Traceability, quality control and uncertainties of AERONET-OC data for satellite ocean color applications G. Zibordi 1, B. Holben 2, M.Gergely 1, M.Talone 1, F.Melin 1 and J.-F. Berthon 1 Earth Observation for Ocean-Atmosphere Interactions Science 2014 (ESA-ESRIN October 28-31, 2014) 1 European Commission, JRC, Ispra, IT 2 National Aeronautics Space Administration, GSFC, Greenbelt, MD Joint Research Centre

AERONET – Ocean Color is a sub-network of the Aerosol Robotic Network (AERONET), relying on modified sun-photometers to support ocean color validation activities with highly consistent time-series of L WN ( ) and  a ( ). G.Zibordi et al. A Network for Standardized Ocean Color Validation Measurements. Eos Transactions, 87: 293, 297, AERONET-OC Autonomous radiometers operated on fixed platforms in coastal regions; Identical measuring systems and protocols, sensors calibrated using a single reference source and method, and data processed with the same code; Standardized products of normalized water-leaving radiance and aerosol optical thickness. Rationale: Joint Research Centre

AERONET-OC (2002-present) NASA manages the network infrastructure (i.e., handles the instruments calibration and, data collection, processing and distribution within AERONET). JRC has the scientific responsibility of the processing algorithms and performs the quality assurance of data products (in addition to the management of 5 out of 15 sites). PIs establish and maintain individual AERONET-OC sites. Current management and responsibilities Joint Research Centre Current sites Planned sites Potential sites

Examples of AERONET-OC Sites Site: AAOT Location: Northern Adriatic Sea Water type: Case-1/Case-2 Period: 2002-present Site: GDLT Location: Northern Baltic Proper Water type: Case-2 Period: 2005-present (summer) Site: AABP Location: Persian Gulf Water type: Case-1 (?) Period: Joint Research Centre G.Zibordi et al. AERONET-OC: a network for the validation of Ocean Color primary radiometric products. Journal of Atmospheric and Oceanic Technology, 26, , 2009.

Validation of MODIS-A radiometric data Joint Research Centre Scatter plots of MODIS-A (MOD-A) versus AERONET-OC (PRS) L WN match-up values at selected center- wavelengths for the GLR site. N indicates the number of match-ups, L WN and rmsd are in units of mW cm −2 μm −1 sr −1, |  | is the mean of absolute percent differences while  is the mean of percent differences, and r 2 is the determination coefficient. The right panel in the second row displays the AERONET-OC L WN spectra utilized to construct match-ups.

Validation of MODIS-A aerosol optical depth data Joint Research Centre Scatter plot of MODIS-A (MOD-A) versus AERONET-OC (PRS)  a match-ups at selected center-wavelengths for the GLR site. N indicates the number of match-ups,  a and rmsd are dimensionless, |  | is the mean of absolute percent differences while  is the mean of percent differences, and r 2 is the determination coefficient. The right panel in the second row displays frequency distributions of α determined with τ a at 785 and 869 nm for MODIS-A, and 667 and 870 nm for PRS. The black characters and lines in the frequency distribution plot, indicate results from the analysis of MODIS-A data while grey characters and solid bars indicate results from the analysis of AERONET-OC data (m is the median and σ the standard deviation).

Intra-annual climatology at GLR for the band-ratios L WN (547)/L WN (488) for AERONET-OC (PRS) an MODIS-A (MOD-A) data, determined with all available data for the period Intra-annual climatology Joint Research Centre The intra-annual climatology confirms the existence of two bio-optical cycles: i. one occurring in winter and affecting the regions starting from the shelf and extending to the open waters; ii. the other mostly affecting the shelf and showing a maximum in spring and a successive one less pronounced in fall.

