1.  Assuming only absorbing trace gas abundance and AOD are retrieved, using CO 2 absorption band alone provides a DOF ~ 1.1, which is not enough to determine the two parameters.  Combining O 2 absorption band and assuming the O 2 concentration is fixed at 0.2095, aerosol information can be obtained from the O 2 band. DOF increases to 1.9, so retrieving CO 2 and AOD simultaneously from normalized spectra is possible. Figure 2. Degree of freedom (DOF) and information content (IC) of retrieval using CO 2 band alone and combining CO 2 and O 2 bands. CLARS Reflected sunlight Direct beam LA basin PBL height Retrieval of CO 2 Mixing Ratios from CLARS Measurements: Correcting Aerosol Induced Biases Qiong Zhang 1, Vijay Natraj 2, Run-Lie Shia 1, Coleen M. Roehl 1, Yuk L. Yung 1, and Stanley P. Sander 2 1 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, USA. 2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA Abstract CLARS measurement bias due to aerosol Conclusions Comparing with TCCON results  We demonstrate that combining O 2 and CO 2 absorption bands can obtain enough information to retrieve CO 2 concentration and aerosol optical depth simultaneously.  Using both TCCON and CLARS Spectralon measurements, which are not affected by scattering, X CO2 along the CLARS West Pasadena measurement light path can be calculated with a simple box model. It is compared with our retrieval results.  Wavelength dependence of aerosol scattering typically causes a higher value of retrieved X CO2. By including aerosol parameters into the retrieval, the bias caused by aerosol scattering can be reduced.  TCCON X CO2 measurements (total column) are combined with CLARS Spectralon (free atmosphere) measurements to provide a comparison for CLARS West Pasadena measurements. We assume that CO 2 is well-mixed in the boundary layer. This approach is insensitive to the boundary layer height.  Compared with the calculated X CO2, CLARS measurements show a larger peak at 2:00 pm, one hour later than the peak in TCCON. Wind field and local emissions may explain the difference.  The high value of CLARS X CO2 is partly due to aerosol scattering. When aerosol effects are considered, the retrieved X CO2 shows a lower peak. DOF and IC analyses CO 2 absorption band alone  When pollution is heavy, we can see the 'U shape' as the concentrations of absorbing gas changes during the daytime.  For the case of West Pasadena, measurement shows a low bias from the morning to the afternoon. The bias can be as large as 13% in the CO 2 absorption band and 17% in the O 2 absorption band (Figure 1).  Biases in the 1.61  m CO 2 band are smaller than that in the 1.27  m O 2 band. Ratioing CO 2 SCD by O 2 SCD cannot cancel the bias. CO 2 and O 2 absorption bands Fit of spectra Figure 1. Variations of the CLARS measurements from the morning to the afternoon, measured SCD vs. geometric SCD for (a) CO 2 and (b) O 2. A, B and C represent morning, noon and afternoon. Unit for CO 2 SCD is scaled by 10 22. Unit for O 2 SCD is scaled by 10 25. Figure 3. Top panel shows the fit of measured spectra (red) and simulated spectra (blue) by the 2S-ESS model in the O 2 absorption band. Radiances are normalized by the maximum value. Middle panel shows the residual of retrieval. Lower panel shows the probability density function (pdf) of the residual (red) and a Gaussian function curve (blue) generated to be compared with the residual pdf.  A two-stream-exact single scattering (2S-ESS) model [Spurr and Natraj, 2011] is used to fit the observed spectra.  We retrieve CO 2 H 2 O and AOD simultaneously. Local aerosol compositions are used. Measurement includes both CO 2 and O 2 bands. O 2 is kept constant in the retrieval.  Angstrom coefficient is obtained from AERONET station at Caltech [Holben et al., 1998].  RMS of the residual is comparable to the instrument SNR and matches the Gaussian distribution. CO 2 band spectra show similar features. Figure 4. Schematic figure of CLARS measurement [Fu et al., 2014]. References: Fu et al., AMT, 2014; Holben et al., Remote Sens. Environ., 1998; Spurr and Natraj, JQSRT, 2011; Wunch et al., Philos. T. R. Soc. A., 2011. Figure 5. X CO2 measured by TCCON at Caltech, CLARS in West Pasadena and Spectralon. Black line shows the calculated X CO2 along the CLARS WP light path. Black marker-line shows X CO2 retrieved by a 2-stream model where the effect of aerosol is taken into account.