Improved algorithms for accurate retrieval of UV/visible diffuse attenuation coefficients in optically complex, inshore waters

Photochemical processes driven by high-energy ultraviolet radiation (UVR) in inshore, estuarine, and coastal waters play an important role in global biogeochemical cycles and biological systems. A key to modeling photochemical processes in these optically complex waters is an accurate description of the vertical distribution of UVR in the water column which can be obtained using the diffuse attenuation coefficients of downwelling irradiance (K_d(λ)). The SeaUV/SeaUVc algorithms (Fichot et al., 2008) can accurately retrieve K_d (λ=320, 340, 380, 412, 443 and 490nm) in oceanic and coastal waters using multispectral remote sensing reflectances (R_rs(λ), SeaWiFS bands). However, SeaUV/SeaUVc algorithms are currently not optimized for use in optically complex, inshore waters, where they tend to severely underestimate K_d(λ). Here, a new training data set of optical properties collected in optically complex, inshore waters was used to re-parameterize the original SeaUV/SeaUVc algorithms, resulting in improved K_d(λ) retrievals for turbid, estuarine waters. Although the updated SeaUV/SeaUVc algorithms perform best in optically complex waters, the original SeaUV/SeaUVc models still perform well in most coastal and oceanic waters. Therefore, we propose a composite set of SeaUV/SeaUVc algorithms, optimized for K_d(λ) retrieval in almost all marine systems, ranging from oceanic to inshore waters. The composite algorithm set can retrieve K_d from ocean color with good accuracy across this wide range of water types (e.g., within a mean relative error of 13% for K_d(340)). A validation step using three independent, in situ data sets indicates that the composite SeaUV/SeaUVc can generate accurate K_d(λ) values at λ=320-490nm from ocean color on a global scale. Taking advantage of the inherent benefits of our statistical methods, we pooled the validation data with the training set, obtaining an optimized composite model for estimating K_d(λ) in UV wavelengths for almost all marine waters. This "optimized composite" set of SeaUV/SeaUVc algorithms will provide the optical community with improved ability to quantify the role of solar UV radiation in photochemical and photobiological processes in the ocean.