Diffuse downwelling attenuation coefficients for Southern Ocean waters for wavelengths 290-700 nm

Diffuse downwelling attenuation coefficients in Napierian form (DAC down) for Southern Ocean waters for wavelengths 290-700 nm. Coefficients for 320-700 nm were calculated directly from depth profiles of in situ irradiance made with an Ocean Optics Jaz spectrophotometer at PAL-LTER Station B, Arthur Harbor, Antarctica. The station is a 75 m deep sampling location about 1 km offshore. Due to the low signal-to-noise ratio in the in situ irradiance observations at wavelengths < 320 nm, coefficients were estimated for 290-320 nm using an exponential model fit to observed data from the 320-370 nm interval:

DAC down =e^(-0.0149 * lamda+3.60)

To achieve minimum boat shadow while making the irradiance profile measurements, the fiber optic cable and light sensor (cosine corrector) were streamed away from the small boat in a direction that was both to windward and toward the sun; the boat was then allowed to drift downwind from the measurement location to a suitable stand-off distance. A series of concurrent surface irradiance measurements made with a LI-COR PAR sensor (model LI-193SA; LI-COR Biosciences, Lincoln, NE, USA) was used to correct the spectra made as part of these profiles for any changes in incident light intensity that occurred between measurements.