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

摘要

Nitrogen oxide (NO) emissions from maritime shipping produceozone (O) and hydroxyl radicals (OH), which in turn destroy methane(CH). The balance between this warming (due to O) and cooling(due to CH) determines the net effect of ship NO onclimate. Previous estimates of the chemical impact and radiative forcing (RF)of ship NO have generally assumed that plumes of ship exhaustare instantly diluted into model grid cells spanning hundreds of kilometers,even though this is known to produce biased results. Here we improve theparametric representation of exhaust-gas chemistry developed in the GEOS-Chemchemical transport model (CTM) to provide the first estimate of RF fromshipping that accounts for sub-grid-scale ship plume chemistry. The CTM nowcalculates O production and CH loss both within and outside theexhaust plumes and also accounts for the effect of wind speed. With theimproved modeling of plumes, ship NO perturbations are smallerthan suggested by the ensemble of past global modeling studies, but if weassume instant dilution of ship NO on the grid scale, the CTMreproduces previous model results. Our best estimates of the RF componentsfrom increasing ship NO emissions by 1 Tg(N) yr aresmaller than that given in the past literature: + 3.4 ±0.85 mW m (1σ confidence interval) from the short-livedozone increase, −5.7 ± 1.3 mW m from the CH decrease, and−1.7 ± 0.7 mW m from the long-lived O decrease thataccompanies the CH change. The resulting net RF is −4.0 ±2.0 mW m for emissions of 1 Tg(N) yr. Due to non-linearityin O production as a function of background NO, RF fromlarge changes in ship NO emissions, such as the increase sincepreindustrial times, is about 20% larger than this RF value for smallmarginal emission changes. Using sensitivity tests in one CTM, we quantifysources of uncertainty in the RF components and causes of the ±30%spread in past model results; the main source of uncertainty is thecomposition of the background atmosphere in the CTM, which is driven by modelformulation (±10 to 20%) and the plausible range of anthropogenicemissions (±10%).