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Elevated levels of formaldehyde (HCHO) along the ship corridors have beenobserved by satellite sensors, such as ESA/ERS-2 GOME (Global OzoneMonitoring Experiment), and were also simulated by global 3-Dchemistry-transport models. In this study, three likely sources of theelevated HCHO levels in the ship plumes as well as their contributions tothe elevated HCHO levels (budget) were investigated using a newly-developedship-plume photochemical/dynamic model: (1) primary HCHO emission fromships; (2) secondary HCHO production via the atmospheric oxidation ofnon-methane volatile organic compounds (NMVOCs) emitted from ships; and (3)atmospheric oxidation of CH4 within the ship plumes. For thisship-plume modelling study, the ITCT 2K2 (Intercontinental Transport andChemical Transformation 2002) ship-plume experiment, which was carried outabout 100 km off the coast of California on 8 May 2002 (11:00 localstandard time), was chosen as a base study case because it is the bestdefined in terms of (1) meteorological data, (2) in-plume chemicalcomposition, and (3) background chemical composition. From multipleship-plume model simulations for the ITCT 2K2 ship-plume experiment case,CH4 oxidation by elevated levels of in-plume OH radicals was found tobe the main factor responsible for the elevated levels of HCHO in the ITCT2K2 ship-plume. More than ~88% of the HCHO for the ITCT 2K2ship-plume is produced by this atmospheric chemical process, except in theareas close to the ship stacks where the main source of the elevated HCHOlevels would be primary HCHO from the ships (due to the deactivation ofCH4 oxidation from the depletion of in-plume OH radicals). Because ofactive CH4 oxidation by OH radicals, the instantaneous chemicallifetime of CH4 (τCH4) decreased to ~0.45 yr inside the shipplume, which is in contrast to τCH4 of ~1.1 yr in the background(up to ~41% decrease) for the ITCT 2K2 ship-plume case. A varietyof likely ship-plume situations at three different latitudinal locationswithin the global ship corridors was also studied to determine theenhancements in the HCHO levels in the marine boundary layer (MBL)influenced by ship emissions. It was found that the ship-plume HCHO levelscould be 19.9–424.9 pptv higher than the background HCHO levels depending onthe latitudinal locations of the ship plumes (i.e., intensity of solarradiation and temperature), MBL stability and NOx emission rates. Onthe other hand, NMVOC emissions from ships were not found to be a primarysource of photochemical HCHO production inside ship plumes due to theirrapid and individual dilution. However, the diluted NMVOCs would contributeto the HCHO productions in the background air

Source identification and budget analysis on elevated levels of formaldehyde within the ship plumes: a ship-plume photochemical/dynamic model analysis