Using a speed-dependent Voigt line shape to retrieve Osub2/sub from Total Carbon Column Observing Network solar spectra to improve measurements of XCOsub2/sub

摘要

High-resolution, laboratory, absorption spectra of the a 1 Δ g ← X 3 Σ g - oxygen ( Osub2/sub ) band measured using cavity ring-down spectroscopy were fitted using the Voigt and speed-dependent Voigt line shapes. We found that the speed-dependent Voigt line shape was better able to model the measured absorption coefficients than the Voigt line shape. We used these line shape models to calculate absorption coefficients to retrieve atmospheric total columns abundances of Osub2/sub from ground-based spectra from four Fourier transform spectrometers that are a part of the Total Carbon Column Observing Network (TCCON). Lower Osub2/sub total columns were retrieved with the speed-dependent Voigt line shape, and the difference between the total columns retrieved using the Voigt and speed-dependent Voigt line shapes increased as a function of solar zenith angle. Previous work has shown that carbon dioxide ( COsub2/sub ) total columns are better retrieved using a speed-dependent Voigt line shape with line mixing. The column-averaged dry-air mole fraction of COsub2/sub ( XCOsub2/sub ) was calculated using the ratio between the columns of COsub2/sub and Osub2/sub retrieved (from the same spectra) with both line shapes from measurements taken over a 1-year period at the four sites. The inclusion of speed dependence in the Osub2/sub retrievals significantly reduces the air mass dependence of XCOsub2/sub , and the bias between the TCCON measurements and calibrated integrated aircraft profile measurements was reduced from 1?% to 0.4?%. These results suggest that speed dependence should be included in the forward model when fitting near-infrared COsub2/sub and Osub2/sub spectra to improve the accuracy of XCOsub2/sub measurements.