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 concentrat ions over their natural 'background' levels, thereby increasing the oxidizi ng power of the atmosphere(2). Fossil-fuel emissions of NOx (refs 3, 4) acc ount for about half of the global NOx source to the atmosphere; other signi ficant sources are from biomass burning(5), soil emissions(6), aircraft exh austs' and lightning(8), all primarily continental. However, ocean-going sh ips burning fossil fuels may also contribute a significant fraction (>10%) to global NOx production(9). Here we use NOx emission data and a high-resol ution chemistry-transport model to estimate that ship NOx emissions result in a more than 100-fold increase in surface NOx concentrations in heavily t raversed ocean regions. This enhancement has a notable effect on modelled s urface ozone and hydroxyl-radical concentrations. In particular, a predicte d fivefold increase in the July hydroxyl-radical burden over the northern A tlantic and Pacific oceans would be expected to reduce the atmospheric life times of reactive greenhouse gases-such as methane-as well as to increase a erosol production rates and cloud reflectivities, therefore exerting a cool ing influence on the climate.