Polycyclic aromatic hydrocarbons (PAHs) and their alkylated-, nitro- and oxy-derivatives in the atmosphere over the Mediterranean and Middle East seas

. Polycyclic aromatic hydrocarbons (PAHs), their alkylated (RPAHs), nitrated (NPAHs) and oxygenated (OPAHs) derivatives are air pollutants. Many of these substances are long-lived, can undergo long-range atmospheric transport and 20 adversely affect human health upon exposure. However, the occurrence and fate of these air pollutants has hardly been studied in the marine atmosphere. In this study, we report the atmospheric concentrations over the Mediterranean Sea, the Red Sea,

Canal is included into the northern Red Sea region; the Gulf of Aden is part of the Arabian Sea. The sampling regions and sampling cruises are shown in Fig. S1 in the Supplement.

Air sampling for analysis of PAHs, OPAHs and NPAHs
The air pollutants were sampled separately in gas and particulate phases in polyurethane foams (PUFs, Molintan a.s., Břeclav, Czech Republic) and on quartz microfibre filters (QFFs, QMA type, Whatman, Sheffield, United Kingdom), respectively, by active air sampling on the observation deck (in the front part of the vessel, around 7.7 m a.s.l. and 55 m away from the stack). The aerosol was sampled as PM 10 (all particles with an aerodynamic equivalent diameter of <10 µm) 105 by a Digitel sampler (DH77, Hegnau, Switzerland). Additionally, PM was collected size-segregated with 6 size fractions (5 stages + backup filter) within PM 10 (PM <0.49µm (backup filter), PM 0.49-0.95µm , PM 0.95-1.5µm , PM 1.5-3µm , PM 3-7µm and PM 7-10µm ) using a high-volume sampler (Baghirra HV 100-P, Prague, Czech Republic) equipped with a cascade impactor inlet (TE-235, Tisch Environmental, Inc., Cleves, USA). All filters were pre-baked at 300 °C for 12 h and the PUFs were pre-cleaned (8 h Soxhlet extraction in acetone and 8 h in dichloromethane (DCM)) before wrapping them into two layers of aluminium 110 foil, placing into zip-lock polyethylene bags and keeping them frozen at -20 °C prior to deployment. After exposure, the samples were wrapped in aluminium foil and kept in polyethylene zip-lock bags at -20 °C during storage. During the whole cruise, 62 air samples (gas and particulate phase) and 30 size-resolved PM samples were collected together with 6 field blanks. Detailed sampling information is provided in Supplement Fig. S1 and in Table S1.

Air sampling for analysis of PAHs and RPAHs 115
45 air (gas and particulate phase) samples for the determination of PAHs and alkylated PAHs were collected on the monkey deck of the research vessel (around 4 m higher and 5 m less far away from the stack compared to the samplers for PAHs, OPAHs and NPAHs) during the campaign, using a high-volume air sampler (GMWL-2000H;General Metal Works, Cleves, USA). In contrast to the Digitel high volume sampler, total suspended particles (TSP) instead of PM 10 were collected. The sampling duration varied from 6 to 24 h and the total volume of each air sample ranged from 318 to 1428 m 3 (Table S1 in the 120 Supplement). Pre-combusted QFFs (3 h at 420 °C) and pre-extracted PUF plugs (8.0 x 7.5 cm, Ziemer, Langerwehe, Germany) were used for the collection of particulate and gaseous phases, respectively. TSP mass was gravimetrically determined. Filters were pre-and post-sampling weighed on a microbalance (KERN GmbH, Balingen, Germany; 1.0 -5 g readability) at constant temperature (21±2 °C) and relative humidity (45±10%) conditions. https://doi.org/10.5194/acp-2022-32 Preprint. Discussion started: 18 January 2022 c Author(s) 2022. CC BY 4.0 License.
The extract was cleaned up using a silica column (5 g of silica, 0.063-0.200 mm, activated at 150 °C for 12 hours, 10% deactivated with water) and 1 g Na 2 SO 4 . The sample was loaded onto the column and the target substances were eluted by 5 mL n-hexane, followed by 50 mL DCM. The volume of the eluate was then reduced by a stream of nitrogen in a TurboVap II (Caliper LifeSciences, Mountain View, USA) concentrator unit and transferred into a GC vial, spiked with p-terphenyl and 135 PCB 121 (syringe standards), and the final volume in the vial was adjusted to 200 μL.
Polycyclic aromatic compounds (PACs) in the sample extracts were analysed at the Trace Analytical Laboratory of the research centre RECETOX at the Masaryk University in Brno, Czech Republic, similar to the method described by Nežiková et al. (2021). The target compounds in this analysis were 26 PAHs, 1S-heterocycle, 1 RPAH, 17 NPAHs and 11 OPAHs. All target PAHs, OPAHs and NPAHs including their acronyms are shown in Table 1. The physico-chemical properties of all 140 targeted compounds are shown in Table S2.
The analysis of PAHs was performed by gas chromatography (GC, 7890A, Agilent, Santa Clara, USA) equipped with a 60 m x 0.25 mm x 0.25 µm Rxi-5Sil MS column (Restek, Bellefonte, USA) coupled to a mass spectrometer (MS, 7000B triple quadrupole, Agilent, Santa Clara, USA). 1 μL of sample was injected splitless at 280 °C with He as carrier gas at a constant flow rate of 1.5 mL min -1 . The GC program was as follows: 80 °C (1 min hold), then heated at a rate of 15 °C min -1 to 180 145 °C, followed by 5 °C min -1 to 310 °C (20 min hold). The MS was operated in positive electron ionization (EI+) mode with selected ion monitoring (SIM). The SIM m/z ratios and the retention times of the targeted PAHs are shown in Table S3a.
NPAHs and OPAHs were analysed by GC atmospheric pressure chemical ionization tandem mass spectrometry (GC-APCI-MS/MS) on a Waters Xevo TQ-S MS (Waters, Mildford, USA) coupled to a GC (GC 7890, Agilent, Santa Clara, USA). The MS was operated under dry source conditions in multiple reactions monitoring (MRM) mode. The GC was fitted with a 30 150 m x 0.25 mm × 0.25 µm Rxi-5Sil MS column (Restek, Bellefonte, USA). The injection of 1 µL of the sample was splitless at 270 °C. He was used as carrier gas at a constant flow rate of 1.5 mL min -1 . The oven temperature program was as follows: 90 °C (1 min hold), then heated at a rate of 40 °C min -1 to 180 °C, followed by 5 °C min -1 to 320 °C (6 min hold). The MRM m/z ratios and the retention times of the targeted OPAHs and NPAHs are given in Table S3b.

RPAHs 155
For alkylated PAHs, particulate and gas-phase samples were extracted and cleaned-up separately following a procedure described in detail elsewhere (Iakovides et al., 2021). Each fraction was reduced to approximately 0.3 mL by rotary evaporation, transferred to 1.1 mL GC vials and further evaporated almost to dryness under a gentle stream of nitrogen at -10 °C to minimize evaporation losses. Prior to GC/MS analysis, a known amount of internal standard mixture (4-20 ng of anthracene-d 10 in iso-octane) was added in each GC vial to assess the analyte recovery in the collected samples. The sample 160 extracts were analysed at the Cyprus Institute (Cyprus). The target compounds in this analysis were phenanthrene (PHE) and 18 RPAHs, which are shown in Table 1.
The analysis was carried out on a GC (7890N GC, Agilent, Santa Clara, USA) equipped with a deactivated fused silica guard column (5 m, Agilent, Santa Clara, USA) followed by a 30 m × 0.25 mm × 0.25 μm fused silica column (DB-5MS, J&W, Santa Clara, USA). The GC was coupled to a mass selective detector (5977B Inert MSD, Agilent, Santa Clara, USA) 165 operating in EI mode. Either 1 or 2 μL of the final extract were injected into the column using a cool-on-column inlet (80 °C constant temperature) with a column flow rate of 1.0 mL/min. The GC oven program was modified to 80 °C initial temperature, hold for 1 min, heated at a rate of 21 °C min -1 to 150 °C, 5 °C min -1 to 300 °C and finally hold for 20 min (54 min total run time). The transfer line was kept at 300 °C, while the MS quadrupole and ion source temperature were held at 150 and 230 °C, respectively. Molecular ions used for the identification are shown in Table S3c. 170 https://doi.org/10.5194/acp-2022-32 Preprint. Discussion started: 18 January 2022 c Author(s) 2022. CC BY 4.0 License.

