The climate impact of ship NO_x emissions: an improved estimate accounting for plume chemistry

Abstract. Nitrogen oxide (NO_x) 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 NO_x on climate. Previous estimates of the chemical impact and radiative forcing (RF) of ship NO_x have 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 O₃ production and CH₄ loss both within and outside the exhaust plumes and also accounts for the effect of wind speed. With the improved modeling of plumes, ship NO_x perturbations are smaller than suggested by the ensemble of past global modeling studies, but if we assume instant dilution of ship NO_x on the grid scale, the CTM reproduces previous model results. Our best estimates of the RF components from increasing ship NO_x emissions by 1 Tg(N) yr⁻¹ are smaller than that given in the past literature: + 3.4 ± 0.85 mW m⁻² (1σ confidence interval) from the short-lived ozone increase, −5.7 ± 1.3 mW m⁻² from the CH₄ decrease, and −1.7 ± 0.7 mW m⁻² from the long-lived O₃ decrease that accompanies the CH₄ change. The resulting net RF is −4.0 ± 2.0 mW m⁻² for emissions of 1 Tg(N) yr⁻¹. Due to non-linearity in O₃ production as a function of background NO_x, RF from large changes in ship NO_x emissions, 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%).