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Article Is Available DA8 Spectrometer, J Atmos. Meas. Tech., 12, 35–50, 2019 https://doi.org/10.5194/amt-12-35-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Using a speed-dependent Voigt line shape to retrieve O2 from Total Carbon Column Observing Network solar spectra to improve measurements of XCO2 Joseph Mendonca1, Kimberly Strong1, Debra Wunch1, Geoffrey C. Toon2, David A. Long3, Joseph T. Hodges3, Vincent T. Sironneau3, and Jonathan E. Franklin4 1Department of Physics, University of Toronto, Toronto, ON, Canada 2Jet Propulsion Laboratory, Pasadena, CA, USA 3National Institute of Standards and Technology, Gaithersburg, MD, USA 4Harvard John A. Paulson School of Engineering and Applied Sciences, Cambridge, MA, USA Correspondence: Joseph Mendonca ([email protected]) Received: 23 February 2018 – Discussion started: 3 April 2018 Revised: 23 August 2018 – Accepted: 17 September 2018 – Published: 3 January 2019 Abstract. High-resolution, laboratory, absorption spectra of infrared CO2 and O2 spectra to improve the accuracy of 1 3 − the a 1g X 6g oxygen (O2) band measured using cav- XCO2 measurements. ity 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 mea- sured absorption coefficients than the Voigt line shape. We 1 Introduction used these line shape models to calculate absorption coeffi- cients to retrieve atmospheric total columns abundances of Accurate remote sensing of greenhouse gases (GHGs) such O2 from ground-based spectra from four Fourier transform as CO2, in the Earth’s atmosphere is important for study- spectrometers that are a part of the Total Carbon Column Ob- ing the carbon cycle to better understand and predict cli- serving Network (TCCON). Lower O2 total columns were mate change. The absorption of solar radiation by O2 in the retrieved with the speed-dependent Voigt line shape, and Earth’s atmosphere is important because it can be used to the difference between the total columns retrieved using the study the properties of clouds and aerosols and to determine Voigt and speed-dependent Voigt line shapes increased as a vertical profiles of temperature and surface pressure. Wallace function of solar zenith angle. Previous work has shown that and Livingston (1990) were the first to retrieve total columns 1 3 − carbon dioxide (CO2) total columns are better retrieved us- of O2 from some of the discrete lines of the a 1g X 6g ing a speed-dependent Voigt line shape with line mixing. The band of O2 centred at 1.27 µm (which will be referred to bel- column-averaged dry-air mole fraction of CO2 (XCO2) was low as the 1.27 µm band) using atmospheric solar absorp- calculated using the ratio between the columns of CO2 and tion spectra from Kitt Peak National Observatory. Mlawer O2 retrieved (from the same spectra) with both line shapes et al. (1998) recorded solar absorption spectra in the near- from measurements taken over a 1-year period at the four infrared (NIR) region to study collision-induced absorption 1 3 − sites. The inclusion of speed dependence in the O2 retrievals (CIA) in the a 1g X 6g band as well as two other O2 significantly reduces the air mass dependence of XCO2, and bands. The spectra were compared to a line-by-line radia- the bias between the TCCON measurements and calibrated tive transfer model and the differences between the measured integrated aircraft profile measurements was reduced from and calculated spectra showed the need for better absorp- 1 % to 0.4 %. These results suggest that speed dependence tion coefficients in order to accurately model the 1.27 µm should be included in the forward model when fitting near- band (Mlawer et al., 1998). Subsequently, spectroscopic pa- rameters needed to calculate the absorption coefficients from Published by Copernicus Publications on behalf of the European Geosciences Union. 36 J. Mendonca et al.: Improving the retrieval of O2 total columns discrete transitions of the 1.27 µm band were measured in fer intensity from one part of the spectral band to another multiple studies (Cheah et al., 2000; Newman et al., 1999, (Lévy et al., 1992), was shown to be prevalent in multiple 2000; Smith and Newnham, 2000), as was collision-induced studies (Predoi-Cross et al., 2008; Tran et al., 2006; Tran absorption (CIA) (Maté et al., 1999; Smith and Newnham, and Hartmann, 2008). Tran and Hartmann (2008) showed 2000), while Smith et al. (2001) validated the work done in that including line mixing when calculating the O2 A-band Smith and Newnham (2000) using solar absorption spectra. absorption coefficients reduced the air mass dependence of The 1.27 µm band is of particular importance to the To- the O2 column retrieved from TCCON spectra. When fit- tal Carbon Column Observing Network (TCCON) (Wunch ting cavity ring-down spectra of the O2 A-band, Drouin et et