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quality control step was to remove days where the standard 
deviation was more than half of the daily average. This was 
meant to identify days with high variability. Very few days 
(N — 2) were removed with this test. These quality control 
criteria were applied per wavelength, which resulted in some 
observations with an incomplete spectrum. 
2.2.3 AErosol RObotic NETwork-Ocean Color 
(AERONET-OC) 
The AErosol RObotic NETwork-Ocean Color (AERONET- 
OC) is a component of AERONET, including sites where 
sun photometers operate with a modified measurement pro 
tocol leading to the determination of the fully normalised 
water-leaving radiance (Zibordi et al., 2006, 2009). The 
result of collaboration between the Joint Research Centre 
(JRC) and NASA, this component has been specifically de 
veloped for the validation of ocean-colour radiometric prod 
ucts. The strength of AERONET-OC is “the production of 
standardised measurements that are performed at different 
sites with identical measuring systems and protocols, cali 
brated using a single reference source and method, and pro 
cessed with the same codes” (Zibordi et al., 2006, 2009). All 
high-quality data (level 2) were acquired from the project 
website for 11 sites: Abu_Al_Bukhoosh (~ 25° N, ~ 53° E), 
COVE_SEAPRISM (~ 36° N, ~ 75° W), Gloria (~ 44° N, 
~ 29° E), Gustav_Dalen_Tower (~ 58° N, ~ 17° E), Helsinki 
Lighthouse (~ 59° N, ~ 24° E), LISCO (~ 40° N, ~ 73° W), 
Lucinda (~ 18° S, ~ 146° E), MVCO (~41°N, ~70°W), 
Palgrunden (~58°N, ~13°E), Venice (~45°N, ~ 12° E) 
and WaveCIS_Site_CSI_6 (~ 28° N, ~ 90° W). The com 
piled variable was rrs. Remote-sensing reflectance was com 
puted from the original fully normalised water-leaving radi 
ance (see Sect. 2.2.2 for definition). The solar irradiance (Fo), 
which is not part of the AERONET-OC data, was computed 
from the Thuillier (2003) solar spectrum irradiance by av 
eraging Fo over a wavelength-centred lOnm window. Data 
were compiled for the exact wavelengths of each record, 
which can change over time for a given site depending on 
the specific instrument deployed. 
2.2.4 SeaWiFS Bio-optical Archive and Storage System 
(SeaBASS) 
The SeaWiFS Bio-optical Archive and Storage System 
(SeaBASS) is one of the largest archives of in situ marine 
bio-optical data (Werdell et al., 2003). It is maintained by 
NASA’s Ocean Biology Processing Group (OBPG) and in 
cludes measurements of optical properties, phytoplankton 
pigment concentrations, and other related oceanographic and 
atmospheric data. The SeaBASS database consists of in situ 
data from multiple contributors, collected using a variety of 
measurement instruments with consistent, community-vetted 
protocols, from several marine platforms such as fixed buoys, 
hand-held radiometers and profiling instruments. Quality 
control of the received data includes a rigorous series of pro 
tocols that range from hie format verification to the inspec 
tion of the geophysical data values (Werdell et al., 2003). Ra 
diometric data were acquired through the Validation search 
tool, which provided in situ data with matchups for particular 
ocean-colour sensors (Bailey and Werdell, 2006). The crite 
ria in the search query were defined to have the minimal hag 
conditions in the satellite data in order to retrieve a greater 
number of matchups and therefore in situ data. Regarding 
phytoplankton pigment data, they were acquired through 
the Pigment search tool, which provides pigment data di 
rectly from the archives. As stated in the SeaBASS website 
(see Pigment tab at http://seabass.gsfc.nasa.gov/seabasscgi/ 
search.cgi), the Pigment search tool was originally designed 
to return only in vitro huorometric measurements, which is 
consistent with our approach, but over time chlorophyll a 
measurements made using other methods (e.g. in situ fluo- 
rometry) were included in the retrieved pigment data. In the 
pigment data used in this work, a large number of in situ flu- 
orometric measurements from continuous underway instru 
ments were identified and discarded. These data were firstly 
identified from cruises with more than 50 observations per 
day and then re-checked on the SeaBASS website to confirm 
whether indeed they were continuous underway measure 
ments. A total of 148 015 such measurements were identified 
and discarded. Given the large volume of this group of data, 
it is possible that some chlorophyll a observations from in 
situ methods may have escaped the scrutiny and made it into 
the final merged dataset. In the future, the SeaBASS plans 
to add ancillary information to the extractions, which will 
enable users to distinguish the different types of chlorophyll 
measurements. The compiled variables from SeaBASS data 
were: rrs, chla_hplc, chla_fluor, aph, adg, bbp and kd. No 
conversion was necessary since all variables were acquired 
in the desired format. 
2.2.5 NASA bio-Optical Marine Algorithm Data set 
(NOMAD) 
The NASA bio-Optical Marine Algorithm Data set (NO 
MAD) is a publicly available dataset compiled by the NASA 
OBPG at the Goddard Space Flight Center. It is a high- 
quality global dataset of coincident radiometric and phyto 
plankton pigment observations for use in ocean-colour algo 
rithm development and satellite-data product-validation ac 
tivities (Werdell and Bailey, 2005). The source bio-optical 
data are the SeaBASS archive; therefore, many dependencies 
exist between these two datasets, which were addressed dur 
ing the merging. The current version (Version 2.0 ALPHA, 
2008) includes data from 1991 to 2007 and an additional 
set of observations of inherent optical properties. The cur 
rent version was used in this work, but with an additional set 
of columns of remote-sensing reflectance corrected for the 
bidirectional effects (Morel and Gentili, 1996; Morel et al., 
2002). This additional set of columns was provided directly