**2. Materials and Methods**

The test ship is a RoPax ferry taking rolling goods and passengers, on a scheduled timetable between the ports and is one of the largest in its category. The engine used for the trials is of model 8LMAN48/60. It is a 9600 kW, four-stroke engine with common rail assisted fuel injection. The scrubber system tested uses seawater, with added sodium hydroxide for increased alkalinity, as process water. Tests on LSFO were conducted in February 2017 and tests on HFO in September 2017.

All sampling is performed through sampling holes cut in the exhaust pipe for the probes. One set of holes are cut on deck 11, upstream of the scrubber and another set on deck 15, downstream of the scrubber. During the measurements at LSFO operations, the set of holes on deck 11 are used for sampling, while both decks are used during the tests with HFO.

Tests with similar scopes are made at multiple steady state engine loads close to these load points: 85%, 75%, 50%, and 34% of maximum continuous rating (MCR), for measurements on LSFO.

76%, 49%, and 32% MCR, for measurements on heavy fuel oil upstream of the scrubber.

76%, 48%, and 41% MCR, for measurements on heavy fuel oil downstream of the scrubber.

The engine loads are equally relevant to everyday operations, although long periods on loads below 35% MCR are avoided.

A number of gases and metrics of particles are included in the measurement scheme. The gas phase pollutants are sampled without dilution, extracting the hot exhaust sample through a heated probe directly to online instruments and adsorbent tubes. A Horiba PG 350 measures sulphur dioxide (SO2), carbon monoxide (CO) and carbon dioxide (CO2) by non-dispersive infrared (NDIR), and nitrogen oxide (NOX) by chemiluminiscence. Raw gas is prior to the instrument conducted through a heated tube with Teflon lining via a ceramic filter to a preparation unit. The tube is heated to 190◦C. The gas preparation unit cools the gas to 4◦C and removes particles by filtration. The dry and particle free gas is used for continuous concentration measurements in the instruments with the interval 1 second. Hydrocarbons are measured with a flame ionisation detector instrument of model Graphite 52M-D. The instrument monitors total hydrocarbon, non-methane hydrocarbon and methane simultaneously. Hydrocarbons are measured as total carbon in unit ppb(V) of CH4 equivalents.

Gaseous SO3/H2SO4 concentrations are sampled in adsorbent glass tubes containing NaCl placed directly inside the exhaust channel. A flow is pumped through the NaCl tube and gas-phase H2SO4 reacts to form sodium sulphate and sodium bisulphate. Any gas-phase SO3 is converted to H2SO4 in the flue gas at temperatures below 500◦C, and is not present at temperatures below 200◦C due to the presence of water vapour in the exhaust [12]. Subsequent analysis of the salt columns by ion chromatography for sulphates is made at the IVL Swedish Environmental Research Institute laboratory in Gothenburg.

Two di fferent devices for exhaust gas dilution are used prior to particle sampling and measurements. The first system is a dilution tunnel designed to comply with ISO standard 8178:2/8178:1 for partial dilution systems. Filtered ambient air is used as dilution gas. The dilution tunnel dilutes the exhaust gas sample to a ratio of ~1:5-20 and allows for minor adjustments of flow through the system. The exact dilution rate is checked by CO2 measurements using a sensor from Servomex in the diluted sample

and comparing with the readings from the Horiba PG 350 in the raw exhaust gas. The second system is a Dekati Fine Particle Sampler (FPS) model 4000, using pressurized and optionally heated air in a two-stage dilution, with an adjustable dilution to a ratio of up to 1:500. A heated probe is used in all measurements with the FPS. The primary di ffusion dilution stage dilutes the extracted exhaust sample with preheated air (set to ~250◦C). The secondary ejector dilution step uses pressurized air of ambient temperature. The dilution ratios are determined from the ratio of CO2 in raw and diluted exhaust gas using a CO2 analyser from LiCor in the diluted sample.