Supporting parameters
Further description of analytical methods and other supporting parameters such as meteorological data, PM 10 mass and 175 concentrations of transition metals, elemental carbon (EC) and organic carbon (OC) can be found in the Supplement. The methods and the resulting data of other additional supporting parameters during the AQABA campaign used in this paper are reported in the following studies: a) The ship exhaust filter, black carbon (BC) and surface PAH concentrations as well as https://doi.org/10.5194/acp-2022-32 Preprint. Discussion started: 18 January 2022 c Author(s) 2022. CC BY 4.0 License. bypassing ships, potentially influencing the sampled air, in Celik et al. (2020), b) O 3 , nitrogen oxides (NO x , i.e. NO+NO 2 ) and OH radicals in , c) O 3 , NO 2 and SO 2 in Eger et al. (2019) and d) NO x and NO y (i.e. NO x +organic and 180 inorganic oxides of nitrogen) in Friedrich et al. (2021). Measurements of OH radicals were done using the HydrOxyl Radical measurement Unit based in fluorescence Spectroscopy (HORUS) instrument (Martinez et al., 2010;Hens et al., 2014), with the Inlet Pre-Injector (IPI) modification (Novelli, et al., 2014). The measurement of the actinic flux was done by a spectral radiometer as described in Meusel et al. (2016). The measurement of polychlorinated biphenyls (PCBs), hexachlorocyclohexanes (HCHs), dichlorodiphenyl-trichloroethane and isomers (DDX), other organochlorine pesticides 185 (drins) was done similar to Lammel et al. (2016).

Aerosol source apportionment
Positive Matrix Factorization (EPA PMF 5.0) was applied to the PM 10 chemical composition using the concentrations of OC, EC, BC and metals in both PMF groups and the sum of PCBs, HCHs, DDX, drins, PAHs, NPAHs and OPAHs only in group 1 and selected individual PAHs, OPAHs and NPAHs in group 2 to obtain source profiles and their contributions. All Digitel 190 high-volume samples were considered in the PMF runs including those with contamination from the ship's stack. The data matrix was prepared in compliance with the procedure described by Polissar et al. (1998). The final matrices had 62 samples with 26 and 30 species in group 1 and 2, respectively.
To estimate the optimal number of sources, the PMF model was run several times with different model settings and 3 to 7 factors tested. The Q values (Q true , Q robust and Q expected/theoretical ), the resulting source profiles, and the scaled residuals were 195 examined. The optimum number of factors was chosen based on an adequate fit of the model to the data, as shown by the scaled residual histograms and physically interpretable results. The most stable solutions were found for 5 factors by extra modelling uncertainties of 26 % and 19 % for group 1 and group 2, respectively. All runs converged, the scale residuals were normally distributed and no swaps were observed with the displacement error analysis, indicating that there was limited rotational ambiguity (Table S9). 200

Air mass origin
Residence time distributions of air mass histories, 10 days backward in time, were studied using the FLEXPART Lagrangian particle dispersion model, with ECMWF meteorological data (0.5°×0.5°, 3-hourly; Seibert and Frank, 2004;Stohl et al., 2005). The output is a measure of the time the computational particles (fictive air parcels) resided in grid cells. Per 24 h sampling time, 100000 particles were released at a height of 100 m a.s.l.. 205

Quality control
More information about the analytical quality assurance such as the filtering of the samples against contamination by the own ship exhaust, the quality control of the analysis of the PAH derivatives (recoveries, blank correction, detection frequencies, limits of quantifications (LOQs)) and a summary of PMF diagnostics is given in the Supplement (Chapter S1.5).

