Influence of vessel characteristics and atmospheric processes on the gas and particle phase of ship emission plumes: In-situ measurements in the Mediterranean Sea and around the Arabian Peninsula

A total of 252 emission plumes of ships operating in the Mediterranean Sea and around the Arabian Peninsula were investigated using a comprehensive dataset of gas- and submicron-particle-phase properties measured during the 2month shipborne AQABA (Air Quality and Climate Change in the Arabian Basin) field campaign in summer 2017. The post-measurement identification of the corresponding ship emission events in the measured data included the determination of the plume sources (up to 38 km away) as well as the plume ages (up to 115 min) and was based on commercially available historical records of the Automatic Identification System. The dispersion lifetime of chemically inert CO2 in the ship emission plumes was determined as 70 x 15 min, resulting in levels indistinguishable from the marine background after 260 x 60 min. Emission factors (EFs) as quantities that are independent of plume dilution were calculated and used for the investigation of influences on ship emission plumes caused by ship characteristics and the combustion process as well as by atmospheric processes during the early stage of exhaust release and during plume ageing. Combustion efficiency and therefore emission factors of black carbon and NOx were identified to depend mostly on the vessel speed and gross tonnage. Moreover, larger ships, associated with higher engine power, were found to use fuel with higher sulfur content and have higher gas-phase SO2, particulate sulfate, particulate organics, and particulate matter EFs. Despite the independence of EFs of dilution, a significant influence of the ambient wind speed on the particle number and mass EFs was observed that can be traced back to enhanced particle coagulation in the case of slower dilution and suppressed vapour condensation on particles in the case of faster dilution of the emission plume. Atmospheric reactions and processes in ship emission plumes were investigated that include NOx and O-3 chemistry, gas-to-particle conversion of NOx and SO2, and the neutralisation of acids in the particle phase through the uptake of ambient gas-phase ammonia, the latter two of which cause the inorganic particulate content to increase and the organic fraction to decrease with increasing plume age. The results allow for us to describe the influences on (or processes in) ship emission plumes quantitatively by parameterisations, which could be used for further refinement of atmospheric models, and to identify which of these processes are the most important ones.