Question: what are nox and where do they come from

title: NOx
text: NO emissions also cause global cooling through the formation of OH radicals that destroy methane molecules , countering the effect of greenhouse gases . The effect can be significant . For instance , according to the OECD `` the large NO emissions from ship traffic lead to significant increases in hydroxyl ( OH ) , which is the major oxidant in the lower atmosphere . Since reaction with OH is a major way of removing methane from the atmosphere , ship emissions decrease methane concentrations . ( Reductions in methane lifetimes due to shipping - based NO emissions vary between 1.5 % and 5 % in different calculations ) . '' `` In summary , most studies so far indicate that ship emissions actually lead to a net global cooling . However , it should be stressed that the uncertainties with this conclusion are large , in particular for indirect effects , and global temperature is only a first measure of the extent of climate change in any event . ''