Modeling the chemical effects of ship exhaust in the cloud-free marine boundary layer

[摘要] 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 theship plume via entrainment of background air was treated as in studies of aircraft emissions and was found to be a very important process thatsignificantly alters model results. We estimated the chemical lifetime (definedas the time when differences between plume and background air are reduced to5% or less) of the exhaust plume of a single ship to be 2 days. Increased concentrations ofNO_x (= NO + NO₂) in the plume air lead to higher catalyticalphotochemical production rates of O₃ and also of OH. Due to increasedOH concentrations in the plume, the lifetime of many species (especiallyNO_x) is significantly reduced in plume air. The chemistry on backgroundaerosols has a significant effect on gas phase chemistry in the ship plume,while partly soluble ship-produced aerosols are computed to only have a verysmall effect. Soot particles emitted by ships showed no effect on gas phasechemistry. Halogen species that are released from sea salt aerosols are slightly increased in plume air. In the early plume stages, however, the mixingratio of BrO is decreased because it reacts rapidly with NO.To study the global effects of ship emissions we used a simple upscaling approachwhich suggested that the parameterization of ship emissions in global chemistrymodels as a constant source at the sea surface leads to an overestimation ofthe effects of ship emissions on O₃ of about 50% and on OH of roughly a factor of 2. The differences aremainly caused by a strongly reduced lifetime (compared to background air) ofNO_xin the early stages of a ship plume.

[学科分类] 大气科学