Improving the Retrieval of XCO2 from Total Carbon Column Network Solar Spectra
Joseph Mendonca, Kimberly Strong, Debra Wunch, Geoffrey Toon, David Long, Joseph T. Hodges, Vincent T. Sironneau
High-resolution absorption spectra of the a^1 ∆_g←X^3 Σ_g^- 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. Total columns of O2 were retrieved from ground-based high-resolution absorption spectra from four Total Carbon Column Observing Network (TCCON) sites using both Voigt and speed-dependent Voigt line shapes to calculate absorption coefficients. We found that less O2 was retrieved with the speed-dependent Voigt line shape, with the difference increasing as a function of solar zenith angle. CO2 total columns from the same spectra were also retrieved using a Voigt line shape and speed-dependent Voigt with line mixing. The column-averaged dry-air mole fraction of CO2 was calculated using the CO2 and O2 columns retrieved with both line shapes from a year of measurements at the four sites and compared. We find that the inclusion of speed-dependence, reduces the airmass dependence of XCO2. The TCCON empirical airmass correction factor for XCO2 derived from a year of measurements from Darwin, Lamont, and Park Falls for XCO2 improved from -0.0071±0.0057 to -0.0012±0.0054 when speed-dependence was included. XCO2 retrieved with the Voigt and speed- dependent Voigt line shapes was compared to aircraft profiles measured at 13 TCCON sites. The bias between the TCCON measurements and the integrated aircraft profile measurements was reduced from 0.9897±0.0005 to 1.0041±0.0005 for XCO2 retrieved with the Voigt and speed- dependent Voigt line shapes respectively. 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.
, Strong, K.
, Wunch, D.
, Toon, G.
, Long, D.
, Hodges, J.
and Sironneau, V.
Improving the Retrieval of XCO2 from Total Carbon Column Network Solar Spectra, Atmospheric Measurement Techniques, [online], https://doi.org/10.5194/amt-12-35-2019, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=923672
(Accessed October 4, 2023)