# A New Algorithm for Simultaneous Retrieval of Aerosols and Marine Parameters
## 2 Data and Methods
### Radial Basis Function Neural Network (RBF-NN)-Training and Evaluation
#### 2.3.2 Training and Evaluation  
In order to obtain accurate results from Equation (24) it is vital to have adequate input data for the RBF-NN training. To train and evaluate the RBF-NN we chose input data relevant for the coastal waters around Rost (67.79\({}^{\circ}\) N, 12.01\({}^{\circ}\) E), Norway, whose location is shown in Figure 1. At this site there are in-situ measurements of marine parameters (chlorophyll concentration, mineral concentration, and CDOM absorption), and the site is not too far from Andenes (69.28\({}^{\circ}\) N, 16.01\({}^{\circ}\) E), Norway, where the Aerosol Robotic Network (AERONET) has provided measurements of aerosol properties for more than 10 years. Marine in-situ measurements at Rost ([PERSON] et al. [46]) have provided values of the chlorophyll concentration, total suspended matter (TSM), and CDOM absorption. The chlorophyll concentration was obtained from collected water samples using High Pressure Liquid Chromatography (HPLC) based on the method of [PERSON] and [PERSON] et al. [47]. The CDOM absorption was determined by converting absorbance spectra recorded by a dual-beam Shimadzu spectrophotometer (UV-1800) of filtered water samples. TSM was obtained by taking the difference between the weights of [PERSON] 47 nm GF/F filter after and before filtration. To obtain the concentration of mineral particles, the chlorophyll concentration was subtracted from the TSM. Aerosol parameter values for Andenes were downloaded from AERONET to obtain the volume concentration of fine-mode particles (V\({}_{f}\) [\(\mu\)m\({}^{3}\)/\(\mu\)m\({}^{2}\)]), the volume concentration of coarse-mode particles (V\({}_{c}\) [\(\mu\)m\({}^{3}\)/\(\mu\)m\({}^{2}\)]), and the total volume concentration of particles (V\({}_{T}\) [\(\mu\)m\({}^{3}\)/\(\mu\)m\({}^{2}\)]). Volume concentration is the fraction of a given volume occupied (filled) by particles. In contrast to mass concentration, volume fraction is independent of the bulk  
Figure 1: Location of the two sites, Rest and Andenes. density of the particle material. In our retrieval algorithm the volume fraction of fine-mode particles is defined as \(f=\mathrm{V}_{f}/\mathrm{V}_{T}\), and the aerosol volume fraction is defined as \(\mathrm{V}_{T}\) divided by the thickness in [\(\mathrm{\mu m}\)] of the aerosol layer, which was assumed to extend from sea level to a height of 2 km. The variation range of each of the marine parameters was determined by considering the values obtained from all cruise measurements around Rost and then selecting the minimum and maximum values. The minimum and maximum values of the aerosol parameters were determined by considering monthly averages of the aerosol volume fraction and the volume fraction of fine-mode particles (\(\mathrm{V}_{f}\)) and then picking the smallest and largest values. The ranges used were: CDOM absorption coefficient (0.11-0.16) m\({}^{-1}\) at 440 nm; chlorophyll concentration (0.7-2.0) mg m\({}^{-3}\); mineral concentration (0.2-0.7) g m\({}^{-3}\); aerosol volume fraction (\(3.75\times 10^{-12}\)-3.24 \(\times 10^{-11}\)); and volume fraction of fine-mode particles (\(\mathrm{V}_{f}\)) (0.80-0.84). After determining the range of each of the five input/retrieval parameters, we used a random number generator to provide 10,000 different random sets of parameter values. These 10,000 sets of parameter values were used as input to a well-tested RTM for a coupled atmosphere-ocean system [14; 32] to simulate measured TOA radiances in the MODIS channels at 412, 442, 487, 530, 554, 666, 746, and 866 nm. In all the TOA radiance simulations we assumed the satellite sensor zenith angle to be at nadir because all the stations at which the in-situ measurements where carried out the satellite sensor zenith varies between 0.2\({}^{\circ}\) to 1.5\({}^{\circ}\).