UV-Vis remote sensing of atmospheric pollutants from a wind turbine platform in the North Sea: the SEMPAS project

Ship emissions comprise up to 15% of global transport pollution. To regulate this pollution, Nitrogen Emission Control Area (NECA) and Sulphur Emission Control Area (SECA) zones have been introduced in the North Sea, which define a threshold for shipping emissions of respectively NO_x and SO_x. The current method of control of these regulations in the Belgian North Sea uses an aircraft equipped with sniffers to fly through the plume. As this is an intensive method, only a limited number of ships can be evaluated. The Ship Emission Monitoring by Passive Absorption Spectroscopy (SEMPAS) project develops a UV-Visible Differential Optical Absorption Spectroscopy (DOAS) instrument to permanently monitor ships from a Belgian offshore windfarm and to complement the aircraft-based measurements.

The instrument is an imaging DOAS system, with a field of view of 0.5x3 degrees corresponding to 3x15 pixels. The imaging functionality is obtained by means of an optical fiber bundle where fibers are organised in a matrix structure. Every fiber provides a separate measurement and corresponds to a pixel in the image that is created. The light is then fed into a grating spectrometer that measures slant columns of both SO₂ and NO₂ in the UV at a spectral resolution of 0.4 nm, based on the DOAS fitting principle. Next to the DOAS system, a second channel uses a Bruker EM27 FTIR instrument to measure CO₂ and SO₂ in emission mode. The aim is to quantify emission factors based on measured ratios of SO₂ to CO₂, NO₂ to CO₂ and SO₂ to NO₂.

The instrument is accompanied by a simple visible camera. An image recognition AI-based algorithm on the live feed of this camera ensures active tracking of passing ships, to increase sensitivity. From the measured image, the plume concentrations can be derived, as well as the background concentrations measured next to the plume.

In addition, the instrument can be set to MAX-DOAS mode, by binning the signals from the fibers to a point measurement with high sensitivity. This mode will be used when no ships are in the area. As the network of MAX-DOAS instruments in a marine environment is limited, this instrument can help in studying the marine atmosphere in more detail.

The instrument’s development is currently being finalised and it will be installed and start operations in spring 2024. The proposed poster presents an instrumental description, including calibration and initial results from test measurements.