al., 2011). TCCON is a ground-based remote sensing al. (2017) found it necessary to use a speed-dependence Voigt network that makes accurate and precise measurements of line shape, which takes into account different speeds at the GHGs for satellite validation and carbon cycle studies. Us- time of collision (Shannon et al., 1986), with line mixing to ing the O2 column retrieved from solar absorption spectra, properly fit the discrete spectral lines of the O2 A-band. the column-averaged dry-air mole fraction of CO2 (XCO2) The need to include non-Voigt effects when calculating ab- has been shown to provide better precision than when us- sorption coefficients for the O2 1.27 µm band was first shown ing the surface pressure to calculate XCO2 (Yang et al., in Hartmann et al. (2013) and Lamouroux et al. (2014). 2002). The O2 column is retrieved from the 1.27 µm band In Hartmann et al. (2013) and Lamouroux et al. (2014), because of its close proximity to the spectral lines used to Lorentzian widths were calculated using the requantised retrieve CO2, thereby reducing the impact of solar tracker classical molecular dynamics simulations (rCMDSs) and mispointing and an imperfect instrument line shape (ILS) used to fit cavity ring-down spectra with a Voigt line shape (Washenfelder et al., 2006a). To improve the retrievals of O2 for some isolated transitions in the O2 1.27 µm band. The from the 1.27 µm band, Washenfelder et al. (2006a) found studies concluded that a Voigt line shape is insufficient for that adjusting the spectroscopic parameters in HITRAN 2004 modelling the spectral lines of the O2 1.27 µm band and (Rothman et al., 2005) decreased the air mass and tempera- that effects such as speed dependence and Dicke narrowing ture dependence of the O2 column. These revised spectro- should be included in the line shape calculation. scopic parameters were included in HITRAN 2008 (Roth- In this study, air-broadened laboratory cavity ring-down man et al., 2009). Atmospheric solar absorption measure- spectra of the O2 1.27 µm band were fitted using a spectral ments from this band taken at the Park Falls TCCON site line shape that takes into account speed dependence. The by Washenfelder et al. (2006a) were the first measurements derived spectroscopic parameters for the speed-dependent to observe the electric-quadrupole transitions (Gordon et al., Voigt line shape were used to calculate absorption coeffi- 2010). Leshchishina et al. (2011, 2010) subsequently used cients when fitting high-resolution solar absorption spectra. cavity ring-down spectra to retrieve spectroscopic parame- XCO2 was calculated from O2 total columns retrieved using ters for the 1.27 µm band using a Voigt spectral line shape the new absorption coefficients and CO2 total columns re- and these parameters were included in HITRAN 2012 (Roth- trieved with the line shape model described in Mendonca et man et al., 2013a). Spectroscopic parameters for the discrete al. (2016). These new XCO2 values were compared to the spectral lines of the O2 1.27 µm band from HITRAN 2016 XCO2 retrieved using the Voigt line shape. Section 2 details (Gordon et al., 2017) are very similar to HITRAN 2012 ex- the formulas used to calculate absorption coefficients using cept that HITRAN2016 includes improved line positions re- different spectral line shapes. In Sect. 3, we describe the re- ported by Yu et al. (2014). trieval of spectroscopic parameters from three air-broadened Extensive spectral line shape studies have been performed cavity ring-down spectra fitted with a speed-dependent Voigt for the O2 A-band, which is centred at 762 nm and used by line shape. For Sect. 4, the speed-dependent line shape along the Greenhouse Gases Observing Satellite (GOSAT) (Yokota with the retrieved spectroscopic parameters is used to fit solar et al., 2009) and the Orbiting Carbon Observatory-2 (OCO- absorption spectra from four TCCON sites and retrieve total 2) satellite (Crisp et al., 2004) to determine surface pressure. columns of O2, which is compared to O2 retrieved using a Studies have shown that the Voigt line shape is inadequate Voigt line shape. In Sect. 5, we investigate the change in the to describe the spectral line shape of the discrete O2 lines air mass dependence of XCO2 with the new O2 retrievals. in the A-band. Dicke narrowing occurs when the motion of In Sect. 6, we discuss our results and their implications for the molecule is diffusive due to collisions changing the ve- remote sensing of greenhouse gases. locity and direction of the molecule during the time that it is excited. This diffusive motion is taken into account by aver- aging over many different Doppler states, resulting in a line 2 Absorption coefficient calculations width that is narrower than the Doppler width (Dicke, 1953).
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