Atmospheric IR Absorption
Representations of atmospheric infrared (ir) light absorption and transmission in atmospheric, climate,
and environmental science are incorrect, because they completely ignore the
composition dependence of measured component ir absorption spectra, which
results in claimed atmospheric trace gas absorption that is orders of
magnitude greater than it actually is. Proper adjustment indicates
that CO2 and CH4 absorbance values in literature ir spectra are too
high by a factor of about 800 and 147,000, respectively, based on the NIST
component ir spectra in our national database.
The Beer-Lambert or Beer's law describes the change in light intensity I of of
unidirectional (x-direction) monochromatic (single wavelength) radiation passed through a path
length p in a homogeneous single-phase sample of a single light-absorbing
species, as measured in a spectrometer, as
A = ε p C,
where absorbance A is defined as log (I0/I)
= log(1/T), Io is the light intensity of wavelength λ into
the sample and I is the reduced intensity after passing through path length
p, ε(λ) is the molar absorption (or extinction) coefficient which is a
function of wavelength (or wavenumber), pressure and temperature for each component, and
C is the single absorbing
component concentration, or molar density, in moles/volume.
Transmittance T is defined as the fraction of light transmitted, or I/I0
= 10-A. The equation is simply an exponential decay
function of light intensity with distance at fixed temperature and pressure,
with an exponent equal to the negative of the extinction coefficient times
the path length times concentration. Absorbance is also proportional
to component mole fraction, since mole fraction and concentration differ by
a fixed factor for an ideal gas. For multicomponent mixtures,
component Transmittance values are additive (Absorbance is not).
Compositions of atmospheric trace gases are made at a high sample
concentration Cs in absorbance measurement in order to see the full
amplitude of the spectrum, and since at the very low atmospheric
concentration Ca, absorbance is virtually immeasurable. So according
to Beer's law, measured absorbance of each component must be scaled by the
ratio Ca/Cs to obtain the true atmospheric absorption at Ca. But this
has always been completely ignored by atmospheric and climate scientists,
who use the the unadjusted measured component spectra at Cs to compute
mixture transmittance at some set of atmospheric conditions.
The National Institute for Standards of Standards and Technology (NIST) is
our national storehouse of chemical and physical measurements including
infrared light absorption. The measured
NIST methane (CH4) ir absorbance spectrum1
is shown below, and is measured at a stated composition of 25% methane (75% nitrogen), or 0.25 mole
fraction, and it has an atmospheric mole fraction of 0.0000017. So
according to Beer's law, it must be scaled by the factor Ca/Cs = 0.0000068,
making it disappear on that scale, and negligible. Components with 0
composition obviously have 0 effect on anything! But climate science
says that methane is a significant contributor to global warming. So
claimed CH4 ir absorbance in the literature must be scaled down by a factor
of about 147,000!
For example, from the below NIST methane spectrum measured at a mole
fraction of 0.25, peak absorbance is about 1.6 at a wavenumber of about 1300
cm-1. So absorbance at 0.0000017 atmospheric mole fraction is equal to
1.6(0.0000068) = 0.00001088 = log(1/T), so transmittance is equal to 10-0.00001088
= 0.99997495, which means that only 0.002505% is absorbed over the 5 cm path
length at atmospheric concentration. Transmittance of that wavelength
claimed based on the unadjusted spectrum is 10-1.6
= 0.025, meaning 97.5% is absorbed over the path length, under-represented
in the literature by a factor of about 39,000. So the presence of
methane in our atmosphere cannot even be detected by infrared spectrography
and it can't possibly appear in any atmospheric ir absorption or emission
spectrum.
The measured
NIST CO2 absorbance spectrum, also shown below, is measured at a stated
mole fraction of 33.33% or 0.3333, while the atmospheric mole fraction is
0.04%, or 0.0004. So the measured CO2 absorbance must be scaled by the
factor 0.0004/0.3333 = 0.0012, or about 1/800.
The measured
NIST water vapor absorbance spectrum, also shown below, is measured at
an undisclosed water composition, counter to NIST standards, but can be
assumed to be saturated water vapor at lab conditions, since any other
mixture would be relatively very difficult to accurately measure and create.
Water vapor composition in air or nitrogen at lab conditions (75 deg. F, 1
atm) is about 2%, or 0.02 mole fraction, which is close to an average
atmospheric value in the troposphere. Where water is significantly
present, all other trace gas effects are completely negligible with respect
to infrared absorption, transmission, and emission. because of its much
greater concentration and its wider absorption.
One can find hundreds of incorrect atmospheric infrared spectra by doing an
image search for "atmospheric ir absorption". All showing any effects
of water plus CO2, CH4, or O3 on the same scale, or even just CO2 and CH4,
are completely wrong. All represent CO2 and CH4 and all trace gas
absorbance spectra at their measured compositions, although those are never
stated or discussed and are completely ignored. Some such
examples of calculated and even claimed measured atmospheric spectra are
shown below the NIST component spectra.
In order to calculate infrared transmittance through the
atmosphere, in addition to composition, temperature and pressure versus
altitude we would also need to know extinction coefficients for all
components εi(λ,P,T). Measurements at other than lab
conditions are difficult and not available. And composition and temperature
profiles are highly variable. But we can ask the question, what would
the transmittance be over much larger path lengths for the peak component
wave numbers using assumed fixed compositions and the known lab condition
extinction coefficients εi(λ,1 atm, 75 deg. F).
Transmittance close to the surface can be well-approximated.

Owner |
COBLENTZ SOCIETY
Collection (C) 2018 copyright by the U.S. Secretary of Commerce
on behalf of the United States of America. All rights reserved. |
Origin |
DOW CHEMICAL
COMPANY |
Source reference |
COBLENTZ NO. 8873 |
Date |
1964 |
State |
GAS (150 mmHg
DILUTED TO A TOTAL PRESSURE OF 600 mmHg WITH N2) |
Instrument |
DOW KBr FOREPRISM |
Instrument parameters |
GRATING CHANGED
AT 5.0, 7.5, 15.0 MICRON |
Path
length |
5 CM |
Resolution |
4 |
Sampling procedure |
TRANSMISSION |
Data
processing |
DIGITIZED BY NIST
FROM HARD COPY (FROM TWO SEGMENTS) |

Owner |
COBLENTZ SOCIETY
Collection (C) 2018 copyright by the U.S. Secretary of Commerce
on behalf of the United States of America. All rights reserved. |
Origin |
DOW CHEMICAL
COMPANY |
Source reference |
COBLENTZ NO. 8753 |
Date |
1964 |
Name(s) |
dioxomethane |
State |
GAS (200 mmHg
DILUTED TO A TOTAL PRESSURE OF 600 mmHg WITH N2) |
Instrument |
DOW KBr FOREPRISM |
Instrument parameters |
GRATING CHANGED
AT 5.0, 7.5, 15.0 MICRON |
Path
length |
10 CM |
Resolution |
4 |
Sampling procedure |
TRANSMISSION |
Data
processing |
DIGITIZED BY NIST
FROM HARD COPY (FROM TWO SEGMENTS) |

Owner |
NIST Standard
Reference Data Program
Collection (C) 2018 copyright by the U.S. Secretary of Commerce
on behalf of the United States of America. All rights reserved. |
Origin |
Sadtler Research
Labs Under US-EPA Contract |
State |
gas |
|