Measurement of Thermal Infrared Radiation Emitted by the Atmosphere Using FTIR Spectroscopy By Narayan Adhikari Charles Woodman 5/11/2010 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 ( 0.7  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. Electromagnetic radiation spectrum 5/11/2010 PHY 360 GammaX-RayUVInfrared Microwaves Radio waves Wavelength (microns)

How do gases interact with IR radiation? Energy states of carbon dioxide molecule Energy states of water molecule  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 CO 2, H 2 O vapor, O 3, CH 4, CFCLs and N 2 O etc. absorb and re-emit IR radiation at different wavelengths. 5/11/2010PHY 360

Black body emission* Black body emission curves at terrestrial temperatures  Planck’s function:  Wien’s displacement law: wavelength (  m ) radiative flux ( W m -2  m -1 ) ( scaled by a factor of ) BB emission curves of the Sun and Earth 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 ). Important !  The Earth and the atmosphere are the major sources of thermal IR radiation. 5/11/2010PHY 360 *Adapted from G.W. Petty 2 nd edition

History of FTIR spectroscopy Earliest application of interference spectroscopy was performed by Ann H. Fizeau in 1862, who used Newton’s rings to study yellow sodium radiation. In 1902, Michelson invented the interferometer in order to study the wavelength of light with more precision. First true interferogram was published in 1911 by H. Rubens and R. W. Wood while studying IR radiation emitted by a Welsbach mantle. P.B. Feglett introduced the concept of FTIR spectroscopy in In 1961 J. Connes published what many consider the ‘definitive’ writing on FTIR spectroscopy. After that, FTIR spectroscopy became more popular, both for scientific research and commercial use. 5/11/20106

FTIR spectroscopy interferogram, I D Fourier transform 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. path difference  = x 1 - x 2 measured interferogram computed spectrum source detector movable mirror beam-splitter fixed mirror X2X2 X1X1 interferogram note: = 1/ (cm -1 ): wavenumber 5/11/2010 PHY 360

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 = (V 1 -V 2 )/(B 1 -B 2 ) a = [ V 1 (B 1 -B 2 ) - B 1 (V 1 -V 2 ) ]/(B 1 -B 2 )  Finally the calibrated target radiance is given by R( ) = [ (B 1 - B 2 ) V + V 1 B 2 - V 2 B 1 ] / (V 1 - V 2 ) FTIR spectrometer hot BB cold BB window mirror Thermistor probe 5/11/2010PHY 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) p2p2 p m surface 0 TsTs T1T1 T2T2 TmTm T top TOA p1p1 psps 5/11/2010PHY 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. 5/11/2010PHY 360 Cloudy Clear

Comparison between FTIR and weather balloon measurements  Brightness temperature (T b ):  For  = 1, T b  physical temperature (T)  For   1, T b  T.  The surface brightness temperature (  280 K) observed by FTIR is very close to that recorded by weather balloon.  The higher brightness temperature in H 2 O 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) 5/11/2010PHY 360

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. 5/11/2010PHY 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. 5/11/2010PHY 360

Thank You ! 5/11/201014