Particulate absorption during POLARSTERN cruise ANT-XXVI/4 (PS75)

Samples were collected during the ANT-XXVI/4 expedition of the RV Polarstern in April/May 2010 along a South-to-North transect through the Atlantic Ocean from Punta Arenas (Chile) to Bremerhaven (Germany) at 32 CTD stations sampling at the surface, chlorophyll max. and 100 depth during noon time and at 57 stations sampling only surface waters through the moonpool of the ship (details see Taylor et al. 2013). Water samples were filtered on GF/F filters, shock-frozen in liquid nitrogen and stored at −80 C until further analysis in the home laboratory. Measurements of the particulate absorption were carried out on a dual-beam UV/VIS spectrophotometer (Cary 4000, Varian Inc.) equipped with a 150 mm integrating sphere (external DRA-900, Varian, Inc. and Labsphere Inc., made from Spectralon (TM)) using a quantitative filterpad technique modified as follows (see e.g. Simis et al., 2005). The filters were placed in the center of the integrating sphere using a center-mount filter holder perpendicular to the light beam. A wavelength scan from 300 to 850 nm with a resolution of 1 nm (slit width 2 nm, scan rate 150 nm min−1 ) was performed, when the reflectance ports were covered with Spectralon(TM) reflectance standards. The baseline was recorded beforehand with a clean, dry filter, and a filter which was soaked for more than 30 min in purified water served as a reference. The absorption coefficient was calculated from optical density (OD) measurements using a path length amplification factor of 4.5 (β = 1/4.5, Taylor et al. 2011) as a [m−1 ] = −ln (T · A · β/V ), where the transmittance T = exp (−OD), V is the filtrated sample volume in m3 and A the filter clearance area in m2 . Results from the original filter gave particulate absorption, ap. More details on the method can be found in Taylor et al. (2011). The data set is also collocated to Bracher (2015) phytoplankton pigment data.