**4. Conclusions**

This study suggests that the reduction of particle mass over an exhaust gas treatment system is not efficient enough to accomplish particle emission levels equal to those from combustion of low sulphur fuel oil. With a traditional marine gasoil or other low sulphur fuel oil as marine fuel, the particle emissions can be expected to be significantly lower than at the use of HFO and a scrubber. The results from our measurement and previous studies sugges<sup>t</sup> particle mass reductions of on average around 40% over exhaust gas scrubbers. Results of measurement studies on particle emission spread largely. This likely reflects the influence of sampling systems on the results. Particularly challenging are measurements in the cold exhaust gas downstream of the exhaust gas scrubber which are not covered by any standard.

The measurements also showed lower specific emissions of BC, EC, PAH-16 at operations on low sulphur fuel oil than downstream of the scrubber at heavy fuel oil operations. The scrubber efficiently reduced concentrations of gaseous SO2, SO3, total hydrocarbons and the heavier fraction of the measured PAHs.

The internationally agreed sulphur regulations for ships aim at reducing emissions to air of particles as well as sulphur oxides. In a wider context, the results from this study indicate that an extensive use of scrubbers may partly cancel the aimed for effect of reduced particle emissions to air that would be achieved by only using fuels with low sulphur content.

Further research on the amount of fuel used in the engines on ships equipped with scrubbers is important to draw conclusions on environmental and health effects of the technology.

**Author Contributions:** Conceptualization, H.W, E.F. and J.M.; investigation, H.W, E.F. and J.M.; writing—original draft preparation, H.W.; writing—review and editing, E.F and J.M. All authors have read and agreed to the published version of the manuscript. Authorship must be limited to those who have contributed substantially to the work reported.

**Funding:** This research was funded by the EU via Connecting Europe Facility and the IVL foundation.

**Acknowledgments:** The captains, chief engineers and crew are gratefully acknowledged for their support during emission measurements. The efforts on board by Håkan Salberg and Kjell Peterson were essential for the work.

**Conflicts of Interest:** The authors declare no conflict of interest.
