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

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.

The Red Sea Deep Water is a potent source of atmospheric ethane and propane

“…[2][3][4]. From their tropospheric background values over the Arabian Sea (defined as the lowest 10% of data points: 0.18 ± 0.02 ppb for ethane and 0.02 ± 0.01 ppb for propane), to a maximum of~50 ppb over the Arabian Gulf, variations in absolute and relative abundance of the NMHCs indicated multiple emission sources 16,17 . The high mixing ratios over the Arabian Gulf and Suez Canal could be attributed to emissions from the intense oil and gas activities and urban centers, respectively 16 .…”

“…In total, 20,000 km of the marine route was covered with an average speed of 3.4 ± 1.8 m s −1 over the course of 60 days. Further information on the AQABA ship campaign can be found elsewhere in the literature 16,17,57 .…”

Missing OH reactivity in the global marine boundary layer

“…Several recent studies provide evidence for an unknown VOC or OVOC emitted into the atmosphere from the ocean. Oceanic sources have also been proposed for butanes and pentanes in some regions (Pozzer et al, 2010) and for methanol (Read et al, 2012). Measurements of biogenic VOCs in coastal waters found monoterpenes, C12-C15 nalkanes, and several higher aldehydes that could contribute to enhanced OH reactivity (Tokarek et al, 2019).…”

Measurements of carbonyl compounds around the Arabian Peninsula indicate large missing sources of acetaldehyde

“…Generally, secondary photochemical formation from various precursors is the main source in those regions. However, several recent studies have shown that acetaldehyde mixing ratios in both the remote marine boundary layer and the free troposphere could not be explained by known photochemistry as implemented in various atmospheric chemistry models, which consistently underestimated the measurements by an order of magnitude or more (Singh et al, 2003;Read et al, 2012;Wang et al, 2019). Several potential additional acetaldehyde sources have been proposed including new hydrocarbon oxidation mechanisms, aerosol related sources and oceanic sources.…”

“…One possible source of acetaldehyde in the remote marine boundary layer is oceanic emission from the photo degradation of colored dissolved organic matter (CDOM) in sea-surface water, where acetaldehyde could be produced together with other low molecular weight carbonyl compounds (Kieber et al, 1990;Zhou and Mopper, 1997;Sinha et al, 2007;Dixon et al, 2013). Nevertheless, due to both limited airborne and seawater measurements of acetaldehyde, the importance of oceanic emission is still under debate (Millet et al, 2010;Wang et al, 2019). In order to better understand the atmospheric budgets of acetaldehyde (and the other carbonyl compounds), it is informative to analyze a dataset of multiple carbonyl compounds in both polluted and clean environments, with influence from marine emissions, varying particulate loadings, and high rates of oxidation as shown in Figure 1, which demonstrates the main formation pathways of acetaldehyde during this campaign.…”