Influence of NOx emissions from ships on tropospheric photochemistry and climate

Emissions of nitrogen oxides (NOx, the sum of NO and NO2) from fossil-fuel burning dominate the NOx burden of the lower troposphere in many regions1. These emissions increase tropospheric ozone and hydroxyl-radical concentrations over their natural ‘background’ levels, thereby increasing the oxidizing power of the atmosphere2. Fossil-fuel emissions of NOx (refs 3, 4) account for about half of the global NOx source to the atmosphere; other significant sources are from biomass burning5, soil emissions6, aircraft exhausts7 and lightning8, all primarily continental. However, ocean-going ships burning fossil fuels may also contribute a significant fraction (>10%) to global NOx production9. Here we use NOx emission data and a high-resolution chemistry–transport model to estimate that ship NOx emissions result in a more than 100-fold increase in surface NOx concentrations in heavily traversed ocean regions. This enhancement has a notable effect on modelled surface ozone and hydroxyl-radical concentrations. In particular, a predicted fivefold increase in the July hydroxyl-radical burden over the northern Atlantic and Pacific oceans would be expected to reduce the atmospheric lifetimes of reactive greenhouse gases—such as methane—as well as to increase aerosol production rates and cloud reflectivities, therefore exerting a cooling influence on the climate.