Occurrence of PAHs and PAH derivatives
The average total (sum of gaseous and particulate phase) concentrations of the pollutants in the different sea regions are shown in Fig. 1. The average total concentrations (range in brackets) of the sum of one pollutant class from all high-volume air samples of the ∑ 16 PAHs, ∑ 27 PAHs (including the S-heterocycle BNT), ∑ 19 RPAHs (range without RET), ∑ 11 OPAHs and ∑ 17 NPAHs were 2.92 ± 3.34 (0.14-17.28) ng m -3 , 2.99 ± 3.35 (0.15-17.34) ng m -3 , 0.85 ± 0.87 (0.19-3.41) ng m -3 , 0.24 ± 215 0.25 (0.04-1.42) ng m -3 and 4.34 ± 7.37 (0.69-46.50) pg m -3 , respectively. All the data is filtered for contamination with the stack of our research vessel (details given in S1.5.1 in the Supplement). The detection frequencies of the compounds in the high-volume samples are shown in Fig. S1. All targeted PAHs, RPAHs and OPAHs were detected at least in one sample.
From the 17 targeted NPAHs, 7 were detected in at least one high-volume sample. All total concentrations of the individual compounds and individual samples can be found in the Supplement, Tables S10-S14. Individual phases' concentrations are 220 presented and discussed in a separate communication.
The spatial distribution of the concentrations of the different substance classes in both legs is shown in Fig. 2. The plot only shows the average concentration of each sampling stretch, which could also be impacted by individual local plumes at distinct times and locations. As visible in Figs. 1 and 2, the cleanest region, with the lowest concentration of all substance classes, was the Arabian Sea in the Indian Ocean. The average air concentrations (in brackets upper and lower estimate when 225 using LOQ instead of LOQ/2 and 0 instead of LOQ/2, respectively) over the Arabian Sea were 0.59 (0.57-0.61) ng m -3 , 0.59 (no significant difference) ng m -3 , 47.8 (24.1-71.4) pg m -3 and 0.89 (0.29-1.49) pg m -3 for the ∑ 27 PAHs, ∑ 19 RPAHs, ∑ 11 OPAHs and ∑ 17 NPAHs, respectively. These concentrations are among the lowest ever reported levels of the PAHs and PAH derivatives. The air masses originated from the Indian Ocean and from parts of Somalia with no significant sources of PAHs or PAH derivatives (Fig. S3d). Similarly, findings from the same campaign for other air pollutants showed the lowest 230 concentration over the Arabian Sea Eger et al., 2019;Pfannerstill et al., 2019;. As shown in Fig. 2 a), c) and e), several samples in the Arabian Sea are missing in the first leg due to rejection as possibly contaminated by the stack of our research vessel Kommandor Iona (detailed overview of rejected samples and method description in Table S4 and Chapter S1.5.1, respectively). The Mediterranean Sea showed the highest average concentration of the ∑ 27 PAHs and ∑ 11 OPAHs, i.e. 4.40 ng m -3 and 0.37 ng m -3 , respectively. As illustrated in Fig. 2, 235 the pollutant concentration over the Mediterranean Sea during the first leg differed from that of the second leg. The concentration of the ∑ 27 PAHs during the first leg (2.20 ng m -3 ) was significantly lower (p<0.05, Student's t-test) than during the second leg (5.18 ng m -3 ). The difference was also significant (p<0.05, Student's t-test) for the ∑ 11 OPAHs. The air mass histories ( Fig. S2 and S3a) reveal that the difference is related to the different origin of the air masses. During the first leg, the sampled air predominantly originated from northern Africa and the western Mediterranean Sea, while during the second 240 leg, the prevailing air masses came from north, transporting polluted air from large parts of Europe, including coastal areas and islands as also reported by . The northerly wind is a typical large-scale circulation pattern in summer https://doi.org/10.5194/acp-2022-32 Preprint. Discussion started: 18 January 2022 c Author(s) 2022. CC BY 4.0 License. over the Mediterranean Sea (Lelieveld et al., 2002). The highest concentrations of the PAH derivatives throughout the entire cruise were found in sample D58 in the Mediterranean Sea close to Sicily. The concentrations of ∑ 11 OPAHs and ∑ 17 NPAHs were 1.42 ng m -3 and 46.5 pg m -3 , respectively. Sample D54, sampled 400 km south-east of Sicily, showed the highest 245 concentration of the ∑ 27 PAHs, especially of the low-molecular-weight PAHs (2-3-ring PAHs). These samples will be evaluated in more detail in Sect. 3.3.4.
The concentration of the ∑ 17 NPAHs was similar during both legs in the Mediterranean Sea. On the one hand, 2-NFLT and 2-NPYR were more abundant during the first leg, possibly due to higher secondary formation in aged air (Atkinson and Arey, 1994;Arey et al., 1986) On the other hand, the primarily emitted 1-NPYR (Atkinson and Arey, 1994) as well as 2-NNAP 250 (having primary and secondary sources, Zhuo et al., 2017;Atkinson and Arey, 1994) had a higher concentration during the second leg. In contrast to the PAHs and OPAHs, the concentration of the ∑ 19 RPAHs in the air over the Mediterranean Sea was higher (not significant, p=0.14, Student's t-test) and not lower during the first leg compared to the second leg. The different result for the RPAHs can firstly be explained by the different sampling intervals of the air sampler for the RPAHs (see Table S1). The RPAHs were not collected at the end of the campaign close to Sicily and Sardinia, where a high burden 255 of PACs was measured. Second, north of Egypt in the Mediterranean Sea, close to the Suez Canal during the first leg, high concentrations of the MPHEs and M 2 PHEs were found, possibly due to intense marine traffic concentrating or even queueing before entering into the Suez Canal.
The average concentration of the ∑ 17 NPAHs over the Mediterranean Sea was 5.23 pg m -3 , which was slightly higher than the average concentration over the northern Red Sea (4.52 pg m -3 ) and the Gulf of Oman (4.37 pg m -3 ) but slightly lower than 260 the average concentration over the Arabian Gulf (6.65 pg m -3 ). The concentration of the ∑ 19 RPAHs over the Mediterranean Sea (0.81 ng m -3 ) was similar to the Gulf of Oman (0.83 ng m -3 ) and lower than over the Arabian Gulf (1.12 ng m -3 ), too.
Similar to the NPAHs and RPAHs, most other air pollutants (e.g. non-methane hydrocarbons, carbonyl compounds, NO x , NO z , O 3 , SO 2 ) measured during the AQABA campaign showed the highest concentration in the Arabian Gulf Eger et al., 2019;Pfannerstill et al., 2019;Friedrich et al., 2021). 265 The concentration of the ∑ 17 NPAHs over the southern Red Sea was 1.68 pg m -3 . Similar as for the ∑ 17 NPAHs, the southern Red Sea was the region with the second lowest concentrations of the ∑ 27 PAHs and the ∑ 11 OPAHs (0.94 ng m -3 and 88.3 pg m -3 , respectively). The pollutant burden was low since the air was predominantly coming from eastern Africa, mainly from Sudan, Eritrea and western and southern parts of Egypt (Fig. S3c), areas with low population and industrial emitter densities.
Air over the northern Red Sea, including the Suez Canal, is more polluted owing to the dense shipping traffic in the canal 270 , the vicinity of the megacity Cairo and the densely populated and urbanised Nile Delta. The total concentration of the ∑ 19 RPAHs over the northern Red Sea was 0.93 ng m -3 . The highest concentration was measured in air close to Jeddah, which was almost one order of magnitude higher polluted than the other samples.  The lower levels of secondarily formed 2-NFLT and 2-NPYR can be explained by significant long-range transport from NO x 310 poor areas, notably northern Africa during the first leg. One decade earlier, in the eastern Mediterranean Sea in summer 2001, Tsapakis and Stephanou (2007) report approximately one order of magnitude higher values, i.e. 29 and 21 pg m -3 for 2-NFLT and 2-NPYR, respectively. Furthermore, they determined 9,10-O 2 ANT and 9-OFLN (34.2 and 46.3 pg m -3 , respectively), which was in the same range as our measurements in the Mediterranean Sea with 95.4 and 36.2 pg m -3 ,

respectively. 315
The concentrations of NPAHs (40, 90 and 60 pg m -3 for 1-NPYR, 2-NFLT and 2-NPYR, respectively) in source regions of the Mediterranean such as Athens (Marino et al., 2000) were approximately three orders of magnitude higher than those in our study over the Mediterranean Sea. This urban to marine background gradient is a lot smaller for the OPAHs compared to the NPAHs. In summer 2013, Alves et al. (2017) found at a suburban site in Athens an air concentration of 9, 28 and 242 pg m -3 for 9,10-O 2 ANT, 9-OFLN and BAN, respectively. This is one order of magnitude lower for 9,10-O 2 ANT, the same 320 magnitude for 9-OFLN and one order of magnitude higher for BAN compared to our results in the Mediterranean Sea. This suggests longer lifetimes or higher formation rates of OPAHs than NPAHs. Harrison et al. (2016) measured PAHs and its derivatives at three sites along the east coast of the Red Sea, in a plume of a major point source (petrochemical complex). 9,10-O 2 ANT had a concentration between 3.15 and 4.02 ng m -3 , which is two orders of magnitude higher than in the particulate phase of samples over the northern Red Sea in our study. The 325 concentration of 5,12-O 2 NAC was between one and two orders of magnitude higher, while the difference was smaller for 7,12-O 2 BAA. The difference of the individual NPAHs concentrations between the measurements at the coast from Harrison et al. and our measurements offshore is even more pronounced. The concentrations of 2-NNAP, 2-NFLT, 1-NPYR, 2-NPYR and 7-NBAA were over three orders of magnitude higher in the plume measured by Harrison and colleagues. In contrast, the PAH concentration was almost similar (for low-molecular-weight PAHs) or only one order of magnitude higher (for high-330 molecular-weight PAHs, i.e. 5-7-ring PAHs) onshore. This, again, points to short atmospheric lifetimes of NPAHs. The OH reaction rate coefficients of PAHs and NPAHs are similar (Table S2, US EPA, 2019), but NPAHs are more prone to photolysis (Fan et al., 1996;Keyte et al., 2013;Wilson et al., 2020). This is furthermore supported by findings that the NPAH/PAH ratios in mid-latitudes are higher in winter than in summer, obviously since the photochemical sink of NPAHs in summer overcompensates the higher formation potential as a source of NPAHs (Nežiková et al., 2021). Nassar et al. 335 (2011) measured PAHs and two NPAHs in the area of Greater Cairo. The concentration of 1-NPYR in the study was around one order of magnitude higher than that in the air measured on the ship over the Suez Canal. In contrast, the concentration of https://doi.org/10.5194/acp-2022-32 Preprint. Discussion started: 18 January 2022 c Author(s) 2022. CC BY 4.0 License. the low-molecular-weight PAHs was in the same range, while the concentrations of high-molecular-weight PAHs offshore were one or more orders of magnitude lower.
Few studies report MPHE and M 2 PHE in the coastal marine atmosphere, the open sea, background and urban sites. Tsapakis 340 and Stephanou (2005) analysed atmospheric samples collected offshore over the eastern Mediterranean Sea and at a background station in north-eastern Crete (Greece) and reported total (gas and particulate phase) concentrations for ∑MPHE of 13.6 ng m -3 and for ∑M 2 PHE of 6.5 ng m -3 , respectively. Mandalakis et al. (2002) reported gas and particulate phase concentrations of 6.07 ng m -3 and 3.17 ng m -3 for ∑MPHE and ∑M 2 PHE, respectively, in the Saronikos Gulf, which is impacted by busy marine traffic and the shipyard industry along the coast (Valavanidis et al., 2008). In the same study,

