Reactive nitrogen around the Arabian Peninsula and in the Mediterranean Sea during the 2017 AQABA ship campaign.

We present shipborne measurements of NO x (≡ NO + NO 2) and NO y (≡ NO x+ gas- and particle-phase organic and inorganic oxides of nitrogen) in summer 2017 as part of the expedition "Air Quality and climate change in the Arabian BAsin" (AQABA). The NO x and NO z (≡ NO y -NO x) measurements, made with a thermal dissociation cavity ring-down spectrometer (TD-CRDS), were used to examine the chemical mechanisms involved in the processing of primary NO x emissions and their influence on the NO y budget in chemically distinct marine environments, including the Mediterranean Sea, the Red Sea, and the Arabian Gulf, which were influenced to varying extents by emissions from shipping and oil and gas production. Complementing the TD-CRDS measurements, NO and NO 2 data sets from a chemiluminescence detector (CLD) were used in the analysis. In all regions, we find that NO x is strongly connected to ship emissions, both via direct emission of NO and via the formation of HONO and its subsequent photolytic conversion to NO. The role of HONO was assessed by calculating the NO x production rate from its photolysis. Mean NO 2 lifetimes were 3.9 h in the Mediterranean Sea, 4.0 h in the Arabian Gulf, and 5.0 h in the Red Sea area. The cumulative loss of NO 2 during the night (reaction with O 3) was more important than daytime losses (reaction with OH) over the Arabian Gulf (by a factor 2.8) and over the Red Sea (factor 2.9), whereas over the Mediterranean Sea, where OH levels were high, daytime losses dominated (factor 2.5). Regional ozone production efficiencies (OPEs; calculated from the correlation between O x and NO z , where O x= O 3+ NO 2) ranged from 10.5 ± 0.9 to 19.1 ± 1.1. This metric quantifies the relative strength of photochemical O 3 production from NO x compared to the competing sequestering into NO z species. The largest values were found over the Arabian Gulf, consistent with high levels of O 3 found in that region (10–90 percentiles range: 23–108 ppbv). The fractional contribution of individual NO z species to NO y exhibited a large regional variability, with HNO 3 generally the dominant component (on average 33 % of NO y) with significant contributions from organic nitrates (11 %) and particulate nitrates in the PM 1 size range (8 %). [ABSTRACT FROM AUTHOR]