Abstract
Abstract. Retrievals of aerosol optical depth (AOD) at 388 nm over the ocean from the Ozone Monitoring Instrument (OMI) two-channel near-UV algorithm (OMAERUV) have been compared with independent AOD measurements. The analysis was carried out over the open ocean (OMI and MODerate-resolution Imaging Spectrometer (MODIS) AOD comparisons) and over coastal and island sites (OMI and AERONET, the AErosol RObotic NETwork). Additionally, a research version of the retrieval algorithm (using MODIS and CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) information as constraints) was utilized to evaluate the sensitivity of the retrieval to different assumed aerosol properties. Overall, the comparison resulted in differences (OMI minus independent measurements) within the expected levels of uncertainty for the OMI AOD retrievals (0.1 for AOD < 0.3, 30 % for AOD > 0.3). Using examples from case studies with outliers, the reasons that led to the observed differences were examined with specific purpose to determine whether they are related to instrument limitations (i.e., pixel size, calibration) or algorithm assumptions (such as aerosol shape, aerosol height). The analysis confirms that OMAERUV does an adequate job at rejecting cloudy scenes within the instrument's capabilities. There is a residual cloud contamination in OMI pixels with quality flag 0 (the best conditions for aerosol retrieval according to the algorithm), resulting in a bias towards high AODs in OMAERUV. This bias is more pronounced at low concentrations of absorbing aerosols (AOD 388 nm ∼ < 0.5). For higher aerosol loadings, 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 and MODIS AODs. In a case study, a detailed examination of the aerosol height from CALIOP and the AODs from MODIS, along with sensitivity tests, was carried out by varying the different assumed parameters in the retrieval (imaginary index of refraction, size distribution, aerosol height, particle shape). It was found that the spherical shape assumption for dust in the current retrieval is the main cause of the underestimate. In addition, it is demonstrated in an example how an 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 retrieval algorithm.
Highlights
Lack of information on absorbing aerosol properties, as well as their horizontal and vertical distribution, have been singled out as one of the major sources of uncertainty in the computation of global radiative forcing (Loeb and Su, 2010; Bond et al, 2013, Gómez-Amo et al, 2014; Wang et al, 2014; Samset and Myhre, 2015)
It is expected that the differences between C6 and C5.1 MODerate-resolution Imaging Spectrometer (MODIS) aerosol optical depth (AOD) products are minimal for the application presented here since all collocations with Ozone Monitoring Instrument (OMI) using MODIS are located within 30◦ from the equator
The third section evaluates the cause of observed underestimation of OMI AODs in certain scenes with dust aerosols
Summary
Lack of information on absorbing aerosol properties (single scattering albedo, SSA, and aerosol absorption optical depth, AOD), as well as their horizontal and vertical distribution, have been singled out as one of the major sources of uncertainty in the computation of global radiative forcing (Loeb and Su, 2010; Bond et al, 2013, Gómez-Amo et al, 2014; Wang et al, 2014; Samset and Myhre, 2015). Comparisons of the OMAERUV retrievals of the AOD and SSA with independent measurements have been made over land sites (Torres et al, 2007, 2013; Ahn et al, 2008, 2014; Jethva et al, 2014), and a number of features have been identified that impact the retrieval, mainly the height of the aerosol layer under observation and subpixel cloud contamination. An examination of the impact of cloud contamination in the retrievals will be important in applications such as transport of dust and pollution across the Atlantic and Pacific basins This requires an adequate characterization of OMI aerosol optical depth retrievals over the ocean.
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