Dependence of apparent optical properties on solar altitude: Experimental results based on mooring data collected in the Sargasso Sea

Abstract

The in situ variability of the vertical attenuation coefficient for downward irradiance (Kd) and radiance reflectance (RL) in response to changing solar altitude has been investigated. Observations were made 18 times per day from a fixed position mooring deployed in the Sargasso Sea, The mooring included instrumentation to measure spectral downwelling irradiance (Ed) and spectral upwelled radiance (Lu) at 15‐ and 35‐m depths. Time series data used for our analysis represent conditions of low, almost constant concentration of chlorophyll‐like pigments in the water and sunny or partly cloudy sky conditions. During sunny days the dominant component of the observed diel variability of the apparent optical properties (AOPs) was caused by the variable solar altitude. The statistical analysis performed on the data indicates a significant correlation between Kd and the cosine of the zenith angle of the direct solar beam after refraction at the air/water boundary (μ). These changes were wavelength dependent; Kd at 412, 443, 490, 510, and 555 nm decreased by 18–30%, while Kd at 665 and 683 nm increased by more than 50% when solar altitude increased from about 17° to 60° (i.e., when μ increased from 0.69 to 0.93). The vertical attenuation coefficient for photosynthetically available radiation (KPAR) decreased by about 25%. The daily amplitudes of similar changes in RL at the blue/green light were much smaller, up to about 4 to 8% at 412, 443,490, and 510 nm and 11% at 555 nm. However, at 665 and 683 nm, RL changed by more than 100%. This large increase of RL and decrease of Kd at 665 and 683 nm with Sun altitude (with μ) can be explained by the fact that the light field at red wavelengths was strongly affected by inelastic processes such as Raman scattering and natural fluorescence. Remote‐sensing techniques rely on the interpretation of reflectance and upwelled radiance ratios. We observed up to 8% changes in reflectance ratios and up to 17% changes in upwelling radiance ratios during the day. These maximum changes were observed for the ratios including Lu or RL at 555 nm, which were affected by inelastic scattering of light. Although our data illustrate the dependence of AOPs on the Sun altitude on a daily timescale, similar relationships are expected to be of importance on a seasonal scale and with latitude.