PAHs
The substance patterns of PAHs in the six different sea regions are shown in Fig. 3a. PHE was by far the most abundant PAH in all regions (average contribution of 49 %), followed by FLN, ACE, FLT and PYR with average contributions of 24 365 %, 10 %, 6 % and 5 %, respectively. It can be noted that the PAH composition patterns were similar in all regions. However, the patterns of the Mediterranean Sea and the southern Red Sea differed from the other regions. The contribution of PHE in the southern Red Sea was higher than the average, while it was lower than the average in the Mediterranean Sea. The opposite could be observed for FLN. The different pattern in the Mediterranean Sea was mainly caused by the samples from the second leg with high influence of aerosols from Europe. 370

RPAHs
The distribution pattern of RPAHs among the campaign regions is presented in Fig. 3b. The RPAHs did not show significant regional differences in the composition pattern. 2-MPHE, 1-MPHE and 3-MPHE were the most abundant alkylated PAH species throughout the campaign, making up 22 %, 16 % and 12 % of the total RPAHs measured. Among the M 2 PHEs, 1,6and 2,9-M 2 PHE were the most abundant compounds. 375

OPAHs
As shown in Fig. 3c, the regional differences between the OPAH composition patterns are more pronounced than the regional average PAH and RPAH patterns. 1,4-O 2 NAP, 9,10-O 2 ANT and 9-OFLN were the most abundant OPAHs, with an average contribution of 35 %, 22 % and 11 %, respectively. The high share of 9,10-O 2 ANT and 9-OFLN was also found at several continental sites (Albinet et al., 2007;Wei et al., 2015;Drotikova et al., 2020;Lammel et al., 380 2020;Jariyasopit et al., 2021;Nežiková et al., 2021). In several of these papers, 1,4-O 2 NAP was not measured. Nežiková et al. (2021) and Jariyasopit et al. (2021) only found small relative amounts of 1,4-O 2 NAP at the continental background site Košetice and in the Athabasca oil sands region in Canada, respectively. In contrast, Wei et al. (2015) and Lammel et al. (2020) found relatively high contributions to the total amount of OPAHs at urban sites in China and in the Czech Republic, respectively. Bandowe et al. (2014) observed higher concentrations of 1,4-O 2 NAP in summer in PM 2.5 compared to the cold 385 season, although the partitioning of the compound will be shifted to the gas phase in summer. This would lead to lower concentrations in summer since the degradation rates of most PACs are expected to be higher in the gas phase (Feilberg et al., 1999;Keyte et al., 2013). They hypothesized that 1,4-O 2 NAP is significantly formed by secondary formation, as also shown by Kautzman et al. (2010) and Keyte et al. (2013). This can be supported by the low winter to summer ratio of 1,4-O 2 NAP despite higher emissions in winter at Košetice (Nežiková et al., 2021). High secondary formation in plumes 390 especially in the Mediterranean Sea and the Arabian Gulf (as explained in Sect. 3.3.4), as well as low reaction rates for the degradation of 1,4-O 2 NAP compared to all other OPAHs (Table S2, Atkinson et al., 1989) might explain the high relative contribution of this quinone in our study. Except for the samples from the Gulf of Oman, 1,4-O 2 NAP always had the highest contribution of 25-40 %. This quinone is frequently reported having a high ability to produce reactive oxygen species (Charrier and Anastasio, 2012;Verma et al., 2015). 395 In the Gulf of Oman, the contribution from high-molecular-weight OPAHs (4-ring OPAHs) was higher compared to the other regions. The composition pattern of the samples from the Arabian Sea differed from the other samples because of a lower share of 9,10-O 2 ANT and a higher share of 1-(CHO)NAP. The same was true for the samples of the first leg in the Mediterranean Sea (see Table S16). 1-(CHO)NAP has been reported prominent among OPAHs from urban and other polluted sites, but not generally (Albinet et al., 2007;2008 (partly); Wei et al., 2015;Tomaz et al., 2016;Lammel et al., 2020 400 (in Kladno)).

