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Formaldehyde (HCHO), hydrogen peroxide (H 2 O 2) and organic hydroperoxides (ROOH) play a key role in atmospheric oxidation processes. They act as sources and sinks for HO x radicals (OH + HO 2), with OH as the primary oxidant that governs the atmospheric self-cleaning capacity. Measurements of these species allow for evaluation of chemistry-transport models which need to account for multifarious source distributions, transport, complex photochemical reaction pathways and deposition processes of these species. HCHO is an intermediate during the oxidation of volatile organic compounds (VOCs) and is an indicator of photochemical activity and combustion-related emissions. In this study, we use in situ observations of HCHO, H 2 O 2 and ROOH in the marine boundary layer (MBL) to evaluate results of the general circulation model EMAC (ECHAM5/MESSy2 Atmospheric Chemistry; European Center HAMburg, Modular Earth Submodel System). The dataset was obtained during the Air Quality and Climate Change in the Arabian Basin (AQABA) ship campaign around the Arabian Peninsula in summer 2017. This region is characterized by high levels of photochemical air pollution, humidity and solar irradiation, especially in the areas around the Suez Canal and the Arabian Gulf. High levels of air pollution with up to 12 ppbv HCHO, 2.3 ppbv ROOH and relatively low levels of H 2 O 2 (≤0.5 ppbv) were detected over the Arabian Gulf. We find that EMAC failed to predict absolute mixing ratios of HCHO and ROOH during high-pollution events over the Arabian Gulf, while it reproduced HCHO on average within a factor of 2. Dry deposition velocities were determined for HCHO and H 2 O 2 at night with 0.77±0.29 cm s -1 for HCHO and 1.03±0.52 cm s -1 for H 2 O 2 over the Arabian Sea, which were matched by EMAC. The photochemical budget of H 2 O 2 revealed elevated HO x radical concentrations in EMAC, which resulted in an overestimation of H 2 O 2 by more than a factor of 5 for the AQABA dataset. The underestimated air pollution over the Arabian Gulf was related to EMAC's coarse spatial resolution and missing anthropogenic emissions in the model. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) and 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 adversely affect human health upon exposure. However, the occurrence and fate of these air pollutants have hardly been studied in the marine atmosphere. In this study, we report the atmospheric concentrations over the Mediterranean Sea, the Red Sea, the Arabian Sea, the Gulf of Oman and the Arabian Gulf, determined during the AQABA (Air Quality and Climate Change in the Arabian Basin) project, a comprehensive ship-borne campaign in summer 2017. The average concentrations of ∑26 PAHs, ∑19 RPAHs, ∑11 OPAHs and ∑17 NPAHs, in the gas and particulate phases, were 2.99 ± 3.35 ng m -3 , 0.83 ± 0.87 ng m -3 , 0.24 ± 0.25 ng m -3 and 4.34 ± 7.37 pg m -3 , respectively. The Arabian Sea region was the cleanest for all substance classes, with concentrations among the lowest ever reported. Over the Mediterranean Sea, we found the highest average burden of ∑26 PAHs and ∑11 OPAHs, while the ∑17 NPAHs were most abundant over the Arabian Gulf (known also as the Persian Gulf). 1,4-Naphthoquinone (1,4-O 2 NAP) followed by 9-fluorenone and 9,10-anthraquinone were the most abundant studied OPAHs in most samples. The NPAH composition pattern varied significantly across the regions, with 2-nitronaphthalene (2-NNAP) being the most abundant NPAH. According to source apportionment investigations, the main sources of PAH derivatives in the region were ship exhaust emissions, residual oil combustion and continental pollution. All OPAHs and NPAHs except 2-nitrofluoranthene (2-NFLT), which were frequently detected during the campaign, showed elevated concentrations in fresh shipping emissions. In contrast, 2-NFLT and 2-nitropyrene (2-NPYR) were highly abundant in aged shipping emissions due to secondary formation. Apart from 2-NFLT and 2-NPYR, benz(a)anthracene-7,12-dione and 2-NNAP also had significant photochemical sources. Another finding was that the highest concentrations of PAHs, OPAHs and NPAHs were found in the sub-micrometre fraction of particulate matter (PM 1). [ABSTRACT FROM AUTHOR]

Formaldehyde (HCHO) is the most abundant aldehyde in the troposphere. While its background mixing ratio is mostly determined by the oxidation of methane, in many environments, especially in the boundary layer, HCHO can have a large variety of precursors, in particular biogenic and anthropogenic volatile organic compounds (VOCs) and their oxidation products. Here we present shipborne observations of HCHO , hydroxyl radical (OH) and OH reactivity (R(OH)), which were obtained during the Air Quality and Climate Change in the Arabian Basin (AQABA) campaign in summer 2017. The loss rate of HCHO was inferred from its reaction with OH, measured photolysis rates and dry deposition. In photochemical steady state, the HCHO loss is balanced by production via OH-initiated degradation of VOCs, photolysis of oxygenated VOCs (OVOCs) and the ozonolysis of alkenes. The slope αeff from a scatter plot of the HCHO production rate versus the product of OH and R(OH)eff (excluding inorganic contribution) yields the fraction of OH reactivity that contributes to HCHO production. Values of αeff varied between less than 2 % in relatively clean air over the Arabian Sea and the southern Red Sea and up to 32 % over the polluted Arabian Gulf (also known as Persian Gulf), signifying that polluted areas harbor a larger variety of HCHO precursors. The separation of R(OH)eff into individual compound classes revealed that elevated values of αeff coincided with increased contribution of alkanes and OVOCs, with the highest reactivity of all VOCs over the Arabian Gulf. [ABSTRACT FROM AUTHOR]

