MARPOL Annex VI sets limits on NOx and SOx emissions from ship exhausts, and prohibits deliberate emissions of ozone depleting substances:
https://www.dieselnet.com/standards/inter/imo.php
Here is a quick and easy guide to Tire II & III SOX and NOX limitations:
https://www.linkedin.com/pulse/imo-tier-iii-2016-limitations-nox-sox-opti̇ma-marine-ltd-/