Abstract
Abstract. In this study, we investigate the feasibility of a multi-pixel scheme in the inversion of aerosol optical properties for multispectral satellite instruments over the ocean. Different from the traditional satellite aerosol retrievals conducted pixel by pixel, we derive the aerosol optical thickness (AOT) of multiple pixels simultaneously by adding a smoothness constraint on the spatial variation of aerosols and oceanic substances, which helps the satellite retrieval, with higher consistency from pixel to pixel. Simulations are performed for two representative oceanic circumstances, open and coastal waters, as well as the land–ocean interface region. We retrieve the AOT for fine, sea spray, and dust aerosols simultaneously using synthetic spectral measurements, which are from the Greenhouse Gases Observing Satellite and Thermal and Near Infrared Sensor for Carbon Observation – Cloud and Aerosol Imager (GOSAT∕TANSO-CAI), with four wavelengths ranging from the ultraviolet to shortwave infrared bands. The forward radiation calculation is performed by a coupled atmosphere–ocean radiative transfer model combined with a three-component bio-optical oceanic module, where the chlorophyll a concentration, sediment, and colored dissolved organic matter are considered. Results show that accuracies of the derived AOT and spectral remote-sensing reflectance are both improved by applying smoothness constraints on the spatial variation of aerosol and oceanic substances in homogeneous or inhomogeneous surface conditions. The multi-pixel scheme can be effective in compensating for the retrieval biases induced by measurement errors and improving the retrieval sensitivity, particularly for the fine aerosols over the coastal water. We then apply the algorithm to derive AOTs using real satellite measurements. Results indicate that the multi-pixel method helps to polish the irregular retrieved results of the satellite imagery and is potentially promising for the aerosol retrieval over highly turbid waters by benefiting from the coincident retrieval of neighboring pixels. A comparison of retrieved AOTs from satellite measurements with those from the Aerosol Robotic Network (AERONET) also indicates that retrievals conducted by the multi-pixel scheme are more consistent with the AERONET observations.
Highlights
Aerosols are one of the largest uncertainty factors in estimations and interpretations of the Earth’s changing energy budget (Boucher et al, 2013)
We focus on the retrieval from the GOSAT/TANSO-Cloud and Aerosol Imager (CAI)
The results indicate that the accuracy of the retrieved aerosol optical thickness (AOT) of each mode is generally improved by using the spatial smoothness constraint, i.e., the multi-pixel method, to correct the retrieval bias induced by measurement errors
Summary
Aerosols are one of the largest uncertainty factors in estimations and interpretations of the Earth’s changing energy budget (Boucher et al, 2013). They exert significant and complex impacts on the radiation process through both direct and indirect effects and have detrimental influences on the air quality and public health. Since the ocean covers more than 70 % of the Earth’s surface, it is indispensable to estimate the aerosol loading over the ocean. Due to the limitation of spatial and temporal coverage from ground-based measurements, satellite remote sensing has been the most efficient approach for observing the variation of aerosols over wide areas and with fine spatiotemporal resolution. Shi et al.: Multi-wavelength and multi-pixel information over the ocean
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