Particulate absorption during Maria S. Merian cruise MSM09/1

Samples for the pigment data set were collected during RV Maria S. Merian cruise MSM9/1 in July/August 2008 in the North Atlantic from Bremen, Germany to St. John's, Newfoundland, Canada (see cruise report Kucera et al. 2014). Sampling was conducted at various stations from a rosette coupled to a conductivity temperature density (CTD) profiler at three to four depths and otherwise sampling regularly every 6 to 8 hours the surface water (approx. 10 m depth) from the ship's moonpool. Water samples were filtered on GF/F filters. Filters were immediately shockfrozen in liquid nitrogen and stored at -80°C until further analysis at the laboratories of the Alfred-Wegener-Institute Helmholtz Centre for Polar and Marine Research (AWI). 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 and Taylor (2017) phytoplankton pigment data.