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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 photochem-
ical activity, especially during the summer months. However,
air chemistry measurements in the region are scarce. In or-
der to assess regional pollution sources and oxidation rates,
the first ship-based direct measurements of total OH reac-
tivity were performed in summer 2017 from a vessel trav-
eling around the peninsula during the AQABA (Air Qual-
ity and Climate Change in the Arabian Basin) campaign.
Total OH reactivity is the total loss frequency of OH rad-
icals due to all reactive compounds present in air and de-
fines 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 maxi-
mum of 32
.
8
±
9
.
6 s
−
1
over the Arabian Gulf (also known as
Persian Gulf) when air originated from large petroleum ex-
traction/processing facilities in Iraq and Kuwait. In the pol-
luted marine regions, OH reactivity was broadly compara-
ble to highly populated urban centers in intensity and com-
position. 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 ex-
traction and processing industry were an important OH sink
with
∼
9 % of total OH reactivity each, whereas NO
x
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 ac-
counted for within the measurement uncertainty (50 %), with
10 dominant trace gases accounting for 20 %–39 % of re-
gional 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 NO
x
- and VOC-attributed OH reactivity was fa-
vorable for ozone formation almost all around the Arabian
Peninsula, which is due to NO
x
and VOCs from ship ex-
hausts and, often, oil/gas production. Therewith, total OH
reactivity measurements help to elucidate the chemical pro-
cesses underlying the extreme tropospheric ozone concentra-
tions observed in summer over the Arabian Basin.
