Measurements of carbonyl compounds around the Arabian Peninsula

11 Volatile organic compounds (VOCs) were measured around the Arabian Peninsula using a research vessel during the AQABA 12 campaign (Air Quality and Climate Change in the Arabian Basin) from June to August 2017. In this study we examine carbonyl 13 compounds (CxHyO), measured by a proton transfer reaction mass spectrometer (PTR-ToF-MS), and present both a regional 14 concentration distribution and a budget assessment for these key atmospheric species. Among the aliphatic carbonyls, acetone had 15 the highest mixing ratios in most of the regions traversed, varying from 0.43 ppb over the Arabian Sea to 4.5 ppb over the Arabian 16 Gulf, followed by formaldehyde (measured by Hantzsch monitor, 0.82 ppb over the Arabian Sea and 3,8 ppb over the Arabian 17 Gulf) and acetaldehyde (0.16 ppb over the Arabian Sea and 1.7 ppb over the Arabian Gulf). Unsaturated carbonyls (C4-C9) varied 18 from 10 to 700 ppt during the campaign, and followed similar regional mixing ratio dependence as aliphatic carbonyls, which were 19 identified as oxidation products of cycloalkanes over polluted areas. An empirical method based on hydrocarbon ratios was applied 20 to investigate the photochemical source strength of the aliphatic carbonyls. While the distribution and relative concentration 21 enhancements of the C3-C8 aliphatic carbonyls could be explained by this method, that of acetaldehyde could not. A smaller but 22 still significant discrepancy was found when comparing measurements to global chemistry-transport model (EMAC) results, with 23 the model underestimating the measured acetaldehyde mixing ratio up to an order of magnitude. Implementing a photolytically 24 driven marine source of acetaldehyde significantly improved the agreement between measurements and model, particularly over 25 the remote regions (e.g. Arabian Sea). However, the newly introduced acetaldehyde source was still insufficient to describe the 26 observations over the most polluted regions (Arabian Gulf and Suez), where model underestimation of primary emissions and 27 biomass burning events are possible reasons. 28

were loaded onto the vessel, containing multiple gas and particle phase measurement instruments as well as a weather station. sccm through 1/8" (0.3175 cm) FEP tubing (~ 10 m in length, insulated and heated to 60 °C) from the main fast air flow and then 104 to the instrument's PEEK (polyether ether ketone) inlet which was likewise heated to 60 °C. The inlet system was shared with total 105 OH reactivity measurement (Pfannerstill et al., 2019).

106
The working principle of PTR-MS has been described in detail in previous studies (Lindinger et al., 1998  117 containing 14 compounds (methanol, acetonitrile, acetaldehyde, acetone, dimethyl sulfide, isoprene, methyl vinyl ketone, 118 methacrolein, methyl ethyl ketone, benzene, toluene, xylene, 1,3,5-trimethylbenzene and α-pinene). It has been previously reported 119 that the sensitivity of some compounds measured by PTR-MS are humidity dependent (de Gouw and Warneke, 2007). As the 120 relative humidity (RH) was expected to be high and varying (marine boundary layer with occasional desert air influence), humidity 121 calibration was combined with 4-point calibration by humidifying the gas mixture at different levels from 0% -100% RH. The data were initially processed by the PTR Analyzer software (Müller et al., 2013) to identify and integrate the peaks. After 124 obtaining the raw data (counts per second for each mass identified), a custom-developed python-based program was used to further 125 process the data to final mixing ratios. For compounds present in the standard gas cylinder, interpolated sensitivities based on the 126 five in-campaign calibrations were applied to derive the mixing ratios; while mixing ratios of the other masses were calculated by 127 using a proton transfer reaction rate constant ( ) of 2.0  10 -9 cm 3 s -1 . The uncertainty associated with the mixing ratios of the 128 calibrated compounds was around 6-17% (see Table S1). For the mixing ratios derived by assuming , the accuracy was around to lose a H2O molecule and fragment to other masses (Buhr et al., 2002;Spanel et al., 2002). Moreover, although both ketones and 137 aldehydes can be produced via atmospheric oxidation processes, ketones tend to have longer atmospheric lifetimes than aldehydes 138 as mentioned in the introduction. Therefore, signals on the exact mass of carbonyl compounds from the PTR-ToF-MS are expected 139 to be dominated by ketones, particularly in regions remote from the sources.

