Using a speed-dependent Voigt line shape to retrieve O<sub>2</sub> from Total Carbon Column Observing Network solar spectra to improve measurements of XCO<sub>2</sub>

Abstract

High-resolution, laboratory, absorption spectra of the a1Δg←X3∑g− oxygen (O2) 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 O2 from ground-based spectra from four Fourier transform spectrometers that are apart of the Total Carbon Column Observing Network (TCCON) Lower O2 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 (CO2) total columns are better retrieved using a speed-dependent Voigt line shape with line mixing. The column-averaged dry-air mole fraction of CO2 (XCO2) was calculated using the ratio between the columns of CO2 and O2 retrieved (from the same spectra) with both line shapes from measurements made over a one-year period at the four sites. The inclusion of speed dependence in the O2 retrievals significantly reduces the airmass dependence of XCO2 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 CO2 and O2 spectra to improve the accuracy of XCO2 measurements.

Overview: Estimating and reporting uncertainties in remotely sensed atmospheric composition and temperature

“…Elster et al (2013) and European Centre for Mathematics and Statistics in Metrology (2019) critically discuss the applicability of the GUM concept to inverse problems. Conversely, QA4EO task team (2010), Merchant et al (2017), and Povey and Grainger (2015), e.g., largely endorse the GUM-based uncertainty concept. The latter authors, however, state that the GUM conventions " [...] apply equally to satellite remote sensing data but represent an impractical ideal that does not help an analyst fully represent their understanding of the uncertainty in their data.…”

“…Line mixing is a high-pressure phenomenon where collisions transfer angular momentum, entailing energy transfer between energetically adjacent transitions (e.g., Armstrong, 1982;Bulanin et al, 1984;Strow and Gentry, 1986;Hartmann and Boulet, 1991;Hartmann et al, 2009;Thompson et al, 2012;Alvarado et al, 2013). The usual line-shape models, such as the Voigt line shape (Voigt, 1912), may not adequately repre-sent the true line shape (e.g., Galatry, 1961;Berman, 1972;Pickett, 1980;Thompson et al, 2012;Long and Hodges, 2012;Mendonca et al, 2019). Issues related to Zeeman coefficients, most relevant in microwave spectroscopy of mesospheric oxygen, are discussed in Larsson et al (2019).…”

Bias Correction of the Ratio of Total Column CH4 to CO2 Retrieved from GOSAT Spectra

“…The airmass dependence of retrieved XCH 4 and XCO 2 has been corrected empirically for satellite data [37,39] and TCCON data [69,70]. Recently, Mendonca et al [71] found that using speed-dependent Voigt line shapes for retrieval of the O 2 total column reduces the airmass dependence of TCCON XCO 2 . ∆ ratio significantly increases with ∆P srf , although the number of data with large ∆P srf is small.…”

Estimating and Reporting Uncertainties in Remotely Sensed Atmospheric Composition and Temperature

“…Line mixing is a high pressure phenomenon where collisions transfer angular momentum, entailing energy transfer between energetically adjacent transitions (e.g., Armstrong, 1982;Bulanin et al, 1984;Strow and Gentry, 1986;Hartmann and Boulet, 1991;Hartmann et al, 2009;Thompson et al, 2012;Alvarado et al, 2013). The usual line-shape models, such as the Voigt lineshape (Voigt, 1912), may not adequately represent the true line shape (e.g., Galatry, 1961;Berman, 1972;Pickett, 1980;Thompson et al, 2012;Long and Hodges, 2012;Mendonca et al, 2019). Issues related to Zeeman coefficients, most relevant 5 in microwave spectroscopy of mesospheric oxygen, are discussed in Larsson et al (2019).…”