AUTHOR'S ACCEPTED DRAFT OF PAPER PRIOR TO COPY-EDITING AND TYPESETTING PLEASE DO NOT DISTRIBUTE REFER TO FINAL VERSION IN JGR WHEN AVAILABLE

28 The spaceborne Advanced Very High Resolution Radiometer (AVHRR) sensor data record is 29 approaching 40 years, providing a crucial asset for studying long-term trends of aerosol 30 properties regionally and globally. However, due to limitations of its channels’ information 31 content, aerosol optical depth (AOD) data from AVHRR over land are still largely lacking. In 32 this paper, we describe a new physics-based algorithm to retrieve aerosol loading over both land 33 and ocean from AVHRR for the first time. The over-land algorithm is an extension of our Sea34 viewing Wide Field-of-view Sensor (SeaWiFS) and Moderate Resolution Imaging 35 Spectroradiometer (MODIS) Deep Blue algorithm, while a simplified version of our Satellite 36 Ocean Aerosol Retrieval (SOAR) algorithm is used over ocean. We compare retrieved AVHRR 37 AOD with that from MODIS on a daily and seasonal basis, and find in general good agreement 38 between the two. For the satellites with equatorial crossing times within two hours of solar noon, 39 the spatial coverage of the AVHRR aerosol product is comparable to that of MODIS, except over 40 very bright arid regions (such as the Sahara), where the underlying surface reflectance at 630 nm 41 reaches the critical surface reflectance. Based upon comparisons of the AVHRR AOD against 42 Aerosol Robotic Network (AERONET) data, preliminary results indicate that the expected error 43 confidence interval envelope is around ±(0.03+15%) over ocean and ±(0.05+25%) over land for 44 this first version of the AVHRR aerosol products. Consequently, these new AVHRR aerosol 45 products can contribute important building blocks for constructing a consistent long-term data 46 record for climate studies. 47