# A New Algorithm for Simultaneous Retrieval of Aerosols and Marine Parameters
## 1 Introduction  
Algorithms for retrieval of atmospheric and marine parameters from measurements of back-scattered radiances at several wavelengths by instruments deployed on earth-orbiting satellites (ocean color data) have been developed over the past few decades. The earliest ocean color remote sensing algorithms were based on a two-step approach. First, an atmospheric correction was carried out to estimate the aerosol optical depth (AOD) at a near-infrared (NIR) channel (865 nm for Sea Wide Field of view (SeaWiFS) sensor), for which the ocean was assumed to be black (the NIR black-pixel approximation (NIR-BPA)). Second, AOD values for wavelengths in the visible range were obtained by extrapolation and used to generate water-leaving radiances [1, 2, 3, 4, 5]. The two-step algorithms employ regression or look-up table matching based on bio-optical models to estimate chlorophyll concentration from visible-channel water-leaving radiances. The weakness ofthis two-step approach is its reliance on the inaccurate NIR-BPA assumption, according to which atmospheric and oceanic radiative transfer processes can be de-coupled [6; 7]. To circumvent the problems associated with the two-step approach a one-step retrieval scheme that accounts for radiative coupling between the atmosphere and the ocean was proposed [8]. Amongst the first investigators to implement the one step retrieval scheme were [PERSON] and [PERSON] [9], [PERSON] et al. [10], [PERSON] et al. [11], and [PERSON] et al. [12]. The algorithms presented in [9; 10] are based on using a bio-optical model of [PERSON] et al. [13] to relate the water leaving reflectance to the water constituents, and on using an aerosol model to determine the contribution of absorbing aerosols to the satellite measured reflectance. Using these algorithms, one obtains a combination of ocean parameters and aerosol model that gives a best fit to the measured reflectance throughout the spectrum. [PERSON] et al. [11] and [PERSON] et al. [12] employed a well-tested radiative transfer model (RTM) for a coupled atmosphere-ocean system [14; 15] to compute radiances at the top of the atmosphere (TOA) and an iterative inversion scheme for simultaneous retrieval of marine and atmospheric parameters. The algorithm in [12] was capable of retrieving two aerosol parameters and one marine parameter, which is appropriate for open-ocean water bodies in which chlorophyll is the main marine retrieval parameter of interest, whereas the algorithm in [11] was capable of retrieving three marine parameters and one aerosol parameter. Even though the algorithms in [11; 12] are more accurate than the two-step algorithms, they can not easily be extended to retrieve more parameters. The algorithm of [PERSON] et al. [16] combines the bio-optical model of [PERSON] and [PERSON] [17] with the spectral optimization algorithm of [PERSON] and [PERSON] [9] to retrieve a set of ocean parameters (CDOM at 443 nm, chlorophyll concentration, and backscattering coefficient of particulate matter at 443 nm) along with estimates of aerosol optical properties. However, this algorithm is based on aerosols having a wavelength-independent, real refractive index, and therefore cannot be applied to absorbing aerosols. [PERSON] et al. [18] developed an atmospheric correction _POLYnomial based algorithm applied to MERIS_ (POLYMER) algorithm which can retrieve ocean color parameters in the whole sun glint pattern. The algorithm employs a method of spectral matching where it first models the spectral reflectance of the atmosphere and sun glint by use of a polynomial and then employs a water reflectance model. [PERSON] et al. [19] developed a method for simultaneous retrieval of two aerosol parameters and three marine parameters based on a linearized, coupled atmosphere-ocean forward RTM [20] and optimal estimation to solve the inverse problem. The retrieved aerosol parameters included the AOD at 865 nm and a bimodal fraction of large vs. small particles.