Geostationary satellite retrievals of aerosol optical thickness during ACE-Asia

Using 30 days of hourly geostationary satellite (GMS5 imager) data and discrete ordinate radiative transfer (DISORT) calculations, aerosol optical thickness (AOT) at 0.67 mm was retrieved over the west Pacific Ocean (20°N–45°N, 110°E–150°E) during the Aerosol Characterization Experiment (ACE-Asia) intensive observation period in April 2001. Different from previous one-channel retrieval algorithms, we have developed a strategy that utilizes in situ and ground measurements to characterize aerosol properties that vary both in space and time. Using Mie calculations and bilognormal size distribution parameters inferred from measurements, the relationship between Angstro ˚ ¨m exponent (a) and the ratio of two volume lognormal modes (g) was obtained. On the basis of spectral AOT values inferred from the Aerosol Robotic Network (AERONET) sites, NASA Ames Airborne Sun photometers (AATS6 and AATS14) and a Sun photometer on board a ship, a successive correction method (SCM) was used to infer the spatial distribution of a in the study area. Comparisons between the satellite-retrieved AOT and AERONET values over four sites show good agreement with linear coefficients (R) of 0.86, 0.85, 0.86, and 0.87. The satellite-derived AOTs are also in good agreement with aircraft (R = 0.87) and ship measurements (R = 0.98). The average uncertainty in our AOT retrievals is about 0.08 with a maximum value of 0.15 mainly due to the assumptions in calibration (±0.05), surface reflectance (±0.01–±0.03), imaginary part of refractive index (±0.05), and SCM-derived a values (±0.02). The monthly mean AOT spatial distribution from GMS5 retrievals in April 2001 clearly shows the transport pattern of aerosols with high AOT near the coast of east Asia and low AOT over the open ocean. Using high temporal resolution satellite data, this paper demonstrates that the diurnal variation in AOT can be retrieved by current generations of geostationary satellites. The next generation of geostationary satellites with better spectral, spatial and radiometric resolution will significantly improve our ability to monitor aerosols and quantify their effects on regional climate.