Abstract. Shipborne measurements of nitryl chloride (ClNO2),
hydrogen chloride (HCl) and sulfur dioxide (SO2) were made during the
AQABA (Air Quality and climate change in the Arabian BAsin) ship campaign in
summer 2017. The dataset includes measurements over the Mediterranean Sea,
the Suez Canal, the Red Sea, the Gulf of Aden, the Arabian Sea, the Gulf of
Oman, and the Arabian Gulf (also known as Persian Gulf) with observed
ClNO2 mixing ratios ranging from the limit of detection to ≈600 pptv. We examined the regional variability in the generation of
ClNO2 via the uptake of dinitrogen pentoxide (N2O5) to
Cl-containing aerosol and its importance for Cl atom generation in a marine
boundary layer under the (variable) influence of emissions from shipping and
the oil industry. The yield of ClNO2 formation per NO3 radical
generated was generally low (median of ≈1 %–5 % depending on the
region), mainly as a result of gas-phase loss of NO3 dominating over
heterogeneous loss of N2O5, the latter being disfavoured by the
high temperatures found throughout the campaign. The contributions of
ClNO2 photolysis and OH-induced HCl oxidation to Cl-radical formation
were derived and their relative contributions over the diel cycle compared.
The results indicate that over the northern Red Sea, the Gulf of Suez, and
the Gulf of Oman the formation of Cl atoms will enhance the oxidation rates
of some volatile organic compounds (VOCs), especially in the early morning.