(48.4 ?g/L) for OL samples and 16.5 ?g/L (261 ?g/L) for a CL sample, respectively. These concentrations were generally lower and varied less but were in the same order of magnitude as in the present study. In another study by Thor et al. (2021), P EPA PAH concentrations for one OL sample of 11.0 ?g/L and for two CL samples of 21.6 and 11.0 ?g/L were found whereas the total of unsubstituted and alkylated PAH was reported only for one CL sample and was 131.5 ?g/L. These concen- trations are also in the same range as reported here. The emission factors (i.e. mass of PAH per engine energy output) for the discharge from OL and CL were calculated based on the specific discharge flowrates (m3/MWh) presented in Table 2. The emission fac- tors for the P 71 PAH in S1–4 ranged from 220 to 6479 mg/MWh in OL- discharge and from 8.5 to 1121 mg/MWh in CL-discharge. This confirms the findings from other studies (Hermansson et al., 2021; Teuchies et al., 2020) that report a generally lower pollutant load in discharge waters from CL than from OL, especially for hydrocarbons. This can be attrib- uted to the loss of naphthalenes due to the intensive recirculation and to the removal efficiency in the treatment step in CL operation. 3.1.3. Dissolved fractions of open loop and closed loop discharge water PAH concentrations in the dissolved fractions of the OL- and CL- discharge water samples were 0.82–24.1 ?g/L PEPA PAH (2.2–105.4 ?g/LP71 PAH) and 0.85–33.5 ?g/LPEPA PAH (2.5–109.3 ?g/LP71 PAH), respectively. Medians for P EPA PAH and P 71 PAH concentra- tions were 12.7 ?g/L (50.8 ?g/L) for OL and 19.9 ?g/L (56.5 ?g/L) for CL. Hence, dissolved concentrations in OL and CL discharge water samples were rather similar with a slight tendency to lower values for OL. Alkylated naphthalenes, phenanthrenes and fluorenes, and the unsubstituted compounds 3 ring-PAH (low molecular weight (LMW) PAH) Phe, Nap, Fl and Ace, dominate the PAH spectrum in the dissolved fraction as expected due to their higher water solubility compared with high molecular weight (HMW) PAH (4 ring-PAH) (Fig. 4). Maximum concentrations of 44 ?g/L and 36 ?g/L are reached by 2-methylnaphtha- lene (2-MNap) for OL and CL, respectively. Phe, Nap and dime- thylnaphthalene isomers show second-highest concentrations in the range of about 5–12 ?g/L. Phe and alkylphenanthrenes have been identified as characteristic compounds in combustion emissions from ships (Anders et al., 2023). HMW PAH only occur at trace levels <0.5 ?g/L in both dissolved OL and CL fractions. However, in CL samples HMW PAH were more frequently detected (61 detections) compared with OL samples (21 detections). It is assumed that not all particles could be separated from the dissolved fractions due to their small size (prob- ably in the nanoparticle range) and as more particles were present in the CL samples, also more detections of HMW PAH adsorbed onto these particles are plausible. Several HMW PAH at trace levels are detected in the CL dissolved fraction of S2, from which hardly any particulate fraction could be collected, so presumably more particles remained in the centrifuged (so-called dissolved) fraction. In the comprehensive data set of Teuchies et al. (2020) only unsub- stituted PAH were reported. Nap occurred in OL and CL discharge water samples at concentrations <14 ?g/L and Phe <17 ?g/L followed by lower concentrations for Fl (<3.7 ?g/L), Flu (<3.3 ?g/L) and Ace (<1.6 ?g/L), in line with the findings in this study. The distribution of the LMW PAH in the dissolved fractions were determined semiquantitatively and summarized as alkylated C1, C2, C3 and C4 derivatives of naphthalenes, phenanthrenes ? anthracenes and fluoranthenes ? pyrenes. The patterns show a clear characteristic bell- shape distribution of petrogenic PAH in the naphthalene group (Fig. 5), which shows highest concentrations in the samples, where the C1- and C2- (methyl-, dimethyl- and ethyl-)naphthalenes dominate the PAH pattern indicating petrogenic origin (Neff et al., 2004). This pattern does not occur as clearly in the phenanthrene ? anthracene group, where the unsubstituted compounds Phe and anthracene (Ant) dominate over the C1- and C2-derivatives and which show significantly lower PAH fractions. This may indicate a more pyrogenic slope-shape distribution and/or may be a result of the reduced water solubilities of the C1- and C2-derivatives in comparison with the unsubstituted compounds. For instance, Achten and Andersson (2015) reported solubilities of 1.15 ?g/ L for Phe, 0.3 ?g/L for C1-phenanthrenes and 0.003–0.007 ?g/L for C2- phenanthrenes which could result in increased Phe concentrations compared with C1- and C2-derivatives in the discharge. The occurrence of dominating alkylated naphthalenes and phenanthrenes in discharge which can originate from oil or coal show that the main source is pet- rogenic. Unburnt fuel from an incomplete combustion in the engine was also indicated by Du et al. (2022). They found a ratio in the scrubber discharge water of alkylated PAH/unsubstituted PAH above 5, which is also generally confirmed by the data of this study where all ratios were around 8 (S1: 8.3, S2: 7.8, S4: 8.1) indicating evenmore petrogenic PAH, except for S3 (3.4) with the lowest PAH concentrations of all sampled Fig. 3. P EPA PAH and P 71 PAH concentrations in OL and CL scrubber discharge water samples including dissolved and particulate fractions of the four studied ships (S1–4). C. Achten et al. Marine Pollution Bulletin 208 (2024) 116790 6