CO 2 Lineshapes Near 2060nm Thinh Q. Bui California Institute of Technology ISMS 2014

Orbiting Carbon Observatory 2 OCO-2 aims to determine CO 2 concentrations to a precision of 1 ppm (0.3%) Satellite measurements will provide answers to important carbon cycle questions, such as: -What are the present CO 2 sinks? -Why are they so variable? -How will they respond to climate change? These satellite measurements will require the most accurate laboratory measurements of spectroscopic parameters.

Spectral Bands Recorded by OCO-2 Spectrometer "weak” "strong" O 2 A-BAND CO 2 CO 2 B-X (0,0) Band nm Provide constraints on surface pressure, optical path length, and cloud/aerosol distribution Most linear with respect to CO 2 column Provide constraints on cloud/aerosol distribution, water vapor column, and temperature profiles

Previous Works Non-Voigt line profiles are necessary to properly model CO 2 lineshapes (“weak” & “strong” regions) I.FTS measurements (Devi et al. (2007) J. Mol. Spec., Predoi-Cross et al. (2007, 2009) J. Mol. Spec.) II.Laser based measurements (Hikida et al. (2005, 2006) J. Mol. Spec., Joly et al. (2008) JQSRT) A. Casa et al. (2009) J. Chem. Phys measured R(12) in the (20012)  (00001) band. Retrieved an unphysical narrowing parameter B. Long et al. (2011) J. Chem. Phys. measured R(16) in the (30012)  (00001). Treating simultaneous Dicke narrowing and speed dependence gave the best fit residuals What line profile model is optimal for CO 2 strong band?

2-4m 2060nm Laser OI Servo Frequency Stabilized HeNe AOM -meter DDG Ringdown Cavity PZT R=99.99% Instrumentation L eff =23.5 km Empty-cavity Absorbing medium Long et al Chem. Phys. Lett., 536, 1-8

Experimental Conditions: ~90ppm CO 2 in synthetic air (no Ar) 7 kPa – 27 kPa ( Torr) total pressure Room temperature Measured Spectra Air-broadened CO 2 Isolated line regime within impact approximation (avoid line-mixing)

Line Profiles for Modeling Spectra Doppler Broadening (Gaussian) Collisional Broadening (Lorentzian) Dicke Narrowing -narrowing of Doppler profile -collisions change velocity, not quantum states - Soft (diffusional) and hard collisions Speed Dependence - narrowing of Lorentz profile, asymmetry (speed-dependent shifts) VOIGT PROFILE (VP) RAUTIAN-SOBELMAN (GALATRY (GP)/ NELKIN-GHATAK (NGP)) SD VOIGT (SDVP) SD NELKIN-GHATAK (SDNGP) Correlated effects Correlated effects – simultaneous velocity & phase changing collisions PARTIALLY- CORRELATED SD NELKIN-GHATAK (pcSDNGP)

Single Spectrum Fits - All parameters are fitted independently, ignoring correlations Black = without speed-dependent collisional shifts, Red = with speed-dependent collisional shifts Observation of simultaneous Dicke narrowing and speed-dependence effects!

opt = diff -  (  - i  ) = Re( opt )+iIm( opt ) Correlation between Dicke and speed-dependent narrowing mechanisms If Dicke narrowing was the only source of line narrowing, Re( opt ) would be linear with pressure Observed quadratic behavior To a good approximation, a w is independent of pressure Fitted a w has negative slope diffusional value observed diff  p)

Multi-spectrum fits - Since speed-dependence and Dicke narrowing parameters are competitive effects, multi-spectrum fits are necessary to remove correlation Fix Doppler widths calculated from measured laboratory temperature Lorentz halfwidths were constrained to be linear with pressure  (p) =  air p Collisional narrowing frequencies were constrained to be linear with pressure diff (p) =  p Residuals look reasonable, what about the fitted parameters?

Removing correlation between Dicke narrowing & speed- dependent by multi-spectrum fits Multi-spectrum fits provided physical results, but Re( opt ) << diff. Recall: opt = diff -  (  - i  ) = Re( opt )+iIm( opt ) First evidence for the correlations between velocity-changing and phase-changing collisions (v-p correlations) in CO 2 spectra! diff  p)

Recall: opt = diff -  (  - i  ) = Re( opt )+iIm( opt ) Fitted v-p correlation parameters (Im( opt ) and a s ) to determine value for the correlation parameter    R(24)0.116(4)0.22(6) R(30)0.1101(12)0.020(17)

Implications for OCO-2 and remote sensing Large systematic uncertainties (up to 5%) from spectroscopy are present in retrieved CO 2 concentration if improper line profiles are used!

Conclusions CO 2 strong band lineshapes display simultaneous Dicke narrowing and speed-dependence (Long et al. 2011), but additional velocity-phase changing correlations & asymmetry are observed at 2.06  m CO 2 pCSDNGP profile (partially correlated SDNGP) was ideal at this isolated line (low pressure) regime Neglecting narrowing effects introduces large deviations (5%) in the measured air-broadening parameter and 1% deviations in the integrated areas for CO 2 Multi-spectrum fitting is necessary to quantify and remove correlations

Milinda Rapusinghe, Daniel Hogan, Mitchio Okumura Caltech David Long, Vincent Sironneau NIST, Gaithersburg Agata Cygan, Roman Ciuryło, Daniel Lisak Nicolaus Copernicus University $$$ NASA & NESSF Fellowship Acknowledgements