We present the aerosol optical depth retrieved from OMI measurements using the multi-wavelengthm algorithm for two different environments: over Western Europe where the aerosols are weakly absorbing and over the Amazon basin where aerosol optical properties are governed by biomass burning. The results are evaluated by comparison with MODIS data. Over Western Europe OMI and MODIS aerosol optical depth are well-correlated, although the multi-wavelength algorithm appears to overestimate the aerosol optical depth values with respect to MODIS . Over the Amazon Basin the datasets are poorly correlated. Ongoing work to constrain the height and improve the retrieval of AOD over the Amazon basin is expected to be finalize by fall 2007. OMI, Aerosols.