212
The level of each aliphatic carbonyls over the Arabian Gulf was comparable to those previously reported for urban areas (Table   213 2), despite these measurements being taken at sea. As the Arabian Gulf is highly impacted by the oil and gas industry, we also

223
In contrast, aliphatic carbonyls had much lower average mixing ratios over the Arabian Sea and the Gulf of Aden especially for 224 C7-C9 carbonyls with mean mixing ratios below the detection limit for most of the time. During the summertime AQABA 225 campaign, the prevailing wind direction over the Arabian Sea was southwest ( Figure S1). Four-day back trajectories indicate the  Table 2). Acetaldehyde was 230 measured at relatively low mixing ratios over the Arabian Sea (median: 0.12 ppb), which is lower than the levels reported in most

233
Over the Gulf of Aden, acetone and MEK had slightly higher mixing ratios than those over the Arabian Sea.

234
The Mediterranean Sea had somewhat higher levels of aliphatic carbonyls than the clean regions (the Arabian Sea and the Gulf of   ). However, the levels of acetaldehyde and MEK were much less compared to the levels reported from urban sites (see Table   245 2). Interestingly, the mean acetaldehyde mixing ratio (0.62 ± 0.59 ppb) over Suez was twice the level found over the Mediterranean 246 Sea, whilst the acetone level was only slightly higher. Besides the local-scale emission and photochemical production contribution 247 to the acetone over Suez, the longer lived acetone could be also transported from the Mediterranean Sea (where acetone was high).

248
Although the mean mixing ratios of aliphatic carbonyls over Suez were lower than those over the Arabian Gulf, larger variations 249 were observed.

250
Over the Red Sea, acetone was the most abundant aliphatic carbonyls followed by formaldehyde and acetaldehyde. The mixing 251 ratios of aliphatic C2-C4 carbonyls over the northern part of the Red Sea were similar to those levels measured in Thompson Farm (a rural site in the US, Table 2). It is worth noticing that the levels of aliphatic carbonyls in the northern part of the Red Sea were 253 almost two times higher than the southern part of the Red Sea. According to the four-day back trajectories reported by   measured will be given in the following section 3.2.2.

275
Regional variability was also observed for aromatic carbonyls with highest levels observed over the Arabian Gulf and Suez, and 276 much lower mixing ratios over the Arabian Sea, Mediterranean Sea and Gulf of Aden (Table 1) [ ] represents the initial mixing ratios of hydrocarbons from the source, which could be calculated using equation (1)

319
The corresponding correlation plots of toluene and benzene for each region can be found in Figure S3. An average value of OH 320 exposure in each area was applied in equation (2)

348
Formaldehyde had a more than 50% contribution from hydrocarbon oxidation over the Red Sea and Gulf of Oman but less than 349 40% over the Arabian Gulf. Over the Suez region, calculated formaldehyde level was even 24% higher than the measured mean

372
We further separated the data into daytime and nighttime and calculated correlations among the carbonyls and other selected 373 species (see Fig. 4b and c). Aliphatic carbonyls were well correlated with each other during the daytime and ozone had a generally 374 good correlation with C2-C7 carbonyls (r > 0.7) during the daytime but a much lower correlation during the night, indicating ozone 375 and carbonyls were co-produced via photochemical oxidation. This further emphasizes the importance of local photochemical 376 production of aliphatic carbonyls over the Arabian Gulf, as suggested in previous section 3.2.1. Meanwhile, as shown in Figure 4 377 (a), the calculated OH exposure was high during the first night in leg 1, where an elevation of acetone mixing ratio was observed 378 while the mixing ratio of acetaldehyde remained relatively constant. With limited OH radical abundance during the nighttime, the 379 increased OH exposure indicates that the air reaching the ship was photochemically processed (aged). Therefore, the increase of 380 acetone was mainly from long-distance transport as acetone has a much longer atmospheric lifetime than acetaldehyde. As the ship     As shown in Figure 2 and Table 1, C6 unsaturated carbonyls displayed higher mixing ratios than any other unsaturated carbonyls 441 over the Arabian Gulf while C5 unsaturated carbonyl was slightly higher than C6 in Suez. Bourtsoukidis