We present shipborne measurements of NO x (≡ NO + NO 2) and NO y (≡ NO x+ gas- and particle-phase organic and inorganic oxides of nitrogen) in summer 2017 as part of the expedition "Air Quality and climate change in the Arabian BAsin" (AQABA). The NO x and NO z (≡ NO y -NO x) measurements, made with a thermal dissociation cavity ring-down spectrometer (TD-CRDS), were used to examine the chemical mechanisms involved in the processing of primary NO x emissions and their influence on the NO y budget in chemically distinct marine environments, including the Mediterranean Sea, the Red Sea, and the Arabian Gulf, which were influenced to varying extents by emissions from shipping and oil and gas production. Complementing the TD-CRDS measurements, NO and NO 2 data sets from a chemiluminescence detector (CLD) were used in the analysis. In all regions, we find that NO x is strongly connected to ship emissions, both via direct emission of NO and via the formation of HONO and its subsequent photolytic conversion to NO. The role of HONO was assessed by calculating the NO x production rate from its photolysis. Mean NO 2 lifetimes were 3.9 h in the Mediterranean Sea, 4.0 h in the Arabian Gulf, and 5.0 h in the Red Sea area. The cumulative loss of NO 2 during the night (reaction with O 3) was more important than daytime losses (reaction with OH) over the Arabian Gulf (by a factor 2.8) and over the Red Sea (factor 2.9), whereas over the Mediterranean Sea, where OH levels were high, daytime losses dominated (factor 2.5). Regional ozone production efficiencies (OPEs; calculated from the correlation between O x and NO z , where O x= O 3+ NO 2) ranged from 10.5 ± 0.9 to 19.1 ± 1.1. This metric quantifies the relative strength of photochemical O 3 production from NO x compared to the competing sequestering into NO z species. The largest values were found over the Arabian Gulf, consistent with high levels of O 3 found in that region (10–90 percentiles range: 23–108 ppbv). The fractional contribution of individual NO z species to NO y exhibited a large regional variability, with HNO 3 generally the dominant component (on average 33 % of NO y) with significant contributions from organic nitrates (11 %) and particulate nitrates in the PM 1 size range (8 %). [ABSTRACT FROM AUTHOR]

Strongly enhanced tropospheric ozone (O3) mixing ratios have been reported in the Arabian Basin, a region with intense solar radiation and high concentrations of O3 precursors such as nitrogen oxides (NOx) and volatile organic compounds (VOCs). To analyze photochemical O3 production in the marine boundary layer (MBL) around the Arabian Peninsula, we use shipborne observations of NO, NO2 , O3 , OH, HO2 , HCHO, the actinic flux, water vapor, pressure and temperature obtained during the summer 2017 Air Quality and Climate in the Arabian Basin (AQABA) campaign, and we compare them to simulation results from the ECHAM-MESSy Atmospheric Chemistry (EMAC) general circulation model. Net O3 production rates (NOPRs) were greatest over both the Gulf of Oman and the northern Red Sea (16 ppbv d -1) and over the Arabian Gulf (32 ppbv d -1). The NOPR over the Mediterranean, the southern Red Sea and the Arabian Sea did not significantly deviate from zero; however, the results for the Arabian Sea indicated weak net O3 production of 5 ppbv d -1 as well as net O3 destruction over the Mediterranean and the southern Red Sea with values of -1 and -4 ppbv d -1 , respectively. Constrained by HCHO/NO2 ratios, our photochemistry calculations show that net O3 production in the MBL around the Arabian Peninsula mostly occurs in NOx -limited regimes with a significant share of O3 production occurring in the transition regime between NOx limitation and VOC limitation over the Mediterranean and more significantly over the northern Red Sea and Oman Gulf. [ABSTRACT FROM AUTHOR]

Shipborne measurements of nitryl chloride (ClNO2), hydrogen chloride (HCl) and sulfur dioxide (SO2) were made during the AQABA (Air Quality and climate change in the Arabian BAsin) ship campaign in summer 2017. The dataset includes measurements over the Mediterranean Sea, the Suez Canal, the Red Sea, the Gulf of Aden, the Arabian Sea, the Gulf of Oman, and the Arabian Gulf (also known as Persian Gulf) with observed ClNO2 mixing ratios ranging from the limit of detection to ≈600 pptv. We examined the regional variability in the generation of ClNO2 via the uptake of dinitrogen pentoxide (N2O5) to Cl-containing aerosol and its importance for Cl atom generation in a marine boundary layer under the (variable) influence of emissions from shipping and the oil industry. The yield of ClNO2 formation per NO3 radical generated was generally low (median of ≈1 %–5 % depending on the region), mainly as a result of gas-phase loss of NO3 dominating over heterogeneous loss of N2O5 , the latter being disfavoured by the high temperatures found throughout the campaign. The contributions of ClNO2 photolysis and OH-induced HCl oxidation to Cl-radical formation were derived and their relative contributions over the diel cycle compared. The results indicate that over the northern Red Sea, the Gulf of Suez, and the Gulf of Oman the formation of Cl atoms will enhance the oxidation rates of some volatile organic compounds (VOCs), especially in the early morning. [ABSTRACT FROM AUTHOR]