1. EXPERIMENTAL AND THEORETICAL STUDY OF WATER-VAPOR CONTINUUM ABSORPTION IN THE THZ REGION FROM 0.3 TO 2.7 THZ V.B. PODOBEDOV, D.F. PLUSQUELLIC, K.M. SIEGRIST AND G.T. FRASER National Institute of Standards and Technology, Gaithersburg, MD 20899. Q. MA NASA Goddard Institute for Space Studies, and Department of Applied Physics and Applied Mathematics, Columbia University, 2880 Broadway, New York, NY 10025. R.H. TIPPING Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL 35487.

2. Spectral Range

3. Continuum absorption Atmospheric studies: Global warming, climate modeling Remote sensing Wave propagations and models Transmission Submillimeter wave spectroscopy Astrophysics: Ground-based and airborne observations Atmospheric attenuation System noise temperature Phase delay

4. H 2 O - N 2 mixture absorption components The total absorbance, A, has two parts which are measured in the experiment as their sum, A(, T) = A L + A C, where the indexes L and C specify a local contribution from the discrete water vapor spectrum and continuum absorption, respectively. The continuum part, A C, may be described by the combination of three components A C (, T) = A self (P 2 H 2 O ) + A foreign (P N 2 P H 2 O ) + A foreign (P 2 N 2 ) In case of wet continuum, the third term will be significantly smaller than the remaining two. Detailed formula accounts for the absorption coefficients and temperature exponents for water vapor and foreign gas separately A C (, T) = [K self ( ) ∙ (300/T) n s + K for ( ) ∙ (300/T) n f ] 2 L, where K self ( ) and K for ( ) are the absorption coefficients, is a frequency and L is a pathlength. K self, n s, K for, n f

5.  (dB/km) = 10. Absorbance/km = 10 4. Absorbance/m Expected absorption at 0.25 % (lower spectrum) and 25 % relative atmospheric humidity at T = 296 K (HITRAN 2004) THz Spectrum Features Strong H 2 O vapor lines Intensity range >10 5 max / min ~10 SWR Pswr=1.1-1.3 , Window limited

6. Measurement of continuum absorbance Local Contribution: Intensity HITRAN04 Linewidth HITRAN04 x 1.11 Lineshape VVW Cut-off ( ± 0 ) 100 cm -1 A@ > 0 0 Experiment: Instrument FTS Resolution 0.03-0.12 cm -1 Pathlength 23.3 m Experiment Model Self Continuum H 2 O vapor at P=2.13 kPa, T=293 K

7. Far-IR Multipass Cell (core) In Out Pathlength: 2.4 – 48 m Input aperture: 60 mm Optics: F/2

8. Experimental absorption spectra P H 2 O =0.67 kPa, P N 2 = 70 kPa, T1=293 K, T2=333 K T stab. H2O in (P, T) stab. Spectrum 1 N2 in (P,T) stab. Spectrum 2 One T- set

9. Experimental continuum absorbance Self P = 2.13 kPa Foreign

10. Fitting the continuum data I. 5 (T) data points II. 8( ) x 5(T) data matrix

11. Continuum absorption coefficients, K self and K for, (dB/km)/(kPa. THz) 2 and temperature exponents, n s and n f, obtained from each transmittance window. Same parameters shown below the dashed line were determined from the complete set of 2 -fitted curves __________________________________________________________________________________________ Window center,K self n s K for n f (cm -1 ) Exp. Exp. Exp. Theory Exp. Theory __________________________________________________________________________________________ 22.53.94 5.40.1780.178 4.15.02 28.34.91 7.80.1780.1884.34.75 34.33.75 6.80.1690.1754.64.83 45.03.94 9.60.1880.1776.34.28 50.33.89 9.50.1860.1775.84.29 66.43.66 8.00.1820.1635.73.94 70.13.89 9.30.1830.1766.24.03 84.12.98 7.90.1720.1725.44.07 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 2 - fitted (Exp.) 3.83(300/T) 8.8 8.8 0.185(300/T) 5.7 - 5.7-

12. Comparison of the continuum absorption coefficients, K self and K for, (dB/km)/(kPa. THz) 2 and temperature exponents, n s and n f, from different studies Reference Rosenkranz A. Bauer J. Pardo CKD Present work Range, THz<0.8<0.35 0.35-1.1**) 0.3-2.7 Experiment Theory *) a)b)c) K self 7.88.9-9.5-4.183.834.00 - n s 7.57.8-8.5-6.98.88.5 - K for 0.2360.25-0.280.260.410.1850.231 0.163-0.188 n f 3.04.5-4.63.03.05.74.8 3.94-5.02 Notes on modeling parameters: *) Range of data for different parameters [6] **) Calculations at 50 cm -1 a) Basic set: cut-off = 100 cm -1, A=0 beyond the cut-off b) Same as in a) except cut-off = 25 cm -1, A=0 beyond the cut-off c) Same as in a)

13. CONCLUSIONS 1. The contributions to the absorbance resulting from the structureless H2O – H 2 O and H 2 O - N 2 continua have been measured in the temperature range from 293 to 333 K with spectral resolution of 0.04 to 0.12 cm -1. 2. Within experimental uncertainty, both the H 2 O - H 2 O and H 2 O – N 2 continua demonstrate nearly quadratic dependencies of absorbance on frequency with some deviation near the 2.5 THz window. 3. The absorption coefficients of 3.83 and 0.185 (dB/km)/(kPa. THz) 2 were measured for self- and foreign-gas continuum, respectively. The corresponding temperature exponents were found to be 8.8 and 5.7. 4. Absorption coefficients and temperature exponents were also found in each THz window.

14. ACKNOWLEDGMENTS This work was supported in part by the NASA Upper Atmospheric Research grant, NNH05AB21I. Two of the authors (Q. Ma and R. H. Tipping) acknowledge financial support from NASA under grants NNG06GB23G, and FCCS-547. Q. Ma wishes to acknowledge financial support from the Biological and Environmental Research Program (BER), U.S. Department of Energy, Interagency Agreement No. DE-AI02-93ER61744.

15. Far-wing cut-off H 2 O 2.13 kPa

16. Self-continuum Absorption H 2 O vapor at P=2.13 kPa

17. Foreign Continuum Absorbance H 2 O/N 2 : 1.43/78.5 0.67/70 kPa

18. Multipass cell efficiency