Regional Y-27632 chemical structure algorithms, based on data measured in situ in a given area, are needed (http://optics.ocean.ru). Such measurements were carried out in the expeditions organised by the Russian State Hydrometeorological University (RSHU) in the summers of 2012 and 2013. The field studies were carried out on the yacht CENTAURUS II during 21–28 July 2012 and 20 July–02 August 2013; 15 stations were set up in 2012 and 26 in 2013. The positions of the station are given on

maps showing the spatial distributions of Chl concentration from MODIS-Aqua data on 22 July 2012 and 27 July 2013 derived by a standard MODIS algorithm (Figure 1a,b). According to these maps, Chl values on the most of stations were < 10 and even 20 mg m−3. In fact, the Chl concentration, directly measured in the study area, varied from 1.2 to 23.7 mg m0−3 in 2012 and from 1.6 to 18.6 mg m−3 in 2013. The Secchi depth varied from 1.8 m in the eastern part of the Gulf of Finland near the Neva Bay to 4.0 m in the open part of the Gulf. Station M2 of 26 July 2013 (Figure 1b) was rejected owing to the inconsistency of the measured Chl value with other values on that day; the remaining 40 stations were used for the derivation of the Chl regional algorithm. The spectral radiance reflectance was measured, the surface irradiance at 554 nm for controlling the illumination conditions continuously recorded and photographs of clouds taken at each station. Some of the stations were located directly selleck kinase inhibitor under the

passing MODIS-Aqua and VIIRS satellite scanners. Such measurements provided us with data for evaluating the atmospheric correction errors. This instrument measures the spectral upwelling radiance just beneath the sea surface and the spectral downwelling irradiance just above the sea surface (Artemiev et al. 2000). The spectral range is 390–700 nm, spectral resolution – 2 nm, the scan time – 15 s. The accuracy of measurement of absolute values of the radiance and irradiance is about 5%. Figure 2 shows the spectroradiometer during measurements. The measurements are taken at ever drift stations. The device drifts with the drogue at

a distance of 30–50 m from the ship to avoid the influence of the ship’s hull, and 20–30 scans are run during 15–20 min. The measurement data are processed with a specially developed computer program. The subsurface radiance reflectance p(λ) is calculated from equation(1) ρ(λ)=πLu(λ)/Ed(λ),ρ(λ)=πLu(λ)/Ed(λ),where Lu(λ)and Ed(λ), are the upwelling radiance and downwelling irradiance just beneath the sea surface. The calculated values of ρ(λ) were used to develop bio-optical algorithms and also to validate of the atmospheric correction algorithms if the measurements were performed simultaneously with satellite observations. Chlorophyll concentration was measured by a spectrophotometric method with 90% aqueous acetone solution. For calculating the chlorophyll aconcentration, data for the wavelengths of 630, 645, 663 and 750 nm were used ( Report 1966).