NPAHs
Similar to the OPAHs, the regional differences in the NPAH composition pattern are more pronounced than the PAH and RPAH patterns. As illustrated in Fig. 3d, the most abundant NPAHs were 2-NNAP, 3-NPHE, 2-NFLT and 1-NPYR, with an average contribution of 45 %, 18 %, 15 % and 12 %, respectively. The contribution of 2-NNAP ranged between one third 405 and half in all regions. However, it was not detected above LOQ in the gas phase of samples from the Arabian Sea. Due to the total detection frequency of >30 %, the values were replaced by LOQ/2 what could lead to an overestimation in this case.
A large fractional contribution of NNAPs, 3-NPHE and 2-NFLT was also found by Lammel et al. (2017) in the eastern Mediterranean Sea. At the continental site in the study from Lammel et al., as well as in other previous studies at continental sites, 2-NFLT, 9-NANT and 1-NNAP were the most abundant NPAHs Albinet et al., 2007;410 2008;Tomaz et al., 2016;Drotikova et al., 2020;Lammel et al., 2020;Nežiková et al., 2021). In our study, 9-NANT had a significant contribution only in the northern Red Sea. We found only a few samples with 9-NANT >LOQ (LOQs in  (Table S6c) or that 9-NANT is prone to photolysis, which could have been high in this campaign because of high solar irradiation. The significance of photodegradation of 9-NANT is also supported by lower contributions in summer 415 compared to the cold season, as found by Tomaz et al. (2016) and Nežiková et al. (2021). However, it can also be caused by seasonal variation in the emission sources or a higher degradation rate in the gas phase based on the significantly lower particulate fraction in summer (Tomaz et al., 2016;Nežiková et al., 2021). The same might be true for 1-NNAP. In contrast to our campaign, several other studies found significant amounts of 1-NNAP in air samples at mid-latitude but continental sites Albinet et al., 2007;Tomaz et al., 2016;Drotikova et al., 2020, Lammel et al., 420 2020Nežiková et al., 2021). The low contribution of 1-NNAP in air over the Mediterranean and around the Arabian Peninsula could also be due to the relatively high LOQ in PUFs (Table S6c) or the photodegradation of 1-NNAP, which is faster than of 2-NNAP, as described by Feilberg et al. (1999). We hypothesize that the comparably low rate constants for the photodegradation as well as for the reaction with OH (Table S2, US EPA, 2019) are one reason for the high relative contribution of 2-NNAP. 2-NNAP is frequently detected in continental sites but mostly with lower relative contributions 425 than 1-NNAP, 9-NANT and 2-NFLT Albinet et al., 2007;Tomaz et al., 2016;Drotikova et al., 2020Nežiková et al., 2021). Only at a remote site in Chile, 2-NNAP was also found to have a very high relative contribution, which was explained by direct emissions or transport assuming a long atmospheric lifetime (Scipioni et al., 2012). The resistance to photochemical degradation can also be supported by the finding from Nežiková et al. (2021) that the relative contribution of 2-NNAP is higher in summer than in winter. However, this can also 430 be due to different emission sources or stronger secondary formation in summer (Zhuo et al., 2017).
The fractional contribution of 1-NPYR is high in the Gulf of Oman and the Arabian Gulf. This can be explained by a significant amount of 1-NPYR in the exhaust of fossil fuel combustion (IARC, 2018;Zhao et al., 2015) and its high abundance near petrochemical industries (Caumo et al., 2018). It is used as a marker for primary emissions since it does not have significant secondary sources (Arey et al., 1986). The relatively short estimated lifetime of 1-NPYR in air due to 435 photodegradation (Feilberg and Nielsen, 2000) and the small reaction rate with OH (Table S2, US EPA, 2019) could explain its low contribution in the Mediterranean Sea, since we sampled relatively aged air samples in that region. The relatively high contribution of 1-NPYR in the Arabian Sea might be due to bypassing ships (Table S18) as we found 1-NPYR highly abundant in the ship exhaust (Sect. 3.3.1). The high contribution of 1-NPYR in samples D40-42, in or close to the Gulf of Aden, is possibly due to pollution from coastal cities in the northeastern province of Somalia. The continental influence of 440 these samples is also shown in the results of the PMF analysis (Fig. 4b). The large contribution of long-range transported aerosols in the Mediterranean Sea is also illustrated by the high contribution of 2-NFLT, which is formed in secondary processes (Arey et al., 1986). The contribution of 2-NFLT is also high in the southern Red Sea and the Arabian Sea, two regions with minor influence of primary emissions but higher fraction of long-range transported aerosols. 3-NPHE, which has primary and secondary sources (Atkinson and Arey, 1994;Heeb et al., 2008;Ringuet et al., 2012a), has an almost similar 445 contribution in all regions. This could be explained by various different sources or a long mean atmospheric lifetime.  Table S18.
Another important source of PAH derivatives were continental emissions. Based on the distribution of residence times of air masses during these sampling times, we could conclude that these emissions mainly came from Europe (especially received 460 in the Mediterranean Sea, but also in the Arabian Gulf), countries around the Arabian Gulf (mainly received there) and Egypt (mainly received in the northern Red Sea). Furthermore, the PACs originated from residual oil combustion. High  Table S4 and Fig. S4) 475 to the stack filtered regional average concentrations, we could show that almost all detected PAHs, NPAHs and OPAHs were elevated in the samples with fresh shipping emissions (Table S17). 1-NPYR showed the highest ratio of contaminated to filtered samples among the NPAHs, while 11-OBaFLN and 1-(CHO)NAP had the highest ratio among the OPAHs. All targeted OPAHs showed a ratio higher than 1. For the NPAHs, only 2-NFLT was not elevated except for a ratio of 5 in the https://doi.org/10.5194/acp-2022-32 Preprint. Discussion started: 18 January 2022 c Author(s) 2022. CC BY 4.0 License.
Arabian Sea. 2-NFLT had been reported to be present in diesel particulate matter Zimmermann et al., 480 2012). However, this was explained by the gas phase formation of 2-NFLT after emission during sample collection.
Surprisingly, the concentration of 2-NPYR was significantly elevated in the fresh shipping emissions. This is unexpected, as 2-NPYR was reported absent in diesel exhaust  and believed to be formed through secondary formation only (Finlayson-Pitts and Pitts, 1999;Wilson et al., 2020). Zhao et al. (2019; found significant amounts of 2-NPYR in ship exhaust gas depending on the fuel type and the engine loading. They report high emissions of this 485 compound, especially with heavy fuel oil use and mainly under low engine speeds. The abundance of 2-NPYR was explained by secondary formation due to higher NO x emissions and higher residence times during these conditions. The results from Zhao et al. (2019; and from our study suggest a very high formation rate of 2-NPYR. According to Keyte et al. (2013) and Wilson et al. (2020), the reaction rate constant of PYR with OH is five times higher for 2-NPYR compared to 2-NFLT but the yield of 2-NPYR is lower. 490 The large contribution of aged shipping emissions to the concentration of 2-NFLT and 2-NPYR (Fig. 5) illustrates the importance of secondary formation of these two NPAHs. In contrast, 1-NPYR is not abundant in the aged shipping emissions showing that there is no significant secondary formation. It has been reported that 1-NPYR arises solely from primary emissions (Bezabeh et al., 2003;Reisen and Arey, 2005). The contribution of aged shipping emissions to the occurrence of 3-NPHE could either be explained by the higher atmospheric half-life of 79 h compared to 2-NFLT, 1-NPYR 495 and 2-NPYR (Table S2, US EPA, 2019) or by secondary formation as previously suggested (Tomaz et al., 2017). All detected OPAHs were abundant in the aged shipping emissions. Their abundance points to a long lifetime or formation in the atmosphere. The relative contributions of 11-OBaFLN, BAN and 7,12-O 2 BAA were relatively small (Fig. 5). For 5,12-O 2 NAC and 1,4-O 2 NAP, the contribution of aged shipping emissions was higher. It was previously reported that from the measured OPAHs, 1,4-O 2 NAP, 1-(CHO)NAP, 9-OFLN, 9,10-O 2 ANT, 1,4-O 2 ANT, 9,10-O 2 PHE, 11-OBaFLN and 7,12-500 O 2 BAA can be formed from the reaction of parent-PAHs with oxidants (Helmig and Harger, 1994;Perraudin et al., 2007;Wang et al., 2007;Gao et al., 2009;Ringuet et al., 2012a;Keyte et al., 2013;Dang et al., 2015). Based on the previous findings from literature and the results of the PM factor "aged shipping emissions", a contribution from secondary formation to the burden of 1,4-O 2 NAP and 7,12-O 2 BAA is likely, in addition to the known secondarily formed substances 2-NPYR and 2-NFLT. Since BAN and 11-OBaFLN have not been found as secondary formation products but highly abundant in primary 505 emissions (Albinet et al., 2007;Ringuet et al., 2012a;Clergé et al., 2019), we hypothesize that their contribution to aged shipping emissions is only due to their atmospheric lifetime. Since the primary emitted 1-NPYR is not abundant in aged shipping emissions, it shows that BAN and 11-OBaFLN have a higher atmospheric lifetime than 1-NPYR. Since the estimated lifetime due to oxidation by OH is higher for 1-NPYR than for the two OPAHs (Table S2,  1-NPYR and 3-NPHE seem to be good tracers for oil combustion, hence the correlation with emissions from the petrochemical industry in the Gulf of Oman and the Arabian Gulf. 1-NPYR and 3-NPHE are known to be emitted during combustion of oil (Streibel et al., 2017). In addition, all OPAHs included into the PMF, except for 1,4-O 2 NAP, originated 515 directly or secondarily from residual oil combustion.
Except for 11-OBaFLN, all considered PAHs, OPAHs and NPAHs are partly from continental pollution (Fig. 4a). The abundance of these air pollutants in continental pollution, including 11-OBaFLN, has been shown in many studies Albinet et al., 2007;Wei et al., 2015;Tomaz et al., 2016;Drotikova et al., 2020;Lammel et al., 2020;Nežiková et al., 2021). The missing contribution of continental pollution to the concentration of 11-OBaFLN might be 520 because of its comparably low atmospheric half-life due to degradation by OH (Table S2, US EPA, 2019). Continental pollution was highly abundant in the Mediterranean Sea (Fig. 4b), where we found the highest concentrations of the OH radical of the entire AQABA campaign. 1,4-O 2 NAP has a comparably high contribution of approx. 50 % by this factor. As explained in Sect. 3.2.3, this might be explained by high relative concentrations at the source, high atmospheric lifetime and secondary formation during the transport of the air to the sampler. In contrast, 1,4-O 2 NAP seems to be significantly less 525 abundant in pollution from the combustion of residual oil (Fig. 4a) and marine diesel (Table S17). However, more research is needed to evaluate this aspect in more detail. All PACs are abundant in desert dust, except for BAP and 2-NPYR. The presence of PAHs and PAH derivatives, especially 1-NPYR and 1,4-O 2 NAP in the factor desert dust (Fig. 4a) may indicate co-emissions of dust and PACs in the region (e.g. close to onshore industries).

