Measurement of Thermal Infrared Radiation Emitted by the

Atmosphere Using FTIR Spectroscopy

By Narayan Adhikari

Charles Woodman5/11/2010 PHY 360

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 PHY 360

What is thermal infrared radiation?

IR radiation: part of EM radiation ( 0.7m – 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.

Electromagnetic radiation spectrum

5/11/2010 PHY 360

Gamma X-Ray UV Infrared Microwaves Radio waves

Wavelength (microns)

How do gases interact with IR radiation?

Energy states of carbon dioxide molecule

Energy states of water molecule

...,2,1,0,2

1

fhEvib

...,2,1,0,8

)1(

2

12

22

l

I

hllIErot

h

EEf initialfinal

hfE

Energy of photon absorbed = difference in energy states

Photon energy:

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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Black body emission*

Black body emissioncurves at terrestrial temperatures

,]1)(exp[

2)(

5

2

Tkch

chTB

B

Planck’s function:

.max constT Wien’s displacement law:

wavelength (m)

rad

iativ

e

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

10

20

40

50

0

60

80

70

90 Sun

T = 5780 K

Earth

T = 288 K

Emission: conversion of internal energy into radiant energy

For a black body: a = 1, = 1

(scaled by a factor of 10-6). Important !

The Earth and the atmosphere are the major sources of thermal IR radiation.

5/11/2010 PHY 360*Adapted from G.W. Petty 2nd edition

FTIR spectroscopy

interferogram, ID

Fouriertransform

spectrum R()

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.

dvvRID

0

)2(cos)(

path difference

= x1 - x2

0

)2(cos dvIR D

measured interferogram

computed spectrum

source

detector

movable mirror

beam-splitterfixed mirror

X2

X1

interferogram

note: = 1/ (cm-1): wavenumber

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Calibration of FTIR spectrometer

Brass Cone

Black Paint

Circulation water in Circulation Water Out

5 cm

30cm

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)

FTIR spectrometer

hot BB

cold BB

window

mirror

Thermistor probe

5/11/2010 PHY 360

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

0

RTs

T1

T2

Tm

TtopTOA

p1

ps

1),(0

dpdp

ppdtpTBR

sp

sv

1

2 32

KT

hc

e

hcpTB

])()(/1exp[)],([exp),( p

p

ss

s

dppqpkgppppt

5/11/2010 PHY 360

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.

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 = 0.04 m and 1 m = 25 cm-1.

5/11/2010 PHY 360

Cloudy

Clear

Comparison between FTIR and weather balloon measurements

Rch

K

chTb 322

1ln

)(

Brightness temperature (Tb):

For = 1, Tb physical temperature (T)

For 1, Tb T.

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.

Surface temperature

brightness temperature of upper atmosphere

Weather balloon sounding at Reno, NV (May 02, 2010: 5.00 am)

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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.

5/11/2010 PHY 360

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.

5/11/2010 PHY 360

Thank You !

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