The study is to analyze changes in monthly-averaged aerosol parameters derived from the SeaWiFS observations over East Asia from January 1998 through December 2004. All the SeaWiFS GAC Level 1 data (4 by 4 km spatial resolution data) that cover the Northeast Asian area were collected and processed by the standard atmospheric correction algorithm released by the SeaWiFS Project to produce daily aerosol optical thickness (AOT) and Angstrom exponent imageries. Cloud screening was applied if AOT at 490 predicted from the aerosol look-up tables embedded in the algorithm exceeded 0.7. From the daily composite images, monthly average AOT and Angstrom exponent values were extracted for each one of the six study areas chosen from the surrounding waters of Japan. The results showed that, although annual mean of AOT did not show any trend, +0.01-0.015 increase in Angstrom exponent in almost all study areas was observed over the study period. This increase is interpreted as 4-5% increase in submicron fraction (SMF), or the ratio of contribution of submicron aerosol particles to the total AOT, and will be interpreted as an increase of submicron particles due to the enhanced anthropogenic activities in East Asia.
SeaWiFS observation of the East-Asian seas during the Aerosol Characterization Experiment-Asia shows large areas of under-estimated or even negative water-leaving radiance in the blue. To investigate it, three match-up stations were analyzed. An iterative radiative transfer simulation was carried out, in an attempt to reproduce the satellite measured top-of-atmosphere reflectance. The resulting water reflectance and aerosol optical thickness (AOT) agreed well with field measurements when the effect of sub-micron absorbing particles was considered in the simulation. The error in the retrieved water reflectance was much decreased, with average values of about 6% at 412nm and 443nm for the three stations. The effect of the Asian dust was also simulated in comparison with that of small absorptive aerosols. The under-estimation could not be solely attributed to Asian dust. It was also found that at one of the station, where the presence of dust aerosols was anticipated, an aerosol model mixed with both dust and soot improved the accuracy of the estimated AOT compared with the case of soot as the only absorptive aerosol. Sub-micron absorbing particles, in addition of the Asian dust, should be considered in the optical remote sensing of East-Asian waters.
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