Dispersion and chemical evolution of ship plumes in the marine boundary layer: Investigation of O₃/NO_<i>y</i>/HO_<i>x</i> chemistry

Abstract

The dispersion and chemical evolution of NO x in ship plumes has been investigated for marine boundary layer (MBL) conditions. This effort has involved combining a plume dispersion algorithm with a time‐dependent photochemical box model. The analysis has considered several factors, all of which can influence the net impact of NO x on the background environment. These include the following: season of the year, latitude of point of release, meteorological setting, and ship NO x emission rate. Reaction rates within a plume were shown to be a nonlinear function of the levels of NO x , leading to relative estimates of ship plume NO x lifetimes that were factors of 2.5–10 times shorter than for ambient marine conditions. The shortened ship‐plume NO x lifetime reflects both elevated daytime levels of OH and nighttime levels of NO 3 and N 2 O 5 , all of which were estimated to be several times larger than those typical of ambient marine conditions. During daylight hours, elevated ship plume OH resulted in the net photochemical production of O 3 , with peak concentrations being 5–65% higher than background values, depending on latitude. The areal integrated O 3 effect, however, is estimated to be quite small due to further plume dilution. In addition, because of the shorter estimated lifetime for NO x , it would seem reasonable that the integrated O 3 production from the current Lagrangian modeling effort would be significantly lower than that predicted by global 3‐D grid models. The current predicted shortened lifetime for NO x is quite significant in terms of assessing a ship plume's impact on background marine levels of NO x . In fact, these results would seem to explain a significant fraction of the overprediction of NO x levels in and near shipping lanes recently estimated using 3‐D Eulerian global models.