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The Physics of The Physics of Atmospheric Gas Atmospheric Gas MeasurementsMeasurements
The 2009 Noble LecturesThe 2009 Noble LecturesUniversity of TorontoUniversity of Toronto
Kelly ChanceKelly ChanceHarvard-Smithsonian Center for AstrophysicsHarvard-Smithsonian Center for Astrophysics
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x
x
(a) Fraunhofer reference spectrum for the NO2 fitting region; (b) Fraunhofer convolved to GOME spectral resolution; (c) = (b) convolved with rotational Raman cross-sections = Ring effect scattering source per molecule; (d) High-pass filtered version of (c) / (b) = DOAS “Ring effect correction.”
Ring effect correction spectrum
Top-of-atmosphere solar spectral irradiance
The high resolution solar spectral irradiance is critical in analyzing atmospheric trace gases:
• Solar lines are source of accurate wavelength calibration (0.0003-0.0004 nm for GOME!)• Determination of the Ring effect• Improved knowledge of instrument slit functions• Correction for spectral undersampling• Photochemistry of Schumann-Runge system
A space-based determination would be an ideal support mission for 12+ international atmospheric missions!
• Range: 240-1000+ nm• FWHM: 0.01 nm or better• Ideal FTS Space Shuttle experiment
High resolution solar reference spectrum
Griffiths and De Haseth, Fourier Transform Infrared Spectroscopy
1
2 max
1
( ) sinc[2 ( ) / ],m
i ii
S c
1
( ) ( ) sinc[2 ( ) / ].m
und i ii
S ILS c
Sampling Contributions to Instrument Line Shape (Slit Function)
The fully (Nyquist) sampled part is
where and max is the spatial sampling on the detector.
m runs over the detector pixels. The undersampled part is
OMI NO2 window:2.98 pixel/FWHM
OMI instrument transfer function for the NO2 fitting region and the Nyquist-sampled and
undersampled portions.
OMI instrument transfer function for the NO2 fitting region and the Nyquist-sampled and
undersampled portions for the hypothetical case where the slit function is sampled to twice the spatial frequency.
GOME
Undersampling corrections are made by:
1. Convolving the high resolution solar reference spectrum with the instrument line shape
2. Resampling this convolved high-resolution to the wavelength-shifted position (the shift between the radiance and irradiance) in two ways:
A. Properly sampled (using the entire available solar reference spectrum)
B. Undersampled (using a representation at the actual instrument sampling)
3. The difference between A and B is the undersampling correction.
Now used in GOME, GOME-2, SCIAMACHY, and OMI
Spectral Undersampling Correction
Chance, K. Analysis of BrO measurements from the Global Ozone Monitoring Experiment. Geophys. Res. Lett. 25, 3335-3338, 1998.
Slijkhuis, S., A. von Bargen, W. Thomas, and K. Chance, Calculation of undersampling correction spectra for DOAS spectral fitting, Proc. ESAMS'99 - European Symposium on Atmospheric Measurements from Space, 563-569, 1999.
Solar Reference Spectrum:
Chance, K.V., and R.J.D. Spurr, Ring effect studies: Rayleigh scattering, including molecular parameters for rotational Raman scattering, and the Fraunhofer spectrum, Applied Optics 36, 5224-5230, 1997.
The End!
Rayleigh Scattering Phase Function
Petty, 2004
ElEr E
GOME BrO fitting: Relative contributions absorption by atmospheric BrO (top) and the Ring effect - the inelastic, mostly rotational Raman, part of the Rayleigh scattering – (bottom).
Size Parameter x Determines Type of Scattering
Petty, 2004
a
x2
a = particle radius