Abstract
A simple but novel study was conducted to investigate whether an imager-type spectroradiometer instrument like MODIS, currently flying on board the Aqua and Terra satellites, or MERIS, which flew on board Envisat, could detect absorbing aerosols if they could measure the $Q$ Stokes parameter in addition to the total radiance $I$, that is if they could also measure the linear polarization of the light. Accurate radiative transfer calculations were used to train a fast neural network forward model, which together with a simple statistical optimal estimation scheme was used to retrieve three aerosol parameters: aerosol optical depth at 869 nm, optical depth fraction of fine mode (absorbing) aerosols at 869 nm, and aerosol vertical location. The aerosols were assumed to be bimodal, each with a lognormal size distribution, located either between 0 and 2 km or between 2 and 4 km in the Earth's atmosphere. From simulated data with 3\% random Gaussian measurement noise added for each Stokes parameter, it was found that by itself the total radiance $I$ at the nine MODIS VIS channels was generally insufficient to accurately retrieve all three aerosol parameters ($\sim$ 15\% to 37\% successful), but that together with the $Q$ Stokes component it was possible to retrieve values of aerosol optical depth at 869 nm to $\pm$ 0.03, single-scattering albedo at 869 nm to $\pm$ 0.04, and vertical location in $\sim$ 65\% of the cases. This proof-of-concept retrieval algorithm uses neural networks to overcome the computational burdens of using vector radiative transfer to accurately simulate top-of-atmosphere (TOA) total and polarized radiances, enabling optimal estimation techniques to exploit information from multiple channels. Therefore such an algorithm could, in concept, be readily implemented for operational retrieval of aerosol and ocean products from moderate or hyperspectral spectroradiometers.
Highlights
The Moderate Resolution Imaging Spectroradiometer (MODIS) is a scientific instrument that was launched into Earth orbit by NASA in 1999 on board the Terra satellite, and in 2002 on board the Aqua satellite
A simple study was conducted to explore the advantage of making use of polarization information. To this end a synthetic dataset was created to simulate the top of the atmosphere (TOA) Stokes parameters I and Q for a range of aerosol optical depths, and optical depth fraction fτa of absorbing fine mode aerosol particles embedded in a background molecular atmosphere
We have explored the feasibility of retrieving accurate values of aerosol optical depth, the fine mode fraction and single-scattering albedo, and the vertical location of absorbing aerosol particles from measurements of the I and Q components of the Stokes vector
Summary
The Moderate Resolution Imaging Spectroradiometer (MODIS) is a scientific instrument that was launched into Earth orbit by NASA in 1999 on board the Terra satellite, and in 2002 on board the Aqua satellite. The instruments measure total radiances at varying spatial resolutions (2 bands at 250 m, 5 bands at 500 m, and 29 bands at 1 km) in 36 spectral bands ranging in wavelength from 0.4 μm to 14.4 μm. Together the two instruments image the entire Earth every 1–2 days. They are designed to provide information about large-scale global dynamics including changes in Earth’s cloud cover, radiation budget, and processes occurring in the oceans, on land, and in the lower atmosphere. The MERIS instrument employs spectrometers that measure reflected sunlight in several spectral bands between 390 and 1,040 nm. Apart from SGLI, which has two bands that can measure polarization, these instruments only measure the total radiance, or the I Stokes parameter
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