The temporal development of regulations for the maximum allowed emission of nitrogen oxides (NO_X). The shipping regulations (Tiers I, II and III) cover a range of allowed emissions depending on the engine’s rated speed in rpm (IMO, 2008a), and are therefore shown as vertical lines in the figure. The Tier I and Tier II regulations apply to ships built from 2000 and 2011, respectively. The Tier III regulations apply only in Nitrogen Emission Control Areas for ships built from 2016. The regulations for land transport refer to heavy vehicles in the European Union (EU, 1991, 2000, 2009). DOI: https://doi.org/10.1525/elementa.167.f3