The dilution tunnel is used prior to filter sampling for determination of PM mass and composition at LSFO operations and at HFO operations upstream of the scrubber while filter sampling downstream of the scrubber is conducted using dilution with the FPS. The choice not to use the dilution tunnel downstream of the scrubber is mainly due to space restrictions and inaccessibility of that measurement point; the dimensions of the dilution tunnel are approximately 1.5 × 1 × 0.3 m and the sampling point downstream of the scrubber is located on deck 15 and accessible only via ladders from six decks below. In order to assure comparability of the two dilution alternatives, filter sampling was conducted at similar engine loads both with the dilution tunnel and FPS at operations on LSFO and on HFO upstream of the scrubber. For particle measurements performed with the online particle instruments, the FPS is always used for the dilution.

Sampling and dilution procedures bring uncertainties to the particle measurements. The first group of uncertainties relates to the representativeness of the extracted partial flow exhaust sample. Both dilution devices use isokinetic probes with adjustable inlet nozzle sizes to achieve near-isokinetic sampling. However, deviations can occur especially for the FPS device as the sample flux changes with the dilution ratio and the set-up does not allow for changes of the nozzle during the experiments. For particles in the typical size-range of the diesel exhaust particles, i.e., with a large part of the particle mass in sizes with particle diameter below 100 nm [13,14], deviation from the isokinetic sampling has only a small impact, and the isokinetic conditions are not required by ISO 8178 standard. Hence, potential influence of parameters that relates to particles' kinetics is not investigated further. Secondly, uncertainties are related to the dilution process a ffecting both condensation and nucleation of semi-volatile species and hence the measured PM mass and number. Achieving ISO 8178 standard sampling downstream of the scrubber is not possible as the exhaust temperature at this point is too low. Our filter sampling experienced deviations from the standard, including a stack temperature downstream of the scrubber that was too low and often lower filter sampling temperatures than prescribed. The FPS device is not designed to fulfil the ISO 8178. We see that sampling with FPS at LSFO combustion results in higher particle emission estimates than when using the dilution tunnel. Contrary to this, the particle emissions at HFO combustion upstream of the scrubber are often indicated to be lower for measurements with the FPS than for measurements with the dilution tunnel. A large variability of the measured emission factors, also when only samples taken with the dilution tunnel are considered, reveals large uncertainties, especially at low engine loads. An analysis of all individual filter samples still indicates agreemen<sup>t</sup> between the sampling systems at the tests with high engine loads. For the low engine load, the individual samples vary more and there is less agreemen<sup>t</sup> between the two sampling systems, see Table A1 in the Appendix A and Figure 1.

At the tests with LSFO and the dilution tunnel, the temperatures at the filters were between 29 ◦C and 33 ◦C, which is close to the dilution air temperature. This is lower than prescribed by ISO 8178. Filter temperatures are not recorded for the other tests. The raw gas transfer line is a 5 meter long tube with Teflon lining, heated to 190 ◦C. Dilution ratios were between 15.9 and 23.8. The ambient temperature was approximately 35 ◦C at tests on HFO at the location upstream of the scrubber and approximately 50 ◦C at the location downstream of the scrubber. We can assume similar temperatures also at the filters. The FPS gives a record of the temperature of the sample leaving the device, and the PM sample temperature was indicated to be between 32 and 50 ◦C during the tests (see the table in the Appendix A for details). The automatic logging of temperature data during the tests were performed in high surrounding temperature and it is probable that the high temperature cause problems with the

dilution system signals. The dilution ratios were however confirmed, also when logging failed, by the CO2 instruments. The transfer lines include approximately 1 meter insulated metal tubing prior to the FPS and 0.5 meter conductive tubing for the diluted sample. Table A1 in the Appendix A includes details on temperatures during sampling.

**Figure 1.** Particulate matter (PM) emission factors calculated from individual filter samples in a comparison between the two sampling systems used for exhaust gas dilution.

Particulate matter mass is sampled as PMtot and PM1.6 on polytetrafluoroethylene (PTFE) filters. Cyclones are used for the PM1.6 sampling, assuring that large particles are removed from the sampling stream before the filter. The cyclones used are primarily designed for a cutoff at particle diameter of 2.5 μm, however, the flow through the cyclone during the tests was higher than the design flow, resulting in a lower cutoff diameter. The size cutoff at particle diameter 1.6 μm is calculated from the actual flow through the system. The gravimetrical analysis of the filter samples was performed at the certified laboratory of IVL. The filters are weighed in a controlled environment before and after sampling. A Mettler Toledo model MT5 balance is used. The balance is calibrated to an uncertainty limit of ±3 μg in the range 0–10 000 μg and ±7 μg in the range 10 000–100 000 μg, our sample weights are in the range 15 000–100 000 μg (Table A1 in the Appendix A).

