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Accounting for Aerosol Scattering in the Remote Sensing of Greenhouse Gas


Zhang, Qiong (2017) Accounting for Aerosol Scattering in the Remote Sensing of Greenhouse Gas. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9XW4GRQ.


This thesis includes three different projects related to the remote sensing of Earth's atmosphere. The first part, comprising Chapter 2 and Chapter 3, focuses on the retrieval of Level 1 product, particularly the effect of aerosol scattering in the remote sensing of greenhouse gases. In Chapter 2, we study the aerosol induced bias in the retrieval of column averaged CO2 mixing ratios (XCO2). Ground based remote sensing data from the California Laboratory for Atmospheric Remote Sensing Fourier Transform Spectrometer (CLARS-FTS) are used. We employ a numerical radiative transfer model to simulate the impacts of neglecting aerosol scattering on the CO2 and O2 slant column densities (SCDs) operationally retrieved from CLARS-FTS measurements. These simulations show that the CLARS-FTS operational retrieval algorithm likely underestimates CO2 and O2 abundances over the LA basin in scenes with moderate aerosol loading. The bias in the CO2 and O2 abundances due to neglecting aerosol scattering cannot be canceled by ratioing each other in the derivation of the operational product of XCO2. We propose a method for approximately correcting the aerosol-induced bias. Results for CLARS XCO2 are compared to the direct-sun XCO2 retrievals from a nearby Total Carbon Column Observing Network (TCCON) station.

In Chapter 3, we explain why large XCO2 retrieval errors are found over deserts in the space borne Orbiting Carbon Observatory-2 (OCO-2) data. We argue that these errors are caused by the surface albedo being close to a critical surface albedo (αc). Over a surface with albedo close to αc, increasing the aerosol optical depth (AOD) does not change the continuum radiance. The spectral signature caused by changing the AOD is identical to that caused by changing the absorbing gas column. The degeneracy in the retrievals of AOD and XCO2 results in a loss of degrees of freedom (DOF) and information content (H). We employ a radiative transfer model to study the physical mechanism of XCO2 retrieval error over a surface with albedo close to αc. Based on retrieval tests over surfaces with different albedos, we conclude that over a surface with albedo close to αc, the XCO2 retrieval suffers from a significant loss of accuracy.

In the Appendix, we put in a Chapter based on my work with Prof. Andrew Thompson on ocean The second part, mainly in Chapter 4, focuses on the application of Level 2 product. In this Chapter, we examine the uncertainties in middle atmospheric HOx chemistry by comparing the Aura Microwave Limb Sound (MLS) OH and HO2 measurements with the simulations of the Caltech-JPL KINETICS photochemical model. The model using the standard chemical kinetics underestimates OH and HO2 concentrations in the mesosphere. To resolve the discrepancies, we use MLS OH and HO2 measurements as benchmark to adjust the involved chemical rate coefficients within reasonable uncertainty ranges with an optimal estimation algorithm. The results show that four key reaction rate constants and the O2 cross section at Lyman-α (121.6 nm) are the most sensitive parameters for determining the HOx profiles. We conclude that the rate coefficient of H + O2 + M → HO2 + M requires a very large adjustment beyond the uncertainty limits recommended in the NASA Data Evaluation, which suggests the need for future laboratory measurements. An alternative explanation is that radiative association plays a significant role in this process, i.e. H + O2 → HO2 + hv, which has never been measured or computed.

In the Appendix, we put in a Chapter based on my work with Prof. Andrew Thompson on ocean submesoscale turbulence.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Aerosol; Greenhouse gas; HOx chemistry
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Environmental Science and Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Yung, Yuk L.
Thesis Committee:
  • Wennberg, Paul O. (chair)
  • Thompson, Andrew F.
  • Frankenberg, Christian
  • Bordoni, Simona
  • Yung, Yuk L.
Defense Date:6 June 2016
Record Number:CaltechTHESIS:06152016-152036651
Persistent URL:
Zhang, Qiong0000-0002-8762-0557
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:9876
Deposited By: Qiong Zhang
Deposited On:27 Jun 2016 17:45
Last Modified:08 Nov 2023 00:36

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