Aerosol profiling using radiometric and polarimetric spectral measurements in the O2 near infrared bands: Estimation of information content and measurement uncertainties

Characterization of aerosol vertical distribution in the planetary boundary layer (PBL) using passive remote sensing requires advances in the current state of the art. To quantify the performance of various passive sensor designs within a common framework we developed an aerosol climatology of the Los Angeles basin and applied observing system simulation experiments (OSSEs) to estimate the information content retrievable from a variety of sensors measuring reflected near-infrared solar radiation. In addition to simulating current and planned satellite sensors, we also characterize the sensitivity of the California Laboratory for Atmospheric Remote Sensing – Fourier Transform Spectrometer (CLARS-FTS), located at Mt. Wilson (1.67 km above sea level), which is utilized in this work as a testbed for aerosol profiling remote sensing. We estimate the impacts of spectral coverage, radiance and polarization, spectral resolution, signal to noise ratio (SNR), and number of viewing angles on the information content and retrieval uncertainties of aerosol profiles in the PBL. We found that by adding high spectral resolution (full-width half-maximum of 3 cm⁻¹ or better), polarimetric measurements with a SNR of at least 212 to radiance measurements with SNR of 300 for both O₂ A and ¹∆ bands, the degrees of freedom for signal (DOFS) of a single CLARS-FTS measurement is raised from 2.1 to 2.8. This improvement is sufficient to simultaneously quantify three key parameters: aerosol optical depth, aerosol peak height, and aerosol layer thickness in the PBL. Current satellite-borne instruments (OCO-2, OCO-3, TEMPO, TROPOMI, and EPIC) and planned instruments (TEMPO, MicroCarb, SPEXone, and MAIA), individually provide a DOFS ≤ 2.25, which is insufficient to simultaneously quantify all three aerosol profiling parameters in the PBL. Joint radiometric and polarimetric measurements of the O₂ A and B bands with 3 cm⁻¹ spectral resolution, SNR of 500 for radiance and 353 for polarization, acquired at three viewing angles, can provide sufficient sensitivity to retrieve the three aerosol parameters simultaneously. The inclusion of high spectral resolution radiometric and polarimetric measurements reduces the required number of viewing angles, which is advantageous when the multiangular data are acquired with a pointable instrument. In this case a larger number of viewing angles reduces the spatial coverage that can be achieved for a given target.