1. By Narayan Adhikari Charles Woodman
1/18/2019
Measurement of Thermal Infrared Radiation Emitted by the Atmosphere Using FTIR Spectroscopy By
Narayan Adhikari
Charles Woodman
5/11/2010

2. Overview Electromagnetic radiation spectrum
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Overview
Electromagnetic radiation spectrum
Interaction of gases with IR radiation
Black body emission
FTIR spectroscopy
FTIR measurements at UNR
Conclusions
Future work
5/11/2010

3. What is thermal infrared radiation?
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What is thermal infrared radiation?
Gamma
X-Ray UV
Infrared Microwaves Radio waves
Wavelength (microns)
Electromagnetic radiation
spectrum
 IR radiation: part of EM radiation ( 0.7m – 1 mm)
 Thermal IR band: 4 – 50 m
 Approx. 99% of the radiation emitted by the Earth and its atmosphere lies in thermal IR band.
5/11/2010

4. How do gases interact with IR radiation?
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How do gases interact with IR radiation?
Energy of photon absorbed = difference
in energy states
 Photon energy:
Energy states of carbon dioxide molecule
Energy states of water molecule
 Vibrational and rotational transitions are associated with weak energy
corresponding to IR and microwave radiation.
 Green-house gases like CO2, H2O vapor, O3, CH4, CFCLs and N2O etc. absorb
and re-emit IR radiation at different wavelengths.
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5. Black body emission* Emission: conversion of internal
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Emission: conversion of internal
energy into radiant energy
Black body emission
curves at terrestrial temperatures
 For a black body: a = 1,  = 1
 Planck’s function:
 Wien’s displacement law:
wavelength (m)
radiative flux ( W m-2 m-1)
( scaled by a factor of 10-6 )
BB emission curves of the Sun and Earth
0.1
0.2
0.4 1 2 4 10 20 50
100 30 40 60 80 70 90
Sun
T = 5780 K
Earth
T = 288 K
(scaled by a factor of 10-6).
Important !
 The Earth and the atmosphere are the major sources
of thermal IR radiation.
5/11/2010
*Adapted from G.W. Petty 2nd edition

6. FTIR spectroscopy FTIR: Fourier transform of infrared radiation.
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FTIR spectroscopy
FTIR: Fourier transform of infrared radiation.
It measures the intensity of the IR radiation emitted by a source.
 It consists of: (a) Michelson interferometer and (b) computer for Fourier transform.
source
detector
movable mirror
beam-splitter
fixed mirror X2 X1
interferogram
path difference
 = x1 - x2
measured interferogram
computed spectrum
note:  = 1/ (cm-1): wavenumber
interferogram, ID
Fourier
transform
spectrum
R()
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7. Calibration of FTIR spectrometer
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Calibration of FTIR spectrometer
hot BB
window
Brass Cone
Black Paint
Circulation water in
Circulation Water Out
5 cm
30cm
mirror
cold BB
Thermistor probe
FTIR spectrometer
Assumed linear model for spectral response:
V() = a() + b () R()
▪ V(): detector voltage
▪ R(): target radiance
▪ R() = B() for perfect black body at temperature T
▪ a() and b() are calibration factors.
With the measurements of cold and hot black
bodies, we obtain a and b as follows:
b = (V1-V2)/(B1-B2)
a = [ V1(B1-B2) - B1(V1-V2) ]/(B1-B2)
Finally the calibrated target radiance is given by
R() = [ (B1 - B2) V + V1B2 - V2B1 ] / (V1 - V2)
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8. Infrared radiative transfer model (non- scattering atmosphere)
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Infrared radiative transfer model (non- scattering atmosphere)
 Radiant intensity at  reaching the sensor at ground is:
where
: Planck’s emission function
(transmittance at )
 K: absorption coefficient of an absorbing gas
 q(p): mixing ratio of the absorbing gas (g/Kg) p2 pm
surface Ts T1 T2 Tm
Ttop
TOA p1 ps
The IR radiation emitted from each layer of the atmosphere suffers partial absorption
and transmission in the lower layers of the atmosphere before reaching the ground.
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9. Measurement of downwelling IR radiation with FTIR at UNR
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Measurement of downwelling IR radiation with FTIR at UNR
Cloudy sky, 01 Apr., 2010 ( 10 am)
Clear sky, 06 Apr., 2010 ( 10 am)
The atmosphere seems to be opaque at
the strong IR absorption bands & FTIR
records the emission from the
atmosphere right by it i.e. the surface.
Atmospheric ‘dirty’ window region for IR
radiation: 800 – 1300 cm-1
The atmosphere is more transparent at
this region and FTIR records emission
from the higher atmosphere.
Cloudiness affects radiance throughout
IR band.
note: 1 cm-1 = m and 1 m = 25 cm-1.
Cloudy
Clear
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10. Comparison between FTIR and weather balloon measurements
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Comparison between FTIR and weather balloon measurements
Brightness temperature (Tb):
 For  = 1, Tb  physical temperature (T)
 For   1, Tb  T.
Surface temperature
brightness temperature
of upper atmosphere
The surface brightness temperature (280 K) observed by FTIR
is very close to that recorded by weather balloon.
The higher brightness temperature in H2O vapor absorption band
infers the abundance of water vapor near the surface as well as the
slight temperature inversion effect.
Weather balloon sounding at Reno, NV
(May 02, 2010: 5.00 am)
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11. Conclusions
A good agreement between FTIR and weather balloon soundings suggests the accuracy of the FTIR.
Since the normal frequency of weather balloon launches is 12h, the FTIR provides much better temporal resolution of the atmospheric features than the weather balloon does.
FTIR sounding data together with satellite sounding data can yield entire tropospheric vertical profiles of temperature and water vapor.
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12. Future work
 With FTIR measurements, we can  retrieve the temperature and humidity profile of the atmospheric boundary layer (ABL).  frequently update the primary meteorological parameters of Reno which will be helpful to: - monitor the air quality by estimating potential air pollution dilution in Reno. - predict daily weather of Reno. - study the diurnal and seasonal variation of air quality in Reno.
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13. Thank You !
5/11/2010