Using Satellites to Measure UV Light in the Ocean

“At all levels of marine and climate research, highly precise data on sunlight is indispensable,” says Astrid Bracher from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). “Climate models are only reliable when we have comprehensive information on the intensity, absorption and penetration depth of sunlight in the oceans, allowing us to make correct statements on the oceans’ important services – like primary production on the part of phytoplankton. In the visible spectrum of light, the sensors now used on numerous satellites offer a good basis of data. In the shortwave ultraviolet range of the spectrum, until recently there was no such basis. As a result, information on the UV light in the oceans had to be indirectly derived from the satellite data on the visible light, which involved empirical methods and often produced imprecise results.”

In the oceans, too, UV light can harm the cells of microalgae and microorganisms. In addition, it can change the chemistry of the oceans: it affects the bioavailability of important trace metals like iron and copper, as well as the exchange of trace gases between the water and atmosphere. “If our goal is that all these effects are precisely reflected, for instance in climate models, we have to do away with the uncertainties in the available data,” says Bracher. “In our study, we show a new method that can achieve exactly that.”

For the study, which was financed by the European Space Agency (ESA), the team of researchers led by Bracher including the study’s first author, Julia Oelker, used data from the TROPOMI spectrometer on board the Sentinel-5 Precursor satellite. Sentinel-5P is an Earth observation satellite launched by the ESA in 2017 as part of the European Union’s Copernicus Programme. Its TROPOMI (TROPOspheric MOnitoring Instrument) sensor is one of the very first satellite sensors to deliver with high spatial resolution data on the ultraviolet spectrum and can detect the UV light reflected back from the oceans with a high spectral resolution of 0.5 nanometres.