You're ignoring the possibility that cars may emit only tiny amounts of sulphur and nitrogen oxides, compared to ships whose emissions are largely made up of th

You're ignoring the possibility that cars may emit only tiny amounts of sulphur and nitrogen oxides, compared to ships whose emissions are largely made up of those gases. That would be consistent with both sentences, although I don't know if it's true. EDIT: Looks like that is indeed true, see this thread: https://news.ycombinator.com/item?id=10716251 https://news.ycombinator.com/item?id=10716251