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