Monitoring shipping emissions in the German Bight using MAX-DOAS measurements

Abstract. A 3-year time series of ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of NO₂ and SO₂ on the island Neuwerk has been analyzed for contributions from shipping emissions. The island is located in the German Bight, close to the main shipping lane (at a distance of 6–7 km) into the river Elbe towards the harbor of Hamburg. Measurements of individual ship plumes as well as of background pollution are possible from this location. A simple approach using the column amounts of the oxygen molecule dimer or collision complex, O₄, for the determination of the horizontal light path length has been applied to retrieve path-averaged volume mixing ratios. An excellent agreement between mixing ratios determined from NO₂ retrievals in the UV and visible parts of the spectrum has been found, showing the validity of the approach. Obtained mixing ratios of NO₂ and SO₂ are compared to co-located in situ measurements showing good correlation on average but also a systematic underestimation by the MAX-DOAS O₄ scaling approach. Comparing data before and after the introduction of stricter fuel sulfur content limits (from 1 to 0.1 %) on 1 January 2015 in the North Sea Emission Control Area (ECA), a significant reduction in SO₂ levels is observed. For situations with wind from the open North Sea, where ships are the only local source of air pollution, the average mixing ratio of SO₂ decreased by a factor of 8, while for NO₂ in the whole time series from 2013 to 2016, no significant change in emissions was observed. More than 2000 individual ship emission plumes have been identified in the data and analyzed for the emission ratio of SO₂ to NO₂, yielding an average ratio of 0.3 for the years 2013/2014 and decreasing significantly, presumably due to lower fuel sulfur content, in 2015/2016. By sorting measurements according to the prevailing wind direction and selecting two angular reference sectors representative for wind from the open North Sea and coast excluding data with mixed air mass origin, relative contributions of ships and land-based sources to air pollution levels in the German Bight have been estimated to be around 40 % : 60 % for NO₂ as well as SO₂ in 2013/2014, dropping to 14 % : 86 % for SO₂ in 2015/2016.