ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-4849-2010 Validation of the GRAPE single view aerosol retrieval for ATSR-2 and insights into the long term global AOD trend over the ocean Thomas G. E. 1 Poulsen C. A. 2 Siddans R. 2 Sayer A. M. 1 Carboni E. 1 Marsh S. H. 1 3 Dean S. M. 1 4 Grainger R. G. 1 Lawrence B. N. 2 Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, UK Space Science and Technology Department, Rutherford Appleton Laboratory, Didcot, Oxfordshire, UK present address: Department of Medical Physics and Bioengineering, Christchurch Hospital, New Zealand present address: National Institute of Water and Atmospheric Research, Wellington, New Zealand 26 05 2010 10 10 4849 4866 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/4849/2010/acp-10-4849-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/4849/2010/acp-10-4849-2010.pdf

The Global Retrieval of ATSR Cloud Parameters and Evaluation (GRAPE) project has produced a global data-set of cloud and aerosol properties from the Along Track Scanning Radiometer-2 (ATSR-2) instrument, covering the time period 1995–2001. This paper presents the validation of aerosol optical depths (AODs) over the ocean from this product against AERONET sun-photometer measurements, as well as a comparison to the Advanced Very High Resolution Radiometer (AVHRR) optical depth product produced by the Global Aerosol Climatology Project (GACP). <br><br> The GRAPE AOD over ocean is found to be in good agreement with AERONET measurements, with a Pearson's correlation coefficient of 0.79 and a best-fit slope of 1.0&plusmn;0.1, but with a positive bias of 0.08&plusmn;0.04. Although the GRAPE and GACP datasets show reasonable agreement, there are significant differences. These discrepancies are explored, and suggest that the downward trend in AOD reported by GACP may arise from changes in sampling due to the orbital drift of the AVHRR instruments.

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