We will present an overview of the development of the High-resolution Metagrating Spectropolarimeter for Aerosol Profiling (HiMAP), a collaborative effort between the University of Iowa and the Jet Propulsion Laboratory (JPL), supported by NASA's Instrument Incubator Program (IIP).
HiMAP represents a major advance in atmospheric remote sensing. Operating over the 680–780 nm spectral range, it combines exceptional spectral resolving power with high-precision measurements of both radiance and the degree of linear polarization of backscattered sunlight from three viewing angles along the satellite or aircraft ground track. By analyzing the Stokes parameters Q and U within the oxygen A and B absorption bands and the adjacent continuum at high spectral resolution, HiMAP is capable of retrieving aerosol layer thickness, peak altitude, and total aerosol loading, thereby providing valuable information on the three-dimensional distribution of aerosols in the troposphere. Unlike stereo-imaging techniques, which generally retrieve aerosol plume height only under high aerosol loading or near source regions, HiMAP is designed to perform under both low- and high-aerosol-loading conditions and can retrieve aerosol layer height, thickness, and, under favorable conditions, the aerosol extinction profile.
In addition to an overview of the instrument development, we will present recent progress in the retrieval algorithm developed for HiMAP. The algorithm represents the aerosol vertical extinction profile as a weighted combination of the three to five leading empirical orthogonal functions (EOFs) obtained through principal component analysis (PCA) of a 15-year global record of aerosol extinction profiles measured by the spaceborne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). This approach reduces the inversion problem to retrieving only a small number of EOF coefficients while retaining the essential variability of aerosol vertical structure.
To support this retrieval framework, a new PCA module has been implemented in the Unified Linearized Vector Radiative Transfer Model (UNL-VRTM) to compute the Jacobians of top-of-atmosphere (TOA) radiance and polarization with respect to the EOF coefficients. These Jacobians enable efficient optimal-estimation retrievals of the aerosol vertical profile. We will also present averaging-kernel analyses to evaluate the vertical sensitivity of aerosol profile retrievals using both synthetic HiMAP observations and measurements from the Tropospheric Monitoring Instrument (TROPOMI).