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Spectroscopy for the Atmospheric Chemistry Experiment (ACE)

Overview of the ACE satellite mission for remote sensing of the Earth's atmosphere, with a focus on Arctic ozone. Discusses the primary instrument ACE-FTS and the use of line mixing and speed-dependence in line shape calculations. Also highlights the challenges and improvements in spectroscopy for molecules like H2O, HNO3, CHF3, CH3Cl, and isotopologues 17O12C16O and 18O13C16O.

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Spectroscopy for the Atmospheric Chemistry Experiment (ACE)

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  1. Spectroscopy for the Atmospheric Chemistry Experiment (ACE) Chris Boone, Kaley Walker, and Peter Bernath HITRAN Meeting June, 2010

  2. Atmospheric Chemistry Experiment • Satellite mission for remote sensing of the Earth’s atmosphere, with a primary focus on Arctic ozone • Developed by the Canadian Space Agency • Launched August 2003, science operations began February 2004 • Operating well, no major problems yet. • Primary instrument ACE-FTS: 0.02 cm-1 resolution, 750-4400 cm-1, ~300:1 SNR.

  3. Line mixing (Voigt) • Rosenkranz first order line mixing (Voigt) • gV,LM is the Voigt function with line mixing, W(z) is the complex probability function, and Y is the line mixing parameter. In the absence of line mixing (Y = 0), only the K(x,y) term contributes to the line shape.

  4. Analytical expressions derived for L(x,y) using the Humlicek algorithm. A paper describing these expressions is about to be submitted to JQSRT.

  5. Line mixing (speed-dependent) • Some methane lines feature both line mixing and speed dependence. • Simple empirical extension of the first order Rosenkranz approximation for line mixing • Assume coupling coefficient Y has no speed dependence

  6. Line mixing plus speed-dependence (continued)

  7. For these lines, speed-dependence appears to be a stronger effect than line mixing. It is the opposite for other CH4 lines in the vicinity.

  8. With CH4 line mixing and speed-dependent Voigt parameters in place (derived from ACE-FTS spectra), we can now retrieve acetone from the ACE-FTS.

  9. Line shape benefits • Analytical, simple, and efficient. The most complicated is line mixing + SDV: requires real parts + imaginary parts of 2 Voigt-type functions. • Well-suited to line-by-line calculations. One extra parameter per line for speed-dependence (2) and one extra parameter per line for line mixing (Y). Extra parameters for temperature dependences? • Not aiming for the truest physical model or the most accurate calculation approach. Aiming for “accurate enough:” a significant improvement over the Voigt function, improved fitting residuals, improved VMRs • Geared toward atmospheric VMR retrievals.

  10. H2O • Obtained a set of 27 lab spectra from Manfred Birk at DLR (23 air-broadening), covering the range 1250-1750 cm-1. • Currently exclude 4 with poorer SNR but will include them in final analysis. • Awaiting a few higher-P measurements. • Analyzing spectra with a speed-dependent Voigt line shape, generating spectroscopic parameters.

  11. Self-broadened spectra

  12. Temperature-dependent Pressure-shift

  13. Difficult Doublets • Pairs of closely spaced H2O lines (same isotopologue, nearly the same E’’, etc.) can often be difficult to fit • Something else going on. Including line mixing improves results, but far from perfect.

  14. H2O in ACE-FTS • Speed-dependent Voigt parameters derived from gas cell measurements improve fitting residuals in ACE-FTS, but problems remain. • Deficiencies in the forward model for H2O in the troposphere. • Forward model employs a 1-km altitude grid. H2O VMR can double over the span of 1 km in the troposphere. • Changing the forward model.

  15. Missing HNO3 H15NO3 HNO3 HNO3 HNO3 Residual spectra full of missing HNO3. O2 continuum N2 continuum

  16. CHF3 • Fluorine budget in the stratosphere is an important measure of anthropogenic activity (unlike Chlorine, few natural sources). • No spectroscopic data available for the molecule. Found a set of lab measurements with various problems. • Used low-resolution measurements from PNNL for absolute calibration, and then Geoff Toon generated a set of pseudo-lines for the molecule.

  17. No CH3OH in this region in HITRAN ACE-FTS window for CH3OH retrievals

  18. Missing CH3Cl Red curve = CH3Cl calculated with HITRAN 2008 CH3Cl excluded from calculation Missing a lot of CH3Cl lines in HITRAN 2008. Looking at the program for this meeting, is this now fixed?

  19. Wish list: CH3OOH K.H. Becker et al, “Tunable diode laser measurements of CH3OOH cross-sections near 1320 cm-1”, Geophys Res Lett, 16, 1367-1370 (1989).

  20. 17O12C16O Is the isotopic differentiation really this large, or are there problems with the intensities of the isotopologue 4 lines?

  21. 18O13C16O

  22. Conclusions • Refining ACE-FTS line shape calculations to improve residuals (and thereby retrievals). • Continuing to search for weak absorbers. • Would especially like spectroscopy for the 3-micron region. • Generating spectroscopic parameters for H2O and CH4 from lab spectra.

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