Abstract
High-resolution, laboratory, absorption spectra of the 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.
Highlights
Accurate remote sensing of greenhouse gases (GHGs) such as CO2, in the Earth’s atmosphere is important for studying the carbon cycle to better understand and predict climate change
Using cavity ring-down spectra measured in the lab, we have shown that the Voigt line shape is insufficient to model the line shape of O2 for the 1.27 μm band, consistent with the results of Hartmann et al (2013) and Lamouroux et al (2014)
This is partly due to the blending of spectral lines and the inability to retrieve the spectroscopic parameters for weak O2 transitions
Summary
Accurate remote sensing of greenhouse gases (GHGs) such as CO2, in the Earth’s atmosphere is important for studying the carbon cycle to better understand and predict climate change. Tran and Hartmann (2008) showed that including line mixing when calculating the O2 A-band absorption coefficients reduced the air mass dependence of the O2 column retrieved from TCCON spectra. The derived spectroscopic parameters for the speed-dependent Voigt line shape were used to calculate absorption coefficients when fitting high-resolution solar absorption spectra. 3, we describe the retrieval of spectroscopic parameters from three air-broadened cavity ring-down spectra fitted with a speed-dependent Voigt line shape. 4, the speed-dependent line shape along with the retrieved spectroscopic parameters is used to fit solar absorption spectra from four TCCON sites and retrieve total columns of O2, which is compared to O2 retrieved using a Voigt line shape.
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