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Atmospheric Composition

Since its establishment in 1994, AERONET has provided high quality data for the validation of satellite-derived aerosol products. World wide, there are over 200 AERONET sites. AERONET not only derives aerosol optical thickness (AOT) from its direct solar radiance measurements, but also retrieves aerosol size distribution, aerosol refractive indices, and other by-products such as aerosol phase function and aerosol single scattering albedo from its sky radiance measurements. The most widely used AERONET product in the remote sensing community has been AOT, although the other products have also played an important role in characterizing the global climatology of aerosol scattering properties. Validation of Aerosol Polarimetry Sensor (APS) aerosol products using currently available AERONET data presents several challenges. The narrow (6 km) ground track of APS limits the spatiotemporal match between APS footprints and AERONET sites in clear sky conditions. In addition, the physical inconsistency between AERONET quantities and APS-retrieved aerosol scattering properties can hamper our scientific understanding when comparing these two datasets. AERONET uses only skylight radiance, but APS uses both radiance and polarization to retrieve aerosol size and refractive indices. As a result, AERONET inversions for aerosol refractive indices and single scattering albedos are only reliable under conditions of high AOT (>0.4 at 0.4μm) and at high solar zenith angle (>50º); APS aerosol retrieval algorithms, however, have no such restrictions. Furthermore, the current AERONET inversion algorithms assume the same complex refractive indices for both fine-mode and coarse-mode aerosols; therefore, these parameters in the standard APS aerosol products cannot be validated using AERONET. Here, we propose an algorithm for AERONET retrieval of aerosol optical properties using both sky radiance and polarization. This algorithm has two advantages: (a) it enhances the accuracy and reliability of AERONET inversions of aerosol optical properties, and (b) it ensures that APS aerosol products can be compared in a physically consistent manner with AERONET quantities, as both data sets are retrieved from a combined used of radiance and polarization. Does AERONET have skylight polarization measurements? The answer is yes; AERONET has measured the polarization since its inception; but the accuracy and the potential of these data have not been fully explored to date. This proposal builds upon a hyperspectral vector radiative transfer model used in one of our previous studies: "High-spectral resolution simulation of polarization of skylight: sensitivity to aerosol vertical profile, Geophys. Res. Lett., 35, L20801, 2008". The improvement will be made through a combined use of the Vector Linearized Discrete Ordinate Radiative Transfer (VLIDORT) and a linearized Mie (LMIE) code to directly compute not only radiance and polarization, but also the Jacobians of skylight radiance and polarization with respect to aerosol microphysical properties including effective radius, effective variance, real and imaginary part of refractive indices at various spectral bands of AERONET. Iterative retrieval of these aerosol properties will be conducted using the optimal estimation technique by minimizing the difference between forward modeled and observed skylight radiances and polarization until the retrieval converges.

Project PI: Jun Wang/University of Nebraska - Lincoln

122C, Avery Hall.
Department of Computer Science & Engineering University of Nebraska - Lincoln, NE 68588

Phone: (402)472-3597

Fax: (402)472-4917

Email: junwang@cse.unl.edu

http://cse.unl.edu/~junwang/

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Started: Sep 27, 2010

Last Activity: Jan 04, 2011

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