Shipborne measurements of total OH reactivity around the Arabian Peninsula and its role in ozone chemistry
Authors: Pfannerstill Eva Y. , Wang Nijing , Edtbauer Achim , Bourtsoukidis Efstratios , Crowley John N. , Dienhart Dirk , Eger Philipp G. , Ernle Lisa , Fischer Horst , Hottmann Bettina , Paris Jean-Daniel , Stönner Christof , Tadic Ivan , Walter David , Lelieveld Jos , Williams Jonathan 
Published: September 2019

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.