Abstract
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 traveling 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 northwestern 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 (50 %), 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.
Highlights
The Arabian Peninsula is a hotspot of global change
Our results show that the ratio of NOx- and volatile organic compound (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
The largest fraction of attributed OH reactivity was provided by oxygenated volatile organic compounds (OVOCs) in almost all AQABA regions (9 %–35 %), which is comparable to previous studies in the oil-impacted region of Houston, where between 11 % and 24 % of the OH sink was due to OVOCs (Mao et al, 2010)
Summary
The Arabian Peninsula is a hotspot of global change. It is already subject to extremes of heat and drought, which are predicted to intensify (Zhang et al, 2005; Wasimi, 2010; Chenoweth et al, 2011; Tanarhte et al, 2012; Lelieveld et al, 2012, 2016; Terink et al, 2013; Donat et al, 2014; Cook et al, 2016), and to photochemical air pollution (Lelieveld et al, 2009; Smoydzin et al, 2012; Abdelkader et al, 2015; Farahat, 2016). The waters surrounding the Arabian Peninsula are key routes of global trade and bottlenecks for marine traffic, another source of air pollution (Endresen, 2003; Eyring, 2005; Boersma et al, 2015; Mertens et al, 2018) that is expected to increase in the future (Eyring et al, 2007). The main regional photochemical oxidation process is initiated by ozone photolysis to O(1D), which reacts with water vapor to form hydroxyl radicals (OH). The abundance of precursors and intense photochemistry make conditions in the Arabian Basin very favorable for photochemical ozone production, leading to extremely high ozone mixing ratios up to 200 ppb (Lelieveld et al, 2009; Smoydzin et al, 2012; Krotkov et al, 2016). VOC oxidation by OH and ozone can generate secondary organic aerosol (Grosjean and Seinfeld, 1989; Kroll and Seinfeld, 2008) with implications for human health (Lelieveld et al, 2015) and regional radiative budgets (Ezhova et al, 2018)
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