Abstract
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.
Highlights
1.1 Shipping – a fast-growing sectorShipping has always been an important mode of transportation throughout the course of history
On this day, elevated levels of NO2 were measured in the morning, corresponding to a polluted air mass coming from land, which appears as an enhanced, slowly varying NO2 background signal below the peaks
For 2015 and 2016, the total mean value declined by two-thirds to 0.15 ± 0.34 ppb. These results clearly show that the stricter limitations on the fuel sulfur content are working and significantly improved air quality in the North Sea coastal regions with respect to SO2
Summary
Shipping has always been an important mode of transportation throughout the course of history. Globalization of markets has lead to an enormous increase in world trade and shipping traffic in the last decades, with growth rates being typically about twice those of the world gross domestic product (GDP) (Bollmann et al, 2010). With a volume of 9.84 billion tons in 2014, shipping accounts for four-fifths of the worldwide total merchandise trade volume (UNCTAD, 2015), as compared to, Published by Copernicus Publications on behalf of the European Geosciences Union. As a result, shipping accounts for a significant part of the emissions from the transportation sector (Eyring et al, 2005b). Despite growth rates being lower compared to those prior to the 2008 economic crisis, seaborne trade is growing faster than the rest of the transportation sector, with an annual growth rate of 3–4 % in the years 2010 to 2014, compared to 2.0–2.6 % for the global merchandise volume (UNCTAD, 2014, 2015). Total fuel consumption and emissions increased as well (Corbett and Koehler, 2003; Eyring et al, 2005a, b, 2010b). Eyring et al (2005b) predicted that future development of shipping emissions will depend more on the usage of new technologies and imposed regulations than on the economic growth rates
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