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The climate impact of ship NOxemissions: An improved estimate accounting for plume chemistry

  • Author(s): Holmes, CD
  • Prather, MJ
  • Vinken, GCM
  • et al.

Nitrogen oxide (NOx) emissions from maritime shipping produce ozone (O3) and hydroxyl radicals (OH), which in turn destroy methane (CH4). The balance between this warming (due to O3) and cooling (due to CH4) determines the net effect of ship NOxon climate. Previous estimates of the chemical impact and radiative forcing (RF) of ship NOxhave generally assumed that plumes of ship exhaust are instantly diluted into model grid cells spanning hundreds of kilometers, even though this is known to produce biased results. Here we improve the parametric representation of exhaust-gas chemistry developed in the GEOS-Chem chemical transport model (CTM) to provide the first estimate of RF from shipping that accounts for sub-grid-scale ship plume chemistry. The CTM now calculates O3production and CH4loss both within and outside the exhaust plumes and also accounts for the effect of wind speed.With the improved modeling of plumes, ship NOxperturbations are smaller than suggested by the ensemble of past global modeling studies, but if we assume instant dilution of ship NOxon the grid scale, the CTM reproduces previous model results. Our best estimates of the RF components from increasing ship NOxemissions by 1 Tg(N) yr-1are smaller than that given in the past literature: +3.4±0.85mWm-2(1σ confidence interval) from the short-lived ozone increase, -5.7±1.3mWm-2 from the CH4decrease, and -1.7±0.7mWm-2 from the long-lived O3decrease that accompanies the CH4change. The resulting net RF is -4.0±2.0mWm-2 for emissions of 1 Tg(N) yr-1. Due to non-linearity in O3production as a function of background NOx, RF from large changes in ship NOxemissions, such as the increase since preindustrial times, is about 20% larger than this RF value for small marginal emission changes. Using sensitivity tests in one CTM, we quantify sources of uncertainty in the RF components and causes of the ±30% spread in past model results; the main source of uncertainty is the composition of the background atmosphere in the CTM, which is driven by model formulation (±10 to 20 %) and the plausible range of anthropogenic emissions (±10 %). © 2014 Author(s).

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