Aerosol optical depth over Northeastern Brazil: A multi-platform intercomparison study

The atmosphere over the Northeastern region of Brazil (NEB) contains a variety of different aerosol types: marine, soil dust, biomass burning, urban pollution, as well as mixed species. However, very few reliable information on aerosols is available for the region. Satellite data provides spatial and temporal coverage in places with low density ground-based observational networks, with Aerosol Optical Depth (AOD) products as most important information on the atmospheric aerosol load. To find out how different AOD products from satellites behave over the NEB region we compared AOD at 550 nm from MODIS retrievals aboard both NASA's Terra and Aqua satellites, and ground-based derived AOD 550 nm from the two Aerosol Robotic Network (AERONET) sites in the region, located in cities of Petrolina and Natal, between 2015 and 2018. We based our analysis on the following MODIS AOD retrievals products: Dark Target (DT), Deep Blue (DB), the combined product (DTDB), DT 3 km resolution product (MxD3k) and MAIAC at 1 km resolution. Additionally, we used AOD from MERRA2 reanalysis for another comparison that may provide additional insight on which products perform best over the region. We also analyze and compare the seasonality of the AOD products to the four biomes of NEB, which are Amazon rainforest, Atlantic Forest, Cerrado and Caatinga (wooded savannah-like) biomes. Petrolina is located in the NEB largest biome, the Caatinga, while Natal is a coastal city in the Atlantic Forest. The results show that DB yields the best results for the Petrolina site. The analyses also show that in all biomes the AOD monthly averages decrease during austral autumn and winter and increase during spring and summer. The compared products show some differences in AOD, even though with similar patterns. MAIAC and DB show very similar values in all four biomes throughout the year, recommending the use of DB and MAIAC for studies in the Caatinga region.