490
(66 ± 12 ppb), where a modest correlation was found between acetaldehyde and ozone over the Arabian Gulf (r 2 =0.54) and no 491 significant correlation over the Red Sea North (r 2 =0.40). However larger correlation coefficients were identified between ozone 492 and other carbonyls over the Arabian Gulf (see Figure S5), which suggests that the correlation was due to atmospheric 493 photochemical production rather than artifacts. Moreover, acetaldehyde was found to have a much worse correlation with ozone 494 during the nighttime compared to the correlation during the daytime over the Arabian Gulf (Figure 4b and c), which also indicates

582
Previous studies have shown that the organic matter fraction was highest in smaller sea spray aerosols and that the aerosols contain

601
Observations of carbonyl compounds around the Arabian Peninsula were investigated in terms of mixing ratios abundance over 602 different areas. Aliphatic carbonyl compounds were generally more abundant than the unsaturated and aromatic carbonyl 603 compounds, and were dominated by low-molecular-weight compounds (carbon number less than five). Aliphatic carbonyl 604 compounds were found at the highest mixing ratios over the Arabian Gulf followed by the Suez region, while the lowest mixing 605 ratios were observed over the Arabian Sea and the Gulf of Aden. Over the Mediterranean Sea, aliphatic carbonyls were low except 606 for acetone that was much higher compared to the levels observed over clean remote areas (i.e. Arabian Sea). The atmospheric 607 composition over the Red Sea showed obvious differences between the northern and the southern part, with higher mixing ratios 608 in the north. Similar region-dependent distributions were observed for unsaturated and aromatic carbonyls. Generally, the mixing 609 ratios of aromatic carbonyl compounds decreased as the carbon number increased. Particularly over the Suez region, benzaldehyde 610 (C7 aromatic carbonyls) was much more abundant than other aromatic carbonyls, indicating direct sources as well as abundant 611 oxidation precursors. For unsaturated carbonyl compounds, C5 and C6 carbonyl compounds dominated the mixing ratio 612 distribution, while the air chemistry highly depends on the chemical structure assignment of those masses.

613
To better understand the air chemistry of aliphatic carbonyl compounds over different regions, we used an empirical method to 614 calculate the levels of carbonyl compounds resulting from OH oxidation of precursor hydrocarbon species. The results indicate 615 that mixing ratios of formaldehyde and C3-C8 carbonyl compounds could, to a large part, be explained by OH initiated 616 photooxidation in each region, especially over the Arabian Gulf and Suez region. This result indicates that photooxidation is a 617 dominant production pathway for formaldehyde and C3-C8 aliphatic carbonyl compounds in these two regions. However, 618 acetaldehyde from hydrocarbon precursors was not sufficient to explain the high mixing ratios observed, indicating the existence 619 of other sources and/or formation pathways. Further case studies showed that the carbonyl compounds produced via photooxidation 620 were highly correlated to the high ozone levels during daytime over the Arabian Gulf while the air chemistry in Suez region was 621 strongly influenced by regional biomass burning. Due to the unexpectedly high loading of m/z 69 (usually assigned as isoprene)

632
The results indicate that the ocean plays an important role in the atmospheric acetaldehyde budget, under both clean and polluted 633 conditions. The underestimated acetaldehyde in the model is significant as it will influence the atmospheric budget of e.g. PAN.

634
As shown in Figure 1, multiple sources and formation pathways need to be considered to better understand the atmospheric budget

647
The authors declare that they have no conflict of interest.

649
We acknowledge the collaboration with the King Abdullah University of Science and Technology (KAUST), the Kuwait Institute