Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 2 3 2002 10.5194/acpd-2-525-2002 http://www.atmos-chem-phys-discuss.net/2/525/2002/ http://www.atmos-chem-phys-discuss.net/2/525/2002/acpd-2-525-2002.html http://www.atmos-chem-phys-discuss.net/2/525/2002/acpd-2-525-2002.pdf 525 575 2002-06-07 Modeling the chemical effects of ship exhaust in the cloud-free marine boundary layer R. von Glasow M. G. Lawrence R. Sander P. J. Crutzen Max-Planck-Institut für Chemie, Atmospheric Chemistry Division, PO Box 3060, 55020 Mainz, Germany Center for Atmospheric Sciences, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093-0221, USA now at Scripps The chemical evolution of the exhaust plumes of ocean-going ships in the cloud-free marine boundary layer is examined with a box model. Dilution of the ship plume via entrainment of background air was treated as in studies of aircraft emissions and was found to be a very important process that significantly alters model results. We estimated the chemical lifetime (defined as the time when differences between plume and background air are reduced to 5% or less) of the exhaust plume of a single ship to be 2 days. Increased concentrations of NO_x in the plume air lead to higher catalytical photochemical production rates of O₃ and also of OH. Due to increased OH concentrations in the plume, the lifetime of many species (especially NO_x) is reduced in plume air. The chemistry on background aerosols has a significant effect on gas phase chemistry in the ship plume, while partly soluble ship-produced aerosols are computed to only have a very small effect. Soot particles emitted by ships showed no effect on gas phase chemistry. Halogen species that are released from sea salt aerosols are slightly increased in plume air. In the early plume stages, however, the mixing ratio of BrO is decreased because it reacts rapidly with NO. To study the global effects of ship emissions we used a simple upscaling approach which suggested that the parameterization of ship emissions in global chemistry models as a constant source at the sea surface leads to an overestimation of the effects of ship emissions of roughly a factor of 2. The differences are caused by a strongly reduced lifetime (compared to background air) of NO_x in the early stages of a ship plume.