Marine fluorescence detected in hyperspectral satellite data
PhD project (2011-2015) by Aleksandra Wolanin
Inelastic processes (e.g., Raman scattering and fluorescence) lead to a redistribution of solar backscattered electromagnetic radiation and shift of the frequency toward higher or lower energy. Inelastic scattering by molecules in the air (mostly N2 and O2) is called the rotational Raman scattering (RRS). In lakes, rivers and oceans there are two dominant inelastic processes: vibrational Raman scattering by water molecules (VRS), and fluorescence of phytoplankton pigments, mainly chlorophyll a, and colored dissolved organic matter (CDOM). Their effect on the backscattered radiation at the top of atmosphere can be identified in the filling-in of Fraunhofer lines (spectrally narrow and often saturated absorption features in the solar spectrum), the process known as the Ring effect.
The filling-in of Fraunhofer lines can observed with satellite-borne passive spectrometers, which have sufficient spectral resolution. We have examined these effects with an approach based on the Differential Optical Absorption Spectroscopy (DOAS) technique. applied to the hyperspectral data from SCIAMACHY instrument on board the ENVISAT satellite, which was in operation from 2002 until 2012. It gives as a unique possibility to observe the fluorescence processes from space. Here, we focus on the chlorophyll a fluorescence, which is directly linked to the physiology of phytoplankton or plant. When chlorophyll molecules absorb light, most of this energy is transformed into chemical energy in a process of photosynthesis. However, a fraction of the energy absorbed is re-emitted as fluorescence. Hence, as a result of its relationship to photosynthetic efficiency, information about chlorophyll fluorescence can be used to assess the productivity and physiological state of phytoplankton.
In our retrieval, we evaluate the filling-in in SCIAMACHY data of the Fraunhofer line Fe I at 684.3 nm, which is located very close to the emission peak of marine fluorescence (∼685 nm), see Fig. 1 left panel. What is more, we can also observe the signal coming from terrestrial vegetation. Subsequently, we have applied the analogous retrieval to another spectral region (∼756 nm, see Fig. 1 right panel), which is close to the second peak of chlorophyll fluorescence (∼740 nm). Here, we can still observe strong fluorescence signal from terrestrial vegetation, but no from oceans – due to the strong absorption of liquid water in this spectral region.
Knowledge about the strength of two fluorescence peaks can be used for several applications, e.g., the determination of the chlorophyll content leaf level or of the canopy structure. It is also the very first time, that the chlorophyll fluorescence from both land and ocean was retrieved with the same approach.
Wolanin A. (2015) Exploitation of hyperspectral satellite data for the detection of fluorescence originating from biological sources. PhD thesis, Department of Physics and Electrical Engeneering, University Bremen, 178 p., https://suche.suub.uni-bremen.de/peid=B88185774&LAN=EN&CID=&index=L&Hitnr=10&dtyp=h&rtyp=a
Wolanin A., Rozanov V., Noel S., Dinter T., Vountas M., Burrows J.P., Bracher A. (2015) Phytoplankton chl-a fluorescence from hyperspectal data. Remote Sensing of Environment 166:243-261
Wolanin A., Dinter T., Rozanov V., Noel S., Burrows J., Vountas M., Bracher A. (2014) Marine chlorophyll fluorescence from high spectrally resolved satellite measurements and its terrestrial application. Proceedings of the 5th International Workshop on Remote Sensing of Vegetation Fluorescence. Paris, France
Wolanin A., Dinter T., Rozanov V., Noel S., Burrows J., Vountas M., Bracher A. (2014) Marine chlorophyll fluorescence from high spectrally resolved satellite data. Proceedings of Ocean Optics XXII. Portland, ME, USA
Wolanin A., Rozanov V., Dinter T., Bracher A. (2015) Detecting CDOM fluorescence using high spectrally resolved satellite data: a model study. In: G. Lohmann, H. Meggers, V. Unnithan, D. Wolf-Gladrow, J. Notholt, A. Bracher (eds.), Towards an Interdisciplinary Approach in Earth System Science, SpringerBriefs in Earth System Sciences, Springer, Heidelberg, Germany. ISBN 978-3-319-13864-0, DOI 10.1007/978-3-319-13865-7, pages 109-121
Figure 1: Annual climatology of chlorophyll fluorescence at 684 nm (top panel) and 756 nm (bottom panel) from SCIAMACHY data