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

Overview Electromagnetic radiation spectrum 1/18/2019 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

What is thermal infrared radiation? 1/18/2019 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

How do gases interact with IR radiation? 1/18/2019 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. 5/11/2010

Black body emission* Emission: conversion of internal 1/18/2019 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

FTIR spectroscopy FTIR: Fourier transform of infrared radiation. 1/18/2019 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() 5/11/2010

Calibration of FTIR spectrometer 1/18/2019 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) 5/11/2010

Infrared radiative transfer model (non- scattering atmosphere) 1/18/2019 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. 5/11/2010

Measurement of downwelling IR radiation with FTIR at UNR 1/18/2019 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. Cloudy Clear 5/11/2010

Comparison between FTIR and weather balloon measurements 1/18/2019 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) 5/11/2010

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

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

Thank You ! 5/11/2010