Global Climatology of Aerosol Optical Thickness and Size for the Period of NOAA-9 Observations

One of the objectives of the Global Aerosol Climatology Project (GACP) is the creation of a global climatology of aerosol properties for the full period of satellite observations. The way to achieve this goal is a creative combination of satellite retrievals, aerosol modeling, and field observations.

This paper presents the results of applying an advanced aerosol retrieval algorithm to the period of AVHRR observations from the NOAA-9 platform (Februalry 1995 - November 1988). The algorithm incorporates a multi-channel cloud screening scheme and an aerosol retrieval scheme based on channel 1 and 2 AVHRR radiances. The aerosol product consists of monthly 1x1 deg averages of the aerosol optical thickness and Angstrom exponent.

We will present the results of aerosol retrievals and discuss their local and global climatological implications. The cloud screening algorithm rejects approximately 75% of the pixels classified by ISCCP as "cloud free". The fact that a significant fraction of AVHRR pixels are not analysed by the cloud and aerosol retrieval algorithms may mean that the definitions of a cloud and an aerosol may need to be more clearly formulated and that both retrieval schemes may miss the periods of most intense aerosol-cloud interactions. The global mean aerosol optical thickness and Angstrom exponent retrieved using the ISCCP post- launch calibartion of channel 1 and the NOAA post-launch calibration of channel 2 are close to 0.15 and 0.9, respectively. These quantities do not show a measurable systematic trend over the period of NOAA-9 observations.