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Summary: The aim of this study was to investigate the light transfer through sea ice with a focus on bio-optical substances both in fast ice and in the drift ice zones in the northern Baltic Sea. The measurements included snow and ice structure, spectral irradiance and photosynthetically active radiation below the sea ice. We also measured the concentrations of the three main bio-optical substances which are chlorophyll-a, suspended particulate matter, and coloured dissolved organic matter (CDOM). These bio-optical substances were determined for melted ice samples and for the underlying sea water. The present study provides the first spectral light transfer data set for drift ice in the Baltic Sea. We found high CDOM absorption values typical to the Baltic Sea waters also within sea ice. Our results showed that the transmittance through bare ice was lower for the coastal fast ice than for the drift ice sites. Bio-optical substances, in particular CDOM, modified the spectral distribution of light penetrating through the ice cover. Differences in crystal structure and the amount of gas inclusions in the ice caused variation in the light transfer. Snow cover on ice was found to be the dominant factor influencing the light field under ice, confirming previous studies. In conclusion, snow cover dominated the amount of light under the ice, but did not modify its spectral composition. CDOM in the ice absorbs strongly in the short wavelengths. As pure water absorbs most in the long wavelengths, the light transfer through ice was highest in the green (549–585 nm).

A Water Quality Monitor (WQM) equipped with a range of oceanographic sensors was deployed from April 2017 to October 2017 in the North Western (NW) Baltic Sea. We assessed here if the data from a moored chlorophyll-a fluorometer can be used to improve satellite validation in coastal waters. Calibrated mooring data and ship-based chlorophyll-a concentrations from 2017 and 2018 were matched with OLCI (Ocean and Land Colour Instrument) data to validate the C2RCC (Case-2 Regional Coast Colour) processor, a locally-adapted version of C2RCC (LA-C2RCC), as well as the POLYMER processor. Using additional mooring data resulted in a substantial increase in paired observations compared to using ship-based data alone (C2RCC; N = 41–63, LA-C2RCC; N = 37–59, POLYMER; N = 108–166). However, the addition of mooring data only reduced the error and bias of the LA-C2RCC (MNB: from 24 % to 22 %, RMSE: from 60 % to 57 %, APD: both 47 %). In contrast, the statistical errors increased with the addition of mooring data both for C2RCC (MNB: −26 % to −33 %, RMSE: 50 %–51 %, APD 84 %–96 %) and for POLYMER (MNB: 26 %–36 %, RMSE: 79 % to 79 %, APD 64 %–64 %). The results indicate that the locally-adapted version of the C2RCC should be used for the area of investigation. These results are most likely also related to the effect of the System Vicarious Calibration (SVC). As opposed to C2RCC, the locally-adapted version had not been vicariously calibrated. The results indicate that SVC is not beneficial for Baltic Sea data and that more work needs to be done to improve SVC for Baltic Sea waters or for other waters with high CDOM absorption. In order to improve the validation capabilities of moored fluorometers in general, they should be strategically placed in waters with representative ranges of chl-a concentrations for the area of research in question.

A five-year dataset of spectral aerosol optical thickness was used to analyse the seasonal variability of aerosol optical properties (the aerosol opticalthickness (AOT) at wavelength λ=500 nm, AOT(500) and the Ångström exponent for the 440-870 nm spectral range,α(440, 870)) over the Baltic Sea and dependence of these optical properties on meteorological factors (wind direction, wind speed and relativehumidity). The data from the Gotland station of the global radiometric network AERONET (Aerosol Robotic Network, http://aeronet.gsfc.nasa.gov)were taken to be representative of the Baltic Sea conditions. Meteorological observations from Fårosund were also analysed. Analysis of the data from 1999 to 2003 revealed a strong seasonal cycle in AOT(500) and α(440, 870). Two maxima of monthly mean values of AOT(500)over the Baltic were observed. In April, an increase in the monthly mean aerosol optical thickness over Gotland most probably resulted from agricultural waste strawburning, mainly in northern Europe and Russia as well as in the Baltic states, Ukraine and Belarus. During July and August, the aerosol optical thickness wasaffected by uncontrolled fires (biomass burning). There was a local minimum of AOT(500) in June. Wind direction, a local meteorological parameter strongly related to air mass advection, is the main meteorological factor influencing the variabilityof aerosol optical properties in each season. The highest mean values of AOT(500) and α(440, 870) occurred with easterly winds in both spring and summer,but with southerly winds in autumn.

In this paper we compare the followingMERIS processors against sea-truthing data:the standard MERIS processor (MEGS 7.4.1), the Case 2Regional processor (C2R) of the German Institutefor Coastal Research (GKSS), and the Case 2Water Properties processor developed at the FreieUniversität Berlin (FUB). Furthermore, the ImprovedContrast between Ocean and Land processor (ICOL), a prototypeprocessor for the correction of adjacency effects from land,was tested on all three processors, and the retrieval of level 2data was evaluated against sea-truthing data before and afterICOL processing.