Long-term Operation of the AERONET-OC Station Palgrunden (Vänern) for validation of Copernicus data

The NASA Aerosol Robotic Network (AERONET) is a global network consisting of approximately 600 autonomous ground-based instruments that measure the optical properties of aerosols (i.e. optical thickness of aerosols and the Angstrom coefficient).

For validation of marine remote sensing data, the network has been further developed to AERONET - Ocean Color (AERONET-OC).

This part of the network consists of instruments that have the extra ability to also measure the water-leaving radiance from the lake surface.The AERONET-OC station ‘Palgrunden’ in Lake Vänern was established by ESA and SNSA in 2008.

It is a vital component for the development and validation of Sentinel-3 (S3) mission data at high latitudes and over boreal lakes with high humic content (highly absorbing waters).

The station is crucial for the Copernicus ocean colour mission as the retrieval of ocean colour products over highly absorbing waters still poses a challenge.

Our Marine Remote Sensing Group (MRSG) at the Department of Ecology, Environment and Plant Sciences (DEEP) has been part of the MERIS validation team since 2001 and is also part of the Sentinel-3 validation team, and the data from the AERONET-OC Palgrunden has already be used for the validation of Sentinel-2 mission data.As the data from AERONET-OC is generally regarded as state-of-the-art for the validation of Ocean Colour (OC) satellite data the Palgrunden station is vital for the success of the Copernicus missions and for validation of Sentinel-3 as well as Sentinel-2 processors and products in Swedish waters.

The satellite processors and products can be improved and validated for Lake Vänern, and thereby become also applicable for other highly absorbing waters, such as the Baltic Sea.

The Pålgrunden station performs high-frequency measurements, and thereby offers a possibility to monitor short- and long-term changes in water quality and atmospheric composition.

The station can thus be considered an important national infrastructure of great relevance for Swedish research and development, and, additionally, for demonstrating that Earth Obeservation data can be a reliable part of freshwater management programs.In 2017, the Swedish National Space Agency decided to fund the Aeronet-OC project as a national infrastructure of international importance through to 2029 which allowed us to continue the measurements.

Since then, we have upgraded the station with a new CIMEL CE-318-T series of instruments with support from the Joint Research Centre (JRC) in Ispra, Italy. The CE-318-T series of instruments is state of the art and the only one accepted for future AERONET-OC deployments.

JRC also give us high quality technical support in terms of instrument calibration in a dedicated optical calibration laboratory, and JRC also provide help with data quality assurance.

NASA supports the project by yearly intercalibration against the same source, and by acting as data host and making the data available via the NASA AERONET-OC web site (https://aeronet.gsfc.nasa.gov/new_web/ocean_color.html).

We propose here a continuation of the long-term funding from the Swedish Natioanl Spave Agency in order to sutain operation and data exploitation of this national infrastructure, and to secure high quality data for the validation of S3 and Sentinel-2 (S2) data throughout the ESA Copernicus mission.

The continuation of the station clearly enhances the societal benefit and application of Copernicus data in Swedish natural waters.

The high quality reference data does not only serve Earth Observation research and fresh water monitoring programs, but can also support many other fields of research, such as climate change and atmospheric research.Overall, the project is relevant to the UN´s global environmental goals, in particular Goal 13 ´Climate action´ and Goal 14 ´Life below water´.

The project also addresses some of the Swedish environmental goals, e.g. ´Reduced climate impact´´ and ´No eutrophication´.