Abstract
Abstract. 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 NOx on climate. Previous estimates of the chemical impact and radiative forcing (RF) of ship NOx 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 NOx perturbations are smaller than suggested by the ensemble of past global modeling studies, but if we assume instant dilution of ship NOx on the grid scale, the CTM reproduces previous model results. Our best estimates of the RF components from increasing ship NOx 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 NOx, RF from large changes in ship NOx 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%).
Highlights
Maritime shipping affects climate through emissions of CO2, nitrogen oxides (NOx ≡ NO + NO2), and SO2, the latter two of which indirectly influence methane, ozone, aerosols, and clouds
Two of these estimates lie within the cluster of literature values and are based on chemical transport model (CTM) that assume instant dilution, while the outlying third estimate based on an earlier version of GEOS-Chem with fixed Ozone Production Efficiency (OPE) demonstrates that plume chemistry significantly influences the climate impact of ships
The non-linear chemistry governing O3 and OH production in emission plumes has been recognized for decades. In spite of this knowledge, global modeling studies of ship Nitrogen oxide (NOx) emissions and their impacts on climate and air quality are usually made under the assumption that emissions are instantly diluted into large grid volumes, which overestimates production of tropospheric O3 and OH
Summary
Maritime shipping affects climate through emissions of CO2, nitrogen oxides (NOx ≡ NO + NO2), and SO2, the latter two of which indirectly influence methane, ozone, aerosols, and clouds. While the non-linear nature of NOx–HOx– O3 chemistry in plumes is well known (e.g., Lin et al, 1988) and numerical techniques have been developed for modeling sub-grid-scale plumes from other sources (e.g., Paoli et al, 2011; Sillman et al, 1990), many global CTMs have continued to use the instant dilution assumption for ship NOx while acknowledging its deficiency. As a result, these models overestimate O3 and OH production by ships and generate biased impacts on climate and air quality. We identify major sources of uncertainty in ship NOx RF using similar methods to our earlier work on aviation NOx (Holmes et al, 2011): by decomposing the RF into factors that can be assessed individually and by reproducing the spread of past results in a single model
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