Abstract
Abstract. Elevated levels of formaldehyde (HCHO) along the ship corridors have been observed by satellite sensors, such as ESA/ERS-2 GOME (Global Ozone Monitoring Experiment), and were also simulated by global 3-D chemistry-transport models. In this study, three likely sources of the elevated HCHO levels in the ship plumes as well as their contributions to the elevated HCHO levels (budget) were investigated using a newly-developed ship-plume photochemical/dynamic model: (1) primary HCHO emission from ships; (2) secondary HCHO production via the atmospheric oxidation of non-methane volatile organic compounds (NMVOCs) emitted from ships; and (3) atmospheric oxidation of CH4 within the ship plumes. For this ship-plume modelling study, the ITCT 2K2 (Intercontinental Transport and Chemical Transformation 2002) ship-plume experiment, which was carried out about 100 km off the coast of California on 8 May 2002 (11:00 local standard time), was chosen as a base study case because it is the best defined in terms of (1) meteorological data, (2) in-plume chemical composition, and (3) background chemical composition. From multiple ship-plume model simulations for the ITCT 2K2 ship-plume experiment case, CH4 oxidation by elevated levels of in-plume OH radicals was found to be the main factor responsible for the elevated levels of HCHO in the ITCT 2K2 ship-plume. More than ~88% of the HCHO for the ITCT 2K2 ship-plume is produced by this atmospheric chemical process, except in the areas close to the ship stacks where the main source of the elevated HCHO levels would be primary HCHO from the ships (due to the deactivation of CH4 oxidation from the depletion of in-plume OH radicals). Because of active CH4 oxidation by OH radicals, the instantaneous chemical lifetime of CH4 (τCH4) decreased to ~0.45 yr inside the ship plume, 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 variety of likely ship-plume situations at three different latitudinal locations within the global ship corridors was also studied to determine the enhancements in the HCHO levels in the marine boundary layer (MBL) influenced by ship emissions. It was found that the ship-plume HCHO levels could be 19.9–424.9 pptv higher than the background HCHO levels depending on the latitudinal locations of the ship plumes (i.e., intensity of solar radiation and temperature), MBL stability and NOx emission rates. On the other hand, NMVOC emissions from ships were not found to be a primary source of photochemical HCHO production inside ship plumes due to their rapid and individual dilution. However, the diluted NMVOCs would contribute to the HCHO productions in the background air.
Highlights
Ship emissions have attracted increasing attention because it is becoming obvious that ship-emitted NOx, SO2 and particles can perturb the atmospheric photochemical cycles and global radiation budget significantly in the marinePublished by Copernicus Publications on behalf of the European Geosciences Union.C
Elevated levels of HCHO in heavy shiptraffic corridors were detected by the ESA/ERS-2 GOME (Global Ozone Monitoring Experiment) sensor (Marbach et al, 2009)
Enhanced levels of formaldehyde (HCHO) along global ship corridors were observed from satellite sensors and were simulated using global 3-D chemistry-transport modelling (CTM) in several previous studies (e.g., Hoor et al, 2009; Marbach et al, 2009)
Summary
Ship emissions have attracted increasing attention because it is becoming obvious that ship-emitted NOx, SO2 and particles can perturb the atmospheric photochemical cycles and global radiation budget significantly in the marine. According to Song et al (2003a) and von Glasow et al (2003), by-passing the nonlinear ship-plume photochemistry can result in overestimated O3, OH and NOx mixing ratios in the 3-D grid-based CTM simulations In this sense, the rates of CH4 destruction calculated by Hoor et al (2009) could be overestimated. There have been some discussions of the HCHO levels (and enhancements) related to the uncertainty in tropospheric HCHO column retrieval from satellites over heavy ship traffic corridors (e.g., Marbach et al, 2009) In this numerical analysis of ship-plume photochemistry, attempts were made to investigate two scientific issues: (1) what is the main (or dominant) atmospheric HCHO generation process, and (2) how much can HCHO be generated or the HCHO mixing ratios be elevated above the background HCHO levels by the atmospheric HCHO generation processes within the ship plumes?. The photochemical modelling mode for the ship-plumes was used
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