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(8) Absorption – Visible and IR

Physics of the Atmosphere II

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Absorption – visible and near-IR

Molecular Oxygen – O2

Electronic ground state transitions of O2 coupled with vibrational–rotational transitions produce weak absorption lines in the near-IR and visible – known as infrared bands and red bands. The most important red bands are the A band centered at 0.762 μm, the B band at 0.688 μm, and the γ band at 0.628 μm.

Even though these absorptions are weak, they are important, since they happen near the peak of the solar spectrum.

The oxygen A band is often used in remote sensing.

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Water Vapor – H2O

has a bent triatomic configuration (with the oxygen atom in the middle) and a bond angle of 104.45°. The distance between the oxygen and hydrogen atoms is 0.958 Å (~ 0.1 nm)

H2O has three fundamental vibration modes. The bending vibration, ν2, has the lowest wave number, ν1 and ν3 have wave numbers about twice that of ν2. The ν2 fundamental band of H2O is centered at 6.25 μm and is important in thermal infrared radiation transfer and remote sensing. The ν1 and ν3 fundamentals of H2O (caused by symmetric and asymmetric stretching) produce bands centered at 3657.05 cm−1 (2.74 μm) and 3755.93 cm−1 (2.66 μm). Together they form a strong band in the solar spectrum, known as 2.7 μm band.

Water vapor is the most important absorber of solar near-IR in the lower Troposphere.

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Water Vapor absorption (credit: B. Stevens & S. Bony). The rotational-vibrational bands have overtones – even in the visible (red) – which are responsible for the (often) blue color of the ocean.

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Carbon Dioxide – CO2

The CO2 molecule has a linear symmetrical configuration, with the carbon atom in the middle and an oxygen atom on each side. The length of theC–O bond is 1.1632 Å. As a result of its symmetry, the CO2 molecule has no permanent dipole moment and no permitted rotation band.

The symmetrical stretch mode, ν1, is radiatively inactive at its fundamental.

The (degenerate) bending mode, ν2, produces the most important band in the thermal infrared: the 15 μm CO2 band (later).

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Absorption – Thermal Infrared

Water Vapor – H2O

We have already heard about the ν2 fundamental band of H2O at 6.25 μm.

The region from 800 to 1200 cm−1, the thermal infrared window, contains a water vapor continuum around 1000 cm−1 (10 μm), which is still not well understood. It may results from the accumulated absorption of the distant wings of water vapor lines, principally in the far-infrared part of the spectrum. This absorption is caused by the collision broadening between absorbing molecules (H2O–H2O) and between absorbing and nonabsorbing molecules (H2O–N2). But here is also some evidence that contributions to continuous absorption may be caused by water dimer [(H2O·H2O)].

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Carbon Dioxide – CO2

Besides the famous ν2 fundamental band at 15 μm, numerous combination bands have been detected. Simultaneous transitions in two of the vibration modes are possible, resulting in weak combination (or difference) frequencies.

There are also numerous hot bands in the 15 μm CO2 band. They are produced by transitions between excited levels and are significant in cooling-rate calculations in the middle atmosphere.

There are also several overtone and combination bands. Absorption near 5 μm is cuased by the 3·ν2 band at 5.2 μm and several combination bands at 4.8 μm.

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Ozone – O3

The ozone molecule has an asymmetric top configuration similar to water vapor. The ν1 and ν3 fundamental vibration modes are centered at 1110 and 1043 cm−1 and constitute the 9.6 μm ozone band.

The ν2 fundamental band, centered at 705 cm−1 (14.27 μm), is masked by the strong CO2 15 μm band. There is also a relatively strong band at 4.75 μm, produced by overtone and combination transitions.

Ben Mills

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Methane – CH4

The CH4 molecule has a spherical top configuration. It has no permanentelectric dipole moment and, hence, no pure rotational spectrum.

There are four fundamental vibration modes. Of these, only ν3 and ν4, centered at 3020.3 and 1306.2 cm−1, are active in the IR. The ν4 fundamental band (7.7 µm) is important in the climatic greenhouse effect. The ν1 and ν2 fundamental bands are inactive.

Methane has also a rich spectrum of overtone and combination bands.

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Nitrous Oxide – N2O

N2O – known as “laughing gas” – has a linear and asymmetric structure, with the configuration NNO. Similar to CO2, it has a single rotational constant and a detectable rotational spectrum. Numerous bands produced by the fundamental, overtone, and combination frequencies exist in the IR. The three fundamental frequencies are centered at 1285.6 cm−1(ν1), 588.8 cm−1 (ν2), and 2223.5 cm−1(ν3). Ben Mills

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Chlorofluorocarbons

Methyl chloride (CH3Cl): ν3 band at 732 cm−1, ν2 band at 1350 cm−1.Dichlorodifluoromethane (CF2Cl2): ν8 region, centered at 1161 cm−1, band at 1095 cm−1. Trichlorofluoromethane (CFCl3): ν1 and ν4 fundamental transitions are active and centered at 848 and 1085 cm−1.Methylchloroform (CH3CCl3): ν2 fundamental band at 1348.5 cm−1.Carbon tetrachloride (CCl4): active ν3 band near 796 cm−1.

All are located in the window region – potential increase may lead to significant greenhouse effect.

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K. N. Liou

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Transmitted Radiation

R.A. Rhode

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