Source attribution by PAHs and alkylated PAHs
The ratio of the particulate concentration of BAP to BAP+BEP is often used as a marker for the ageing of atmospheric particles since photodegradation of BAP is faster than of BEP (Tobiszewski and Namieśnik, 2012). A concentration ratio BAP/(BAP+BEP) of less than 0.5 indicates photochemically aged aerosols. The ratio was <0.5 in all regions and ≈0.5 in the Arabian Sea (see Fig. S5). The somewhat elevated ratio in the Arabian Sea might be caused by local ship plumes (for 540 number of encounters see Table S18; identification based on Celik et al. (2020)) and other offshore emissions, which contributed to the mostly long-range transported and aged air pollution in the region. This is also supported by Bourtsoukidis et al. (2019) studying non-methane hydrocarbons during the AQABA campaign.
The relatively low ratios in all other regions might be explained by the low amount of primary sources of air pollutants on sea except for ship traffic and some emissions from the offshore oil and gas industry. Thus, the pollution from urban and 545 industrial areas, which are located mostly on the coast, is already slightly aged when reaching the sampler on the ship depending on the proximity to the emission sources. This could also be the explanation why the second-highest regional average values were found in the southern and the northern Red Sea receiving the emissions from the nearby coast as well as from the intense ship traffic in the region. The lowest regional average BAP/(BAP+BEP) values were detected in the Gulf of Oman and the Arabian Gulf. Air mass histories of sample D33 showed that a significant amount of aerosols came from less 550 populated areas of Iran with a low amount of primary emissions (Fig. S3f, . The results in the Mediterranean Sea can be divided into the first leg with a lower BAP/(BAP+BEP) ratio due to the prevalent westerly winds bringing aged air from Africa and from the sea and the second leg with higher ratios due to pollution from close European coastal areas and islands. The samples D58 and D49-52, close to Sicily and the Greek islands, respectively, showed the highest BAP/(BAP+BEP) ratios. 555 The ratio of ΣMPHE/PHE <1 indicates pyrogenic origin of PAHs for most days in the Red Sea, while a ratio of ΣMPHE/PHE >1 indicates petrogenic origin, i.e. from unburned fuel (Gogou et al., 1996), which occurred during the period from the 8-9 July 2017 in the northern Red Sea. Findings by Bourtsoukidis et al. (2020) could tentatively provide an explanation for the high ratio of ΣMPHE/PHE observed, namely degassing from the Red Sea Deep water.

Source attribution by NPAHs and OPAHs
Similar to the ratio of BAP/(BAP+BEP), the ratio of 2-NFLT/1-NPYR can indicate the photochemical age of aerosols. A ratio <5 shows the predominance of combustion sources, while a higher ratio indicates photochemically aged aerosols (Ciccioli et al., 1996). As illustrated in Fig. 6a, the highest regional average ratio of 2-NFLT/1-NPYR but also with the 575 highest absolute and relative standard deviation was found over the Mediterranean Sea, followed by the southern and the northern Red Sea. However, the ratio was in only two samples (D1 and D58, collected over the Mediterranean Sea) higher than 5. In contrast, the ratio of BAP to BEP suggests aged aerosols in several samples as explained in Sect. 3.2.2.
The reason for the low incidence of high ratios could be that the concentrations of atmospheric oxidants OH and NO 3 radicals as well as NO 2 in some sea regions during the campaign were low 580 Friedrich et al., 2021). The difference might be caused by different oxidants being responsible for degradation of BAP and formation of 2-NFLT. BAP, which is predominantly in the particulate phase, is mainly degraded by heterogeneous reaction with ozone (Shiraiwa et al., 2009), while 2-NFLT is mainly formed by homogeneous reaction of FLT with OH or NO 3 (Atkinson and Arey, 1994;Reisen and Arey, 2005) and subsequent reaction with NO 2 . Ozone concentrations varied between 20 ppbv (in the Arabian Sea) and 150 ppbv (in the Arabian Gulf), while the variation of NO x was higher (from 50 pptv in 585 Arabian Sea to more than 10 ppbv in the northern Red Sea and the Arabian Gulf) Friedrich et al., 2021).
The highest NO x mixing ratios were found in the Northern Red Sea and the Gulf region, especially close to the Suez Canal, Kuwait and Fujairah Friedrich et al., 2021). NO 2 concentrations are generally significantly smaller in the marine environment than on land, as shown by satellite data (Roşu et al., 2019) due to the short lifetime (Schaub et al., 2007;Shah et al., 2020) and missing sources for NO x on sea except for ship traffic and the offshore oil and gas industry. Friedrich 590 et al. (2021) calculated NO 2 lifetimes of less than 6h during the AQABA campaign, which means that land-based NO x emissions will be degraded before reaching the sampler for several samples, especially in parts of the Mediterranean Sea.
Missing primary sources and high degradation due to high OH radical concentrations explain the low NO x mixing ratios over the Mediterranean Sea (Friedrich et al., 2021). However, NO 2 is crucial for the formation of 2-NFLT competing with O 2 to either form NPAHs or OPAHs, respectively (Kamens et al., 1994;Finlayson-Pitts and Pitts, 1999;Atkinson and Arey, 595 2007).
The average 2-NFLT/1-NPYR ratio in air sampled over the Mediterranean Sea was significantly higher (p<0.05, Student's ttest) than in the air from Arabian Sea and the Gulf of Oman, respectively. Similarly, the ratio over the northern Red Sea was significantly higher (p<0.05, Student's t-test) than over both Gulf regions. In polluted air near the coast (e.g. as found in the Red Sea and at the beginning and the end of the campaign in the Mediterranean Sea) and in plumes (in samples D1, D30, 600 D48 and D58), the 2-NFLT/1-NPYR ratio was high. These samples explain the high average 2-NFLT/1-NPYR ratio in the https://doi.org/10.5194/acp-2022-32 Preprint. Discussion started: 18 January 2022 c Author(s) 2022. CC BY 4.0 License.
Mediterranean Sea and the northern Red Sea. The ratio can rise during transport of the pollutants. However, the formation of 2-NFLT slows down or stops probably due to low concentrations of the parent-PAH (FLT), the atmospheric oxidants OH and NO 3 or NO 2 since formation of 2-NFLT depends on these reactants (Wilson et al., 2020). This was also shown by Lammel et al. (2017), who found much larger yields of 2-NFLT and 2-NPYR in the marine background with urban influence 605 compared to the marine background without significant pollution sources.
In contrast to the regions with polluted air, a low ratio was observed in air over the Arabian Sea and in the parts of the Mediterranean Sea far from the coast. Due to these samples with low 2-NFLT/1-NPYR ratios in the Mediterranean Sea, the standard deviation of the regional average ratio is the highest among all sea regions. The average ratio in the Arabian Sea was only 0.36 (taking LOQ/2 values of 1-NPYR into account). This points to primary sources (as indicated in Sect. 3.3.2) 610 and/or very low NO 2 concentrations in the sampled air masses, as shown by Friedrich et al. (2021). When secondary formation far away from sources is not significant anymore, the differences in characteristic time for chemical (which is primarily photolysis) and physical sinks (which is primarily particle deposition) determine the ratio 2-NFLT/1-NPYR. One influencing factor might be the difference in deposition velocity of the two compounds due to the different particulate fractions, which is lower for 2-NFLT (not shown, gas-particle partitioning is studied in a separate paper). A lower particulate 615 mass fraction of 2-NFLT might lead to a slower deposition of this compound compared to 1-NPYR, which would lead to higher ratios. In contrast, a lower ratio would be the result of the faster degradation of 2-NFLT by OH and ozone compared to the degradation of 1-NPYR (Table S2, USEPA, 2019). In contrast, 1-NPYR is probably more prone to photodegradation, although the photodegradation rates strongly depend on the aerosol composition (Feilberg and Nielsen, 2000). The removal and degradation rates are reported to be approximately similar (Kamens et al., 1994;Fan et al., 1996;Feilberg and Nielsen, 620 2000;Albinet et al., 2008). However, this may not be the case in this study due to exceptionally low particulate mass fractions of the PACs due to the high temperature and the low EC and OC concentrations in the aerosols (Table S15). If degradation of 2-NFLT plays a larger role in the investigated regions, the ratio of 2-NFLT/1-NPYR would decrease with time, when there is no formation of 2-NFLT. This might be another explanation for the low 2-NFLT/1-NPYR ratios in some sea regions. However, more research is needed on the exact kinetics influencing the ratio, especially the photolysis rate 625 coefficients. At continental sites, the ratio of 2-NFLT/1-NPYR mostly increases with distance to the emission source due to the significant formation of 2-NFLT (Ciccioli et al., 1996;Nežiková et al., 2021).  Since 2-NPYR is almost entirely formed by the reaction of PYR with OH radicals during daytime, while 2-NFLT can be formed by daytime reaction with OH as well as by nighttime reaction with NO 3 , the ratio of 2-NFLT/2-NPYR can reveal the predominant formation pathway of NPAHs (Feilberg et al., 2001;. The main formation pathway 635 during the campaign was the OH radical initiated formation, since the average concentration ratio of 2-NFLT/2-NPYR was 15.1 ± 11.6. This is close to a ratio of 5-10, suggesting the predominant formation of NPAHs by OH radicals and far from a ratio of >100, which would indicate reactions mainly involving the NO 3 radical. This result is similar to the findings by Tang et al. (2014) at a remote site on a Japanese peninsula. Lammel et al. (2017) determined even lower 2-NFLT/2-NPYR ratios in the eastern Mediterranean Sea, also pointing to a predominant NPAH formation by hydroxyl radicals. 640 As illustrated in Fig. 6b, the lowest average ratio (9.5) was found in the northern Red Sea, while the ratio of 15.8 (first leg: 22.3; second leg: 13.4) in the Mediterranean Sea was the highest regional average value. The high value in the Mediterranean Sea during the first leg was due to two exceptionally high values in samples D1 and D5. The formation of these NPAHs will be largely determined by the accumulated nighttime NO 2 and the actinic flux during the day, the air mass had been exposed to prior sampling. For example, samples D52 and D56 had relatively high ratios since the aerosols have 645 picked up NO x emissions from the urban areas of Athens and Istanbul (D52) or Rome and Naples (D56) in a previous night, which can be converted to the NO 3 radical by the reaction with ozone. Whereas the samples D50 and D54, which had not picked up NO x emissions from particular source areas within ≈48h, did not show a high 2-NFLT/2-NPYR ratio.