Elemental and organic carbon (EC/OC) content on particles are determined by sampling using filter holders with pre-heated double quartz filters (Pall, Tissuequarz). Backup filters are used to correct for the positive sampling artefact from condensation of volatile organics on the filter. A filter section with a total area of 2.01 cm<sup>2</sup> is cut out of the filter for the analysis of OC and EC by a thermal–optical method (EC/OC analyser Model 4, Sunset Laboratory, USA) using the EUSAAR\_2 temperature protocol [15]. The analyses of filters are conducted by Laboratory of Aerosols Chemistry and Physics; Institute of Chemical Process Fundamentals, in Prague. Reported uncertainties by the laboratory for their analyses are 10% for OC and 20% for EC.

An on-line electromobility-based instrument (TSI EEPS 3090) measures the number concentrations of particles between 5.6 nm and 560 nm in diameter in 32 size channels at 10 Hz. The instrument is used at HFO operations, upstream and downstream of the scrubber. There are no EEPS results for the LSFO fuel due to EEPS instrument failure during this fuel testing. A thermodenuder of model Dekati ELA 423 is used to vaporize volatile particles from the sample. We can thereby identify total number concentrations and size distributions of solid particles. The thermodenuder is heated to

300 ◦C in order to vaporize the volatile fraction of particles including many organics and sulphate. The use of a thermodenuder has been shown to also cause a loss of solid particles in the denuder through thermophoresis and diffusion depends on particle size, temperature, and velocity through the thermodenuder. Size dependent particle losses in the thermodenuder are calculated according to the manufacturer's instructions.

Magee Scientific's Aethalometer AE33, with continuous measurement of the attenuation of transmitted light at eight wavelengths is used for the detection of black carbon (BC) content. Measurement of absorption at 880 nm is interpreted as concentration of BC; in this context the BC refer to equivalent black carbon (eBC) as recommended in [16].

A schematic of the scrubber system and the instrument and sampling setup is presented in Figure 2.

**Figure 2.** Schematic of the scrubber system and the instrument and sampling setup.

Uncertainties in the on-line instruments are expressed as coefficients of variance of average values. The variations in dilution ratios are assumed to add more uncertainty to these results.

PAHs are sampled from the undiluted exhaust by an adsorbent for subsequent Soxhlet extraction and analysis by high-performance liquid chromatography (HPLC) in the IVL laboratory in Gothenburg. Particle-bound PAHs are collected on filters coupled in series to glass columns with XAD7 and polyurethane foam (PUF) absorbent for capture of gas phase PAH. This method gives an analysis of combined content of PAHs in gaseous and particulate form. Contents of USEPA's PAH-16 priority pollutants are analysed. These include naphthalene, acenaphthylene, acenaphthene, fluorene, phenantrene, anthracene, fluoranthene, pyrene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, dibenz[a, h]anthracene, benzo[ghi]perylene, and indeno[1,2,3-cd]pyrene.

Exhaust gas flow and emission factors of gases are calculated based on the carbon balance method as specified in ISO 8178:4. Fuel oil samples were sent for analysis of density, viscosity, calorific value and elemental content to the Saybolt Laboratory in Gothenburg. Analyses of the lubrication oil were also made. The density and viscosity of the HFO were significantly higher than those of the LSFO. The LSFO was still too viscous and dense to qualify as marine distillate oil (DMX-DFB) according to ISO 8217:2017 standard for marine fuels. The results from the analyses of the LSFO, the HFO, and the lube oil are presented in Table 1. Sulphur dioxide emissions are calculated from the sulphur content of fuel except for measurement downstream of the scrubber, since this presents a more reliable value than SO2 measurements with the gas analyser.


**Table 1.** Fuel and lube oil analyses.
