331 Journal of Oceanography, Vol. 62, pp. 331 to 337, 2006 Keywords: ⋅ Ocean color, ⋅ remote sensing, ⋅ aerosols, ⋅ atmospheric correction, ⋅ chlorophyll. * Corresponding author. E-mail: RFrouin@UCSD.edu Copyright©The Oceanographic Society of Japan/TERRAPUB/Springer Retrieval of Chlorophyll-a Concentration via Linear Com- bination of ADEOS-II Global Imager Data ROBERT FROUIN 1 *, PIERRE-YVES DESCHAMPS 2 , LYDWINE GROSS-COLZY 1 , HIROSHI MURAKAMI 3 and TAKASHI Y. NAKAJIMA 4 1 Scripps Institution of Oceanography, University of California San Diego, 8810 La Jolla Shores Drive, La Jolla, CA 92037, U.S.A. 2 Laboratoire d’Optique Atmosphérique, Université des Sciences de Lille, 59655 Villeneuve d’Ascq, France 3 Earth Observation Research Center, Japan Aerospace Exploration Agency, Harumi, Chuo-ku, Tokyo 104-6023, Japan 4 Department of Network and Computer Engineering, Tokai University, Tomiyaga, Shibuya-ku, Tokyo 151-0063, Japan (Received 17 September 2005; in revised form 17 December 2005; accepted 19 December 2005) Top-of-atmosphere reflectance measured above the ocean in the visible and near in- frared, after correction for molecular scattering, may be linearly combined to re- trieve surface chlorophyll-a abundance directly, without explicit correction for aero- sol scattering and absorption. The coefficients of the linear combination minimize the perturbing effects, which are modeled by a polynomial, and they do not depend on geometry. The technique has been developed for Global Imager (GLI) spectral bands centered at 443, 565, 667, and 866 nm, but it is applicable to other sets of spec- tral bands. Theoretical performance is evaluated from radiation-transfer simulations for a wide range of geophysical and angular conditions. Using a polynomial with ex- ponents of –2, –1, and 0 to determine the coefficients, the residual influence of the atmosphere on the linear combination is within ±0.001 in most cases, allowing chloro- phyll-a abundance to be retrieved with a root-mean-squared (RMS) error of 8.4% in the range 0.03–3 mgm –3 . Application of the method to simulated GLI imagery shows that estimated and actual chlorophyll-a abundance are in agreement, with an aver- age RMS difference of 32.1% and an average bias of –2.2% (slightly lower estimated values). The advantage of the method resides in its simplicity, flexibility, and rapidity of execution. Knowledge of aerosol amount and type is avoided. There is no need for look-up tables of aerosol optical properties. Accuracy is adequate, but depends on the polynomial representation of the perturbing effects and on the bio-optical model se- lected to relate the linear combination to chlorophyll-a abundance. The sensitivity of the linear combination to chlorophyll-a abundance can be optimized, and the method can be extended to the retrieval of other bio-optical variables. ocean can be considered black (i.e., totally absorbing), and extrapolating the estimated radiance to shorter wave- lengths. The retrieved water-leaving radiance is then re- lated to chlorophyll concentration using a bio-optical model. This approach has been successful, and it is em- ployed in the operational processing of data from major satellite ocean-color missions. Other, more recent algo- rithms (André and Morel, 1991; Land and Haigh, 1996; Fraser et al ., 1997; Gordon et al ., 1997; Zhao and Nakajima, 1997; Chomko and Gordon, 1998) attempt to determine aerosol properties and pigment concentration simultaneously, in a single step. Through systematic vari- 1. Introduction Standard algorithms to estimate phytoplankton chlo- rophyll concentration from space (e.g., Gordon, 1978, 1997; Viollier et al ., 1980; Gordon and Wang, 1994; Fukushima et al., 1998; Antoine and Morel, 1999; Gao et al., 2000) aim at correcting accurately for atmosphere and surface effects. The procedure consists of estimating the aerosol radiance in the red and near infrared where the