PAHs and derivatives in photochemically aged pollution
A high ratio of the secondarily formed PAH derivatives 2-NPYR and 2-NFLT (Arey et al., 1986;650 Reisen and Arey, 2005) to their parent PAHs indicates long-range transported aerosols with a significant concentration of the atmospheric oxidants OH and NO 3 as well as NO 2 . Similar to the ratio of 2-NFLT/1-NPYR, the highest ratios were observed in air over the Mediterranean Sea (especially during the first leg in aged aerosols). We determined the highest ratios in sample D1 at the beginning of the campaign close to Sardinia and Sicily. Another high ratio was found in sample D30 in the Arabian Gulf. As already revealed by , photochemically aged air reached the ship from the first night of 655 the first leg in the Arabian Gulf (28 July 2017 16:00 UTC) until the 30 July 2017 at 00:00 UTC (see air mass histories in Fig.   7a), as evidenced by high mixing ratios of some carbonyl compounds such as acetone. After that, the air was dominated by fresh emissions, while approaching Kuwait. According to the distribution of residence times of air masses, the air arrived from northwest with influence of several oil fields and refineries in that region (Fig. 7b and S3f;Bourtsoukidis et al., 2019;Pfannerstill et al., 2019;. Thus, the samples from the first leg in the Arabian Gulf were affected by fresh 660 emissions as well as photochemically aged air. Apart from 2-NFLT/FLT and 2-NPYR/PYR, several other PAH derivatives to parent-PAH concentration ratios (e.g. 1,4-O 2 NAP/NAP, 9-OFLN/FLN and 9,10-O 2 ANT/ANT) were also elevated in sample D30, showing the high contribution of photochemically aged air. In addition, the results indicate that these PAH derivatives are secondarily formed or significantly slower degraded and deposited than their parent PAH.
Another sample with a high ratio of secondarily formed NPAHs is sample D48 in the northern Red Sea nearby the Suez 665 Canal. Similar to the first night in the Arabian Gulf,   and comparably high production rates of the NO 3 radical in this sea region, as reported by Eger et al. (2019). In addition, sample D48 is also affected by primary emissions, e.g., from oil refineries and shipping emissions . This is supported by the PMF, suggesting aged shipping emissions as well as continental pollution as the major sources.
Generally, low and high NPAH/PAH and OPAH/PAH ratios coincided with NO x and radical availabilities. The highest 675 concentrations of PAHs (D54) as well as of NPAHs and OPAHs (D58) were found in air masses carrying continental pollution, sampled in the Mediterranean Sea (from south-east Europe, covering major urban areas including Thessaloniki and Istanbul as well as from Sicily, Corsica, Sardinia and parts of continental Italy, respectively; Figs. 7c and d). While night-time sample D54 corresponded to low NO x and low OH and NO 3 radical concentrations, 24 h sample D58 corresponded to high OH and NO x concentrations Friedrich et al., 2021).

Significance of OPAHs and NPAHs photochemical sources
We found a significant positive correlation of 7,12-O 2 BAA with the ratio of 2-NFLT/1-NPYR (r=0.83, p<0.05), which is typically used as an indicator for the contribution of PAH derivatives formed from oxidative reactions. Several laboratory studies have shown that 7,12-O 2 BAA is formed from the heterogeneous reaction of BAA with O 3 alone and also with the combination of O 3 and NO 2 (Gao et al., 2009;Ringuet et al., 2012a, b). The formation of 7,12-O 2 BAA from the 690 photochemical reaction of BAA has also been reported (Jang and McDow, 1997;Shen et al., 2007). In our current study, the quinone weakly, not significantly correlated with ozone (r=0.25, p=0.11), the OH concentration (r=0.29, p=0.11) and the actinic flux (r=0.23, p=0.14). The weak correlation of the ratio of 7,12-O 2 BAA and the parent-PAH BAA with the actinic flux (r=0.28, p=0.07) was the strongest correlation among all PAH derivative/PAH ratios. However, the secondary formation of 7,12-O 2 BAA is expected to be only a part of the total burden of this quinone. We also found correlations with primary 695  Lin et al. (2015) reported that around 30 % of 7,12-O 2 BAA in atmospheric PM sampled in Beijing was secondarily formed. Lin and colleagues (2015) also found significant secondary formation of 3-NPHE and 7-NBAA. Tomaz et al. (2017) even suggested 3-NPHE to be used as a marker for secondary formation from PHE. However, 3-NPHE is not or only to trace amounts formed by homogeneous reactions with OH or NO 3 (Atkinson and Arey, 1994;Helmig and Harger, 1994;Lee and 700 Lane, 2010). Though, Ringuet et al. (2012a) reported the formation of 3-NPHE and 7-NBAA by heterogeneous formation with atmospheric oxidants. Liu et al. (2019) found a correlation between 2-NFLT and 7-NBAA. We observed a significant correlation of the ratio 2-NFLT/1-NPYR with 3-NPHE (r=0.44, p<0.05) but not with 7-NBAA (r=0.01, p=0.91). 7-NBAA was positively correlated with the concentration of the NO 3 radical and NO 2 (r=0.60, p<0.05 and r=0.67, p<0.05, respectively), but not with the OH radical. 3-NPHE showed a weak correlation with NO 2 (r=0.28, p=0.1) and the actinic flux 705 (0.25, p=0.11). Based on the high emission factors of 3-NPHE and 7-NBAA in diesel combustion (Heeb et al., 2008;Zhao et al., 2019), we expect primary pollution as the major source of these compounds and only a minor contribution from secondary formation. This is supported by Zhuo et al. (2017), who report a contribution from secondary formation of only 3-10 % to the total concentration of 3-NPHE in the city Nanjing in eastern China.
1-(CHO)NAP was already reported to be secondarily formed by ozonolysis from ACY, 1-methylnaphthalene and possibly 725 other precursors within hours (Dang et al., 2015). It is significantly correlating with the ratio of 2-NFLT/1-NPYR (r=0.42, p<0.05) and shows a very weak, not significant correlation with ozone (r=0.11, p=0.47). 5,12-O 2 NAC showed a very weak correlation with ozone (r=0.18, p=0.25). However, we are not aware of any study showing secondary formation of this quinone. In contrast, 11-OBbFLN can be formed by the reaction of the parent PAH with ozone (Ringuet et al., 2012a). However, we did not find any correlation. Thus, there is no clear indication for significant secondary formation of 11-730 OBaFLN, 11-BbOFLN and BAN, based on the correlation analysis.

