**1. Introduction**

Regulations ban the use of marine residual fuels with sulphur mass content above 0.5% from 2020, in all ocean areas. The sulphur limits in emission control areas (SECAs) at 0.1% m/m will remain. The regulation further permits the use of exhaust gas cleaning systems (EGCS) that reduce the SO2 concentration in the exhaust to levels equivalent to those from the regulated fuel sulphur limits [1]. The same regulation that limits the marine fuel sulphur content also aims at reducing particle emissions. A fundamental difference compared to the sulphur limit is that the permitted particle emission levels are not quantified but relies on the decrease of particle emissions with fuel sulphur content [2]. The emissions of particles after the exhaust gas scrubbing are accordingly neither regulated to a standard level nor is a specific measurement standard decided. The installation and operation of exhaust gas scrubbers on ships is often an attractive choice compared to operating a ship on refined fuel such as marine gasoil (MGO), from an economic perspective, see for example reference [3]. The classification society DNV GL estimates that approximately 4100 ships will be equipped with one or more EGCSs in 2021, based on statistics from a number of scrubber manufacturers [4]. Dry and liquid bulk carriers, container ships, cruise ships and roll-on/roll-off (RoRo) ships are the ship types most represented in the statistics. The engine power and fuel consumption of these ships are more important aspects than number of ships from an environmental perspective. A complete inventory on these parameters is not available. It seems though that large ships and high power engines are fitted with scrubbers to a larger extent than is the case for smaller ships.

Di fferent scrubber designs exist. Open sea water scrubbers utilize the natural alkalinity of sea water and keep a high flow of process water in order to reduce SO2 in the exhaust. The wash water is discharged to sea, most often without substantial treatment. In closed-loop scrubber designs, the process water is fresh- or seawater with an added alkaline chemical, often sodium hydroxide. The water is recirculated in the system and the sodium hydroxide neutralizes the sulphuric acid, see [5] for chemical reactions. Small portions of the process water are extracted from the process and passed through a water treatment unit before being discharged to the sea. This is often referred to as bleed-o ff water. The treatment produces a residue sludge that is brought ashore to a port reception facility. Open systems can use over 100 times more scrubber fluid per kWh output of the engine than a closed loop system. Washwater discharge criteria set minimum pH, maximum polycyclic aromatic hydrocarbons (PAH) level, maximum nitrate level, and maximum turbidity level with guidelines given in Resolution 259(68) of the Marine Environmental Protection Committee of the International Maritime Organization (IMO). The same criteria are valid for e ffluents from closed-loop systems as for the open systems. An important aspect for all types of exhaust gas scrubbers is the relocation of substances in the exhaust gas from air to the marine environment when scrubber systems are employed [6].

Quantification and characterization of particle emissions from marine engines fitted with exhaust gas scrubbers are interesting from environmental and health perspectives. The e ffects of an extensive use of scrubbers could significantly impact particulate air emissions from shipping. A limited number of previous studies have been published with the purpose to quantify and communicate the potential to reduce harmful exhaust gases and particles by using marine exhaust gas scrubbers; in total, specific emissions from six marine engines equipped with scrubbers have been found in scientific literature [7–11]. The ability of scrubbers to e fficiently abate SO2 is unchallenged while the e ffect of the exhaust gas scrubbing process on particle mass concentration varies significantly between the studies. Some measurements have indicated 75% reduction and more [7,9] and other studies conclude on more moderate particle reductions or even increases in particles over the scrubber [8,11]. A measured increase in particle mass over a wet scrubber has been explained as an increase in salt particles originating from salt in the process water and as an increase in sulphate particles due to low temperatures and high humidity after the scrubber [8]. Two peer reviewed studies comprise results from four engines in total and indicate between 7% and 75% particulate matter mass (PM) reduction over the scrubber of which one study presents the higher extreme, and the other find reductions below 45% from all tests [7,11]. The engine types, fuel oil, exhaust gas systems, and scrubber design di ffer between the two studies and could explain the di fferences in results.

Factors that influence particle removal in the scrubber include the design of the scrubber and scrubber process, and the composition of the exhaust gas particles [10]. The occurrence of semi-volatile species in the exhaust can be an important parameter since the temperatures in the scrubber process and during sampling will influence whether these species are solid or in gas phase. The sampling system setup could potentially be an important contributor to the large di fferences in results in di fferent studies.

Only few studies have reported on the e ffects of scrubbers on specific particle contents and the variations are high. Elemental carbon (EC) concentration reductions range from 50% to 81% and reductions of condensable organic carbon (OC) concentrations of 13%–41% and 73%, in [7] and [8] respectively, and the reductions presented in [11] appear to be within this range however not quantified. Particulate sulphate increased over the scrubber in [8]. PAH contents were analysed by [7] who observed a significant reduction of the tested PAHs over the scrubber. There is a need for more measurement data on particle emissions in order to assess the potential environmental and health impacts of the increased use of scrubbers in shipping.

This paper presents the emission reduction potential of a closed-loop scrubber and compares emissions with those from combustion of low sulphur fuel oil (LSFO). LSFO is sometimes referred to as ultra-low sulphur fuel oil (ULSFO) or hybrid oil. There is no standard specifying density or viscosity for the LSFO fuels, and the name can be collectively used for marine fuels that do not fulfil the marine distillate specifications for e.g. viscosity and density in the ISO 8217 standard, but that are below the regulated limit for sulphur content in fuel. In this case the limit is 0.1%, and the LSFOs became available on the market shortly after the sulphur SECA regulations became e ffective. It costs less than MGO and is, therefore, the preferable choice among some ship operators. With the 0.5% global sulphur limit it is likely that hybrid fuels will be common also outside SECAs.

Our measurements cover both gases and particulate matter. Particle contents have been analysed for organic and elemental carbon, elements, and sulphur. The PAH content in particle and gas phase was analysed, and the SO3/H2SO4 content in gas phase was sampled. Furthermore, online instruments were used to measure gas concentrations, black carbon concentrations and particle number concentrations (PN). Analyses of the scrubber water and ecotoxicology tests were part of the same campaign but are reported separately [6].
