Around 15 per cent of global anthropogenic NOx and 5–8 per cent of global SO2 emissions are attributable to ocean-going ships. Because 70 per cent of ship emissions are estimated to occur within 400 km of land, ships have the potential to contribute significantly to air quality degradation in coastal areas. Despite this, international ship emissions are not well quantified and are one of the least regulated sources of air pollution.
A recent in-depth literature review with the aim of quantifying the impacts of shipping emissions on urban air quality in coastal areas in Europe, concluded among other things that in European coastal areas, shipping emissions contribute 1–7 per cent of PM10 levels, 1–14 per cent of PM2.5, and at least 11 per cent of PM1 in ambient air. Contributions from shipping to ambient NO2 levels range between 7 and 24 per cent, with the highest values being recorded in the Netherlands and Denmark.
Impact of maritime transport emissions on coastal air quality in Europe (March 2014). By Mar Viana et. al. Published in Atmospheric Environment.