Mass size distributions
The highest concentrations of PAHs, OPAHs and NPAHs are found in the sub-micrometre fraction of particulate matter, PM 1 . Fig. 8 shows the campaign average mass size distributions of the PAHs and PAH derivatives. The mass size distributions of PAHs, NPAHs and OPAHs are mainly unimodal given the coarse size resolution of the impactor with 6 size ranges within PM 10 . The maximum was found in particles with an aerodynamic diameter <0.49 μm. For the sum of PAHs, 740 four samples showed an apparently unimodal distribution with a maximum at a particle diameter of 0.49-0.95 µm in the accumulation mode instead of the lowest particle size. In addition, three samples (two in the Arabian Gulf and one in the Arabian Sea) showed a bimodal distribution with maxima in particles with an aerodynamic diameter <0.49 µm and of 0.95-1.5 µm. For the sum of NPAHs, only one sample (in the Mediterranean Sea) showed an apparently unimodal distribution with a maximum in another aerodynamic particle diameter range than <0.49 μm (0.49-0.95 µm). Since we did not resolve 745 the <0.49 μm size fraction, more modes in the sub-micrometre fraction, as found by di Filippo et al. (2010) cannot be excluded.
The ratio between the concentrations in particles <0.49 μm compared to the concentrations in coarse mode PM particles is greater for high-molecular-weight PACs compared to low-molecular-weight PACs and higher for PAH derivatives compared to the parent-PAHs. This can be explained by the lower vapour pressure of PAH derivatives and high-molecular-weight 750 PAHs compared to the parent-PAHs and low-molecular-weight PAHs. Compounds with lower vapour pressure are less subject to redistribution across particles sizes during transport (Degrendele et al., 2014). The process of redistribution is more effective that the pollutants reach higher particle size fractions than the process of coagulation of particles to form larger particles, which would transfer low vapour pressure PACs to bigger particle size fractions. The dependency of the vapour pressure on the mass median diameter was only found for PAHs and was not significant. This can partly be explained 755 by limited explanatory power of the mass median diameter in this study due to low concentrations of e.g. the 3-ring PAHs and some NPAHs, leading to no detectable amount in coarse mode particles. The NPAHs generally had a low concentration in our study and the low-molecular-weight PAHs were not abundant in PM since these substances are preferable in the gas phase. The campaign average mass median diameters of the target compounds are shown in the Supplement Table S19.
Since the process of redistribution depends on time, a shift of the mass median diameter to larger particles sizes is found for 760 aged aerosols (see exemplary Fig. S6). For instance, Lammel et al. (2017) found two maxima for the 4-ring PAHs at a marine background site (same cascade impactor as the one used in this study). The second maximum was explained by aged

Conclusions
For the first time, PAHs and their derivatives were measured in the marine environment around the entire Arabian Peninsula in a comprehensive ship campaign. The atmospheric average concentrations of ∑ 27 PAHs, ∑ 19 RPAHs, ∑ 11 OPAHs and ∑ 17 NPAHs in the gas and particulate phase were 2.85 ± 3.35 ng m -3 , 0.83 ± 0.87 ng m -3 , 0.24 ± 0.25 ng m -3 and 4.34 ± 7.37 775 pg m -3 , respectively. The lowest burden of all targeted pollutant classes was observed in the Arabian Sea with concentrations among the lowest ever reported, followed by the southern Red Sea. The highest average concentrations of the PAHs and the OPAHs were detected in the Mediterranean Sea, while the NPAHs were most abundant in the Arabian Gulf. It was observed that the regional differences in the composition patterns of the NPAHs and OPAHs were more pronounced than those of the PAHs and RPAHs. 1,4-O 2 NAP, 9-OFLN and 9,10-O 2 ANT were the most abundant OPAHs. The NPAH composition pattern 780 was dominated by a high contribution of 2-NNAP, followed by 1-NPYR, which was highly abundant in the Gulf region.
Source apportionment showed that the PAHs and their nitrated and oxygenated derivatives mainly originated from fresh and 785 aged shipping emissions. All OPAHs and NPAHs except 2-NFLT, which were frequently detected during the campaign, showed elevated concentrations in fresh shipping emissions. 1-NPYR among the NPAHs and 11-OBaFLN and 1-(CHO)NAP among the OPAHs showed the highest relative increase in their concentration. In contrast, 2-NFLT and 2-NPYR were highly abundant in aged shipping emissions due to secondary formation. 1-NPYR, 3-NPHE and several OPAHs had a significant contribution from residual oil combustion. PAH derivatives were clearly enriched in long-range transported 790 plumes from polluted regions in Egypt, the Arabian Gulf, and southern and eastern Europe. Throughout the campaign, the https://doi.org/10.5194/acp-2022-32 Preprint. Discussion started: 18 January 2022 c Author(s) 2022. CC BY 4.0 License.
highest concentrations of PAHs, OPAHs and NPAHs were found in the sub-micrometre fraction of particulate matter (PM 1 ).
Due to redistribution, the mass median diameter was shifted to higher values in long-range transported aerosols.
Data availability. The data used in this study are archived and distributed through the KEEPER service of the Max Planck 795 Digital Library (https://keeper.mpdl.mpg.de, last access: 12 December 2021) and have been available from August 2019 to all scientists agreeing to the AQABA protocol.
Supplement. The supplement related to this article is available online at: 800 Author contributions. MW evaluated the data and wrote the manuscript. GL supervised this study. MI and RP did the sampling. MK, MI and ES provided the data about alkylated PAHs and wrote this part. PPo performed the PMF and wrote the part. BN created the FLEXPART Lagrangian particle dispersion model results. JK provided the data about metals. PK and PPr performed the PAH, NPAH and OPAH sample preparation and analysis. IT created GPS plots and provided NO x and O 3 data. NF, PE and JNC contributed measurements of NO 2 , NO X , NO y O 3 and SO 2 . RR and ST provided OH radical 805 data. JW was involved in the discussion about the sources. The stack filter and information about bypassing ships as well as BC and surface PAH concentrations were provided by FD and SC. HH took responsibility for the scientific coordination of the field campaign on board the research vessel. JL designed the AQABA campaign. GL designed this study, supported by ES and UP. All authors contributed to data interpretation and manuscript revision and approved the submitted version.

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Competing interests. The authors declare that they have no conflict of interest.
Acknowledgements. We thank Hays Ships Ltd, Captain Pavel Kirzner and the Kommandor Iona's ship crew for the great support. We would like to thank Marcel Dorf and Claus Koeppel for the organization of the campaign, Hartwig Harder for the management on board, and all other participants and supporters of the campaign. We thank Benedikt Steil for processing 815 meteorological data and Jan Schuladen for the data about the actinic flux. We also thank Abdulaziz al Senafi (Kuwait Inst. of Scientific Research). In addition, we thank Ondrej Sanka for the assistance in the plotting of sampling stretches. This