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The Physics of The Physics of Atmospheric Gas Atmospheric Gas MeasurementsMeasurements
2. 2. Atmospheric physics as Atmospheric physics as applied to data analysis applied to data analysis
algorithmsalgorithms
Kelly ChanceKelly ChanceHarvard-Smithsonian Center for AstrophysicsHarvard-Smithsonian Center for Astrophysics
The Horiba Jobin Yvon Company has an excellent website giving a tutorial on the optics of spectroscopy: http://www.horiba.com/us/en/scientific/products/optics-tutorial/
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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
More scattering:
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
Chance, K., T.P. Kurosu, and C.E. Sioris, Undersampling correction for array-detector based satellite spectrometers, Applied Optics 44, 1296-1304, 2005.
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.
Chance, K., T.P. Kurosu, and C.E. Sioris, Undersampling correction for array-detector based satellite spectrometers, Applied Optics 44, 1296-1304, 2005.
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