Each individual radiometer is calibrated at NASA-GSFC using an integrating sphere. Sample radiometers are re-calibrated at the JRC for quality assessment using an FEL-1000 Watts quartz-halogen lamp, and Spectralon 99% reflectance plaques. All calibrations are traceable to the National Institute for Standards and Technology (NIST). G.Zibordi, B.Holben, I.Slutsker, D.Giles, D.D’Alimonte, F.Mélin, J.-F. Berthon, D. Vandemark, H.Feng, G.Schuster, B.Fabbri, S.Kaitala, J.Seppälä. AERONET-OC: a network for the validation of Ocean Color primary radiometric products. Journal of Atmospheric and Oceanic Technology, 26, , Traceability JRC calibration rely on a NIST traceable FEL-C 1000W lamp and a 99% Spectralon Reflectance panel. NASA calibration rely on GSFC integrating sphere. NASA-JRC differences in absolute radiance calibrations are generally better  2% in the nm spectral interval and  3% beyond 700 nm. This result has been largely confirmed by an inter-comparison involving NIST in addition to NASA and JRC.

AERONET-OC products are classified at different levels: o Level 1.0->  L WN ( ) determined from complete measurement sequences. o Level 1.5->  Cloud screened aerosol optical thickness data exist; o  Replicate sky and sea radiance measurements exhibit low variance; o  Empirical thresholds are satisfied (e.g., exceedingly negative o values or high reflectance in the near infrared); o Level 2.0->  Pre- and post-deployment calibration coefficients exhibit o justifiable differences within 5%; o  L WN ( ) spectral shapes are consistent based on statistical o approaches (i.e., high statistical representativeness within the data set itself (self-consistency) or non-anomalous features with respect to a reference set of quality-assured data (relative-consistency)) ; o  A final spectrum-by-spectrum screening is passed. G.Zibordi, B.Holben, I.Slutsker, D.Giles, D.D’Alimonte, F.Mélin, J.-F. Berthon, D. Vandemark, H.Feng, G.Schuster, B.Fabbri, S.Kaitala, J.Seppälä. AERONET-OC: a network for the validation of Ocean Color primary radiometric products. Journal of Atmospheric and Oceanic Technology, 26, , D.D’Alimonte and G.Zibordi. Statistical assessment of radiometric measurements from autonomous systems. IEEE Transactions in Geoscience and Remote Sensing, 44, 1-11, Quality Control

L WN uncertainties Joint Research Centre Considering the measurement equation for L WN L WN = L W C A C Q with L W = L T -  L i The combined standard uncertainty of the normalized water-leaving radiance u(L WN ) is the composition in quadrature of any independent uncertainty due to sources  i affecting L WN. where L T is the total radiance measured above the sea surface, L i is the sky radiance,  is the surface reflectance, C Q removes the dependence from the viewing geometry and bidirectional effects, and C A removes the basic dependence on sun zenith, atmosphere and sun-earth distance. M. Gergely and G. Zibordi, “Assessment of AERONET L WN uncertainties,” Metrologia 51, 40–47 (2014).

G.Zibordi and K.J.Voss, Field Radiometry and Ocean Color Remote Sensing. In Oceanography from Space, revisited. V.Barale, J.F.R.Gower and L.Alberotanza Ed.s, Springer, Dordrecht, pp , Relative combined uncertainties u(L WN )/L WN (%) and (in square brackets) combined standard uncertainties u(L WN ) and median L WN (mW cm −2 sr −1 μm −1 ), respectively, at different λ (nm) for various AERONET-OC sites. Joint Research Centre L WN relative uncertainties at different AERONET-OC sites GUM

Conclusions AERONET-OC is an operational network delivering globally distributed and cross-site consistent measurements of  a and L WN in coastal and occasionally at open sea sites. Qualifying element is the capability of delivering both,  a and L WN based on metrology principles and standardization of its components. Major application is the validation of satellite ocean color primary data products. However, it also offers the capability of supporting climatological studies on atmospheric and marine processes related to primary or derived quantities. Joint Research Centre Thanks