Spectroscopy for the Atmospheric Chemistry Experiment (ACE)

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Spectroscopy for the Atmospheric Chemistry Experiment (ACE)

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Spectroscopy for the Atmospheric Chemistry Experiment (ACE)

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Spectroscopy for the Atmospheric Chemistry Experiment (ACE)

Chris Boone, Kaley Walker, and Peter Bernath

HITRAN Meeting

June, 2010

- 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.

- 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.

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

- 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

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.

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.

- 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.

- 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.

Self-broadened spectra

- 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.

- 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.

H15NO3

HNO3

HNO3

HNO3

Residual spectra

full of missing

HNO3.

O2 continuum

N2 continuum

- 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.

No CH3OH in this region in HITRAN

ACE-FTS window for CH3OH retrievals

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?

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

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

- 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.