Improvement of aerosol optical properties retrieval over asia using visible and UV measurements from geostationary satellites

다국어 초록 (Multilingual Abstract)

Two algorithms to retrieve aerosol optical properties (AOPs) over Asia were developed in this study. Continuous monitoring of aerosol amount and transport over the region plays significant role in improving the prediction of air quality change as well as in understanding radiative effect of aerosol. From the reason, a single channel algorithm using visible measurement of a Meteorological Imager (MI) onboard a geostationary orbit satellite, Communication, Ocean, and Meteorological Satellite (COMS), was developed to retrieve aerosol optical depth (AOD) over Asia. The single channel algorithm is based on widely used look-up table (LUT) approach for inversion, and clear sky composite method for surface reflectance estimation. Since the algorithm has limitation in detecting aerosol optical type, the seasonally dependent aerosol model was adopted to calculate the LUTs. The aerosol models consists of refractive indices and volume size distribution with integrated from long-term measurements of Aerosol Robotic Network (AERONET) sun-photometer.
For quantitative validation of the algorithm, the AOD from the single channel algorithm was compared with the result of the AERONET direct measurement. The comparison showed good agreement especially in spring. However, accuracy of the algorithm has fundamental limitation in the aerosol type selection as mentioned. Thus, the uncertainty induced by aerosol model assumption was analyzed, and improved by modifying inversion dataset. Furthermore, an effect of existence of background aerosol was corrected to improve the accuracy of surface reflectance estimation. Similarly as other retrieval uncertainties, inversion error in the LUT approach and instrument calibration error was analyzed in quantitative manner. Besides, critical reflectance method was introduced to overcome the limitation in aerosol type selection.
In spite of the advantage of the continuous monitoring of AOD change, lack of information about scattering property results in increaseing uncertainty in understanding radiative effects of aerosol over Asia. From the reason, a multi-channel algorithm utilizing near ultra-violet (UV) measurements was developed to retrieve AOD and single scattering albedo (SSA). The algorithm used the optimal estimation (OE) method to reduce the retrieval uncertainty induced by assumption of aerosol loading height. The algorithm retrieved a priori states of retrieval values by using two channel LUT approach, and the a priori states were applied to find optimized solution to minimize difference between measured and simulated spectrum. The algorithm was developed for the planned Geostationary Environment Monitoring Spectrometer (GEMS) measurement to be launched in 2018, but was tested by using the measurement of Ozone Monitoring Instrument (OMI). An advantage of the UV-Vis algorithm lies in the self-sufficiency in a priori state. However, the large sensitivity to the a priori value in the retrieval of aerosol loading height is one of limitation of the algorithm.
This study focused on the improvement of the satellite measurement of aerosol information over Asia, and presents the two aerosol retrieval algorithms developed by using UV-Vis and visible measurement. After the launch of GEMS, continuous monitoring of AOPs from multi-geostationary measurements is expected to be realized. The results can contribute to estimate its effects on regional climate change, and also can be applied to improve the air quality forecast via data assimilation system.