SEMPAS: A UV-visible Spectroscopy Instrument for Monitoring Shipping Emissions and Atmospheric Composition on the North Sea

Abstract. Ship emissions contribute significantly to global transport pollution, foremost NO_x and SO_x. To mitigate this issue, emission regulated areas have been established, such as in the North Sea. Monitoring systems are essential to ensure consistent compliance to the emission limits. The Ship Emission Monitoring with Passive Absorption Spectroscopy (SEMPAS) instrument has been developed to provide operational monitoring of individual ships; as they transit past the instrument's on-shore or off-shore installation location. SEMPAS applies the UV-visible Differential Optical Absorption Spectroscopy (DOAS) technique to identify SO₂ and NO₂ in the UV, serving as qualitative proxies for NO_x and SO_x emissions. Through an optical fiber bundle organised as a matrix, a coarse 2D scene of the ship plume is captured. Robust housing protects the hardware from the marine environment. The instrument is equipped with a camera, coupled with image recognition software and an Automatic Identification System (AIS) receiver to identify individual ships. The vessels are actively tracked along their trajectory, to perform multiple consecutive measurements of a single ship. In the absence of nearby ships, the instrument operates as a Multi-Axis (MAX-)DOAS system providing regular measurements of the vertical profiles of NO₂ and HCHO. These can be used for atmospheric composition research, as well as satellite validation in a marine environment. In this study, first shipping results are presented from a test campaign in Zeebrugge, Belgium. The ship tracking algorithm is stable, while NO₂ and SO₂ measurements show large variations. A good agreement is found between MAX-DOAS measurements and satellite observations, with discrepancies dominated by local emissions from the port environment.