MARPOL Annex VI establishes the Regulations for the Prevention of Air Pollution from Ships, including the requirements for control of emissions from ships. Reg. 13 deals with Nitrogen Oxides (NOx) emission from shipboard marine diesel engines.
Engine International Air pollution Prevention Certificate - EIAPP Certificate
Parent Engine Certification (certification of the first engine of a series of identical engines)
Member Engine Certification (certification of engines whose parent engine has been already certified)
Details are found in NOx Technical Code.