The reactive nitrogen budget throughout the diel cycle investigated via thermal dissociation cavity ring-down spectroscopy

Total reactive nitrogen (NOy) denominates a group of atmospheric nitrogen species which consists of primary emitted NOx (= NO + NO2) and of the products of NOx sequestering (NOz). The processing of NOx into NOz during the ageing process of an air mass is responsible for the transport of harmful NOx into regions of the planet where anthropogenic emissions are sparse. The reactive nitrogen budget is undergoing constant changes due to increased efforts in mitigating NOx emissions. NOy species contribute strongly to the production and loss of HOx radicals, and therefore to the oxidation capacity of the atmosphere. This work emphasises on a) the development and laboratory characterisation of a new instrument capable of following the partitioning between NOx and NOz, b) the reactive nitrogen budget in the Mediterranean Sea, the Red Sea, and the Arabian Gulf through measurements from a ship campaign, and c) chamber study results on the efficiency of organic nitrate formation following the nighttime isoprene oxidation by the nitrate radical NO3. An instrument was developed which detects NOx and NOy at 40 pptv detection limits (2σ confidence and 1 minute averaging). Complete thermal dissociation to NOx was demonstrated for all relevant NOz species. The NOx detection was validated against a chemiluminescence detector under demanding and versatile field conditions. Memory effects were found to bias measurements of particulate nitrates when the activated carbon denuder is exposed to water, with potential strong implications for other instruments employing this method. Median NOz mixing ratios of up to 1.5 ppbv were observed in the Arabian Gulf, originating from high temperatures, strong photochemical activities, and the unique atmospheric mixture involving hydrocarbons related to petroleum production. An assessment of the NOx source strengths under background conditions revealed the photolysis of nitrous acid (HONO) as an important source, presumably associated with the processing of ship plume black carbon to nitrate particles. In all regions of the AQABA campaign, shifts in the NOy composition were successfully explained by varying air mass origins, using the corresponding back trajectories. The ratio between NOz and NOy mixing ratios is overall a valuable tracer for the processing state of an air parcel. Investigations of the nighttime isoprene oxidation in a chamber study resulted in organic nitrate yields (relative to the lost isoprene) between 77 % and 109 %. These numbers are in good agreement with previous publications and highlight the key role of NO3 in the oxidation of biogenic volatile organic compounds (BVOCs), leading ultimately to the formation of secondary organic aerosol