The Sensitivity of SeaWiFS Ocean Color Retrievals to Aerosol Amount and Type

• Published in: Journal of atmospheric and oceanic technology 2016-06, Vol.33 (6), p.1185-1209
• Main Authors: Kahn, Ralph A., Sayer, Andrew M., Ahmad, Ziauddin, Franz, Bryan A.
• Format: Article
• Language: English
• Subjects: Aerosol effects; Aerosol measurements; Aerosol optical depth; Aerosol transport; Aerosols; Albedo; Algorithms; Anomalies; Anthropogenic factors; Atmospheric correction; Atmospherics; Biological effects; Calibration; Carbon; Chlorophyll; Chlorophyll a; Color; Critical components; Dust; Earth Resources And Remote Sensing; Ecosystem biology; Ecosystems; Field of view; Marine; Ocean color; Ocean colour; Oceans; Optical analysis; Optical thickness; Radiance; Reflectance; Retrieval; Sea-viewing Wide Field-of-view Sensor; Seasons; Seawater; Sensitivity analysis; Spacecraft; Wavelengths
• ISSN: 0739-0572; 1520-0426
• DOI: 10.1175/JTECH-D-15-0121.1

As atmospheric reflectance dominates top-of-the-atmosphere radiance over ocean, atmospheric correction is a critical component of ocean color retrievals. This paper explores the operational Sea-viewing Wide Field-of-View Sensor (SeaWiFS) algorithm atmospheric correction with approximately 13 000 coincident surface-based aerosol measurements. Aerosol optical depth at 440 nm (AOD(sub 440)) is overestimated for AOD below approximately 0.1-0.15 and is increasingly underestimated at higher AOD; also, single-scattering albedo (SSA) appears overestimated when the actual value less than approximately 0.96.AOD(sub 440) and its spectral slope tend to be overestimated preferentially for coarse-mode particles. Sensitivity analysis shows that changes in these factors lead to systematic differences in derived ocean water-leaving reflectance (Rrs) at 440 nm. The standard SeaWiFS algorithm compensates for AOD anomalies in the presence of nonabsorbing, medium-size-dominated aerosols. However, at low AOD and with absorbing aerosols, in situ observations and previous case studies demonstrate that retrieved Rrs is sensitive to spectral AOD and possibly also SSA anomalies. Stratifying the dataset by aerosol-type proxies shows the dependence of the AOD anomaly and resulting Rrs patterns on aerosol type, though the correlation with the SSA anomaly is too subtle to be quantified with these data. Retrieved chlorophyll-a concentrations (Chl) are affected in a complex way by Rrs differences, and these effects occur preferentially at high and low Chl values. Absorbing aerosol effects are likely to be most important over biologically productive waters near coasts and along major aerosol transport pathways. These results suggest that future ocean color spacecraft missions aiming to cover the range of naturally occurring and anthropogenic aerosols, especially at wavelengths shorter than 440 nm, will require better aerosol amount and type constraints.