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Retrievals of aerosol optical depth (AOD) at 388 nm over the ocean from theOzone Monitoring Instrument (OMI) two-channel near-UV algorithm (OMAERUV)have been compared with independent AOD measurements. The analysis wascarried out over the open ocean (OMI and MODerate-resolution ImagingSpectrometer (MODIS) AOD comparisons) and over coastal and island sites (OMIand AERONET, the AErosol RObotic NETwork). Additionally, a research versionof the retrieval algorithm (using MODIS and CALIOP (Cloud-Aerosol Lidar with OrthogonalPolarization) information as constraints) was utilized to evaluate the sensitivity of the retrieval to different assumed aerosolproperties.  Overall, the comparison resulted in differences (OMI minus independentmeasurements) within the expected levels of uncertainty for the OMI AODretrievals (0.1 for AOD < 0.3, 30 % for AOD > 0.3). Usingexamples from case studies with outliers, the reasons that led to theobserved differences were examined with specific purpose to determine whetherthey are related to instrument limitations (i.e., pixel size, calibration) oralgorithm assumptions (such as aerosol shape, aerosol height).  The analysis confirms that OMAERUV does an adequate job at rejecting cloudyscenes within the instrument's capabilities. There is a residual cloudcontamination in OMI pixels with quality flag 0 (the best conditions foraerosol retrieval according to the algorithm), resulting in a bias towardshigh AODs in OMAERUV. This bias is more pronounced at low concentrations ofabsorbing aerosols (AOD 388 nm  ∼  < 0.5). For higher aerosolloadings, the bias remains within OMI's AOD uncertainties.  In pixels where OMAERUV assigned a dust aerosol model, a fraction of them(< 20 %) had retrieved AODs significantly lower than AERONET andMODIS AODs. In a case study, a detailed examination of the aerosol heightfrom CALIOP and the AODs fromMODIS, along with sensitivity tests, was carried out by varying the differentassumed parameters in the retrieval (imaginary index of refraction, sizedistribution, aerosol height, particle shape). It was found that thespherical shape assumption for dust in the current retrieval is the maincause of the underestimate. In addition, it is demonstrated in an example howan incorrect assumption of the aerosol height can lead to an underestimate.Nevertheless, this is not as significant as the effect of particle shape.These findings will be incorporated in a future version of the retrievalalgorithm

The role of cloud contamination, aerosol layer height and aerosol model in the assessment of the OMI near-UV retrievals over the ocean