The Physics of Atmospheric Gas Measurements The 2009

The Physics of Atmospheric Gas Measurements The 2009 Noble Lectures University of Toronto Kelly Chance Harvard-Smithsonian Center for Astrophysics

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Ring effect correction spectrum (a) Fraunhofer reference spectrum for the NO 2 fitting region; (b) Fraunhofer convolved to GOME spectral resolution; (c) = (b) convolved with rotational Raman cross-sections = Ring effect scattering source per molecule; (d) Highpass filtered version of (c) / (b) = DOAS “Ring effect correction. ”

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. 00030. 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

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 NO 2 window: 2. 98 pixel/FWHM OMI instrument transfer function for the NO 2 fitting region and the Nyquist-sampled and undersampled portions.

OMI instrument transfer function for the NO 2 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 Br. O measurements from the Global Ozone Monitoring Experiment. Geophys. Res. Lett. 25, 33353338, 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 El Er E Petty, 2004

GOME Br. O fitting: Relative contributions absorption by atmospheric Br. O (top) and the Ring effect - the inelastic, mostly rotational Raman, part of the Rayleigh scattering – (bottom).

Size Parameter x Determines Type of Scattering a = particle radius Petty, 2004