Solar & Infrared Spectra

Heating of earth by the sun and cooling by radiating to space are a function of the spectra of the following:  incoming sunlight, infrared radiated to space, and the gases in the atmosphere on both.  This page describes these spectra and their effects.

Also explained is why computation of transmission of earth’s infrared to space requires use of a packaged computer program.

A spectrum shows the distribution of specific properties as a function of wavelength or frequency.   Here we are discussing light (electromagnetic radiation).  The full light spectrum is given below:

spectrum

Figure 1

A nanometer (nm), also called a millimicron (mμ) is one billionth of a meter.  Spectra are also presented by frequency, which is usually in units of wavenumbers (cm-1).  Frequency increases as wavelength decreases.  A frequency of 10,000 wavenumbers corresponds to 1 micron or 1,000 mμ wavelength.  To get frequency In microns, divide wavenumber into 10,000.

BLACKBODIES

A blackbody absorbs and emits all the light it can at a specific temperature and wavelength.   A perfect blackbody has an absorptivity of 1 and an emissivity of 1.   Minimum value for each is 0.   The spectrum of the sun at its surface is close to that of a blackbody of 5700 C.  Heat leaving the earth’s surface has a spectrum corresponding to an average blackbody temperature of 15C.   Below is a spectrum of solar light incident at the top of our atmosphere (yellow) and received by the earth’s surface (red).  The visible part of the spectrum is bound by the two horizontal dotted lines.  The difference in the two curves is the effect of the atmosphere:

800px-Solar_spectrum_ita_svg

Figure 2: Solar Spectra

It is seen that the atmosphere absorbs some of the suns light (converting to heat) due to scattering and various gases: O2, water vapor, and CO2.  These gases, and others, also absorb some of the heat radiated from the earth, contributing to the greenhouse effect.   The spectrum below shows the effects of various gases to the greenhouse effect.

 

Absorption Spectra of Atmospheric Gases and Emission Spectra of Earth and Sun

Figure 3 Absorption Spectra of Atmospheric Gases and Emission Spectra of Earth and Sun

Notice in the green curve (CO2) in Figure 3 above.  A value of 100% means that all the light at the given wavelength is absorbed.  The major absorption band at about 15 microns shows that the effect of CO2 in that band is maximized.  The only CO2 bands that can effect of greenhouse effect are those small bands to the left of the large band.   Notice also that water vapor (blue curve) absorbs more of the earths radiation than CO2.

BEER’S LAW AND ABSORPTION SPECTRA

The amount of incident light transmitted by a layer of a substance (T) at a given wavelength (λ) is exponentially dependent upon the path the light travels (l) and the concentration of each component (c) of the substance.  Mathematically, Beer’s Law is expressed as follows:

Tλ = exp-(l * Σac)

Where ais the absorption strength of component i at wavelength λ

In the case of the atmosphere, the path L is constant, so transmission of infrared light it is a function of concentration only.   In the case of a gas, the concentration is proportional to the partial pressure (p) of the gas.  So the for the atmosphere, Beer’s Law can be expressed as

Tλ = exp-(Σap)

WHY COMPUTER COMPUTATION OF ATMOSPHERIC TRANSMISSION IS NECESSARY (ModTrantm is used herein).

The partial pressure does decrease with altitude, so computation of Tλ is very difficult.  Compounded with that is the absorption spectrum of gaseous CO2, typical of polyatomic gases, which looks like the following:

CO2 spectrum

Figure 4 Absorption Spectrum of CO2 Gas, 13-15 micron band

 

 

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