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The impact of a SECA on measured SO2 concentrations

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The Sulphur Emission Control Area (SECA) in the North Sea that became effective in the course of 2007 has reduced the sulphur emissions from Marine shipping in four steps, the last being in 2015. This has resulted in lower emissions and lower ambient concentrations of sulphur dioxide (SO2) and particulate matter (PM). The impact of this measure has mainly been studied by modelling. A few studies exist where the change in ambient SO2 concentrations in coastal or port monitoring sites was established by monitoring. These reports mainly looked at the 2015 step.

For the port of Rotterdam a 2005-2015 emission trend was compiled (Amsterdam 2008-2015). The emissions reductions in SO2 and PM are shown in Table S1. Data in italics-blue are revised or estimated (for a full explanation see Annex 2 of the report). The data show an increase in CO2 emissions, indicating an increase of fuel use by the shipping sector. The number of ships visiting the port has gone down in the last couple of years, indicating that average ships’ size has increased. The assumed emission reductions as a consequence of the SECA are clearly visible in 2007-2009, 2009-2010 and 2014-2015. Overall, in the port of Rotterdam area the SO2 marine shipping emissions were reduced by  5.7 kton/y. In the same period the industrial emissions in Rotterdam were reduced by  19.4 kton/y. Since the shipping emissions are released at a lower altitude their direct impact on the air quality is higher than that of the land based sources.

The average SO2 concentration changes due to shipping and industry in Rotterdam are shown in Table S2 of the report. It should be noted that large changes in the shipping emissions occur in coastal waters and on the Dutch part of the North Sea. Long range transport of these emissions does
influence coastal concentrations (and in particular secondary PM formation; see below) as well. The changes in the so called background in Table S2 of the report are related to North Sea shipping and industrial sources in the Netherlands and abroad. For the Dutch locations where an impact of the SECA measures on the SO2 concentration could be established, the average concentration change of each SECA step are shown in Table S3 of the report. The last two steps seem somewhat larger in Amsterdam compared to Rotterdam, but no explanation was found.

A literature review of some modelled studies revealed that low sulphur fuels had an impact on the PM concentrations. Over large areas (100 or more km inward) a reduction of secondary PM can be observed. A study into the 2010 changes of the shipping emissions reported a 10% change for the Netherlands in the PM contribution of marine shipping. In the immediate vicinity of the coast and ports lower concentration reductions also occur due to the lower emissions of primary PM. The reduction of the shipping contribution to secondary PM is not proportional to the reduction of the sulphur emissions. Though substantially less ammonium sulphate aerosol is formed, a small increase of ammonium nitrate can be observed. However, there is a net benefit from the sulphur reduction: less secondary inorganic aerosol is formed. The implementation of a NECA should further reduce secondary PM attributed to shipping as some of the modelling studies show. The impact of policies aimed at marine shipping is usually assessed with models. This study shows that verification measurements are sometimes possible. To improve the quality of the modelling assessments, shipping emission inventories would have to be improved. The variation between emission data is currently substantial. It could be interesting, in retrospect, to do a modelling study into the impact of all four SECA steps, now that all actual emissions are known. This would reveal to what extend the SECA has contributed to the downward trend of secondary PM in the past decade.

Conclusions
Air quality monitoring can be used to assess the impact of the SECA measures on SO2. For other pollutants other research methods will be needed.
Air quality modelling is a common way to assess the impact of air quality measures. This has the advantage that impacts over large areas can be assessed. In the case of shipping, the modelling would benefit from better emission databases as the currently available datasets vary widely. A modelling study could be done to assess in retrospect the impact of all four SECA steps.

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