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 regions'. These emissions increase tropospheric ozone and hydroxyl-radical concentrations over their natural 'background' levels, thereby increasing the oxidizing power of the atmosphere(2). 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 burning(5), soil emissions(6), aircraft exhausts' and lightning(8), all primarily continental. However, ocean-going ships burning fossil fuels may also contribute a significant fraction (>10%) to global NOx production(9). 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.