Improving the Retrieval of XCO2 from Total Carbon Column Network Solar Spectra

Published

January 3, 2019

Author(s)

Joseph Mendonca, Kimberly Strong, Debra Wunch, Geoffrey Toon, David Long, Joseph T. Hodges, Vincent T. Sironneau

Abstract

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.

Citation

Atmospheric Measurement Techniques

Volume

12

Pub Type

Journals

Download Paper

https://doi.org/10.5194/amt-12-35-2019

Local Download

Keywords

Remote sensing, Fourier-transform infrared spectroscopy, Laboratory spectroscopy, Oxygen

Citation

Mendonca, J. , Strong, K. , Wunch, D. , Toon, G. , Long, D. , Hodges, J. and Sironneau, V. (2019), 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 September 14, 2024)