Abstract
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.
Highlights
The AQABA (Air Quality and climate change in the Arabian BAsin) campaign was designed to study air quality and climate in a region that is likely to be heavily impacted by future climate change with increasing frequency and intensity of droughts, heatwaves, and associated Aeolian dust and pollution emissions (Lelieveld et al, 2012)
Along the ship track we observed a large variability in ClNO2 mixing ratios with nocturnal maxima ranging from below the detection limit over the Arabian Sea to a few hundred parts per trillion by volume over the Gulf of Oman, the northern part of the Red Sea, the Gulf of Suez, and the Mediterranean Sea close to Sicily, Italy, with a campaign maximum of ≈ 600 pptv observed over the Gulf of Suez
The overall ClNO2 production efficiency, i.e. the yield of ClNO2 per NO3 molecule formed in the reaction of NO2 with O3, was generally low and highly variable within individual nights and between different regions with values of 2.9, 2.7, 2.1, 4.7, 4.7, 2.0, and 0.8 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, respectively
Summary
The AQABA (Air Quality and climate change in the Arabian BAsin) campaign was designed to study air quality and climate in a region (eastern Mediterranean and Middle East) that is likely to be heavily impacted by future climate change with increasing frequency and intensity of droughts, heatwaves, and associated Aeolian dust and pollution emissions (Lelieveld et al, 2012). As the Arabian Gulf already suffers from some of the most polluted air on Earth with O3 levels regularly greater than 100 ppbv (Lelieveld et al, 2009), one aspect of the campaign was to investigate the factors that contribute to high levels of air pollution in the region. This includes the impact of reactive chlorine chemistry resulting from the interactions of pollutant emissions from ships and petrochemical activity with sea salt, under conditions influenced by intense photochemistry and high temperatures during summer. ClNO2 has a lifetime of more than 30 h in the nocturnal marine boundary layer (Osthoff et al, 2008) but is rapidly photolysed after sunrise
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