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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 28 May 2019

Research article | 28 May 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Shipborne measurements of total OH reactivity around the Arabian Peninsula and its role in ozone chemistry

Eva Y. Pfannerstill1, Nijing Wang1, Achim Edtbauer1, Efstratios Bourtsoukidis1, John N. Crowley1, Dirk Dienhart1, Philipp G. Eger1, Lisa Ernle1, Horst Fischer1, Bettina Hottmann1, Jean-Daniel Paris2, Christof Stönner1, Ivan Tadic1, David Walter1, Jos Lelieveld1,3, and Jonathan Williams1 Eva Y. Pfannerstill et al.
  • 1Atmospheric Chemistry and Multiphase Chemistry Departments, Max Planck Institute for Chemistry, 55128 Mainz, Germany
  • 2Laboratoire des Sciences du Climat et de l’Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, Gif-sur-Yvette, France
  • 3Energy, Environment and Water Research Center, The Cyprus Institute, Nicosia, Cyprus

Abstract. The Arabian Peninsula is characterized by high and increasing levels of photochemical air pollution. Strong solar irradiation, high temperatures and large anthropogenic emissions of reactive trace gases result in intense photochemical activity, especially during the summer months. However, air chemistry measurements in the region are scarce. In order to assess regional pollution sources and oxidation rates, the first ship-based direct measurements of total OH reactivity were performed in summer 2017 from a vessel travelling around the peninsula during the AQABA (Air Quality and Climate Change in the Arabian Basin) campaign. Total OH reactivity is the total loss frequency of OH radicals due to all reactive compounds present in air and defines the local lifetime of OH, the most important oxidant in the troposphere. During the AQABA campaign, the total OH reactivity ranged from below the detection limit (5.4 s−1) over the north-western Indian Ocean (Arabian Sea) to a maximum of 32.8 ± 9.6 s−1 over the Arabian Gulf (also known as Persian Gulf) when air originated from large petroleum extraction/processing facilities in Iraq and Kuwait. In the polluted marine regions, OH reactivity was broadly comparable to highly populated urban centers in intensity and composition. The permanent influence of heavy maritime traffic over the seaways of the Red Sea, Gulf of Aden and Gulf of Oman resulted in median OH sinks of 7.9–8.5 s−1. Due to the rapid oxidation of direct volatile organic compound (VOC) emissions, oxygenated volatile organic compounds (OVOCs) were observed to be the main contributor to OH reactivity around the Arabian Peninsula (9–35 % by region). Over the Arabian Gulf, alkanes and alkenes from the petroleum extraction and processing industry were an important OH sink with ~ 9 % of total OH reactivity each, whereas NOx and aromatic hydrocarbons (~ 10 % each) played a larger role in the Suez Canal, which is influenced more by ship traffic and urban emissions. We investigated the number and identity of chemical species necessary to explain the total OH sink. Taking into account ~ 100 individually measured chemical species, the observed total OH reactivity can typically be accounted for within the measurement uncertainty, with 10 dominant trace gases accounting for 20–39 % of regional total OH reactivity. The chemical regimes causing the intense ozone pollution around the Arabian Peninsula were investigated using total OH reactivity measurements. Ozone vs. OH reactivity relationships were found to be a useful tool for differentiating between ozone titration in fresh emissions and photochemically aged air masses. Our results show that the ratio of NOx- and VOC-attributed OH reactivity was favorable for ozone formation almost all around the Arabian Peninsula, which is due to NOx and VOCs from ship exhausts and, often, oil/gas production. Therewith, total OH reactivity measurements help to elucidate the chemical processes underlying the extreme tropospheric ozone concentrations observed in summer over the Arabian Basin.

Eva Y. Pfannerstill et al.
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Eva Y. Pfannerstill et al.
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Short summary
The Arabian Peninsula is a global hotspot of ozone pollution. Our measurements, made on a ship in summer 2017, indicate underlying reasons: Despite being at sea, we observed ozone-forming reactive trace gases (measured as so-called total OH reactivity) comparable to highly populated urban regions in amount and composition. This is due to strong emissions from oil extraction and ship traffic. These emissions were quickly converted to ozone due to intense solar irradiation and high temperatures.
The Arabian Peninsula is a global hotspot of ozone pollution. Our measurements, made on a ship...