Abstract
Abstract. Single scattering albedo (SSA) represents a unique identification of aerosol type and can be a determinant factor in the estimation of aerosol radiative forcing. However, SSA retrievals are highly uncertain due to cloud contamination and aerosol composition. The recent improvement in the SSA retrieval algorithm has combined the superior cloud-masking technique of the Moderate Resolution Imaging Spectroradiometer (MODIS) and the higher sensitivity of the Ozone Monitoring Instrument (OMI) to aerosol absorption. The combined OMI–MODIS algorithm has only been validated over a small spatial and temporal scale. The present study validates the algorithm over global oceans for the period from 2008 to 2012. The geographical heterogeneity in the aerosol type and concentration over the Atlantic Ocean, the Arabian Sea and the Bay of Bengal was useful to delineate the effect of aerosol type on the retrieval algorithm. We also noted that OMI overestimated SSA when absorbing aerosols were present closer to the surface. We attribute this overestimation to data discontinuity in the aerosol height climatology derived from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite. OMI uses predefined aerosol heights over regions where CALIPSO climatology is not present, leading to the overestimation of SSA. The importance of aerosol height was also studied using the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model. The results from the joint retrievals were validated using cruise-based measurements. It was seen that OMI–MODIS SSA retrievals performed better than the OMI only retrieval over the Bay of Bengal during winter, when the aerosols are present closer to the surface. Discrepancy between satellite retrievals and cruise measurements was seen when elevated aerosols were present which might not have been detected by the cruise instruments.
Highlights
Aerosols of different types are spatially distributed in a heterogeneous manner and at different altitudes in the atmosphere
Along with the aerosol products retrieved at different heights, the final set of AOD/Single scattering albedo (SSA)/absorption aerosol optical depth (AAOD) retrievals in the OMAERUV product is reported at the mean aerosol layer height (ALH) provided by the 30month long averaged climatology developed using Ozone Monitoring Instrument (OMI)– CALIOP combined observations (Torres et al, 2013)
SSA is highly sensitive to the aerosol composition, size and the wavelength at which the aerosol interacts with radiation
Summary
Aerosols of different types are spatially distributed in a heterogeneous manner and at different altitudes in the atmosphere. The quality of OMI (Ozone Monitoring Instrument) SSA retrievals is affected by sub-pixel cloud contamination (due to the larger footprint size of 13 km × 24 km) and uncertainty in the assumption of spectral surface albedo (Torres et al, 2007) To counter these problems and uncertainties in the OMI SSA retrieval (Table 2), Satheesh et al (2009) used retrievals from multiple satellites; they used combined retrievals from OMI–MODIS (Moderate Resolution Imaging Spectroradiometer) as each of these sensors have their own strengths and both fly within ∼ 7–8 min of each other in the A-train constellation (Stephens et al, 2002). Gassó and Torres (2016) performed a detailed analysis of the OMI UV product retrievals over oceans and island sites, comparing the OMI retrieved AOD with MODIS and AERONET (Aerosol Robotic Network) AODs They used the OMI–MODIS algorithm for two particular cases (only) over and near Africa to understand how the assumption of aerosol height and shape affected AOD and SSA retrievals.
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