1. IMPACT OF ATMOSPHERIC CLUTTER ON DOPPLER-LIMITED GAS SENSORS IN THE SUBMILLIMETER/TERAHERTZ IVAN R. MEDVEDEV, CHRISTOPHER F. NEESE, FRANK C. DE LUCIA, GRANT M. PLUMMER

2. Objectives We assess the effects of atmospheric spectral ‘clutter’ on the performance of high resolution THz chemical point sensors Approaches We simulated transmission spectra of clean and polluted atmosphere and compare it with the spectra of several analytes. Spectra were simulated using synthetic spectra based on catalogs HITRAN, JPL, CDMS as well as experimentally measured spectra of individual species. APPLIED OPTICS / Vol. 50, No. 18 / 20 June 2011

3. A 210-270 GHz SMM/THz gas sensor based on CW electronic and frequency multiplication technologies RB07 – “Sensors across the spectrum” Christopher Neese Optics Letters, 35(10), 1533-1535 (2010)

4. In the worst case scenario non-methane hydrocarbons spectra at 1ppm will fill all of 10 5 resolution elements of the sensor, effectively altering the baseline A large fraction of non-methane hydrocarbons have zero or small dipole moments and have dense spectra due to smaller rotational constants and spectra of low lying vibrational states. PAN is unstable and most likely will not survive injection into the sensor, but if it did the spectral ‘dilution’ argument would apply

5. NameFormulaType Cyanogen Chloride ClCNLinear Carbonyl Sulfide OCSLinear AcetonitrileCH 3 CNSymmetric Top AcroleinC 2 H 3 CHOAsymmetric Rotor AcrylonitrileC 2 H 3 CNAsymmetric Rotor Ethylene Oxide C2H4OC2H4OAsymmetric Rotor Analytes Spectral Completeness Most of the species used here (H 2 O, SO 2, CO, N 2 O, NO, NO 2, O 3, NH 3, HCOOH, HNO 2, HNO 3, and H 2 CO) have relatively sparse spectra in terms of the number of Doppler-limited resolution elements occupied. The catalogue data used to simulate the spectra are adequately complete. For example catalogue spectrum of HNO 3,with one of the denser spectral patterns, contains analysis of vibrational states up to 9 kT (factor of ~5000 below GS). Six analytes were chosen, that are representative of several spectral types, varying in line density and strengths, and appear on lists of toxic industrial chemicals (TICs)

6. Spectral Simulations Spectra of H 2 O, SO 2, CO, N 2 O, NO, NO 2, O 3, NH 3, HCOOH, and H 2 CO were simulated from Hitran Database Spectra of HNO 2, HNO 3 were simulated based on JPL Database Spectra of 6 analytes were simulated based on experimental data

7. Clean vs Polluted Troposphere

8. Distribution of peak absorption coefficients for six gases at 1 ppt concentration, as well as for clean and polluted atmospheres.

9. 1 ppb of OCS (black) in the clean troposphere (red) 1 ppb of CH 3 CN (black) in the clean troposphere (red) 1 ppb of acrylonitrile (black) in a clean troposphere (red) Clean troposphere clutter

10. Polluted troposphere clutter 1 ppt of acrylonitrile (black) in a region of maximum clutter in the polluted troposphere (red). 1 ppt of Acrylonitrile (black) in a region of less clutter in the polluted troposphere (red). 1 ppb of ClCN (green), OCS (black), and CH 3 CN (blue) in a polluted troposphere (red)

11. Effect of Pressure-Broadening Doppler limited 10 mtorr, 10 MHz/torr 1ppt of acrylonitrile (black) in the polluted troposphere

12. Conclusions The much smaller Doppler widths in the SMM/THz combined with the fortuitous spectral characteristics of the major (H 2 O and CO 2 ) and many of the minor atmospheric constituents significantly reduce the clutter limit for point sensors in this spectra region relative to that of the Op/IR. In absolute terms the clutter limit ranges from << 1 ppt for favorable target species in the clean troposphere to ~1 ppt for less favorable species in a standard polluted atmosphere Because this limit is so low, the real challenges for Doppler-limited SMM/THz sensors will not be atmospheric clutter, but rather sensitivity and the potential for interference among large molecules in the analytical mixture.