Aerosol retrievals from AVHRR radiances: effects of particle nonsphericity and absorption and an updated long-term global climatology of aerosol properties

Abstract

The paper describes and discusses long-term global retrievals of aerosol properties from channel-1 and -2 Advanced Very High-Resolution Radiometer (AVHRR) radiances. We reconfirm the previously reached conclusion that the nonsphericity of dust-like and dry sea salt aerosols can lead to very large errors in the retrieved optical thickness if one mistakenly applies the scattering model for spherical particles. Comparisons of single-scattering albedo and A ̊ ngström exponent values retrieved from the AVHRR data and those measured in situ at Sable Island indicate that the currently adopted value 0.003 can be a reasonable choice for the imaginary part of the aerosol refractive index in the global satellite retrievals. Several unexpected features in the long-term satellite record indicate a serious problem with post-launch calibration of channel-2 radiances from the NOAA-11 spacecraft. We solve this problem by using a simple re-calibration procedure removing the observed artifacts and derive a global climatology of aerosol optical thickness and size over the oceans for the period extending from July 1983 to December 1999. The global monthly mean optical thickness and A ̊ ngström exponent of tropospheric aerosols show no significant trends over the entire period and oscillate around the average values 0.145 and 0.75, respectively. The Northern Hemisphere mean optical thickness systematically exceeds that averaged over the Southern Hemisphere. The AVHRR retrieval results during the period affected by the Mt. Pinatubo eruption are consistent with the retrievals of the stratospheric aerosol optical thickness based on Stratospheric Aerosol and Gas Experiment Data (SAGE). Time series of the aerosol optical thickness and A ̊ ngström exponent derived for four separate geographic regions exhibit varying degrees of seasonal variability controlled by local meteorological events and/or anthropogenic activities.