main carbon dioxide absorption band, which decreases Ft and hence increases global warming. Note that the area of the main absorption band, if the carbon dioxide concentration is less than 16 times its present value, is bounded by 475 cm^-1, 875 cm^-1, the 215K black body line, the 290 K line and the water vapor absorption in the absence of carbon dioxide.

It is possible that carbon dioxide in the atmosphere combines with condensed water vapor droplets to form H2CO3, a wide band absorbing species. When there are no clouds there is no H2CO3, and hence this wide band absorption disappears. However, this wide band absorption may be negligible compared to water vapor absorption.Reference:Carbonic acid production in H2O:CO2 ices

As the concentration of carbon dioxide increases, Ftw(W) decreases at all the CO2 absorption lines (475 cm^-1 to 875 cm^-1) causing Ft to decrease. This decrease in Ft forces the temperature Ta to increase, and is the direct cause of global warming due to carbon dioxide.

Recall that greenhouse warming is given by:
(Ta - Te) / Te = {[(1 - Fr) / Ft]^.25 - 1}
The effect of adding ozone (O3) to the atmosphere is to decrease Ft. Hence adding ozone has the effect of increasing greenhouse warming. However, ozone is kept in a constant equilibrium maintained by solar radiation. This equilibrium is in balance provided that there are no catalysts present such as chlorine atoms that destroy ozone. The total amount of ozone present is so small that only the main absorption band has any practical effect. The other absorption lines are too weak.

The main ozone absorption band of appears to have rounded edges. There is a rounded edge in the range 963 cm^-1 to 996 cm^-1. There strong absorptions at 1029 cm^-1 and 1056 cm^-1. There is another rounded edge in the range 1067 cm^-1 to 1080 cm^-1.

Ozone is catalyticly destroyed by chloro-fluorocarbon gases in the atmosphere. Due to the earth's magnetic field holes in the ozone layer tend to occur near the earth's magnetic poles. These holes allow more solar UV energy to reach the ground, contributing to melting of polar ice.

Ozone is not evenly distributed through the atmosphere. Ozone is concentrated in the upper atmosphere where it is made by interaction of UV light with oxygen. Since the total amount of ozone in the atmosphere is very small, its weak broad band absorption lines have little effect on Ft.

METHANE AND NITROUS OXIDE:
Methane (CH4) has an absorption band in the range 1200 cm^-1 to 1400 cm^-1 (8.3 um to 7.14 um). Nitrous oxide (N2O) has a narrow absorption line at 600 cm^-1 (16.6 um) and an absorption band in the range 1200 cm^-1 to 1350 cm^-1 (8.3 um to 7.4 um).
The nitrous oxide absorption is coincident with the carbon dioxide and methane absorption bands, so when carbon dioxide and methane are both present the contribution of nitrous oxide is difficult to distinguish. Methane and nitrous oxide both have the potential to make a small contribution to greenhouse warming.